Audio Decoder, Audio Encoder, Method for Decoding an Audio Signal, Method for
Encoding an Audio Signal, Computer Program and Audio Signal
Description
Background of the Invention
Embodiments according to the invention are related to an audio decoder, an audio encoder,
a method for decoding an audio signal, a method for encoding an audio signal and a
corresponding computer program. Some embodiments are related to an audio signal.
Some embodiments according to the invention are related to an audio encoding/decoding
concept, in which a side information is used for resetting a context of an entropy
encoding/decoding.
Some embodiments are related to the control of the reset of an arithmetic coder.
Traditional audio coding concepts include an entropy coding scheme (for example for
encoding spectral coefficients of a frequency domain signal representation) in order to
reduce redundancy. Typically, entropy coding is applied to quantized spectral coefficients
for frequency domain based coding schemes or quantized time domain samples for time
domain based coding schemes. These entropy coding schemes typically make use of
transmitting a code word in combination with an according code book index, which
enables a decoder to look up a certain code book page for decoding an encoded
information word corresponding to the transmitted code word on said code book page.
For details regarding such an audio coding concept, reference is made, for example, to
international standard ISO/IEC 14496-3:2005(E), part 3: audio, part 4: general audio
coding (GA)-AAC, Twin VQ, BSAC, in which the so called concept for "entropy/coding"
is described.
However, it has been found that a significant overhead in bitrate is produced by the need
for a regular transmission of a detailed code book selection information (e.g. sectcb).
Accordingly, it is an object of the present invention to create a bitrate-efficient concept for
adapting a mapping rule of an entropy decoding to the signal statistics.

Summary of the Invention
This objective is achieved by an audio decoder according to claim 1, an audio encoder
according to claim 12, a method for decoding an audio signal according to claim 11, a
method for encoding an audio signal according to claim 16, a computer program according
to claim 17 and an encoded audio signal according to claim 18.
An embodiment according to the invention creates an audio decoder for providing a
decoded audio information on the basis of an encoded audio information. The audio
decoder comprises a context-based entropy decoder configured to decode the entropy-
encoded audio information in dependence on a context, which context is based on a
previously decoded audio information in a non-reset state of operation. The entropy
decoder is configured to select a mapping information (e.g. a cumulative frequencies table,
or a Huffmann-codebook) for deriving the decoded audio information from the encoded
audio information in dependence on the context. In addition, the context-based entropy
decoder also comprises a context resetter configured to reset the context for selecting the
mapping information to a default context, which is independent from the previously
decoded audio information, in response to a side information of the encoded audio
information.
This embodiment is based on finding that in many cases it is bitrate-efficient to derive the
context, which determines the mapping of an entropy-encoded audio information onto a
decoded audio information (for example by examining a code book, or by determining a
probability distribution) in dependence on a context which is based on previously decoded
audio information items, as accordingly, correlations within the entropy-encoded audio
information can be exploited. For example, if a certain spectral bin comprises a large
intensity in the first audio frame, then there is a high probability that the same spectral bin
again comprises a large intensity in the next audio frame following the first audio frame.
Thus, it becomes apparent that a selection of the mapping information on the basis of the
context allows for a reduction of the bitrate when compared to a case in which a detailed
information for the selection of a mapping information for deriving the decoded audio
information from the encoded audio information is transmitted.
However, it has also been found that a derivation of the context from the previously
decoded audio information sometimes results in situations in which a mapping information
(for deriving the decoded audio information from the encoded audio information)' is
chosen, which is significantly inappropriate and thus results in an unnecessarily high bit

demand for encoding the audio information. This situation would occur, if for example, the
spectral energy distribution of subsequent audio frames differ significantly, such that new
spectral energy distribution within the subsequent audio frame deviates strongly from the
distribution which would be expected on the basis of a knowledge of the spectral
distribution within the previous audio frame.
According to a key idea of the invention, in such cases, in which the bitrate would be
significantly degraded by the choice of an inappropriate mapping information (for deriving
the decoded audio information from the encoded audio information), the context is reset in
response to a side information of the encoded audio information, thereby achieving the
selection of a default mapping information (being associated with the default context)
which in turn results in a moderate bit consumption for an encoding/decoding of the audio
information.
To summarize the above, it is the key idea of the present invention that a bitrate-efficient
encoding of an audio information can be achieved by combining a context-based entropy
decoder, which normally (in a non-reset state of operation) uses a previously encoded
audio information for deriving a context and for selecting a corresponding mapping
information, with a side-information-based reset mechanism for resetting the context,
because such a concept brings along a minimum effort for maintaining an appropriate
decoding context, which is well-adapted to the audio content in a normal case (when the
audio content fulfills the expectations used for the design of the context-based selection of
a mapping rule) and avoids an excessive increase of the bitrate in an abnormal case (when
the audio content strongly deviates from said expectations).
In a preferred embodiment, the context resetter is configured to selectively reset the
context-based entropy decoder at a transition between subsequent time portions (e.g. audio
frames) having associated spectral data of the same spectral resolution (e.g. number of
frequency bins). This embodiment is based on the finding that a reset of the context may
have advantageous effect (in terms of reducing the required bitrate) even if the spectral
resolution remains unchanged. In other words, it has been found that it should be possible
to perform a reset of the context independent from a change of the spectral resolution,
because it has been found that the context may be inappropriate even if it is not necessary
to change the spectral resolution (for example, by switching from a "long window" per
frame to a plurality of "short windows" per frame). In other words, it has been found that a
context may be inappropriate (which raises the desire to reset the context) even in a
situation in which it is not desirable to change from a low temporal resolution (e.g. long

window, in combination with high spectral resolution) to a high temporal resolution (e.g.
short windows, in combination with a small spectral resolution).
In a preferred embodiment, the audio decoder is configured to receive, as the encoded
audio information, an information describing spectral values in a first audio frame and in a
second audio frame subsequent to the first audio frame. In this case, the audio decoder
preferably comprises a spectral-domain-to-time-domain transformer configured to overlap-
and-add a first windowed time domain signal, which is based on the spectral values of the
first audio frame, and a second windowed time domain signal, which is based on the
spectral values of the second audio frame. The audio decoder is configured to separately
adjust a window shape of a window for obtaining the first windowed time domain signal
and of a window for obtaining the second windowed time domain signal. The audio
decoder is also preferably configured to perform, in response to the side information, a
reset of the context between a decoding of the spectral values of the first audio frame and a
decoding of the spectral values of the second audio frame, even if the second window
shape is identical to the first window shape, such that the context used for decoding the
encoded audio information of the second audio frame is independent of the decoded audio
information of the first audio frame in the case of a reset.
This embodiment allows for a reset of the context between a decoding (using mapping
information selected on the basis of the context) of spectral values of the first audio frame
and a decoding (using mapping information selected on the basis of the context) of spectral
values of the second audio frame, even if windowed time domain signals of the first and
second audio frames are overlapped-and-added, and even if identical window shapes are
selected for deriving the first windowed time domain signal and the second windowed time
domain signal from the spectral values of the first audio frame and the second audio frame.
Thus, the reset of the context may be introduced as an additional degree of freedom, which
can be applied by the context resetter even between a decoding of spectral values of
closely-related audio frames, the windowed time domain signals of which are derived
using identical window shapes and are overlapped-and-added.
Thus, it is preferred that the reset of context is independent from used window shapes and
also independent from the fact that windowed time domain signals of subsequent frames
belong to a contiguous audio content, i.e. are overlapped-and-added.
In a preferred embodiment, the entropy decoder is configured to reset, in response to side
information, the context between the decoding of audio information of adjacent frames of

the audio information having identical frequency resolutions. In this embodiment, a reset
of the context is performed independent from a change of the frequency resolution.
In yet another preferred embodiment, the audio decoder is configured to receive a context-
reset side information for signaling a reset of the context. In this case, the audio decoder is
also configured to additionally receive a window-shape side information to adjust the
window shapes of windows for obtaining the first and second windowed time signals
independent from performing the reset of the context.
In a preferred embodiment, the audio decoder is configured to receive, as the side
information for resetting the context, a one-bit context reset flag per audio frame of the
encoded audio information. In this case, the audio decoder is preferably configured to
receive, in addition to the context reset flag, a side information describing a spectral
resolution of spectral values represented by the encoded audio information or a window
length of a time window, for windowing time domain values represented by the encoded
audio information. The context resetter is configured to perform a reset of the context in
response to the one-bit context-reset-flag at a transition between two audio frames of the
encoded audio information representing spectral values of identical spectral resolutions. In
this case, the one-bit context reset-flag typically results in a single reset of the context
between a decoding of encoded audio information of subsequent audio frames.
In another preferred embodiment, the audio decoder is configured to receive, as a side
information for resetting the context, a one-bit context to reset-flag per audio frame of the
encoded audio information. Also, the audio decoder is configured to receive an encoded
audio information comprising of a plurality of sets of spectral values per audio frame (such
that a single audio frame is subdivided into multiple sub frames, to which individual short
windows may be associated). In this case, the context-based entropy decoder is configured
to decode the entropy-decoded audio information of a subsequent set of spectral values of a
given audio frame in dependence on a context, which context is based on a previously
decoded audio information of a preceding set of spectral values of the given audio frame in
a non-reset state of operation. However, the context resetter is configured to reset the
context to the default context before a decoding of a first set of spectral values of the given
audio frame and between a decoding of any two subsequent sets of spectral values of the
given audio frame in response to the one-bit context reset flag (i.e. if, and only if, the one-
bit context reset flag is active), such that an activation of the one-bit context reset flag of
the given audio frame causes a multiple-times resetting of the context when decoding the
multiple sets of spectral values of the audio frame.

This embodiment is based on the finding that in this typically inefficient, in terms of
bitrate, to perform only a single reset of the context in an audio frame comprising a
plurality of "short windows," for which individual sets of spectral values are encoded.
Rather an audio frame comprising multiple sets of spectral values typically comprises a
strong discontinuity of the audio content, such that it is advisable, in order to reduce the
bitrate, to reset the context between each of the subsequent sets of spectral values. Such a
solution has been found to be more efficient than a one-time reset of the context (for
example, only at the beginning of the frame) and than individually signaling (e.g. using
extra one-bit flags) multiple context reset times within the (multiple-short-window) frame.
In a preferred embodiment, the audio decoder is configured to also receive a grouping side
information when using so-called "short windows" (i.e. transmitting multiple sets of
spectral values, which are overlapped-and-added using multiple short windows being
shorter than an audio frame). In this case, the audio decoder is preferably configured' to
group two or more of the sets of spectral values for a combination with a common scale
factor information in dependence on the grouping side information. In this case, the
context-resetter is preferably configured to reset the context to the default context between
a decoding of sets of spectral values grouped together in response to the one-bit context
reset flag. This embodiment is based on the finding that, in some cases, there may be a
strong variation of the decoded audio values (e.g. decoded spectral values) of a grouped
sequence of sets of spectral values, even if the initial scale factors are applicable to the
subsequent sets of spectral values. For example, if there is a steady yet significant
frequency variation between subsequent sets of spectral values, the scale factors of the
subsequent sets of spectral values may be equal (for example, if the frequency variation
does not exceed a scale factor band), while it is nevertheless appropriate to reset the
context at the transition between the different sets of spectral values. Thus, the described
embodiment allows for a bitrate efficient encoding and decoding even in the presence of
such frequency-variation audio signal transitions. Also, this concept still allows for good
performance when encoding rapid volume changes in the presence of strongly correlated
spectral values. In this case, a reset of the context can be avoided by deactivating the
context-reset-flag, even though different scale factors may be associated with subsequent
set of spectral values (which are not grouped together in this case, because the scale factors
differ).
In another embodiment, the audio decoder is configured to receive, as the side information
for resetting the context, a one-bit context reset flag per audio frame of the encoded audio
information. In this case, the audio decoder is also configured to receive, as the encoded
audio information, a sequence of encoded audio frames, the sequence of encoded frames

comprising a linear-prediction-domain audio frame. The linear-prediction-domain audio
frame comprises, for example, a selectable number of transform-coded-excitation portions
for exciting a linear-prediction-domain audio synthesizer. The context-based entropy
decoder is configured to decode spectral values of the transform-coded-excitation portions
in dependence on a context, which context is based on a previously-decoded audio
information in a non-reset state of operation. The context resetter is configured to reset, in
response to the side information, the context to the default context before a decoding of a
set of spectral values of a first transform-coded-excitation portion of a given audio frame,
while omitting a reset of the context to the default context between a decoding of sets of
spectral values of different transform-coded-excitation portions of (i.e. within) the given
audio frame. This embodiment is based on the finding that a combination of a context-
based decoding and a context reset brings along a reduction of the bitrate when encoding a
transform-coded-excitation for a linear-prediction-domain audio synthesizer. In addition, it
has been found that a temporal granularity for resetting the context when encoding a
transform-coded-excitation typically can be chosen larger than a temporal granularity of
resetting the context in the presence of a transition (short windows) of a pure frequency
domain encoding (e.g. an Advanced-Audio-Coding-type audio coding).
In another preferred embodiment, the audio decoder is configured to receive an encoded
audio information comprising a plurality of sets of spectral values per audio frame. In this
case, the audio decoder is also preferably configured to receive a grouping side
information. The audio decoder is configured to group two or more of the sets of spectral
values for a combination with a common scale factor information in dependence on the
grouping side information. In the preferred embodiment, the context resetter is configured
to reset the context to the default context in response to (i.e. in dependence on) the
grouping side information. The context resetter is configured to reset the context between a
decoding of sets of spectral values of subsequent groups, and to avoid to reset the context
between a decoding of sets of spectral values of a single group (i.e. within a group). This
embodiment of the invention is based on the finding that it is not necessary to use a
dedicated context reset side information if there is a signaling of sets of spatial values
having high similarity (and being grouped together for this reason). In particular, it has
been found that there are many cases in which it is appropriate to reset the context
whenever the scale factor data change (for example at a transition from one set of spectral
values to another set of spectral values within a window, particularly if the sets of spectral
values are not grouped, or at a transition from one window to another window). If
however, it is desired to reset the context between two sets of spectral values, to which the
same scale factors are associated, it is still possible to enforce the reset by signaling the
presence of a new group. This brings along the price of retransmitting identical scale

