Audio Codec Supporting Time-Domain and Frequency-Domain Coding Modes
Description
The present invention is concerned with an audio codec supporting time-domain and
frequency-domain coding modes.
Recently, the MPEG USAC codec has been finalized. USAC (Unified speech and audio
coding) is a codec which codes audio signals using a mix of AAC (Advanced audio
coding), TCX (Transform Coded Excitation) and ACELP (Algebraic Code-Excited Linear
Prediction). In particular, MPEG USAC uses a frame length of 1024 samples and allows
switching between AAC-like frames of 1024 or 8x128 samples, TCX 1024 frames or
within one frame a combination of ACELP frames (256 samples), TCX 256 and TCX 512
frames.
Disadvantageously, the MPEG USAC codec is not suitable for applications necessitating
low delay. Two-way communication applications, for example, necessitate such short
delays. Owing to the USAC frame length of 1024 samples, USAC is not a candidate for
these low delay applications.
In WO 201 147950, it has been proposed to render the USAC approach suitable for lowdelay
applications by restricting the coding modes of the USAC codec to TCX and ACELP
modes, only. Further, it has been proposed to make the frame structure finer so as to obey
the low-delay requirement imposed by low-delay applications.
However, there is still a need for providing an audio codec enabling low coding delay at an
increased efficiency in terms of rate/distortion ratio. Preferably, the codec should be able to
efficiently handle audio signals of different types such as speech and music.
Thus, it is an objective of the present invention to provide an audio codec offering lowdelay
for low-delay applications, but at an increased coding efficiency in terms of, for
example, rate/distortion ratio compared to USAC.
This object is achieved by the subject matter of the pending independent claims.
A basic idea underlying the present invention is that an audio codec supporting both, timedomain
and frequency-domain coding modes, which has low-delay and an increased
coding efficiency in terms of rate/distortion ratio, may be obtained if the audio encoder is
configured to operate in different operating modes such that if the active operating mode is
a first operating mode, a mode dependent set of available frame coding modes is disjoined
to a first subset of time-domain coding modes, and overlaps with a second subset of
frequency-domain coding modes, whereas if the active operating mode is a second
operating mode, the mode dependent set of available frame coding modes overlaps with
both subsets, i.e. the subset of time-domain coding modes as well as the subset of
frequency-domain coding modes. For example, the decision as to which of the first and
second operating mode is accessed, may be performed depending on an available
transmission bitrate for transmitting the data stream. For example, the decision's
dependency may be such that the second operating mode is accessed in case of lower
available transmission bitrates, while the first operating mode is accessed in case of higher
available transmission bitrates. In particular, by providing the encoder with the operating
modes, it is possible to prevent the encoder from choosing any time-domain coding mode
in case of the coding circumstances, such as determined by the available transmission
bitrates, being such that choosing any time-domain coding mode would very likely yield
coding efficiency loss when considering the coding efficiency in terms of rate/distortion
ratio on a long-term basis. To be more precise, the inventors of the present application
found out that suppressing the selection of any time-domain coding mode in case of
(relative) high available transmission bandwidth results in a coding efficiency increase:
while, on a short-term basis, one may assume that a time-domain coding mode is currently
to be preferred over the frequency-domain coding modes, it is very likely that this
assumption turns out to be incorrect if analyzing the audio signal for a longer period. Such
longer analysis or look-ahead is, however, not possible in low-delay applications, and
accordingly, preventing the encoder from accessing any time-domain coding mode
beforehand enables the achievement of an increased coding efficiency.
In accordance with an embodiment of the present invention, the above idea is exploited to
the extent that the data stream bitrate is further increased: While it is quite bitrate
inexpensive to synchronously control the operating mode of encoder and decoder, or does
not even cost any bitrate as the synchronicity is provided by some other means, the fact
that encoder and decoder operate and switch between the operating modes synchronously
may be exploited so as to reduce the signaling overhead for signaling the frame coding
modes associated with the individual frames of the data stream in consecutive portions of
the audio signal, respectively. In particular, while a decoder's associator may be
configured to perform the association of each of the consecutive frames of the data stream
with one of the mode-dependent sets of the plurality of frame-coding modes dependent on
a frame mode syntax element associated with the frames of the data stream, the associator
may particularly change the dependency of the performance of the association depending
on the active operating mode. In particular, the dependency change may be such that if the
active operating mode is the first operating mode, the mode-dependent set is disjoined to
the first subset and overlaps with the second subset, and if the active operating mode is the
second operating mode, the mode-dependent set overlaps with both subsets. However, less
strict solutions increasing the bitrate are by exploiting knowledge on the circumstances
associated with the currently pending operating mode are, however, also feasible.
Advantageous aspects of embodiments of the present invention are the subject of the
dependent claims.
In particular, preferred embodiments of the present invention are described in more detail
below with respect to the figures among which
Fig. 1 shows a block diagram of an audio decoder according to an embodiment;
Fig. 2 shows a schematic of a bijective mapping between a the possible values of
the frame mode syntax element and the frame coding modes of the mode
dependent set in accordance with an embodiment;
Fig. 3 shows a block diagram of a time-domain decoder according to an
embodiment;
Fig. 4 shows a block diagram of a frequency-domain encoder according to an
embodiment;
Fig. 5 block diagram of an audio encoder according to an embodiment;
Fig. 6 shows an embodiment for time-domain and frequency-domain encoders
according to an embodiment.