factors, but might be advantageous if a missing reset of the context would significantly
degrade the coding efficiency. Nevertheless, an evaluation of the grouping side
information for the reset of the context may be an efficient concept to avoid the need to
transmit a dedicated context reset side information while still allowing a reset of the
context whenever appropriate. In those cases in which the context must (or should) be reset
even when the same scale factor information could be used, there is a penalty in terms of
bitrate (caused by the need to use an additional group and retransmit the scale factor
information), which penalty in bitrate may be compensated by a bitrate reduction in other
frames.
Another embodiment according to the invention creates an audio encoder for providing an
encoded audio information on the basis of an input audio information. The audio encoder
comprises a context-based entropy encoder configured to encode a given audio information
of the input audio information in dependence on a context, which context is based on-an
adjacent audio information, temporarily or spatially adjacent to the given audio
information, in a non-reset state of operation. The context-based entropy encoder is also
configured to select a mapping information, for deriving the encoded audio information
from the input audio information, in dependence on the context. The context-based entropy
encoder also comprises a context resetter configured to reset the context for selecting the
mapping information to a default context, which is independent from the previously
decoded audio information, within a continuous piece of input audio information in
response to the occurrence of a context reset condition. The context-based entropy encoder
is also configured to provide a side information of the encoded audio information
indicating the presence of a context reset conditional. This embodiment according to the
invention is based on the finding that the combination of a context-based entropy encoding
and on occasional reset of the context, which is signaled by an appropriate side
information, allows for a bitrate-efficient encoding of an input audio information.
In a preferred embodiment, the audio encoder is configured to perform a regular context
reset at least once per n frames of the input audio information. It has been found that a
regular context reset brings along the chance to synchronize to an audio signal very
rapidly, because a reset of the context introduces a temporal limitation of inter-frame
dependencies (or at least contributes to such a limitation of the inter-frame dependences).
In another preferred embodiment, the audio encoder is configured to switch between a
plurality of different coding modes (for example, frequency domain encoding mode and
linear-prediction-domain encoding mode). In this case, the audio encoder may preferably
be configured to perform a context reset in response to a change between two coding

modes. This embodiment is based on the finding that the change between two coding
modes is typically connected with a significant change of the input audio signal, such that
there is typically only a very limited correlation between the audio content before the
switching of the coding mode and after the switching the coding mode.
In another preferred embodiment, the audio encoder is configured to compute or estimate a
first number of bits required for encoding a certain audio information (e.g. a specific frame
or portion of the input audio information, or at least one or more specific spectral values of
the input audio information) of the input audio information in dependence on a non-reset
context, which non-reset context is based on an adjacent audio information temporarily or
spectrally adjacent to the certain audio information, and compute or estimate a second
number of bits required for encoding the certain audio information using the default
context (e.g. the state of the context to which the context is reset). The audio encoder is
further configured to compare the first number of bits and the second number of bits- to
decide whether to provide the encoded audio information corresponding to the certain
audio information on the basis of the non-reset context or on the basis of the default
context. The audio encoder is also configured to signal the result of said decision using the
side information. This embodiment is based on the finding that it is sometimes difficult to
decide a priori whether it is advantageous, in terms of bitrate, to reset the context. A reset
of the context may result in a selection of a mapping information (for deriving the encoded
audio information from a certain input audio information), which is better suited (in terms
of providing a lower bitrate) for the encoding of the certain audio information or worse-
suited (in terms of providing a higher bitrate) for encoding the certain audio information.
In some cases, it has been found to be advantageous to decide, whether or not to reset the
context, by determining the number of bits required for the encoding using both variants,
with and without resetting the context.
Further embodiments according to the invention create a method for providing a decoded
audio information on the basis of an encoded audio information, and a method for
providing an encoded audio information on the basis of an input audio information.
Further embodiments according to the invention create corresponding computer programs.
Further embodiments according to the invention create an audio signal.
Brief Description of the Figures

Embodiment according to the invention will subsequently be described taking reference to
the enclosed figures, in which:
Fig. 1 shows a block schematic diagram of an audio decoder, according to an
embodiment of the invention;
Fig. 2 shows a block schematic diagram of an audio decoder, according to another
embodiment of the invention;
Fig. 3a shows a graphical representation, in the form of a syntax representation, of
information comprised by a frequency domain channel stream, which can be
provided by the inventive audio encoder and which can be used by the
inventive audio decoder;
Fig. 3b shows a graphical representation, in the form of a syntax representation,'of
information representing arithmetically coded spectral data of the frequency
domain channel stream of Fig. 3a;
Fig. 4 shows a graphical representation, in the form of a syntax representation, of
arithmetically coded data, which may be comprised by the arithmetically
coded spectral data represented in Fig. 3b, or by the transform-coded-
excitation data represented in Fig. 1 lb;
Fig. 5 shows a legend defining information items and help elements used in the
syntax representations of Figs. 3a, 3b and 4;
Fig. 6 shows a flow chart of a method for processing an audio frame, which can be
used in an embodiment of the invention;
Fig. 7 shows a graphical representation of a context for a calculation of a state for
selecting a mapping information;
Fig. 8 shows a legend of data items and help elements used for arithmetically
decoding an arithmetically encoded spectral information, for example using
the algorithm of Figs. 9a to 9f;
Fig. 9a shows a pseudo program code - in a C-language like form - of a method for
resetting a context of an arithmetic coding;

Fig. 9b shows a pseudo program code of a method for mapping a context of an
arithmetic decoding between frames or windows of identical spectral
resolution and also between frames or windows of different spectral
resolution;
Fig. 9c shows a pseudo program code of a method for deriving a state value from a
context;
Fig. 9d shows a pseudo program code of a method for deriving an index of a
cumulative frequencies table from a value describing the state of the
context;
Fig. 9e shows a pseudo program code of a method for arithmetically decoding
arithmetically encoded spectral values;
Fig. 9f shows a pseudo program code of a method for updating the context
subsequent to a decoding of a tuple of spectral values;
Fig. 10a shows a graphical representation of a context reset in the presence of audio
frames having associated therewith "long windows" (one long window per
audio frame);
Fig. 10b shows a graphical representation of a context reset for audio frames having
associated therewith a plurality of "short windows" (e.g. eight short windows
per audio frame);
Fig. 10c shows a graphical representation of a context reset at a transition between a
first audio frame having associated therewith a "long start window" and an
audio frame having associated therewith a plurality of "short windows;"
Fig. 11a shows a graphical representation, in the form of a syntax representation, of
information comprised by a linear prediction-domain channel stream;
Fig. lib shows a graphical representation, in the form of a syntax representation, of
information comprised by a transform coded-excitation coding, which
transform-coded-excitation coding is part of the linear-prediction-domain
channel stream of Fig. 11a;

Figs, 11c and 11d show a legend defining information items and help elements used in
the syntax representations of Figs. 11a and 11b;
Fig. 12 shows a graphical representation of a context reset for audio frames
comprising a linear-prediction-domain excitation coding;
Fig. 13 shows a graphical representation of a context reset based on grouping-
information;
Fig. 14 shows a block schematic diagram of an audio encoder, according to an
embodiment of the invention;
Fig. 15 shows a block schematic diagram of an audio encoder, according to another
embodiment of the invention;
Fig. 16 shows a block schematic diagram of an audio encoder, according to another
embodiment of the invention;
Fig. 17 shows a block schematic diagram of an audio encoder, according to yet
another embodiment of the invention;
Fig. 18 shows a flow chart of a method for providing a decoded audio information,
according to an embodiment of the invention;
Fig. 19 shows a flow chart of a method for providing an encoded audio information,
according to an embodiment of the invention;
Fig. 20 shows a flow chart of a method for a context-dependent arithmetic decoding
of tuples of spectral values, which can be used in the inventive audio
decoders; and
Fig. 21 shows a flow chart of a method for a context-dependent arithmetic encoding
of tuples of spectral values, which can be used in the inventive audio
encoders.
Detailed Description of the Embodiments

1. Audio decoder
1.1 Audio decoder - generic embodiment
Fig. 1 shows a block schematic diagram of an audio decoder, according to an embodiment
of the invention. The audio decoder 100 of Fig. 1 is configured to receive an entropy-
encoded audio information 110 and to provide, on the basis thereof, a decoded audio
information 112. The audio decoder 100 comprises a context-based entropy decoder 120,
which is configured to decode the entropy-encoded audio information 110 in dependence
on a context 122, which context 122 is based on a previously decoded audio information in
a non-reset state of operation. The entropy decoder 120 is also configured to select a
mapping information 124, for deriving the decoded audio information 112 from the
encoded audio information 110, in dependence on the context 122. The context-based
entropy decoder 120 also comprises a context resetter 130, which is configured to receive a
side information 132 of the entropy-encoded audio information 110 and to provide a
context reset signal 134 on the basis thereof. The context resetter 130 is configured to reset
the context 122 for selecting the mapping information 124 to a default context, which is
independent from the previously decoded audio information, in response to a respective
side information 132 of the entropy-encoded audio information 110.
Thus, in operation, the context resetter 130 resets the context 122 whenever it detects a
context-reset side information (e.g. a context reset flag) associated with the entropy-
encoded audio information 110. A reset of the context 122 to the default context may have
the consequence that a default mapping information (e.g. a default Huffmann-codebook, in
the case of a Huffmann coding, or a default (cumulative) frequency information
"cumfreq" in the case of an arithmetic coding) is selected for deriving the decoded audio
information 112 (e.g. decoded spectral values a,b,c,d) from the entropy-encoded audio
information 110 (comprising, e.g. encoded spectral values a,b,c,d).
Accordingly, in a non-reset state of operation, the context 122 is affected by previously
decoded audio information, for example spectral values of previously decoded audio
frames. Consequently, the selection of the mapping information (which is performed on the
basis of the context), for decoding a current audio frame (or for decoding one or more
spectral values of the current audio frame), is typically dependent on decoded audio
information of a previously decoded frame (or a previously decoded "window").
In contrast, if the context is reset (i.e. in a context reset state of operation), the impact of
the previously decoded audio information (e.g. decoded spectral values) of a previously
decoded audio frame onto the selection of the mapping information, for decoding a current

audio frame, is eliminated. Thus, after a reset, the entropy decoding of the current audio
frame (or at least of some spectral values) is typically no longer dependent on the audio
information (e.g. spectral values) of the previously decoded audio frame. Nevertheless, a
decoding of an audio content (e.g. one or more spectral values) of the current audio frame
may (or may not) comprise some dependencies on previously decoded audio information
of the same audio frame.
Accordingly, the consideration of the context 122 may improve the mapping information
124 used for deriving the decoded audio information 112 from the encoded audio
information 110 in the absence of a reset condition. The context 122 may be reset if the
side information 132 indicates a reset condition in order to avoid the consideration of an
inappropriate context, which would typically result in an increased bitrate. Accordingly,
the audio decoder 100 allows for a decoding of an entropy-encoded audio information with
a good bitrate efficiency.
1.2 Audio decoder-Unified-Speech-and-Audio-Coding (USAC) embodiment
1.2.1 Decoder overview
In the following, an overview will be given over an audio decoder, which allows for a
decoding of both frequency-domain encoded audio content and linear-prediction-domain
encoded audio content, thereby allowing for the dynamic (e.g. frame-wise) choice of the
most appropriate coding mode. It should be noted that the audio decoder discussed in the
following combines frequency-domain decoding and linear-prediction-domain decoding.
However, it should be noted that the functionalities that are discussed in the following can
be used separately in a frequency-domain audio decoder and a linear-prediction-domain
audio decoder.
Fig. 2 shows an audio decoder 200, which is configured to receive an encoded audio signal
210 and to provide, on the basis thereof, a decoded audio signal 212. The audio decoder
200 is configured to receive a bitstream representing the encoded audio signal 210. The
audio decoder 200 comprises a bitstream demultiplexer 220, which is configured to extract
different information items from the bitstream representing the encoded audio signal 210.
For example, a bitstream multiplexer 220 is configured to extract frequency-domain
channel stream data 222, including, for example, so-called "arithdata" and a so-called
"arith_reset_flag", and linear-prediction-domain channel stream data 224 (including, for
example, so-called "arithdata" and a so-called "arithresetflag") from the bit stream
representing the encoded audio signal 200, whichever is present within the bitstream. Also,
the bitstream demultiplexer is configured to extract additional audio information and/or

side information from the bitstream representing the encoded audio signal 200, for
example, linear-prediction-domain control information 226, frequency-domain control
information 228, domain-selection information 230 and post processing control
information 232. The audio decoder 200 also comprises an entropy decoder/context
resetter 240, which is configured to entropy-decode entropy-encoded frequency-domain
spectral values or entropy-encoded linear-prediction-domain transform-coded-excitation
stimulus spectral values. The entropy decoder/context resetter 240 is sometimes also
designated as "a noiseless decoder" or "arithmetic decoder," because it typically performs
a lossless decoding. The entropy decoder/context resetter 240 is configured to provide
frequency-domain decoded spectral values 242 on the basis of the frequency-domain
channel stream data 222, or linear-prediction-domain transform-coded-excitation (TCX)
stimulus spectral values 244 on the basis of the linear-prediction-domain channel stream
data 224. Thus, the entropy decoder/context resetter 240 may be configured to be used both
for the decoding of the frequency-domain spectral values and the linear-prediction-domain
transform-coded-excitation stimulus spectral values, whichever is present in the bitstream
for the current frame.
The audio decoder 200 also comprises a time domain signal reconstruction. In the case of a
frequency-domain encoding, the time domain signal reconstruction may for example,
comprise an inverse quantizer 250, which receives the frequency-domain decoded spectral
values provided by the entropy decoder 240 and to provide, on the basis thereof, inversely
quantized frequency-domain decoded spectral values to a frequency-domain-to-time-
domain audio signal reconstruction 252. The frequency-domain-to-time-domain audio
signal reconstruction may be configured to receive the frequency-domain control
information 228 and, optionally, additional information (like, for example, control
information). The frequency-domain-to-time-domain audio signal reconstruction 252 may
be configured to provide, as an output signal, a frequency-domain coded time domain
audio signal 254. Regarding the linear prediction domain, the audio decoder 200 comprises
a linear-prediction-domain-to-time-domain audio signal reconstruction 262, which is
configured to receive the linear-prediction-domain transform-coded-excitation stimulus
decoded spectral values 244, the linear-prediction-domain control information 226 and
optionally, additional linear-prediction-domain information (for example coefficients of
the linear prediction models, or an encoded version thereof), and to provide, on the basis
thereof, a linear-prediction-domain coded time domain audio signal 264.
The audio decoder 200 also comprises a selector 270 for selecting between the frequency-
domain coded time domain audio signal 254 and the linear-prediction-domain coded time
domain audio signal 264 in dependence on the domain selection information 230, to decide

whether the decoded audio signal 212 (or a temporal portion thereof) is based on the
frequency-domain coded time domain audio signal 254 or the linear-prediction-domain
coded time domain audio signal 264. At the transition between the domains, a cross fade
may be performed by the selector 270 to provide the selector output signal 272. The
decoded audio signal 212 may be equal to the selector audio signal 272, or may preferably
be derived from the selector signal 272 using an audio signal postprocessor 280. The audio
signal postprocessor 280 may take into consideration the post processing control
information 232 provided by the bitstream demultiplexer 220.
To summarize the above, the audio decoder 200 may provide the decoded audio signal 212
on the basis of either the frequency-domain channel stream data 222 (in combination with
possible additional control information), or the linear-prediction-domain channel stream
data 224 (in combination with additional control information), wherein the audio decoder
200 may switch between the frequency-domain and the linear-prediction-domain using the
selector 270. The frequency-domain coded time domain audio signal 254 and the linear-
prediction-domain coded time domain audio signal 264 may be generated independently
from each other. However, the same entropy decoder/context resetter 240 may be applied
(possibly in combination with different, domain-specific mapping information, like
cumulative frequencies tables) for the derivation of the frequency domain decoded spectral
values 242, which form the basis of the frequency-domain coded time domain audio signal
254, and for the derivation of the linear-prediction-domain transform-coded-excitation
stimulus decoded spectral values 244, which form the basis for the linear-prediction-
domain coded time-domain audio signal 264.
In the following, details regarding the provision of the frequency-domain decoded spectral
values 242 and regarding the provision of the linear-prediction-domain transform-coded-
excitation stimulus decoded spectral values 244 will be discussed.
It should be noted that details regarding the derivation of the frequency-domain coded time
domain audio signal 254 from frequency-domain decoded spectral values 242 can be found
in international standard ISO/IEC 14496-3:2005, part 3: audio, part 4: general audio coding
(GA)-AAC, Twin VQ, BSAC, and the documents referenced therein.
It should also be noted that details regarding the computation of the linear-prediction-
domain coded time-domain audio signal 264 on the basis of the linear-prediction-domain
transform-coded-excitation stimulus decoded spectral values 244 may for example, be
found in the international standards 3GPP TS 26.090, 3GPP TS 26.190 and 3GPP TS
26.290.

Said standards also comprise information regarding some of the symbols used in the
following.
1.2.2 Frequency-domain channel stream decoding
In the following, it will be described, how the frequency-domain decoded spectral values
242 can be derived from the frequency-domain channel stream data, and how the inventive
context reset is involved in this calculation.
1.2.2.1 Data structures of frequency domain channel stream
In the following, the relevant data structures of a frequency domain channel stream will be
described taking reference to Figs. 3a, 3b, 4 and 5.
Fig. 3a shows a graphical representation, in the form of a table, of the syntax of the
frequency domain channel stream. As can be seen, the frequency domain channel stream -
may comprise a "globalgain" information. In addition, the frequency domain channel
stream may comprise scale factor data ("scale_factor_data"), which define scale factors for
different frequency bins. Regarding the global gain and the scale factor data, and their
usage, reference is made to international standard ISO/TEC 14496-3(2005), part 3, sub part
4, and to the documents referenced therein.
The frequency domain channel stream may also comprise arithmetically coded spectral
data ("acspectraldata") which will be explained in detail in the following. It should be
noted that the frequency-domain channel stream may comprise additional optional
information, like noise filling information, configuration information, time warp
information and temporal noise shaping information, which are not of relevance for the
present invention.
In the following, details regarding the arithmetically coded spectral data will be discussed
taking reference to Figs. 3b and 4. As can be seen in Fig. 3b, which shows a graphical
representation in the form a table, of the syntax of the arithmetically coded spectral data
"acspectraldata," the arithmetically coded spectral data comprise a context reset flag
"arith_reset_fiag" for resetting the context for the arithmetic decoding. Also, the
arithmetically coded spectral data comprise one or more blocks of arithmetically encoded
data "arith_data." It should be noted that an audio frame, which is represented by the
syntax element "fd_channel_stream" may comprise one or more "windows," wherein the

number of windows is defined by the variable "numwindows." It should be noted that one
set of spectral values (also designated as "spectral coefficients") are associated with each of
the windows of an audio frame, such that an audio frame comprising num_windows
windows comprises numwindows sets of spectral values. Details regarding the concept of
having multiple windows (and multiple sets of spectral values) within a single audio frame
are described, for example, in the international standard ISO/TEC 14493-3(2005), part 3,
sub part 4.
Taking reference again to Fig. 3, it can be concluded that the arithmetically coded spectral
data "ac_spectral_data" of a frame, which are included in the frequency-domain channel
stream "fd_channel_stream," comprise one (single) context reset flag "arith_reset_flag"
and one (single) block of arithmetically coded data "arithdata," if a single window is
associated with the audio frame represented by the present frequency domain channel
stream. In contrast, the arithmetically coded spectral data of a frame comprise a single
context rest flag "arithresetflag" and a plurality of blocks of arithmetically encoded data
"arithdata" if the current audio frame (associated with the frequency-domain channel
stream) comprises multiple windows (i.e. num_windows windows).
Taking reference now to Fig. 4, the structure of a block of arithmetically encoded data
"arith_data" will be discussed taking reference to Fig. 4, which shows a graphic
representation of the syntax of the arithmetically encoded data "arithdata." As can be seen
from Fig. 4, the arithmetically encoded data comprise arithmetically encoded data of, for
example, lg/4 encoded tuples (wherein lg is the number of spectral values of the current
audio frame, or of the current window). For each tuple, an arithmetically encoded group
index "acodng" is included in the arithmetically coded data "arithdata." The group index
ng of a tuple of quantized spectral values a,b,c,d is, for example, arithmetically encoded (at
the encoder side) in dependence on a cumulative frequencies table, which is selected in
dependence on a context, as will be discussed later on. The group index ng of the tuple is
arithmetically coded, wherein a so-called "arithmetic escape" ("ARITHESCAPE") nlay
be used in order to extend the possible range of values.
In addition, for groups of 4-tuples having a cardinal larger than one, an arithmetic
codeword "acodne" for decoding the index ne of the tuple within the group ng may be
included within the arithmetically encoded data "arithdata." The codeword "acod_ne"
may be encoded, for example, in dependent from a context.

In addition, one or more arithmetically encoded code words "acodr" encoding one or
more of the least significant bits of the values a,b,c,d of the tuple may be included in the
arithmetically encoded data "arithdata."
To summarize, the arithmetically encoded data "arithdata" comprise one (or in the
presence of an arithmetic escape sequence, more) arithmetic codeword "acod_ng" for
encoding a group index ng taking into account a cumulative frequencies table having index
pki. Optionally (depending on the cardinal of the group designated by the group index ng)
the arithmetically encoded data also comprise an arithmetic codeword "acodne" for
encoding an element index ne. Optionally, the arithmetically encoded data may also
comprise one or more arithmetic code words for encoding one or more least significant
bits.
The context, which determines the index (e.g. pki) of the cumulative frequencies table used
for the encoding/decoding of the arithmetic codeword "acod_ng" is based on context data
q[0], q[l], qs not shown in Fig. 4 but discussed below. The context information q[0], q[l], qs is
either based on a default value, if the context reset flag "arith_reset_flag" is active prior to the
encoding/decoding of a frame or window, or based on previously encoded/decoded spectral values
(e.g. values a,b,c,d) of a previous window (if the current frame comprises a window preceding the
presently considered window) or a previous frame (if the current frame comprises only one
window, or if the first window within the current frame is considered). Details regarding the
definition of the context can be seen in the pseudo code section labeled "obtain inter-window
context information" of Fig. 4, wherein reference is also made to the definition of the procedures
"arithresetcontext" and "arithmapcontext" described in detail with reference to Figs. 9a and 9d
below. It should also be noted, that the pseudo code portions labeled "compute state of context" and
"obtain index pki of cumulative frequencies table" serve to derive an index "pki" for selecting the
"mapping information" in dependence on the context, and could be replaced by other functions for
selecting the "mapping information" or "mapping rule" in dependence on the context. The
functions "arith_get_context" and " arith_get_pk" will be discussed in more detail below.
It has been noted that the initialization of the context, which is described in the section "obtain-
inter-window context information" is performed once (and preferably only once) per audio frame
(if the audio frame comprises only one window) or once (and preferably only once) per window (if
the current audio frame comprises more than one window).
Accordingly, a reset of the entire context information q[0], q[l], qs (or the alternative initialization
of the context information q[0] on the basis of the decoded spectral values of the previous frame (or
previous window)) is preferably performed only once per block of arithmetically encoded data (i.e.

only once per window if the present frame comprises only one window, or only once per window,
if the present frame comprises more than one window).
In contrast, the context information q[l] (which is based on the previously decoded spectral values
of the current frame or window) is updated upon completion of a decoding of a single tuple" of
spectral values a, b, c, d, for example as defined by the procedure "arithupdatecontext."
For further details regarding the payloads of the "spectral noiseless coder" (i.e. for encoding the
arithmetically encoded spectral values) reference is made to the definitions as given in the tables of
Fig. 5.
To summarize, spectrum coefficients (e.g. a, b, c, d) from both the „linear prediction domain"
coded signal 224 and the "frequency-domain" coded signal 222 are scalar quantized and then
noiselessly coded by an adaptively context dependent arithmetic coding (for example an encoder
providing the entropy coded audio signal 210). The quantized coefficients (e.g. a, b, c, d) are
gathered together in 4-tuples before being transmitted (by the encoder) from the lowest-frequency
to the highest-frequency. Each 4-tuple is split into the most significant 3-bits (one bit for the sign
and 2 for the amplitude) wise plane and the remaining less significant bit-planes. The most
significant 3-bits wise plane is coded according to its neighborhood (i.e. taking into consideration
the "context") by means of the group index, ng, and the element index, ne. The remaining less
significant bit-planes are entropy coded without considering the context. The indexes ng and ne and
the less significant bit-planes form the samples of the arithmetic coder (which are evaluated by the
entropy decoder 240). Details regarding the arithmetic coding will be described below in the
section 1.2.2.2.
1.2.2.2 Method for decoding of the frequency-domain channel stream
In the following, the functionality of the context-based entropy decoder 120, 240,
comprising the context resetter 130, will be described in detail taking reference to Figs. 6,
7,8,9a-9f and 20.
It should be noted that it is the function of the context-based entropy decoder to reconstruct
(decode) an entropy decoded (preferably arithmetically decoded) audio information (e.g.
spectral values a, b, c, d of a frequency-domain representation of the audio signal, or of a
linear-prediction-domain transform-coded-excitation representation of the audio signal) on
the basis of an entropy encoded (preferably arithmetically encoded) audio information (e.g.
encoded spectral values). The context-based entropy decoder (comprising the context

resetter) may for example be configured to decode spectral values a, b, c, d encoded as
described by the syntax shown in Fig. 4.
It should also be noted that the syntax shown in Fig. 4 may be considered as a decoding
rule, in particular when taken in combination with the definition of Figs. 5, 7, 8 and 9a-9f
and 20, such that the decoder is generally configured to decode information encoded
according to Fig. 4.
Taking reference now to Fig. 6, which shows a flow chart of a simplified decoding
algorithm for the processing of an audio frame or of a window within an audio frame, the
decoding will be described. The method 600 of Fig. 6 may comprise a step 610 of
obtaining an inter-window context information. For this purpose, it may be checked
whether the context reset flag "arith_reset_flag" is set for the current window (or current
frame, if the frame only comprises one window). If the context reset flag is set, the context
information may be reset in step 612, for example by executing the function
"arithresetcontext" discussed below. In particular, the portion of the context information
describing the coded values of a previous window (or previous frame) may be set'to
default value (e.g. 0 or -1) in the step 612. In contrast, if it is found that the context reset
flag is not set for the window (or frame), context information from a previous frame (or a
window) may be copied, or mapped, to be used for determining (or influencing) the
context for the decoding of the arithmetically encoded spectral values of the present
window (or frame). The step 614 may correspond to the execution of the function
"arithmapcontext." When executing said function, the context may be mapped even if
the current frame (or window) and the previous frame (or window) comprise different
spectral resolutions (even though this functionality is not absolutely required).
Subsequently, a plurality of arithmetically encoded spectral values (or tuples of such
values) may be decoded by performing steps 620, 630, 640 one or more times. In step 620,
a mapping information (for example a Huffmann codebook, or a cumulative frequencies
table "cum_freq") is selected on the basis of the context as established in step 610 (and
optionally updated in the step 640). The step 620 may comprise a one-or-more step method
for determining the mapping information. For example, the step 620 may comprise a step
622 of computing the state of the context on the basis of the context information (e.g. q[0],
q[l]). The computation of the state of the context may for example be performed by the
function "arithgetcontext," which is defined below. Optionally, an auxiliary mapping
may be performed (for example as seen in the pseudo code portion labeled "compute state
of context" of Fig. 4). Further, the step 620 may comprise a sub-step 624 of mapping the
state of the context (e.g. the variable t as shown in the syntax of Fig. 4) to an index (for