With regard to the description of the figures it is noted that descriptions of elements in one
figure shall equally apply to elements having the same reference sign associated therewith
in another figure, as not explicitly taught otherwise.
Fig. 1 shows an audio decoder 10 in accordance with an embodiment of the present
invention. The audio decoder comprises a time-domain decoder 12 and a frequencydomain
decoder 14. Further, the audio decoder 10 comprises an associator 16 configured to
associate each of consecutive frames 18a- 18c of a data stream 20 to one out of a modedependent
set of a plurality 22 of frame coding modes which are exemplarily illustrated in
Fig. 1 as A, B and C. There may be more than three frame coding modes, and the number
may thus be changed from three to something else. Each frame 18a-c corresponds to one of
consecutive portions 24a-c of an audio signal 26 which the audio decoder is to reconstruct
from data stream 20.
To be more precise, the associator 16 is connected between an input 28 of decoder 10 on
the one hand, and inputs of time-domain decoder 12 and frequency-domain decoder 14 on
the other hand so as to provide same with associated frames 18a-c in a manner described in
more detail below.
The time-domain decoder 12 is configured to decode frames having one of a first subset 30
of one or more of the plurality 22 of frame-coding modes associated therewith, and the
frequency-domain decoder 14 is configured to decode frames having one of a second
subset 32 of one or more of the plurality 22 of frame-coding modes associated therewith.
The first and second subsets are disjoined to each other as illustrated in Fig. 1. To be more
precise, the time-domain decoder 12 has an output so as to output reconstructed portions
24a-c of the audio signal 26 corresponding to frames having one of the first subsets 30 of
the frame-coding modes associated therewith, and the frequency-domain decoder 14
comprises an output for outputting reconstructed portions of the audio signal 26
corresponding to frames having one of the second subset 32 of frame-coding modes
associated therewith.
As is shown in Fig. 1, the audio decoder 10 may have, optionally, a combiner 34 which is
connected between the outputs of time-domain decoder 12 and frequency-domain decoder
14 on the one hand and an output 36 of decoder 10 on the other hand. In particular,
although Fig. 1 suggests that portions 24a-24c do not overlap each other, but immediately
follow each other in time t, in which case combiner 34 could be missing, it is also possible
that portions 24a-24c are, at least partially, consecutive in time t, but partially overlap each
other such as, for example, in order to allow for time-aliasing cancellation involved with a
lapped transform used by frequency-domain decoder 14, for example, as it is the case with
the subsequently-explained more detailed embodiment of frequency-domain decoder 14.
Prior to further prosecuting with the description of the embodiment of Fig. 1, it should be
noted that the number of frame-coding modes A-C illustrated in Fig. 1 is merely
illustrative. The audio decoder of Fig. 1 may support more than three coding modes. In the
following, frame-coding modes of subset 32 are called frequency-domain coding modes,
whereas frame-coding modes of subset 30 are called time-domain coding modes. The
associator 16 forwards frames 15a-c of any time-domain coding mode 30 to the timedomain
decoder 12, and frames 18a-c of any frequency-domain coding mode to frequencydomain
decoder 14. Combiner 34 correctly registers the reconstructed portions of the audio
signal 26 as output by time-domain and frequency-domain decoders 2 and 4 so as to be
arranged consecutively in time t as indicated in Fig. 1. Optionally, combiner 34 may
perform an overlap-add functionality between frequency-domain coding mode portions 24,
or other specific measures at the transitions between immediately consecutive portions,
such as an overlap-add functionality, for performing aliasing cancellation between portions
output by frequency-domain decoder 14. Forward aliasing cancellation may be performed
between immediately following portions 24a-c output by time-domain and frequencydomain
decoders 12 and 14 separately, i.e. for transitions from frequency-domain coding
mode portions 24 to time-domain coding mode portions 24 and vice-versa. For further
details regarding possible implementations, reference is made to the more detailed
embodiments described further below.
As will be outlined in more detail below, the associator 16 is configured to perform the
association of the consecutive frames 18a-c of the data stream 20 with the frame-coding
modes A-C in a manner which avoids the usage of a time-domain coding mode in cases
where the usage of such time-domain coding mode is inappropriate such as in cases of high
available transmission bitrates where time-domain coding modes are likely to be inefficient
in terms of rate/distortion ratio compared to frequency-domain coding modes so that the
usage of the time-domain frame-coding mode for a certain frame 18a- 18c would very
likely lead to a decrease in coding efficiency.
Accordingly, the associator 16 is configured to perform the association of the frames to the
frame coding modes dependent on a frame mode syntax element associated with the
frames 18a-c in the data stream 20. For example, the syntax of the data stream 20 could be
configured such that each frame 18a-c comprises such a frame mode syntax element 38 for
determining the frame-coding mode, which the corresponding frame 18a-c belongs to.