example designated "pki") of a mapping information (for example designating a row or
column of the cumulative frequencies table). For this purpose, it is, for example, possible
to evaluate the function "arith_get_pk." To summarize, the step 620 allows to map the
current context (q[0],q[l]) onto an index (e.g. pki) describing which mapping information
(out of a plurality of discreet sets of mapping information) should be used for the entropy
decoding (e.g. arithmetic decoding). The method 600 also comprises a step 630 of entropy
decoding of encoded audio information (for example the spectral values a, b, c, d) using
the selected mapping information (for example one cumulative frequencies table out of a
plurality of cumulative frequencies tables) to obtain a newly decoded audio information
(e.g. spectral values a, b, c, d). For entropy decoding the audio information, the function
"arithdecode" explained in detail below, may be used.
Subsequently, the context may be updated in the step 640 using the newly decoded audio
information (for example using one or more spectral values a, b, c, d). For example, a
portion of the context representing previously encoded audio information of the present
frame or window (e.g. q[l]) may be updated. For this purpose, the function
"arith_update_context" detailed below may be used.
As mentioned above, steps 620, 630, 640 may be repeated.
Entropy decoding the encoded audio information may comprise using one or more
arithmetic code words (e.g. "acodng," "acod_ne" and/or "acod_r") comprised by the
entropy encoded audio information 222, 224, for example as represented in Fig. 4.
In the following, an example of the context considered for the state calculation (state of the
context) will be described taking reference to Fig. 7. Generally speaking, it can be said that
the spectral noiseless coding (and the corresponding spectral noiseless decoding) is used
(for example in the encoder) to further reduce the redundancy of the quantized spectrum
(and is used in the decoder to reconstruct the quantized spectrum). The spectrum noiseless
coding scheme is based on an arithmetic coding in conjunction with a dynamically adapted
context. The noiseless coding is set by the quantized spectral values (e.g. a, b, c, d) and
uses context dependent cumulative frequencies tables (e.g. cum_freq) derived from, for
example, 4 previously decoded neighboring 4-tuples. Here, neighborhood in both time and
frequency is taken into account, as illustrated in Fig. 7. The cumulative frequencies tables
(which are selected in dependence on the context) are then used by the arithmetic encoder
to generate a variable length binary code (and also by the arithmetic decoder in order to
decode the variable length binary code).

Taking reference now to Fig. 7, it can bee seen that a context for decoding a 4-tuple to
decode 710 is based on a 4-tuple 720 already decoded and adjacent in frequency to the 4-
tuple 710 to decode and associated with the same audio frame or window like the 4-tuple
710 to decode. In addition, the context of the 4-tuple to decode 710 is also based on three
additional 4-tuples 730a,730b,730c already decoded and associated with an audio frame or
window preceding the audio frame or window of the 4-tuple 710 to decode.
Regarding the arithmetic encoding and arithmetic decoding, it should be noted that the
arithmetic coder produces a binary code for a given set of symbols (e.g. spectral values a,
b, c, d) and their respective probabilities (as defined, for example, by the cumulative
frequencies tables). The binary code is generated by mapping a probability interval, where
a set of symbols (e.g. a,b,c,d) lies, to a code word. Inversely, the set of samples in (e.g. a,
b, c, d) are derived from the binary code by an inverse mapping, wherein the probability of
the samples (e.g. a, b, c, d) is taken into account (for example by selecting a mapping
information, like a cumulative frequencies distribution, on the basis of the context). In the
following, the decoding process, i.e. the process of arithmetic decoding, which may be
performed by the context based entropy decoder 120 or by the entropy decoder/context
resetter 240, and which has been generally described taking reference to Fig. 6, will be
explained taking reference to Fig. 9a-9f.
For this purpose, reference is made to the definitions shown in the table of Fig. 8. In the
table of Fig. 8, definitions of data, variables and help elements used in the pseudo program
codes of Figs. 9a-9f are defined. Reference is also made to the definitions of Fig. 5 and to
the above discussion.
Regarding the decoding process, it can be said that 4-tuples of quantized spectral
coefficients are noiselessly coded (by the encoder) and transmitted (via a transmission
channel or storage medium between the encoder and the decoder discussed here) starting
from the lowest-frequency coefficient and progressing to the highest-frequency coefficient.
The coefficients from the advanced audio coding (AAC) (i.e. the coefficients of the
frequency-domain channel stream data) are stored in an array
"x_ac_quant[g][win][sfb][bin]," in the order of transmission of the noiseless coding code
word is such that when they are decoded in the order received and stored in the array, [bin]
if the most rapidly incrementing index and [g] is the most slowly incrementing index.
Within a codeword the order of decoding is a, b, c,d.

The coefficient from the transform-coded-excitation (TCX) (e.g. of the linear-prediction-
domain channel stream data) are stored directly in the array "x_tcx_invquant[win][bin],
and the order of the transmission of the noiseless coding code words is such that when they
are decoded in the order received and stored in the array, bin if the most rapidly
incrementing index and win if the most slowly incrementing index. Within a codeword, the
order of decoding is a, b, c, d.
First, the flag "arith_reset_flag" is evaluated. The flag "arith_reset_flag" determines if the
context must be reset. If the flag is TRUE, the function "arith_reset_context," which is
shown in the pseudo program code representation of Fig. 9a if called. Otherwise, when the
"arith_reset_flag" is FALSE, a mapping is done between the past context (i.e. the context
determined by decoded audio information of the previously decoded window or frame) and
the current context. For this purpose, the function "arithmapcontext," which is
represented in the pseudo program code representation of Fig. 9b, is called (thereby
allowing for the reuse of the context even if the previous frame or window comprises a
different spectral resolution). However, it should be noted that the call of the function
"arithmapcontext" should be considered as being optional.
The noiseless decoder (or entropy decoder) outputs 4-tuples of signed quantized spectral
coefficients. At first, the state of the context is calculated based on the four previously
decoded groups "surrounding" (or, more precisely, neighboring) the 4-tuple to decode (as
shown in Fig. 7 at reference numerals 720,730a,730b,730c). The state of the context is
given by the function "arith_get_context()," which is represented by the pseudo program
code representation of Fig. 9c. As can be seen, the function "arithgetcontext" allocates a
context state value s to the context in dependence on the values "v" (as defined in the
pseudo program code of Fig. 9f).
Once the state s is known, the group to which belongs the most significant 2-bits wise
plane of 4-tuple is decoded using the function "arith_decode()" fed with (or configured to
use) the appropriate (selected) cumulative frequencies table corresponding to the context
state. The correspondence is made by the function "arith_get_pk()" which is represented
by the pseudo code representation of Fig. 9d.
To summarize, the functions "arith_get_context" and "arith_get_pk" allow the obtain a
cumulative frequencies table index pki on the basis of the context (namely q[0][l+i],
q[l][l+i-l], q[s][l+i-l], q[0][l+i+l]). Thus, it is possible to select mapping information
(namely one of the cumulative frequencies tables) in dependence on the context.

Then (once the cumulative frequencies table is selected) the "arith_decode()" function is
called with the cumulative frequencies table corresponding to the index returned by the
"arith_get_pk()." The arithmetic decoder is an integer implementation generating tag with
scaling. The pseudo C-code shown in Fig. 9e describes the used algorithm.
Taking reference to the algorithm "arithdecode" shown in Fig. 9e, it should be noted that
it is assumed that an appropriate cumulative frequencies table is selected on the basis of the
context. It should also be noted that the algorithm "arithdecode" conducts the arithmetic
decoding using the bits (or bit sequences) "acod_ng," "acod_ne" and "acod¬r" defined in
Fig. 4. Also, it should be noted that the algorithm "arithdecode" may use a cumulative
frequencies table "cum_freq" defined by the context for a decoding of a first occurrence of
the bit sequence "acodng" related to a tuple. However, additional occurrences of the bit
sequences "acod_ng" for the same tuple (which may follow after and arithescape-
sequence) may for example be decoded using a different cumulative frequencies table or
even a default cumulative frequencies table. Further, it should be noted that the decoding
of the bit sequences "acod_ne" and "acodr" may be performed using appropriate
cumulative frequencies table, which may be independent from the context. Thus, to
summarize, a context-dependent cumulative frequencies table may be applied (unless the
context is reset, such that a context-reset-state is reached and a default cumulative
frequencies table is used) for decoding of the arithmetic codeword "acod_ng" for decoding
the group index (at least until an arithmetic escape is recognized).
This can be seen when considering the graphic representation of the syntax of "arithdata",
which is given in Fig. 4, when seen in combination with the pseudo program code of the
function "arithdecode" given in Fig. 9e. An understanding of the decoding can be
obtained on the basis of an understanding of the syntax of "arithdata."
While the decoded group index ng is the "escape" symbol, "ARITHESCAPE," an
additional group index ng is decoded and the variable lev is incremented by two. Once the
decoded group index is not the escape, "ARITHESCAPE," the number of elements, mm,
within the group and the group offset, og, are deduced by looking up to the table
"dgroupsf]":

The element index ne is then decoded by calling the function "arith_decode()" with the
cumulative frequencies table (arith_cf_ne+((mm* (mm-!))»1)[]. Once the element index

is decoded, the most significant 2-bits wise plane of the 4-tuple can be derived with the
table "dgvector[]:"

The remaining bit planes (for example the least significant bits) are then decoded from the
most significant to the lowest significant level by calling lev times "arith_decode()" with
the cumulative frequencies table "arith_cf_r[]" (which is a predefined cumulative
frequencies table for the decoding of the least significant bits, and which may indicate
equal frequencies of the bit combinations). The decoded bit plane r permits to refine the
decode 4-tuple by the following way:

Once the 4-tuple (a, b, c, d) is completely decoded, the context tables q and qs are updated
by calling the function "arith_update_context()," which is represented by the pseudo
program code representation of Fig. 9f.
As can be seen from Fig. 9f, the context representing the previously decoded spectral
values of the current window or frame, namely q[l], are updated (for example each time a
new tuple of spectral values is decoded). In addition, the function "arithupdatecontext"
also comprises a pseudo code section for updating the context history qs, which is
performed only once per frame or window.
To summarize, the function "arith_update_context" comprises two main functionalities,
namely to update the context portion (e.g. q[l]) representing previously decoded spectral
values of the current frame of window, as soon as a new spectral value of the current frame
or window is decoded, and to update the context history (e.g. qs) in response to the
completion of the decoding of a frame or window, such that the context history qs can be
used to derive a context portion (e.g. q[0]) which represents an "old" context when
decoding the next frame or window.

As can be seen in the pseudo program code representation of Figs. 9a and 9b, the context
history (e.g. qs) is either discarded, namely in the case of a context reset, or used for
obtaining the "old" context portion (e.g. q[0]), namely if there is not context reset, when
proceeding to the arithmetic decoding of a next frame or window.
In the following, the method of arithmetic decoding will be briefly summarized taking
reference to Fig. 20, which illustrates a flowchart of the embodiment of the decoding
scheme. In step 2005, corresponding to step 2105, the context is derived on the basis of tO,
t1, t2 and t3. In step 2010, the first reduction level lev() is estimated from the context, and
the variable lev is set to lev(). In the following step 2015, the group ng is read from the
bitstream and the probability distribution for decoding ng is derived from the context. In
step 2015, the group ng can then be decoded from the bitstream. In step 2020 it is
determined whether the ng equals 544, which corresponds to the escape value. If so, the
variable lev can be increased by 2 before returning to step 2015. In case this branch is used
for the first time, i.e., if lev==lev0, the probability distribution respectively the context can
be accordingly adapted, respectively discarded if the branch is not used for the first time, in
line with the above described context adaptation mechanism. In case the group index ng is
not equal to 544 in step 2020, in a following step 2025 it is determined whether the number
of elements in a group is greater than 1, and if so, in step 2030, the group element ne is
read and decoded from the bitstream assuming a uniform probability distribution. The
element index ne is derived from the bitstream using arithmetic coding and a uniform
probability distribution. In step 2035 the literal codeword (a,b,c,d) is derived from ng and
ne, for example, by a look-up process in the tables, for example, refer to dgroups[ng] and
acod_ne[ne]. In step 2040 for all lev missing bitplanes, the planes are read from the
bitstream using arithmetic coding and assuming a uniform probability distribution. The
bitplanes can then be appended to (a,b,c,d) by shifting (a,b,c,d) to the left and adding the
bitplane bp: ((a,b,c,d)«=l)|=bp. This process may be repeated lev times. Finally in step
2045 the 4-tuple q(n,m), i.e.(a,b,c,d) can be provided.
1.2.2.3 Course of the decoding
In the following, the course of the decoding will be briefly discussed for different scenarios
taking reference to Figs. 10a-10d.
Fig. 10a shows a graphical representation of the course of the decoding for an audio frame
being frequency-domain encoded using a so-called "long window." Regarding the
encoding, reference is made to International Standard IOC/IEC 14493-3(2005), part 3, sub-

part 4. As can be seen, the audio contents of a first frame 1010 are closely related, and the
time-domain signals reconstructed for the audio frames 1010, 1012 are overlapped-and-
added (as defined in said standard). One set of spectral coefficients is associated to each of
the frames 1010,1012, as is known from the above referenced standard. Further, a novel 1-
bit context reset flag ("arith_reset_flag") is associated with each of the frames 1010, 1012.
If the context reset flag associated with the first frame 1010 is set, the context is reset (e.g.
according to the algorithm shown in Fig. 9a) prior to the arithmetic decoding of the set of
spectral values of the first audio frame 1010. Similarly, if the 1-bit context reset flag of the
second audio frame 1012 is set, the context is reset, to be independent from the spectral
values of the first audio frame 1010, before decoding the spectral values of the second
audio frame 1012. Thus, by evaluating the context reset flag, is possible to reset the context
for decoding the second audio frame 1012, even though the first audio frame 1010 and the
second audio frame 1012 are closely related in that the windowed time domain audio
signals derived from the spectral values of said audio frames 1010, 1012 are overlapped
and added, and even though identical window shapes are associated with the first and
second audio frame 1010, 1012.
Taking reference now to Fig. 10b, which shows a graphical representation of the decoding
of an audio frame 1040 having associated therewith a plurality of (for example 8) short
windows, a reset of the context for this case will be described. Again, there is a single 1-bit
context reset flag associated with the audio frame 1040, even though a plurality of short
windows are associated with the audio frame 1040. Regarding the short windows, it should
be noted that one set of spectral values is associated with each of the short windows, such
that the audio frame 1040 comprise a plurality of (for example 8) sets of (arithmetically
encoded) spectral values. However, if the context reset flag is active, the context will be
reset before the decoding of the spectral values of the first window 1042a of the audio
frame 1040 and between the decoding of the spectral values of any subsequent frames
1042b-1042h of the audio frame 1040. Thus, once again, the context is reset between a
decoding of the spectral values of two subsequent windows, the audio contents of which
are closely related (in that they are overlapped and added), and even though the subsequent
windows (e.g. windows 1042a, 1042b) comprise identical window shapes associated
therewith. Also, it should be noted that the context is reset during the decoding of a single
audio frame (i.e. between the decoding of different spectral values of a single audio frame).
Also, it should be noted that a single bit context reset flag calls a multiple reset of the
context if a frame 1040 comprises a plurality of short windows 1042a-1042h.
Taking reference now to Fig. 10c, which shows a graphical representation of a context
reset in the presence of a transition from audio frames being associated with long windows

(audio frame 1070 and preceding audio frames) to one or more audio frames being
associated with a plurality of short windows (audio frame 1072). It should be noted that the
context reset flag allows for a signaling of the need to reset the context independent from a
signaling of the window shape. For example, the entropy decoder may be configured to be
capable of obtaining the spectral values of a first window 1074a of the audio frame 1072
using a context, which is based on spectral values of the audio frame 1070, even though the
window shape of the "window" (or, more precisely, frame portion or "subframe"
associated with a short window) 1074a is substantially different from the window shape of
the long window of the audio frame 1070, and even though the spectral resolution of the
short window 1074a is typically smaller than the spectral resolution (frequency resolution)
of the long window of the audio frame 1070. This can be obtained by the mapping of the
context between windows (or frames) of different spectral resolution, which is described
by the pseudo program code of Fig. 9b. However, the entropy decoder is at the same time
capable of resetting the context between the decoding of the spectral values of the long
window of the audio frame 1070 and the spectral values of the first short window 1074a of
the audio frame 1072, if it is found that the context reset flag of the audio frame 1072 is
active. The reset of the context is in this case performed by an algorithm, which has been
described with reference to the pseudo program code of Fig. 9a.
To summarize the above, the evaluation of the context reset flag provides the inventive
entropy decoder with a very large flexibility. In a preferred embodiment, the entropy
decoder is capable of:
• using a context, which is based on a previously decoded frame or window of a
different spectral resolution when decoding (the spectral values of) a current frame
or window; and
• selectively resetting, in response to the context reset flag, the context between a
decoding of (spectral values of) frames or windows having different window
shapes and/or different spectral resolutions; and
• selectively resetting, in response to the context reset flag, the context between a
decoding of (spectral values of) frames or windows having the same window shape
and/or spectral resolution.
In other words, the entropy decoder is configured to perform the context reset independent
from a change of the window shape and/or spectral resolution, by evaluating the context
reset side information separate from the window shape/spectral resolution side
information.

1.2.3 Linear prediction domain channel stream decoding
1.2.3.1 Linear prediction domain channel stream data
In the following, the syntax of a linear-prediction-domain channel stream will be described
taking reference to Fig. 11a, which shows a graphical representation of the syntax of a
linear-prediction-domain channel stream, and also to Fig. lib, which shows a graphical
representation of the syntax of a transform-coded-excitation coding (tcx_coding) and also
to Figs. 1 lc and lid which show a representation of definitions and data elements used in
the syntax of the linear-prediction-domain channel stream.
Taking reference now to Fig. 11a the overall structure of the linear-prediction-domain
channel stream will be discussed. The linear-prediction-domain channel stream show in
Fig. 11a comprises a number of configuration information items, like, for example,
"acelpcoremode" and "lpdmode." Regarding the meaning of the configuration
elements, and the overall concept of the linear-prediction-domain coding, reference is
made to International Standard 3GPP TS 26.090, 3GPP TS 26.190 and 3GPP TS 26.290,
Furthermore, it should be noted that he linear-prediction-domain channel stream may
comprise up to four "blocks" (having indices k=0 to k=3) which comprise either an
ACELP-encoded excitation or a transform-coded-excitation (which may itself be
arithmetically coded). Taking reference again to Fig. 11a, it can be seen that the linear-
prediction-domain channel stream comprises, for each of the "blocks," a ACELP stimulus
encoding or a TCX stimulus encoding. As the ACELP stimulus encoding is not relevant
for the present invention, a detailed discussion will be omitted and reference will be made
to the above international standards regarding this issue.
Regarding the TCX stimulus encoding, it should be noted that different encodings are used
for encoding a first TCX "block" (also designated as "TCX frame") of the current audio
frame and for the encoding of any subsequent TCX "blocks" (TCX frames) of the current
audio frame. This is indicated by the so-called "first tcx flag," which indicates if the
currently processed TCX "block" (TCX frame) is the first in the present frame (also
designated as "super frame" in the terminology of linear-prediction-domain coding).
Taking reference now to Fig. 1 lb, it can be seen that the encoding of a transform-coded-
excitation "block" (tcx frame) comprises an encoded noise factor ("noisefactor") and an
encoded global gain ("global_gain"). In addition, if the presently considered tcx "block" is
the first tcx "block" within the currently considered audio frame, the encoding of the
currently considered tcx comprises a context reset flag ("arith_reset_flag"). Otherwise, i.e.

if the presently considered tcx "block" is not the first tcx "block" of the current audio
frame, the encoding of the current tcx "block" does not comprise such a context reset flag,
as can be seen from the syntax description of Fig. 1 lb. Furthermore, the encoding of the
tcx stimulus comprises arithmetically encoded spectral values (or spectral coefficients)
"arith_data", which are encoded in accordance with the arithmetic coding already
explained with reference to Fig. 4 above.
The spectral values representing the transform-coded-excitation stimulus of a first tcx
"block" of an audio frame are encoded using a reset context (default context) if the context
reset flag ("arith_reset_flag") of said tcx "block" is active. The arithmetically encoded
spectral values of a first tcx "block" of an audio frame are encoded using a non-reset
context if the context reset flag of said audio frame is inactive. The arithmetically encoded
values of any subsequent tcx "blocks" (subsequent to the first tcx "block") of an audio
frame are encoded using a non-reset context (i.e. using a context derived from a previous
tcx block). Said details regarding the arithmetic encoding of the spectral values (or spectral
coefficients) of the transform-coded-excitation can be seen in Fig. 11b when taken in
combination with Fig. 11a.
1.2.3.2 Method for decoding of the transform-coded-excitation spectral values
The transform-coded-excitation spectral values, which are arithmetically encoded, can be
decoded taking into account the context. For example, if the context reset flag of a tcx
"block" is active, the context may be reset, for example, in accordance with the algorithm
shown in Fig. 9a, before decoding the arithmetically encoded spectral values of the tcx
"block" using the algorithm described with reference to Fig. 9c-9f. In contrast, if the
context reset flag of a tcx "block" is inactive, the context for decoding may be determined
by the mapping (of the context history from a previously decoded tcx block) described
with reference to Fig. 9b, or by deriving the context from the previously decoded spectral
values in any other form. Also, the context for the decoding of the "subsequent" tcx
"blocks", which are not the first tcx "block" of an audio frame, may be derived from
previously decoded spectral values of previous tcx "blocks."
For the decoding of tcx excitation stimulus spectral values, the decoder may therefore use
the algorithm, which has been explained, for example, with reference to Fig. 6, 9a-9f and
20. However, the setting of the context reset flag ("arith_reset_flag") is not checked for
every tcx "block" (which corresponds to a "window"), but only for the first tcx "block" of

an audio frame. For the subsequent tcx "blocks" (which correspond to "windows") it may
be assumed that the context shall not be reset.
Accordingly, the tcx excitation stimulus spectral value decoder may be configured to
decode spectral values encoded according to the syntax shown in Figs. 1 lb and 4.
1.2.3.3 Course of the decoding
In the following, a decoding of a linear-prediction-domain excitation audio information
will be described taking reference to Fig. 12. However, the decoding of the parameters
(e.g. of parameters of the linear predictor excited by the stimulus or excitation) of the
linear-prediction-domain signal synthesizer will be neglected here. Rather, the focus of the
following discussion is put on the decoding of the transform-coded-excitation stimulus
spectral values.
Fig. 12 shows a graphical representation of the encoded excitation for exciting a linear-
prediction-domain audio synthesizer. The encoded stimulus information is shown for
subsequent audio frames 1210, 1220, 1230. For example, the first audio frame 1210
comprises a first "block" 1212a which comprises an ACELP-encoded stimulus. The audio
frame 1210 also comprises three "blocks" 1212b,1212c,1212d comprising transform-coded
excitation stimulus, wherein the transform-coded-excitation stimulus of each of the TCX
"blocks" 1212B, 1212C, 1212D comprises a set of arithmetically encoded spectral values.
In addition, the first TCX block 1212B of the frame 1210 comprises a context reset flag
"arithresetflag". The audio frame 1220 comprises, for example, four TCX "blocks"
1222A-1222D, wherein the first TCX block 1222A of the frame 1220 comprises a context
reset flag. The audio frame 1230 comprises a single TCX block 1232, which itself
comprises a context reset flag. Accordingly, there is one context reset flag per audio frame
comprising one or more TCX blocks.
Accordingly, when decoding the linear-prediction-domain stimulus shown in Fig. 12, the
decoder will check whether the context reset flag of the TCX block 1212B is set and reset
the context prior to the decoding of the spectral values of the TCX block 1212B, in
dependence on the state of the context reset flag. However, there will be no reset of the
context between arithmetic decoding of these spectral values of the TCX blocks 1212B,
and 1212C, independent from the state of the context reset flag of the audio frame 1210.
Similarly, there will be no reset of the context between the decoding of the spectral values
of the TCX blocks 1212C, and 1212D. However, the decoder will reset the context before
the decoding of the spectral values of the TCX block 1222A in dependence on the status of

the context reset flag of the audio frame 1222 and will not conduct a reset of the context
between the decoding of the spectral values of the TCX blocks 1222 A and 1222B, 1222B
and 1222C, 1222C and 1222D. However, the decoder will perform a reset of the context
prior to decoding of the spectral values of the TCX block 1232 in dependence on the status
of the context reset flag of the audio frame 1230.
It should be also noted that an audio stream may comprise a combination of frequency-
domain audio framed and linear prediction-domain audio frames, such that the decoder
may be configured to properly decode such an alternating sequence. At a transition
between different encoding modes (frequency-domain vs. linear prediction domain), a reset
of the context may or may not be enforced by the context resetter.
1.3. Audio Decoder - Third Embodiment
In the following another audio decoder concept will be described, which allows for a
bitrate-efficient resetting of the context even in the absence of a dedicated context reset
side information.
It has been found that the side information, which accompanies the entropy encoded
spectral values, can be exploited for deciding whether to reset the context for entropy-
decoding (e.g. arithmetical decoding) of the entropy encoded spectral values.
An efficient concept for resetting the context of the arithmetic decoding has been found for
audio frames in which sets of spectral values associated with a plurality of windows are
comprised. For example, the so-called "advanced audio coding" (also briefly designated as
"AAC"), which is defined in the international standard ISO/IEC 14496-3:2005, part 3,
subpart 4, uses audio frames comprising eight sets of spectral coefficients, wherein each
set of spectral coefficients is associated to one "short window". Accordingly, eight short
windows are associated with such an audio frame, wherein the eight short windows are
used in an overlap-and-add procedure for overlapping-and-adding windowed time domain
signals reconstructed on the basis of the sets of spectral coefficients. For details, reference
is made to said international standard. However, in an audio frame comprising a plurality
of sets of spectral coefficients, two or more of the sets of spectral coefficients may be
grouped, such that common scale factors are associated with the grouped sets of spectral
coefficients (and are applied thereto in the decoder). The grouping of sets of spectral
coefficients may for example be signaled using a grouping side information (e.g.
"scalefactorgrouping" bits). For details, reference is made, for example, to ISO/IEC

14496-3:2005(E), part 3, subpart 4, tables 4.6, 4.44, 4.45, 4.46 and 4.47. Nevertheless, in
order to provide a full understanding, reference is made to the above-mentioned
international standard in its entirety.
However, in an audio decoder according to an embodiment of the invention, the
information regarding the grouping of different sets of spectral values (for example, by
associating them with common scale spectral values) may be used for determining when to
reset the context for the arithmetic encoding/decoding of the spectral values. For example,
an inventive audio decoder according to the third embodiment might be configured to reset
the context of the entropy decoding (e.g. of a context-based Huffmann-decoding of a
context-based arithmetic decoding, as described above) whenever it is found that there is a
transition from one group of sets of encoded spectral values to another group of sets of
spectral values (to which other group of sets new scale factors are associated).
Accordingly, rather than using a context reset flag, the scale factor grouping side
information may be exploited to determine when to reset the context of the arithmetic
decoding.
In the following, an example of this concept will be explained taking reference to Fig. 13,
which shows a graphical representation of a sequence of audio frames and the respective
side information. Fig. 13 shows a first audio frame 1310, a second audio frame 1320 and a
third audio frame 1330. The first audio frame 1310 may be a "long window" audio frame
within the meaning of ISO/IEC 14493-3, part 3, subpart 4 (for example of type
"LONGJSTARTWINDOW"). A context reset flag may be associated with the audio
frame 1310 to decide whether the context for an arithmetic decoding of spectral values of
the audio frame 1310 should be reset, which context reset flag would be considered
accordingly by the audio decoder.
In contrast, the second audio frame is of type "EIGHT_SHORT_SEQUENCE" and may
accordingly comprise eight sets of encoded spectral values. However, the first three sets of
encoded spectral values may be grouped together to form one group (to which a common
scale factor information is associated) 1322a. Another group 1322b may be defined by a
single set of spectral values. A third group 1322C may comprise two sets of spectral values
associated therewith, and a fourth group 1322D may comprise another two sets of spectral
values associated therewith. The grouping of sets of spectral values of the audio frame
1320 may be signaled by the so-called "scalefactorgrouping" bits defined, for example,
in table 4.6 of the above-referenced standard. Similarly, the audio frame 1340 may
comprise four groups 1330A, 1330B, 1330C, 1330D.