Further, the associator 16 is configured to operate in an active one of a plurality of
operating modes, or to select a current operating mode out of a plurality of operating
modes. Associator 16 may perform this selection depending on the data stream or
dependent on an external control signal. For example, as will be outlined in more detail
below, the decoder 10 changes its operating mode synchronously to the operating mode
change at the encoder and in order to implement the synchronicity, the encoder may signal
the active operating mode and the change in the active one of the operating modes within
the data stream 20. Alternatively, encoder and decoder 0 may be synchronously
controlled by some external control signal such as control signals provided by lower
transport layers such as EPS or RTP or the like. The control signal externally provided
may, for example, be indicative of some available transmission bitrate.
In order to instantiate or realize the avoidance of inappropriate selections or an
inappropriate usage of time-domain coding modes as outlined above, the associator 16 is
configured to change the dependency of the performance of the association of the frames
18 to the coding modes depending on the active operating mode. In particular, if the active
operating mode is a first operating mode, the mode dependent set of the plurality of frame
coding modes is, for example, the one shown at 40, which is disjoint to the first subset 30
and overlaps the second subset 32, whereas if the active operating mode is a second
operating mode, the mode dependent set is, for example, as shown at 42 in Fig. 1 and
overlaps the first and second subsets 30 and 32.
That is, in accordance with the embodiment of Fig. 1, the audio decoder 10 is controllable
via data stream 20 or an external control signal so as to change its active operating mode
between a first one and a second one, thereby changing the operation mode dependent set
of frame coding modes accordingly, namely between 40 and 42, so that in accordance with
one operating mode, the mode dependent set 40 is disjoint to the set of time-domain coding
modes, whereas in the other operating mode the mode dependent set 42 contains at least
one time-domain coding mode as well as at least one frequency-domain coding mode.
In order to explain the change in the dependency of the performance of the association of
the associator 16 in more detail, reference is made to Fig. 2, which exemplarily shows a
fragment out of data stream 20, the fragment including a frame mode syntax element 38
associated with a certain one of frames 18a to 18c of Fig. 1. In this regard, it is briefly
noted that the structure of the data stream 20 exemplified in Fig. 1 has been applied merely
for illustrative purposes, and that a different structure may be applied as well. For example,
although the frames 18a to 18c in Fig. 1 are shown as simply-connected or continuous
portions of data stream 20 without any interleaving therebetween, such interleaving may be
applied as well. Moreover, although Fig. 1 suggests that the frame mode syntax element 38
is contained within the frame it refers to, this is not necessarily the case. Rather, the frame
mode syntax elements 38 may be positioned within data stream 20 outside frames 18a to
18c. Further, the number of frame mode syntax elements 38 contained within data stream
20 does not need to be equal to the number of frames 18a to 18c in data stream 20. Rather,
the frame mode syntax element 38 of Fig. 2, for example, may be associated with more
than one of frames 18a to 18c in data stream 20.
In any case, depending on the way the frame mode syntax element 38 has been inserted
into data stream 20, there is a mapping 44 between the frame mode syntax element 38 as
contained and transmitted via data stream 20, and a set 46 of possible values of the frame
mode syntax element 38. For example, the frame mode syntax element 38 may be inserted
into data stream 20 directly, i.e. using a binary representation such as, for example, PCM,
or using a variable length code and/or using entropy coding, such as Huffman or arithmetic
coding. Thus, the associator 16 may be configured to extract 48, such as by decoding, the
frame mode syntax element 38 from data stream 20 so as to derive any of the set 46 of
possible values wherein the possible values are representatively illustrated in Fig. 2 by
small triangles. At the encoder side, the insertion 50 is done correspondingly, such as by
encoding.
That is, each possible value which the frame mode syntax element 38 may possibly
assume, i.e. each possible value within the possible value range 46 of frame mode syntax
element 38, is associated with a certain one of the plurality of frame coding modes A, B
and C. In particular, there is a bijective mapping between the possible values of set 46 on
the one hand, and the mode dependent set of frame coding modes on the other hand. The
mapping, illustrated by the double-headed arrow 52 in Fig. 2, changes depending on the
active operating mode. The bijective mapping 52 is part of the functionality of the
associator 16 which changes mapping 52 depending on the active operating mode. As
explained with respect to Fig. 1, while the mode dependent set 40 or 42 overlaps with both
frame coding mode subsets 30 and 32 in case of the second operating mode illustrated in
Fig. 2, the mode dependent set is disjoint to, i.e. does not contain any elements of, subset
30 in case of the first operating mode. In other words, the bijective mapping 52 maps the
domain of possible values of the frame mode syntax element 38 onto the co-domain of
frame coding modes, called the mode dependent set 50 and 52, respectively. As illustrated
in Fig. 1 and Fig. 2 by use of the solid lines of the triangles for the possible values of set
46, the domain of bijective mapping 52 may remain the same in both operating modes, i.e.
the first and second operating mode, while the co-domain of bijective mapping 52 changes
as is illustrated and described above.
However, even the number of possible values within set 46 may change. This is indicated
by the triangle drawn with a dashed line in Fig. 2. To be more precise, the number of
available frame coding modes may be different between the first and second operating
mode. If so, however, the associator 16 is in any case still implemented such that the codomain
of bijective mapping 52 behaves as outlined above: there is no overlap between the
mode dependent set and subset 30 in case of the first operating mode being active.
Stated differently, the following is noted. Internally, the value of the frame mode syntax
element 38 may be represented by some binary value, the possible value range of which
accommodates the set 46 of possible values independent from the currently active
operating mode. To be even more precise, associator 16 internally represents the value of
the frame syntax element 38 with a binary value of a binary representation. Using this
binary values, the possible values of set 46 are sorted into an ordinal scale so that the
possible values of set 46 remain comparable to each other even in case of a change of the
operating mode. The first possible value of set 46 in accordance with this ordinal scale may
for example, be defined to be the one associated with the highest probability among the
possible values of set 46, with the second one of possible values of set 46 continuously
being the one with the next lower probability and so forth. Accordingly, the possible values
of frame mode syntax element 38 are thus comparable to each other despite a change of the
operating mode. In the latter example, it may occur that domain and co-domain of bijective
mapping 52, i.e. the set of possible values 46 and the mode dependent set of frame coding
modes remains the same despite the active operating mode changing between the first and
second operating modes, but the bijective mapping 52 changes the association between the
frame coding modes of the mode dependent set on the one hand, and the comparable
possible values of set 46 on the other hand. In the latter embodiment, the decoder 10 of
Fig. 1 is still able to take advantage of an encoder which acts in accordance with the
subsequently explained embodiments, namely by refraining from selecting the
inappropriate time-domain coding modes in case of the first operating mode. By
associating more probable possible values of set 46 solely with frequency-domain coding
modes 32 in case of the first operating mode, while using the lower probable possible
values of set 46 for the time-domain coding modes 30 only during the first operating mode,
while changing this policy in case of the second operating mode results in a higher
compression rate for data stream 20 if using entropy coding for insertion/extraction of
frame mode syntax element 38 into/from data stream 20. In other words, while in the first
operating mode, none of the time-domain coding modes 30 may be associated with a
possible value of set 46 having associated therewith a probability higher than the
probability for a possible value mapped by mapping 52 onto any of the frequency-domain
coding modes 32, such a case exists in the second operating mode where at least one timedomain
coding mode 30 is associated with such a possible value having associated
therewith a higher probability than another possible value associated with, according to
mapping 52, a frequency-domain coding mode 32.
The just mentioned probability associated with possible values 46 and optionally used for
encoding/decoding same may be static or adaptively changed. Different sets of probability
estimations may be used for different operating modes. In case of adaptively changing the
probability, context-adaptive entropy coding may be used.
As illustrated in Fig. 1, one preferred embodiment for the associator 16 is such that the
dependency of the performance of the association depends on the active operating mode,
and the frame mode syntax element 38 is coded into and decoded from the data stream 20
such that a number of the differentiable possible values within set 46 is independent from
the active operating mode being the first or the second operating mode. In particular, in the
case of Fig. 1 the number of differentiable possible values is two, as also illustrated in Fig.
2 when considering the triangles with the solid lines. In that case, for example, the
associator 6 may be configured such that if the active operating mode is the first operating
mode, the mode dependent set 40 comprises a first and a second frame coding mode A and
B of the second subset 32 of frame coding modes, and the frequency-domain decoder 14,
which is responsible for these frame coding modes, is configured to use different timefrequency
resolutions in decoding the frames having one of the first and second frame
coding modes A and B associated therewith. By this measure, one bit, for example, would
be sufficient to transmit the frame mode syntax element 38 within data stream 20 directly,
i.e. without any further entropy coding, wherein merely the bijective mapping 52 changes
upon a change from the first operating mode to the second operating mode and vice versa.
As will be outlined in more detail below with respect to Figs. 3 and 4, the time-domain
decoder 1 may be a code-excited linear-prediction decoder, and the frequency-domain
decoder may be a transform decoder configured to decode the frames having any of the
second subset of frame coding modes associated therewith, based on transform coefficient
levels encoded into data stream 20.
For example, see Fig. 3. Fig. 3 shows an example for the time-domain decoder 12 and a
frame associated with a time-domain coding mode so that same passes time-domain
decoder 12 to yield a corresponding portion 24 of the reconstructed audio signal 26. In
accordance with the embodiment of Fig. 3 - and in accordance with the embodiment of
Fig. 4 to be described later - the time-domain decoder 1 as well as the frequency-domain
decoder are linear prediction based decoders configured to obtain linear prediction filter
coefficients for each frame from the data stream 12. Although Figs. 3 and 4 suggest that
each frame 18 may have linear prediction filter coefficients 16 incorporated therein, this is
not necessarily the case. The LPC transmission rate at which the linear prediction
coefficients 60 are transmitted within the data stream 2 may be equal to the frame rate of
frames 18 or may differ therefrom. Nevertheless, encoder and decoder may synchronously
operate with, or apply, linear prediction filter coefficients individually associated with each
frame by interpolating from the LPC transmission rate onto the LPC application rate.