However, the audio frames 1320,1330 may, for example, not comprise a dedicated context
reset flag. For entropy decoding the spectral values of the audio frame 1320, the decoder
may, for example unconditionally or in dependence on a context reset flag, reset the
context before decoding the first set of spectral coefficients of the first group 1322A.
Subsequently, the audio decoder may avoid resetting the context between the decoding of
different sets of the spectral coefficients of the same group of the spectral coefficients.
However, whenever the audio decoder detects the beginning of a new group within the
audio frame 1320 comprising a plurality of groups (of sets of spectral coefficients), the
audio decoder may reset the context for the entropy decoding of the spectral coefficients.
Thus, the audio encoder may effectively reset the contexts for decoding of the spectral
coefficients of the first group 13 22A, before the decoding of the spectral coefficients of the
second group 1322B, before the decoding of the spectral coefficients of the third group
1322C, and before the decoding of the spectral coefficients of the fourth group 1322D.
Accordingly, a separate transmission of a dedicated context reset flag may be avoided
within such audio frames in which there are a plurality of sets of spectral coefficients.
Accordingly, the extra bit load produced by the transmission of the grouping bits may at
least partly be compensated by the omission of the transmission of a dedicated context
reset flag in such a frame, which may be unnecessary in some applications.
To summarize, a reset strategy has been described which can be implemented as a decoder
feature (and also as an encoder feature). The strategy described here does not need the
transmission of any additional information (like a dedicated side information for resetting
the context) to a decoder. It uses the side information already sent by the decoder (e.g. by
an encoder providing an AAC encoded audio stream corresponding to the above industry
standard). As it is described herein, the change of content within the signal (audio signal)
can happen from frame to frame of, for example, 1024 samples. In this case, we have
already the reset flag which can control the context-adaptive coding and mitigate the
impact on its performance. However, within a frame of 1024 samples, the content can
change as well. In such a case, when an audio coder (for example according to the unified
speech and audio coding "USAC") uses a frequency domain (FD) coding, the decoder will
usually switch to short blocks. In short blocks, grouping information is sent (as discussed
above) which already gives information about the position of a transition or a transient (of
the audio signal). Such information can be reused for resetting the context, as discussed in
this section.
On the other hand, when an audio coder (like, for example, according to the unified speech
and audio coding "USAC") uses linear prediction domain (LPD) coding, a change of

content will affect the selected coding modes. When different transform-coding-excitations
occur within one frame of 1024 samples, a context mapping may be used, as described
above. (See, for example, the context mapping of Fig. 9D). It was found to be a better
solution than to reset the context every time a different transform-coded excitation is
selected. As the linear-prediction-domain coding is very adaptive, the coding mode
changes constantly and a systematic reset will penalize greatly the coding performance.
However, when an ACELP is selected, it will be advantageous to reset the context for the
next transform coded excitation (TCX). The selection of ACELP between transform coded
excitations is a strong indication that a great change in the signal happened.
In other words, taking reference, for example, to Fig. 12, the context reset flag preceding
the first TCX "block" of an audio frame when using a linear prediction main coding may
be omitted, however, totally or selectively, if there is at least one ACELP-coded stimulus
within the audio frame. The decoder may, in this case, be configured to reset the context if
a first TCX "block" following an ACELP "block" is identified, and to omit a reset of the
context between a decoding of spectral values of subsequent TCX "blocks".
Also, optionally, the decoder may be configured to evaluate a context reset flag, for
example once per audio frame, if a TCX block is preceding the parent audio frame, to
allow for a reset of the context even in the presence of an extended segments of TCX
"blocks".
2. Audio Encoder
2.1. Audio encoder - basic concepts
In the following, the basic concept of a context-based entropy encoder will be discussed in
order to facilitate the understanding of the specific procedures for the reset of the context
which will be discussed in detail in the following.
Noiseless coding can be based on quantized spectral values and may use context dependent
cumulative frequency tables derived from, for example, four previously decoded
neighbouring tuples. Fig. 7 illustrates another embodiment. Fig. 7 shows a time frequency
plane, wherein along the time axis three time slots are indexed n, n-1 and n-2. Furthermore,
Fig. 7 illustrates four frequency or spectral bands which are labelled by m-2, m-1, m and
m+1. Fig. 7 shows within each time-frequency slot boxes, which represent tuples of

samples to be encoded or decoded. Three different types of tuples are illustrated in Fig. 7,
in which round boxes having a dashed or dotted border indicate remaining tuples to be
encoded or decoded, rectangular boxes having a dotted border indicate previously encoded
or decoded tuples and grey boxes with a solid border indicate previously en/decoded
tuples, which are used to determine the context for the current tuple to be encoded or
decoded.
Note that the previous and current segments referred to in the above described
embodiments may correspond to a tuple in the present embodiment, in other words, the
segments may be processed band wise in the frequency or spectral domain. As illustrated
in Fig. 76, tuples or segments in the neighbourhood of a current tuple (i.e. in the time and
the frequency or spectral domain) may be taken into account for deriving a context.
Cumulative frequency tables may then be used by the arithmetic coder to generate a
variable length binary code. The arithmetic coder may produce a binary code for a given
set of symbols and their respective probabilities. The binary code may be generated by
mapping a probability interval, where the set of symbols lies, to a codeword.
In the present embodiment context based arithmetic coding may be carried out on the basis
of 4-tuples (i.e. on four spectral coefficient indices), which are also labelled q(n,m), or
q[m][n], representing the spectral coefficients after quantization, which are neighboured in
the frequency or spectral domain and which are entropy coded in one step. According to
the above description, coding may be carried out based on the coding context. As indicated
in Fig. 7, additionally to the 4-tuple, which is coded (i.e. the current segment) four
previously coded 4-tuples are taken into account in order to derive the context. These four
4-tuples determine the context and are previous in the frequency and/or previous in the
time domain.
Fig. 21a shows a flow-chart of a USAC (USAC = Universal Speech and Audio Coder)
context dependent arithmetic coder for the encoding scheme of spectral coefficients. The
encoding process depends on the current 4-tuple plus the context, where the context is used
for selecting the probability distribution of the arithmetic coder and for predicting the
amplitude of the spectral coefficients. In Fig. 21a the box 2105 represents context
determination, which is based on tO, t1, t2 and t3 corresponding to q(n-l, m), q(n,m-l), q
(n-l,m-l) and q (n-l,m+l).
Generally, in embodiments the entropy encoder can be adapted for encoding the current
segment in units of a 4-tuple of spectral coefficients and for predicting an amplitude range
of the 4-tuple based on the coding context.

In the present embodiment the encoding scheme comprises several stages. First, the literal
codeword is encoded using an arithmetic coder and a specific probability distribution. The
codeword represents four neighbouring spectral coefficients (a,b,c,d), however, each of a,
b, c, d is limited in range:

Generally, in embodiments the entropy encoder can be adapted for dividing the 4-tuple by
a predetermined factor as often as necessary to fit a result of the division in the predicted
range or in a predetermined range and for encoding a number of divisions necessary, a
division remainder and the result of the division when the 4-tuple does not lie in the
predicted range, and for encoding a division remainder and the result of the division
otherwise.
In the following, if the term (a,b,c,d), i.e. any coefficient a, b, c, d, exceeds the given range
in this embodiment, this can in general be considered by dividing (a,b,c,d) as often by a
factor (e.g. 2 or 4) as necessary, for fitting the resulting codeword in the given range. The
division by a factor of 2 corresponds to a binary shifting to the right-hand side, i.e.
(a,b,c,d)» 1. This diminution is done in an integer representation, i.e. information may be
lost. The least significant bits, which may get lost by the shifting to the right, are stored and
later on coded using the arithmetic coder and a uniform probability distribution. The
process of shifting to the right is carried out for all four spectral coefficients (a,b,c,d).
In general embodiments, the entropy encoder can be adapted for encoding the result of the
division or the 4-tuple using a group index ng, the group index ng referring to a group of
one or more code words for which a probability distribution is based on the coding context,
and an element index ne in case the group comprises more than one codeword, the element
index ne referring to a codeword within the group and the element index can be assumed
uniformly distributed, and for encoding the number of divisions by a number of escape
symbols, an escape symbol being a specific group index ng only used for indicating a
division and for encoding the remainders of the divisions based on a uniform distribution
using an arithmetic coding rule. The entropy encoder can be adapted for encoding a
sequence of symbols into the encoded audio stream using a symbol alphabet comprising
the escape symbol, and group symbols corresponding to a set of available group indices, a
symbol alphabet comprising the corresponding element indices, and a symbol alphabet
comprising the different values of the remainders.

In the embodiment of Fig. 21a, the probability distribution for encoding the literal
codeword and also an estimation of the number of range-reduction steps can be derived
from the context. For example, all code words, in a total 84 = 4096, span in total 544
groups, which consist of one or more elements. The codeword can be represented in the
bitstream as the group index ng and the group element ne. Both values can be coded using
the arithmetic coder, using certain probability distributions. In one embodiment the
probability distribution for ng may be derived from the context, whereas the probability
distribution for ne may be assumed to be uniform. A combination of ng and ne may
unambiguously identify a codeword. The remainder of the division, i.e. the bit-planes
shifted out, may be assumed to be uniformly distributed as well.
In Fig. 21a, in step 2110, the 4-tuple q(n,m), that is (a,b,c,d) or the current segment is
provided and a parameter lev is initiated by setting it to 0. In step 2115 from the context,
the range of (a,b,c,d) is estimated. According to this estimation, (a,b,c,d) may be reduced
by lev() levels, i.e. divided by a factor of 2lev0. The lev() least significant bitplanes are
stored for later usage in step 2150.
In step 2120 it is checked whether (a,b,c,d) exceeds the given range and if so, the range of
(a,b,c,d) is reduced by a factor of 4 in step 2125. In other words, in step 2125 (a,b,c,d) are
shifted by 2 to the right and the removed bitplanes are stored for later usage in step 2150.
In order to indicate this reduction step, ng is set to 544 in step 2130, i.e. ng = 544 serves as
an escape codeword. This codeword is then written to the bitstream in step 2155, where for
deriving the codeword in step 2130 an arithmetic coder with a probability distribution
derived from the context is used. In case this reduction step was applied the first time, i.e.
if lev==lev0, the context is slightly adapted. In case the reduction step is applied more than
once, the context is discarded and a default distribution is used further on. The process then
continues with step 2120.
If in step 2120 a match for the range is detected, more specifically if (a,b,c,d) matches the
range condition, (a,b,c,d) is mapped to a group ng, and, if applicable, the group element
index ne. This mapping is unambiguously, that is (a,b,c,d) can be derived from ng and ne.
The group index ng is then coded by the arithmetic coder, using a probability distribution
arrived for the adapted/discarded context in step 2135. The group index ng is then inserted
into the bitstream in step 2155. In a following step 2140, it is checked whether the number
of elements in the group is larger than 1. If necessary, that is if the group indexed by ng
consists of more than one element, the group element index ne is coded by the arithmetic
coder in step 2145, assuming a uniform probability distribution in the present embodiment.