As shown in Fig. 3, the time-domain decoder 12 may comprise a linear prediction
synthesis filter 62 and an excitation signal constructor 64. As shown in Fig. 3, the linear
prediction synthesis filter 62 is fed with the linear prediction filter coefficients obtained
from data stream 12 for the current time-domain coding mode frame 18. The excitation
signal constructor 64 is fed with a excitation parameter or code such as a codebook index
66 obtained from data stream 12 for the currently decoded frame 18 (having a time-domain
coding mode associated therewith). Excitation signal constructor 64 and linear prediction
synthesis filter 62 are connected in series so as to output the reconstructed corresponding
audio signal portion 24 at the output of synthesis filter 62. In particular, the excitation
signal constructor 64 is configured to construct an excitation signal 68 using the excitation
parameter 66 which may be, as indicated in Fig. 3, contained within the currently decoded
frame having any time-domain coding mode associated therewith. The excitation signal 68
is a kind of residual signal, the spectral envelope of which is formed by the linear
prediction synthesis filter 62. In particular, the linear prediction synthesis filter is
controlled by the linear prediction filter coefficients conveyed within data stream 20 for the
currently decoded frame (having any time-domain coding mode associated therewith), so
as to yield the reconstructed portion 24 of the audio signal 26.
For further details regarding a possible implementation of the CELP decoder of Fig. 3,
reference is made to known codecs such as the above mentioned USAC [2] or the AMRWB+
codec [1], for example. According to latter codecs, the CELP decoder of Fig. 3 may
be implemented as an ACELP decoder according to which the excitation signal 68 is
formed by combining a code/parameter controlled signal, i.e. innovation excitation, and a
continuously updated adaptive excitation resulting from modifying a finally obtained and
applied excitation signal for an immediately preceding time-domain coding mode frame in
accordance with a adaptive excitation parameter also conveyed within the data stream 12
for the currently decoded time-domain coding mode frame 18. The adaptive excitation
parameter may, for example, define pitch lag and gain, prescribing how to modify the past
excitation in the sense of pitch and gain so as to obtain the adaptive excitation for the
current frame. The innovation excitation may be derived from a code 66 within the current
frame, with the code defining a number of pulses and their positions within the excitation
signal. Code 66 may be used for a codebook look-up, or otherwise - logically or
arithmetically - define the pulses of the innovation excitation - in terms of number and
location, for example.
Similarly, Fig. 4 shows a possible embodiment for the frequency-domain decoder 14. Fig.
4 shows a current frame 18 entering frequency-domain decoder 14, with frame 18 having
any frequency-domain coding mode associated therewith. The frequency-domain decoder
14 comprises a frequency-domain noise shaper 70, the output of which is connected to a
retransformer 72. The output of the re-transformer 72 is, in turn, the output of frequencydomain
decoder 14, outputting a reconstructed portion of the audio signal corresponding to
frame 8 having currently been decoded.
As shown in Fig. 4, data stream 20 may convey transform coefficient levels 74 and linear
prediction filter coefficients 76 for frames having any frequency-domain coding mode
associated therewith. While the linear prediction filter coefficients 76 may have the same
structure as the linear prediction filter coefficients associated with frames having any timedomain
coding mode associated therewith, the transform coefficient levels 74 are for
representing the excitation signal for frequency-domain frames 18 in the transform domain.
As known from USAC, for example, the transform coefficient levels 74 may be coded
differentially along the spectral axis. The quantization accuracy of the transform
coefficient levels 74 may be controlled by a common scale factor or gain factor. The scale
factor may be part of the data stream and assumed to be part of the transform coefficient
levels 74. However, any other quantization scheme may be used as well. The transform
coefficient levels 74 are fed to frequency-domain noise shaper 70. The same applies to the
linear prediction filter coefficients 76 for the currently decoded frequency-domain frame
18. The frequency-domain noise shaper 70 is then configured to obtain an excitation
spectrum of an excitation signal from the transform coefficient levels 74 and to shape this
excitation spectrum spectrally in accordance with the linear prediction filter coefficients
76. To be more precise, the frequency-domain noise shaper 70 is configured to dequantize
the transform coefficient levels 74 in order to yield the excitation signal's spectrum. Then,
the frequency-domain noise shaper 70 converts the linear prediction filter coefficients 76
into a weighting spectrum so as to correspond to a transfer function of a linear prediction
synthesis filter defined by the linear prediction filter coefficients 76. This conversion may
involve an ODFT applied to the LPCs so as to turn the LPCs into sprectral wheighting
values. Further details may be obtained from the USAC standard. Using the weighting
spectrum the frequency-domain noise shaper 70 shapes - or weights - the excitation
spectrum obtained by the transform coefficient levels 74, thereby obtaining the excitation
signal spectrum. By the shaping/weighting, the quantization noise introduced at the
encoding side by quantizing the transform coefficients is shaped so as to be perceptually
less significant. The retransformer 72 then retransforms the shaped excitation spectrum as
output by frequency domain noise shaper 70 so as to obtain the reconstructed portion
corresponding to the just decoded frame 18.
As already mentioned above, the frequency-domain decoder 14 of Fig. 4 may support
different coding modes. In particular, the frequency-domain decoder 14 may be configured
to apply different time-frequency resolutions in decoding frequency-domain frames having
different frequency-domain coding modes associated therewith. For example, the
retransform performed by retransformer 72 may be a lapped transform, according to which
consecutive and mutually overlapping windowed portions of the signal to be transformed
are subdivided into individual transforms, wherein retransforming 72 yields a
reconstruction of these windowed portions 78a, 78b and 78c. The combiner 34 may, as
already noted above, mutually compensate aliasing occurring at the overlap of these
windowed portions by, for example, an overlap-add process. The lapped transform or
lapped retransform of retransformer 72 may be, for example, a critically sampled
transform/retransform which necessitates time aliasing cancellation. For example,
retransformer 72 may perform an inverse MDCT. In any case, the frequency-domain
coding modes A and B may, for example, differ from each other in that the portion 18
corresponding to the currently decoded frame 18 is either covered by one windowed
portion 78 - also extending into the preceding and succeeding portions thereby yielding
one greater set of transform coefficient levels 74 within frame 18, or into two consecutive
windowed sub-portions 78c and 78b - being mutually overlapping and extending into, and
overlapping with, the preceding portion and succeeding portion, respectively - thereby
yielding two smaller sets of transform coefficient levels 74 within frame 18. Accordingly,
while decoder and frequency-domain noise shaper 70 and retransformer 72 may, for
example, perform two operations - shaping and retransforming - for frames of mode A,
they manually perform one operation per frame of frame coding mode B for example.
The embodiments for an audio decoder described above were especially designed to take
advantage of an audio encoder which operates in different operating modes, namely so as
to change the selection among frame coding modes between these operating modes to the
extent that time-domain frame coding modes are not selected in one of these operating
modes, but merely in the other. It should be noted, however, that the embodiments for an
audio encoder described below would also - at least as far as a subset of these
embodiments is concerned - fit to an audio decoder which does not support different
operating modes. This is at least true for those encoder embodiments according to which
the data stream generation does not change between these operation modes. In other words,
in accordance with some of the embodiments for an audio encoder described below, the
restriction of the selection of frame coding modes to frequency-domain coding modes in
one of the operating modes does not reflect itself within the data stream 12 where the
operating mode changes are, insofar, transparent (except for the absence of time-domain
frame coding modes during one of these operating modes being active). However, the
especially dedicated audio decoders according to the various embodiments outlined above
form, along with respective embodiments for an audio encoder outlined above, audio
codecs which take additional advantage of the frame coding mode selection restriction
during a special operating mode corresponding, as outlined above, to special transmission
conditions, for example.
Fig. 5 shows an audio encoder according to an embodiment of the present invention. The
audio encoder of Fig. 5 is generally indicated at 100 and comprises an associator 102, a
time-domain encoder 104 and a frequency-domain encoder 106, with associator 102 being
connected between an input 108 of audio encoder 100 on the one hand and inputs of timedomain
encoder 104 and frequency-domain encoder 106 on the other hand. The outputs of
time-domain encoder 104 and frequency-domain encoder 106 are connected to an output
10 of audio encoder 100. Accordingly, the audio signal to be encoded, indicated at 1 2 in
Fig. 5, enters input 108 and the audio encoder 100 is configured to form a data stream 114
therefrom.
The associator 102 is configured to associate each of consecutive portions 1 6a to 116c
which correspond to the aforementioned portions 24 of the audio signal 112, with one out
of a mode dependent set of a plurality of frame coding modes (see 40 and 42 of Figs. 1 to
4)·
The time-domain encoder 104 is configured to encode portions 116a to 116c having one of
a first subset 30 of one or more of the plurality 22 of frame coding modes associated
therewith, into a corresponding frame 118a to 118c of the data stream 114. The frequencydomain
encoder 106 is likewise responsible for encoding portions having any frequency -
domain coding mode of set 32 associated therewith into a corresponding frame 118a to
18c of data stream 114.
The associator 102 is configured to operate in an active one of a plurality of operating
modes. To be more precise, the associator 102 is configured such that exactly one of the
plurality of operating modes is active, but the selection of the active one of the plurality of
operating modes may change during sequentially encoding portions 116a to 116c of audio
signal 112.
In particular, the associator 102 is configured such that if the active operating mode is a
first operating mode, the mode dependent set behaves like set 40 of Fig. 1, namely same is
disjoint to the first subset 30 and overlaps with the second subset 32, but if the active
operating mode is a second operating mode, the mode dependent set of the plurality of
encoding modes behaves like mode 42 of Fig. 1, i.e. same overlaps with the first and
second subsets 30 and 32.
As outlined above, the functionality of the audio encoder of Fig. 5 enables to externally
control the encoder 100 such that same is prevented from disadvantageously selecting any
time-domain frame coding mode although the external conditions, such as the transmission
conditions, are such that preliminarily selecting any time-domain frame coding frame
would very likely yield a lower coding efficiency in terms of rate/distortion ratio when
compared to restricting the selection to frequency-domain frame coding modes only. As
shown in Fig. 5, associator 102 may, for example, be configured to receive an external
control signal 120. Associator 102 may, for example, be connected to some external entity
such that the external control signal 120 provided by the external entity is indicative of an
available transmission bandwidth for a transmission of data stream 14. This external
entity may, for example, be part of an underlying lower transmission layer such as lower in
terms of the OSI layer model. For example, the external entity may be part of an LTE
communication network. Signal 122 may, naturally, be provided based on an estimate of
an actual available transmission bandwidth or an estimate of a mean future available
transmission bandwidth. As already noted above with respect to Figs. 1 to 4, the "first
operating mode" may be associated with available transmission bandwidths being lower
than a certain threshold, whereas the "second operating mode" may be associated with
available transmission bandwidths exceeding the predetermined threshold, thereby
preventing the encoder 100 from choosing any time-domain frame coding mode in
inappropriate conditions where the time-domain coding is very likely to yield more
inefficient compression, namely if the available transmission bandwidths is lower than a
certain threshold.