Following step 2145, the element group index ne is inserted into the bitstream in step 2155.
Finally, in step 2150, all stored bitplanes are coded using the arithmetic coder, assuming a
uniform probability distribution. The coded stored bitplanes are then also inserted into the
bitstream in step 2155.
To summarize the above, an entropy encoder, in which the context reset concepts
described in the following can be used, receives one or more spectral values and provides a
code word, typically of variable length, on the basis of the one or more received spectral
values. The mapping of the received spectral values onto the code word is dependent on an
estimated probability distribution of code words, such that, generally speaking, short code
words are associated with spectral values (or combinations thereof) having a high
probability and such that long code words are associated with spectral values (or
combinations thereof) having a low probability. The context is taken into consideration in
that it is assumed that the probability of the spectral values (or combinations thereof) is
dependent on previously encoded spectral values (or combinations thereof). Accordingly,
the mapping rule (also designated "mapping information" or "codebook" or "cumulative
frequencies table") is selected in dependence on the context, i.e. on the previously encoded
spectral values (or combinations thereof). However, the context is not always considered.
Rather, the context is sometimes reset by the "context reset" functionality described herein.
By resetting the context, it can be considered that the spectral values (or combinations
thereof) to be currently encoded differ strongly from what would be expected on the basis
of the context.
2.2 Audio Encoder-Embodiment of Fig. 14
In the following, an audio encoder will be described taking reference to Fig. 14, which is
based on the basic concepts described before. The audio encoder 1400 in Fig. 14 comprises
an audio processor 1410, which is configured to receive an audio signal 1412 and to
perform an audio processing, for example, a transformation of the audio signal 1410 from
the time domain to the frequency domain, and a quantization of the spectral values
obtained by the time-domain to frequency-domain transformation. Accordingly, the audio
processor provides quantized spectral coefficients (also designated as spectral values)
1414. The audio encoder 1400 also comprises a context-adaptive arithmetic coder 1420,
which is configured to receive the spectral coefficients 1414 and the context information
1422, which context information 1422 can be used for selecting mapping rules for mapping

spectral values (or combinations thereof) onto code words, which are an encoded
representation of these spectral values (or combinations thereof). Accordingly, the context-
adaptive arithmetic coder 1420 provides encoded spectral values (encoded coefficients)
1424. The encoder 1400 also comprises a buffer 1430 for buffering previously encoded
spectral values 1414, because the previously encoded spectral values 1432 provided by the
buffer 1430 have an impact on the context. The encoder 1400 also comprises a context
generator 1440, which is configured to receive the buffered, previously encoded
coefficients 1432 and to derive the context information 1422 (for example a value "PKI"
for selecting a cumulative frequencies table or a mapping information for the context-
adaptive arithmetic coder 1420) on the basis thereof. However, the audio encoder 1400
also comprises a reset mechanism 1450 for resetting the context. The resetting mechanism
1450 is configured to determine when to reset the context (or context information)
provided by the context generator 1440. The reset mechanism 1450 may optionally act on
the buffer 1430, to reset the coefficients stored in or provided by the buffer 1430, or on the
context generator 1440, to reset the context information provided by the context generator
1440.
The audio encoder 1400 of Fig. 14 comprises a reset strategy as an encoder feature. The
reset strategy triggers at the encoder side a "reset flag", which can be considered as a
context reset side information, which is sent every frame of 1024 samples (time domain
samples of the audio signal) on one bit. The audio encoder 1400 comprises a "regular
reset" strategy. According to this strategy, the reset flag is regularly activated, thereby
resetting the context used in the encoder and also the context in an appropriate decoder
(which processes the context reset flag as described above).
The advantage of such a regular reset is to limit the dependence of the coding of the
present frame from the previous frames. Resetting the context every n-frames (which is
achieved by the counter 1460 and the reset flag generator 1470) allows the decoder to
resynchronize its states with the encoder even when an error of transmission occurs. The
decoded signal can then be recovered after a reset point. Further, the "regular reset"
strategy allows the decoder to randomly access at any reset points of the bitstream without
considering the past information. The interval between the reset points and the coding
performance is a trade-off, which is made at the encoder according to the targeted receiver
and the transmission channel characteristics.
2.3 Audio encoder - embodiment of Fig. 15

In the following, another reset strategy as an encoder feature will be described. The
following strategy triggers at the encoder side the reset flag which is sent every frame of
1024 samples on 1-bit. In the embodiment of Fig. 15, the reset is triggered by the coding
characteristics.
As can be seen in Fig. 15, the audio encoder 1500 is very similar to the audio encoder
1400, such that identical means and signals are designated with identical reference
numerals and will not be explained again. However, the audio encoder comprises a
different reset mechanism 1550. The context reset mechanism 1550 comprises a coding
mode change detector 1560 and a reset flag generator. The coding mode change detector
detects a change in the coding mode and instructs the reset flag generator 1570 to provide
the (context) reset flag. The context reset flag also acts on the context generator 1440,'or
alternatively or in addition, on the buffer 1430 to reset the context. As mentioned above,
the reset is trigged by the coding characteristics. In a switched coder, like the unified
speech and audio coder (USAC), different coding modes can happen and be successive.
The context is then difficult to deduce because the time/frequency resolution of the present
frame can differ from the resolution of the previous ones. It is the reason why it exists in
USAC a context mapping mechanism which permits to recover a context even when the
resolution changes between two frames. However, some coding modes differ so much
from each other that even a context mapping may not be efficient. A reset is then required.
For example, in a unified speech and audio coder (USAC) such a reset may be triggered
when going from/to frequency domain coding to/from linear-prediction-domain coding. In
other words, a context reset of the context-adaptive arithmetic coder 1420 may be
performed and signalled whenever the coding mode changes between frequency domain
coding and linear prediction domain coding. Such a reset of the context may be signalised
or not by a dedicated context reset flag. However, alternatively, a different side
information, for example side information indicating the coding mode, may be exploited at
the decoder side to trigger the reset of the context.
2.4. Audio encoder - embodiment of Fig. 16
Fig. 16 shows a block schematic diagram of another audio encoder, which implements yet
another reset strategy as an encoder feature. The strategy triggers at the encoder side the
reset flag which is sent every frame of 1024 samples on 1 bit.

The audio encoder 1600 of Fig. 16 is similar to the audio encoders 1400, 1500 of Figs. 14
and 15, such that identical features and signals are designated with identical reference
numerals. However, the audio encoder 1600 comprises two context-adaptive arithmetic
coders 1420, 1620 (or is at least capable of encoding the spectral values 1414 to be
currently encoded using two different encoding contexts). For this purpose, an advanced
context generator 1640 in configured to provide context information 1642, which is
obtained without a reset of the context, for the first context-adaptive arithmetic encoding
(for example in the context-adaptive arithmetic encoder 1420), and to provide a second
context information 1644, which is obtained by applying a reset of the context, for a
second encoding of the spectral values to be currently encoded (for example in the context-
adaptive arithmetic encoder 1620). A bit counter/comparison 1660 determines (or
estimates) the number of bits required for the encoding of the spectral value using a non-
reset context and also determines (or estimates) the number of bits required for encoding
the spectral values to be currently encoded using a reset context. Accordingly, the bit
counter/comparison 1660 decides whether it is more advantageous, in terms of bitrate, to
reset the context or not. Accordingly, the bit counter/comparison 1660 provides an active
context reset flag in dependence on whether it is advantageous, in terms of bitrate, to reset
the context or not. Further, the bit counter/comparison 1660 selectively provides the
spectral values encoded using a non-reset context or the spectral values encoded using a
reset context as an output information 1424, again in dependence on whether a non-reset
context or a reset context results in a lower bitrate.
To summarize the above, Fig. 16 shows an audio encoder which uses a closed-loop
decision to decide whether to activate or not to activate the reset flag. Thus, the decoder
comprises a reset strategy as an encoder feature. The strategy triggers at the encoder -side
the reset flag, which is sent every frame of 1024 samples on one bit.
It has been found that sometimes the characteristics of a signal change abruptly from frame
to frame. For such non-stationary parts of the signal, the context from the past frame is
often meaningless. Furthermore, it has been found that it can be more penalizing than
beneficial to take into account the past frames in the context adaptive coding. A solution is
then to trigger the reset flag when it happens. A way to detect such a case is to compare the
decoding efficiency when both the reset flag is on and off. The flag value corresponding to
the best coding is then used (to determine the new state of the encoder context) and
transmitted. This mechanism was implemented in the unified speech and audio coding
(US AC), and the following average gain of performance was measured:
12 kbps mono: 1.55 bit/frame (max: 54)


2.5. Audio Encoder - Embodiment of Fig. 17
In the following, another audio encoder 1700 will be described taking reference to Fig. 17.
The audio encoder 1700 is similar to the audio encoders 1400, 1500, and 1600 of Figs. 14,
15 and 16, such that identical reference numerals will be used to designate identical means
and signals.
However, the audio encoder 1700 comprises a different reset flag generator 1770, when
compared to the other audio encoders. The reset flag generator 1770 receive a side
information, which is provided by the audio processor 1410 and provides, on the basis
thereof, the reset flag 1772, which is provided to the context generator 1440. However, it
should be noted that the audio encoder 1700 avoids to include the reset flag 1772 into the
encoded audio stream. Rather, only the audio processor side information 1780 is included
into the encoded audio stream.
The reset flag generator 1770 may, for example, be configured to derive the context reset
flag 1772 from the audio processor side information 1780. For example, the reset flag
generator 1770 may evaluate a grouping information (already described above) to decide
whether to reset the context. Thus, the context may be reset between an encoding of
different groups of sets of spectral coefficients, as explained, for example, for the decoder
taking reference to Fig. 13.
Accordingly, the encoder 1700 uses a reset strategy, which may be identical to a reset
strategy at a decoder. However, the reset strategy may avoid the transmission of a
dedicated context reset flag. In other words, the reset strategy described here does not need
the transmission of any additional information to the decoder. It uses the side information
which is already sent to the decoder (for example, a grouping side information). It should
be noted here that for the present strategy, identical mechanisms for determining whether

to reset the context or not are used at the encoder and at the decoder. Accordingly,
reference is made to the discussion with respect to Fig. 13.
2.6. Audio Encoder - Further Remarks
First of all, it should be noted that different reset strategies discussed herein, for example,
in section 2.1. to 2.5, can be combined. In particular, the reset strategies as an encoder
feature, which have been discussed with reference to Figs. 14-16 can be combined.
However, the reset strategy discussed with reference Fig. 17 can also be combined with the
other reset strategies, if desired.
In addition, it should be noted that the reset of the context at the encoder side should occur
synchronously with the reset of the context at the decoder side. Accordingly, the encoder is
configured to provide the context reset flag discussed above at the time (or for the frames,
or windows) discussed above (e.g. with reference to Figs. 10a-10c, 12 and 13), such that
the discussion of the decoder implies a corresponding functionality of the encoder
(regarding the generation of the context reset flag). Similarly, the discussion of the
functionality of encoder corresponds to the respective functionality of the decoder in most
cases.
3. Method for Decoding an Audio Information
In the following, a method for providing a decoded audio information on the basis of
encoded audio information will be briefly discussed taking reference to Fig. 18. Fig. 18
shows such a method 1800. The method 1800 comprises a step 1810 of decoding the
entropy-encoded audio information taking into account a context, which is based on a
previously decoded audio information, in a non-reset state of operation. Decoding the
entropy-encoded audio information comprises selecting 1812 a mapping information for
deriving the decoded audio information from the encoded audio information in dependence
on the context and using 1814 the selected mapping information for deriving a portion of
the decoded audio information. Decoding the entropy-encoded audio information also
comprises resetting 1816 the context for selecting the mapping information to a default
context, which is independent from the previously decoded audio information, in response
to a side information, and using 1818 the mapping information, which is based on the
default context, for deriving a second portion of the decoded audio information.
The method 1800 can be supplemented by any of the functionalities discussed herein
regarding the decoding of an audio information, also regarding the inventive apparatus.

4. Method for Encoding an Audio Signal
In the following, a method 1900 for providing an encoded audio information on the basis
of an input audio information will be described taking reference to Fig. 19.
The method 1900 comprises encoding 1910 a given audio information of the input audio
information in dependence on a context, which context is based on an adjacent audio
information, temporally or spectrally adjacent to the given audio information, in a non-
reset state of operation.
The method 1900 also comprises selecting 1920 a mapping information, for deriving the
encoded audio information from the input audio information, in dependence on the context.
Also, the method 1900 comprises resetting 1930 the context for selecting the mapping
information to a default context, which is independent from the previously decoded audio
information, within a contiguous piece of input audio information (e.g. between decoding
two frames, the time domain signals of which are overlapped-and-added) in response to the
occurrence of a context reset condition.
The method 1900 also comprises providing 1940 a side information (e.g. a context reset
flag, or a grouping information) of the encoded audio information indicating the presence
of such a context reset condition.
The method 1900 can be supplemented by any of the features and functionalities described
herein with respect to the inventive audio encoding concept.
5. Implementation Alternatives
Although some aspects have been described in the context of an apparatus, it is clear that
these aspects also represent a description of the corresponding method, where a block or
device corresponds to a method step or a feature of a method step. Analogously, aspects
described in the context of a method step also represent a description of a corresponding
block or item or feature of a corresponding apparatus.

The inventive encoded audio signal can be stored on a digital storage medium or can be
transmitted on a transmission medium such as a wireless transmission medium or a wired
transmission medium such as the Internet.
Depending on certain implementation requirements, embodiments of the invention can be
implemented in hardware or in software. The implementation can be performed using a
digital storage medium, for example a floppy disk, a DVD, a Blue-Ray, a CD, a ROM, a
PROM, an EPROM, an EEPROM or a FLASH memory, having electronically readable
control signals stored thereon, which cooperate (or are capable of cooperating) with a
programmable computer system such that the respective method is performed. Therefore,
the digital storage medium may be computer readable.
Some embodiments according to the invention comprise a data carrier having
electronically readable control signals, which are capable of cooperating with a
programmable computer system, such that one of the methods described herein is
performed.
Generally, embodiments of the present invention can be implemented as a computer
program product with a program code, the program code being operative for performing
one of the methods when the computer program product runs on a computer. The program
code may for example be stored on a machine readable carrier.
Other embodiments comprise the computer program for performing one of the methods
described herein, stored on a machine readable carrier.
In other words, an embodiment of the inventive method is, therefore, a computer program
having a program code for performing one of the methods described herein, when the
computer program runs on a computer.
A further embodiment of the inventive methods is, therefore, a data carrier (or a digital
storage medium, or a computer-readable medium) comprising, recorded thereon, the
computer program for performing one of the methods described herein.
A further embodiment of the inventive method is, therefore, a data stream or a sequence of
signals representing the computer program for performing one of the methods described
herein. The data stream or the sequence of signals may for example be configured to be
transferred via a data communication connection, for example via the Internet.