It should be noted, however, that the control signal 120 may also be provided by some
other entity such as, for example, a speech detector which analyzes the audio signal to be
reconstructed, i.e. 112, so as to distinguish between speech phases, i.e. time intervals,
during which a speech component within the audio signal 2 is predominant, and nonspeech
phases, where other audio sources such as music or the like are predominant within
audio signal 112. The control signal 120 may be indicative of this change in speech and
non-speech phases and the associator 102 may be configured to change between the
operating modes accordingly. For example, in speech phases the associator 102 could enter
the aforementioned "second operating mode" while the "first operating mode" could be
associated with non-speech phases, thereby obeying the fact that choosing time-domain
frame coding modes during non-speech phases very likely results in a less-efficient
compression.
While the associator 102 may be configured to encode a frame mode syntax element 122
(compare syntax element 38 in Fig. 1) into the data stream 114 so as to indicate for each
portion 116a to 116c which frame coding mode of the plurality of frame coding modes the
respective portion is associated with, the insertion of this frame mode syntax element 122
into a data stream 114 may not depend on the operating mode so as to yield the data stream
20 with the frame mode syntax elements 38 of Figs. 1 to 4. As already noted above, the
data stream generation of data stream 1 4 may be performed independent from the
operating mode currently active.
However, in terms of bitrate overhead, it is to be preferred if the data stream 114 is
generated by the audio encoder 100 of Fig. 5 so as to yield the data stream 20 discussed
above with respect to the embodiments of Figs. 1 to 4, according to which the data stream
generation is advantageously adapted to the currently active operating mode.
Accordingly, in accordance with an embodiment of the audio encoder 100 of Fig. 5 fitting
to the embodiments described above for the audio decoder with respect to Figs. 1 to 4, the
associator 102 may be configured to encode the frame mode syntax element 122 into the
data stream 114 using the bijective mapping 52 between the set of possible values 46 of the
frame mode syntax element 122 associated with a respective portion 116a to 116c on the
one hand, and the mode dependent set of the frame coding modes on the other hand, which
bijective mapping 52 changes depending on the active operating mode. In particular, the
change may be such that if the active operating mode is a first operating mode, the mode
dependent set behaves like set 40, i.e. same is disjoint to the first subset 30 and overlaps
with the second subset 32, whereas if the active operating mode is the second operating
mode the mode dependent set is like set 42, i.e. it overlaps with both the first and second
subsets 30 and 32. In particular, as already noted above, the number of possible values in
the set 46 may be two, irrespective of the active operating mode being the first or second
operating mode, and the associator 102 may be configured such that if the active operating
mode is the first operating mode, the mode dependent set comprises frequency-domain
frame coding modes A and B, and the frequency-domain encoder 106 may be configured
to use different time-frequency resolutions in encoding respective portions 6a to 116c
depending on their frame coding being mode A or mode B.
Fig. 6 shows an embodiment for a possible implementation of the time-domain encoder
104 and a frequency-domain encoder 106 corresponding to the fact already noted above,
according to which code-excited linear-prediction coding may be used for the time-domain
frame coding mode, while transform coded excitation linear prediction coding is used for
the frequency-domain coding modes. Accordingly, according to Fig. 6 the time-domain
encoder 104 is a code-excited linear-prediction encoder and the frequency-domain encoder
06 is a transform encoder configured to encode the portions having any frequency-domain
frame coding mode associated therewith using transform coefficient levels, and encode
same into the corresponding frames 118a to 118c of the data stream 14.
In order to explain a possible implementation for time-domain encoder 104 and frequencydomain
encoder 106, reference is made to Fig. 6. According to Fig. 6, frequency-domain
encoder 106 and time-encoder 104 co-own or share an LPC analyzer 130. It should be
noted, however, that this circumstance is not critical for the present embodiment and that a
different implementation may also be used according to which both encoders 104 and 106
are completely separated from each other. Moreover, with regard to the encoder
embodiments as well as the decoder embodiments described above with respect to Figs. 1
and 4, it is noted that the present invention is not restricted to cases where both coding
modes, i.e. frequency-domain frame coding modes as well as time-domain frame coding
modes, are linear prediction based. Rather, encoder and decoder embodiments are also
transferable to other cases where either one of the time-domain coding and frequencydomain
coding is implemented in a different manner.
Coming back to the description of Fig. 6, the frequency-domain encoder 106 of Fig. 6
comprises, besides LPC analyzer 130, a transformer 132, an LPC-to-frequency domain
weighting converter 134, a frequency-domain noise shaper 136 and a quantizer 138.