A further embodiment comprises a processing means, for example a computer, or a
programmable logic device, configured to or adapted to perform one of the methods
described herein.
A further embodiment comprises a computer having installed thereon the computer
program for performing one of the methods described herein.
In some embodiments, a programmable logic device (for example a field programmable
gate array) may be used to perform some or all of the functionalities of the methods
described herein. In some embodiments, a field programmable gate array may cooperate
with a microprocessor in order to perform one of the methods described herein. Generally,
the methods are preferably performed by any hardware apparatus.
The above described embodiments are merely illustrative for the principles of the present
invention. It is understood that modifications and variations of the arrangements and the
details described herein will be apparent to others skilled in the art. It is the intent,
therefore, to be limited only by the scope of the impending patent claims and not by the
specific details presented by way of description and explanation of the embodiments
herein.

we claim:
1. An audio decoder (100;200) for providing a decoded audio information (112;212)
on the basis of an entropy encoded audio information (110;210, 222,224), the audio
decoder comprising:
a context-based entropy decoder (120;240) configured to decode the entropy-
encoded audio information (110;210,222,224) in dependence on a context
(q[0],q[l]), which context is based on a previously-decoded audio information in a
non-reset state-of-operation;
wherein the. context-based entropy decoder (120;240) is configured to select a
mapping information (cum_freq[pki]), for deriving the decoded audio information
(112;212) from the encoded audio information, in dependence on the context
(q[0],q[l]);and
wherein the context-based entropy decoder (120;240) comprises a context resetter
(130) configured to reset (arith_reset_context) the context (q[0],q[l]) for selecting
the mapping information to a default context, which default context is independent
from the previously-decoded audio information (qs), in response to a side
information (132; arith_reset_flag) of the encoded audio information (110;210).
2. The audio decoder (100;200) according to claim 1, wherein the context resetter
(130) is configured to selectively reset the context-based entropy decoder (120;240)
between a decoding of subsequent time portions (1010,1012) of the encoded audio
information (110;210) having associated spectral data of the same spectral
resolution.
3. The audio decoder (100;200) according to claim 1 or claim 2, wherein the audio
decoder is configured to receive, as a component of the encoded audio information
(110;210,222,224), an information describing spectral values in a first audio frame
(1010) and in a second audio frame (1012) subsequent to the first audio frame;
wherein the audio decoder comprises a spectral-domain-to-time-domain
transformer (252;262) configured to overlap-and-add a first windowed time domain
signal, which is based on the spectral values of the first audio frame (1010), and a
second windowed time domain signal, which is based on the spectral values of the
second audio frame (1012), to derive the decoded audio information (112;212);

wherein the audio decoder is configured to separately adjust window shapes of a
window for obtaining the first windowed time domain signal and of a window for
obtaining a second windowed time domain signal; and
wherein the audio decoder is configured to perform, in response to the side
information (132; arithresetflag), a reset (arithresetcontext) of the context
(q[0],q[l]) between a decoding of the spectral values of the first audio frame (1010)
and a decoding of the spectral values of the second audio frame (1012), even if the
second window shape is identical to the first window shape,
such that the context used for decoding the encoded audio information of the
second audio frame (1012) is independent from the decoded audio information of
the first audio frame (1010) if the side information indicates to reset the context.
4. The audio decoder (100;200) according to claim 3, wherein the audio decoder is
configured to receive a context-reset side information (132;arith_reset_flag) for
signaling a reset of the context; and
wherein the audio decoder is configured to additionally receive a window-shape
side information (window_sequence, window_shape); and
wherein the audio decoder is configured to adjust the window shapes of windows
for obtaining the first and second windowed time domain signals independent from
performing the reset of the context.
5. The audio decoder (100;200) according to one of claims 1 to 4,
wherein the audio decoder is configured to receive, as the side information for
resetting the context (132;arith_reset_flag), a one-bit context reset flag per audio
frame of the encoded audio information; and
wherein the audio decoder is configured to receive, in addition to the context reset
flag, a side information describing a spectral resolution of spectral values
represented by the encoded audio information (110;210,222,224) or a window
length of a time window for windowing time domain values represented by the
encoded audio information; and

wherein the context resetter (130) is configured to perform a reset of the context,- in
response to the one-bit context-reset flag, between a decoding of spectral values
(242,244) of two audio frames of the encoded audio information representing
spectral values of identical spectral resolutions or window lengths.
6. The audio decoder (100;200) according to one of claims 1 to 5, wherein the audio
decoder is configured to receive, as the side information (132;arith_reset_flag) for
resetting the context, a one-bit context reset flag per audio frame of the encoded
audio information;
wherein the audio decoder is configured to receive an encoded audio information
(110;210,22,224) comprising a plurality of sets of spectral values
(1042a,1042b,... 1042h) per audio frame (1040);
wherein the context-based entropy decoder (120;240) is configured to decode the
entropy-encoded audio information of a subsequent set of spectral values (1042b)
of a given audio frame (1040) in dependence on a context (q[0],q[l]), which
context is based on a previously-decoded audio information (q[0]) of a preceding
set (1042a) of spectral values of the given audio frame (1040), in a non-reset state
of operation; and
wherein the context resetter (130) is configured to reset the context (q[0],q[l]) to
the default context before a decoding of a first set (1042a) of spectral values of the
given audio frame (1040) and between a decoding of any two subsequent sets
(1042a-1042h) of spectral values of the given audio frame (1040) in response to the
one-bit context reset flag (132; arith_reset_flag),
such that an activation of the one-bit context reset flag (132;arith_reset flag) of the
given audio frame (1040) causes a multiple-time resetting of the context (q[0],q[l])
when decoding the multiple sets (1042a-1042h) of spectral values of the audio
frame (1040).
7. The audio decoder (100;200) according to claim 6, wherein the audio decoder is
configured to also receive a grouping side information (scalefactorgrouping); and
wherein the audio decoder is configured to group two or more of the sets (1042a-
1042h) of spectral values for a combination with a common scale factor

information in dependence on the grouping side information
(scale_factor_grouping); and
wherein the context resetter (130) is configured to reset the context (q[0],q[l]) to
the default context between a decoding of two sets (1042a, 1042b) of spectral values
grouped together in response to the one-bit context-reset flag (132;arith_reset_flag).
8. The audio decoder (100;200) according to one of claims 1 to 7,
wherein the audio decoder is configured to receive, as the side information for
resetting the context, a one-bit context reset flag (132;arith_reset_flag) per audio
frame;
when the audio decoder is configured to receive, as the encoded audio information,
a sequence (1070,1072) of encoded audio frames, the sequence of encoded audio
frames comprising single-window frames (1070) and multi-window frames (1072);
wherein the entropy decoder (120) is configured to decode entropy-encoded
spectral values of a multi-window audio frame (1072) following a previous single-
window audio frame (1070) in dependence on a context, which context is based on
a previously-decoded audio information of the previous single window audio frame
(1070) in a non-reset state of operation;
wherein the entropy decoder (120) is configured to decode entropy-encoded
spectral values of a single-window audio frame following a previous multi-window
audio frame (1072) in dependence on a context, which context is based on a
previously-decoded audio information of the previous multi-window audio frame
(1072) in a non-reset state of operation;
wherein the entropy decoder (120) is configured to decode entropy-encoded
spectral values of a single-window audio frame (1012) following a previous single-
window audio frame (1010) in dependence on a context, which context is based on
a previously-decoded audio information of the previous single-window audio frame
(1010) in a non-reset state of operation;
wherein the entropy-decoder (120) is configured to decode entropy-encoded
spectral values of a multi-window audio frame following a previous multi-window
audio frame (1072) in dependence on a context, which context is based on a

previously-decoded audio information of the previous multi-window audio frame
(1072) in a non-reset state of operation;
wherein the context resetter (130) is configured to reset the context (q[0],q[l])
between a decoding of entropy-encoded spectral values of subsequent audio frames
in response to a one-bit context reset flag (132; arith_reset_flag); and
wherein the context resetter (130) is configured to additionally reset, in the case of
a multi-window audio frame, the context (q[0],q[l]) between a decoding of
entropy-encoded spectral values associated with different windows of the multi-
window audio frame in response to the one-bit context reset flag.
9. The audio decoder (100;200) according to one of claims 1 to 8, wherein the audio
decoder is configured to receive, as the side information (132;arith_reset_flag) for
resetting the context (q[0],q[l]), a one-bit context reset flag per audio frame of the
encoded audio information (110;210,224), and
to receive, as the encoded audio information, a sequence of encoded audio frames
(1210,1220,1230), the sequence of encoded audio frames comprising a linear-
prediction-domain audio frame (1210,1220,1230);
wherein the linear-prediction-domain audio frame comprises a selectable number of
transform-coded-excitation portions
(1212b,1212c,1212d!1222a,1222b,1222c,1222d,1232) for exciting a linear-
prediction-domain audio synthesizer (262); and
wherein the context-based entropy decoder (120;240) is configured to decode
spectral values of the transform-coded-excitation portions in dependence on a
context (q[0],q[l]), which context is based on a previously-decoded audio
information in a non-reset of operation; and
wherein the context-resetter (130) is configured to reset, in response to the side
information (132;arith_reset_flag), the context (q[0],q[l]) to the default context
before a decoding of a set of spectral values of a first transform-coded-excitation
portion (1212b,1222a,1232) of a given audio frame (1210,1220,1230), while
omitting a reset of the context to the default context between a decoding of sets of
spectral values of different transform-coded-excitation portions

(1212b,1212c,1212d; 1222a,1222b,1222c,1222d) of the given audio frame
(1210,1220,1230).
10. The audio decoder (100;200) according to one of claims 1 to 9, wherein the audio
decoder is configured to receive an encoded audio information comprising a
plurality of sets of spectral values per audio frame (1320,1330); and
wherein the audio decoder is configured to also receive a grouping side information
(scale_factor_grouping); and
wherein the audio decoder is configured to group
(1322a,1322c,1322d,1330c,1330d) two or more of the sets of spectral values for a
combination with a common scale factor information in dependence on the
grouping side information;
wherein the context resetter (130) is configured to reset the context (q[0],q[l]) to
the default context in response to the grouping side information
(scale_factor_grouping); and
wherein the context resetter (130) is configured to reset the context (q[0],q[l])
between a decoding of sets of spectral values of subsequent groups, and to avoid to
reset the context between a decoding of sets of spectral values of a single group.
11. A method (1800) for providing a decoded audio information on the basis of an
encoded audio information, the method comprising:
decoding (1810) the entropy-encoded audio information taking into account a
context, which is based on a previously-decoded audio information in a non-reset
state of operation,
wherein decoding the entropy-encoded audio information comprises selecting
(1812) a mapping information for deriving the decoded audio information from the
encoded audio information, in dependence on the context, and using (1814) the
selected mapping information for deriving a first portion of the decoded audio
information; and
wherein decoding the entropy-encoded audio information also comprises resetting
(1816) the context for selecting the mapping information to a default context, which

is independent from the previously-decoded audio information, in response to a side
information, and using (1818) the mapping information, which is based on the
default context, for decoding a second portion of the decoded audio information. •
12. An audio encoder (1400; 1500; 1600; 1700) for providing an encoded audio
information (1424) on the basis of an input audio information (1412), the audio
encoder comprising:
a context-based entropy encoder (1420,1440,1450; 1420,1440,1550;
1420,1440,1660; 1420,1440,1770) configured to encode a given audio information
of the input audio information (1412) in dependence on a context (q[0],q[l]), which
context is based on an adjacent audio information, temporally or spectrally adjacent
to the given audio information, in a non-reset state of operation;
wherein the context-based entropy encoder (1420,1440,1450; 1420,1440,1550;
1420,1440,1660; 1420,1440,1770) is configured to select a mapping information
(cum_freq[pki]) for deriving the encoded audio information (1424) from the input
audio information (1412), in dependence on the context; and
wherein the context-based entropy encoder comprises a context resetter (1450;
1550; 1660; 1770) configured to reset the context for selecting the mapping
information to a default context within a contiguous piece of input audio
information (1412), in response to the occurrence of a context reset condition; and
wherein the audio encoder is configured to provide a side information (1480; 1780)
of the encoded audio information (1424) indicating the presence of a context reset
condition.
13. The audio encoder (1400) according to claim 12, wherein the audio encoder is
configured to perform a regular context reset at least once per n frames of the input
audio information.
14. The audio encoder (1500) according to claim 12 or 13, wherein the audio encoder is
configured to switch between a plurality of different coding modes, and wherein the
audio encoder is configured to perform a context reset in response to a change
between two coding modes.

15. The audio encoder (1600) according to one of claims 12 to 14, wherein the audio
encoder is configured to compute or estimate a first number of bits required for
encoding a certain audio information of the input audio information (1212) in
dependence on a non-reset context (1642), which non-reset context is based on an
adjacent audio information, temporally or spectrally adjacent to the certain audio
information, and to compute or estimate a second number of bits required for
encoding the certain audio information using the default context (1644); and
wherein the audio encoder is configured to compare the first number of bits and the
second number of bits to decide whether to provide the encoded audio information
(1424) corresponding to the certain audio information on the basis of the non-reset
context (1642) or the default context (1644), and to signal the result of said decision
using the side information (1480).
16. A Method for providing an encoded audio information (1424) on the basis of an
input audio information (1412), the method comprising:
encoding (1910) a given audio information of the input audio information in
dependence on a context, which context is based on an adjacent audio information,
temporally or spectrally adjacent to the given audio information, in a non-reset state
of operation,
wherein encoding the given audio information in dependence on the context
comprises selecting (1920) a mapping information, for deriving the encoded audio
information from the input audio information, in dependence on the context.
resetting (1930) the context for selecting the mapping information to a default
context within a contiguous piece of input audio information in response to the
occurrence of a context reset condition; and
providing (1940) a side information of the encoded audio information indicating the
presence of the context reset condition.
17. A computer program for performing the method according to claim 11 or claim 16,
when the computer program runs on a computer.
18. An encoded audio signal, the encoded audio signal comprising:

an encoded representation (arithdata) of a plurality of sets of spectral values,
wherein a plurality of the sets of spectral values are encoded in dependence on an
non-reset context, which is dependent on a respective preceding set of spectral
values;
wherein a plurality of the sets of spectral values are encoded in dependence on a
default context, which is independent from a respective preceding set of spectral
values; and
wherein the encoded audio signal comprises a side information (arith_reset_flag)
signaling if a set of spectral coefficients is encoded in dependence on a non-reset
context or in dependence on the default context.

An audio decoder for providing a decoded audio information on the basis of an entropy
encoded audio information comprises a context-based entropy decoder configured to
decode the entropy-encoded audio information in dependence on a context, which context
is based on a previously-decoded audio information in a non-reset state-of-operation. The
context-based entropy decoder is configured to select a mapping information, for deriving
the decoded audio information from the encoded audio information, in dependence on the
context. The context-based entropy decoder comprises a context resetter configured to
reset the context for selecting the mapping information to a default context, which default
context is independent from the previously-decoded audio information, in response to a
side information of the encoded audio information.