Transformer 132, frequency domain noise shaper 136 and quantizer 138 are serially
connected between a common input 140 and an output 142 of frequency-domain encoder
106. The LPC converter 134 is connected between an output of LPC analyzer 130 and a
weighting input of frequency domain noise shaper 136. An input of LPC analyzer 130 is
connected to common input 140.
As far as the time-domain encoder 104 is concerned, same comprises, besides the LPC
analyzer 130, an LP analysis filter 144 and a code based excitation signal approximator
146 both being serially connected between common input 140 and an output 8 of timedomain
encoder 104. A linear prediction coefficient input of LP analysis filter 144 is
connected to the output of LPC analyzer 130.
In encoding the audio signal 112 entering at input 140, the LPC analyzer 130 continuously
determines linear prediction coefficients for each portion 116a to 116c of the audio signal
112. The LPC determination may involve autocorrelation determination of consecutive -
overlapping or non-overlapping - windowed portions of the audio signal - with performing
LPC estimation onto the resulting autocorrelations (optionally with previously subjecting
the autocorrelations to Lag windowing) such as using a (Wiener-)Levison-Durbin
algorithm or Schur algorithm or other.
As described with respect to Figs. 3 and 4, LPC analyzer 130 does not necessarily signal
the linear predication coefficients within data stream 1 4 at an LPC transmission rate equal
to the frame rate of frames 118a to 118c. A rate even higher than that rate may also be
used, generally, LPC analyzer 130 may determine the LPC information 60 and 76 at an
LPC determination rate defined by the above mentioned rate of autocorrelations, for
example, based on which the LPCs are determined. Then, LPC analyzer 130 may insert the
LPC information 60 and 76 into the data stream at an LPC transmission rate which may be
lower than the LPC determination rate, and TD and FD encoders 104 and 106, in turn, may
apply the linear prediction coefficients with updating same at an LPC application rate
which is higher than the LPC transmission rate, by interpolating the transmitted LPC
information 60 and 76 within frames 118a to 118c of data stream 114. In particular, as the
FD encoder 106 and the FD decoder, apply the LPC coefficients once per transform, the
LPC application rate within FD frames may be lower than the rate at which the LPC
coefficients applied in the TD encoder/decoder are adapted/updated by interpolating from
the LPC transmission rate. As the interpolation may also be performed, synchronously, at
the decoding side, the same linear prediction coefficients are available for time-domain and
frequency-domain encoders on the one hand and time-domain and frequency-domain
decoders on the other hand. In any case, LPC analyzer 130 determines linear-prediction
coefficients for the audio signal 112 at some LPC determination rate equal to or higher
than the frame rate and inserts same into the data stream at a LPC transmission rate which
may be equal to the LPC determination rate or lower than that. The LP analysis filter 144
may, however, interpolate so as to update the LPC analysis filter at an LPC application rate
higher than the LPC transmission rate. LPC converter 134 may or may not perform
interpolation so as to determine LPC coeffiencts for each transform or each LPC to spectral
weighting conversion necessary. In order to transmit the LPC coefficients, same may be
subject to quantization in an appropriate domain such as in the LSF/LSP domain.
The time-domain encoder 104 may operate as follows. The LP analysis filter may filter
time-domain coding mode portions of the audio signal 112 depending on the linear
prediction coefficient output by LPC analyzer 130. At the output of LP analysis filter 144,
an excitation signal 150 is thus derived. The excitation signal is approximated by
approximator 146. In particular, approximator 146 sets a code such as codebook indices or
other parameters to approximate the excitation signal 150 such as by minimizing or
maximizing some optimization measure defined, for example, by a deviation of excitation
signal 150 on the one hand and the synthetically generated excitation signal as defined by
the codebook index on the other hand in the synthesized domain, i.e. after applying the
respective synthesis filter according to the LPCs onto the respective excitation signals. The
optimization measure may optionally be perceptually emphasized deviations at
perceptually more relevant frequency bands. The innovation excitation determined by the
code set by the approximator 146, may be called innovation parameter.
Thus, approximator 146 may output one or more innovation parameters per time-domain
frame coding mode portion so as to be inserted into corresponding frames having a timedomain
coding mode associated therewith via, for example, frame mode syntax element
122. The frequency-domain encoder 106, in turn, may operate as follows. The transformer
132 transforms frequency-domain portions of the audio signal 112 using, for example, a
lapped transform so as to obtain one or more spectra per portion. The resulting
spectrogram at the output of transformer 132 enters the frequency domain noise shaper 136
which shapes the sequence of spectra representing the spectrogram in accordance with the
LPCs. To this end, the LPC converter 134 converts the linear prediction coefficients of
LPC analyzer 130 into frequency-domain weighting values so as to spectrally weight the
spectra. This time, the spectral weight is performed such that an LP analysis filter's
transfer function results. That is, an ODFT may be, for example, used so as to convert the
LPC coefficients into spectral weights which may then be used to divide the spectra output
be transformer 132, whereas multiplication is used at the decoder side.
Thereinafter, quantizer 138 quantizes the resulting excitation spectrum output by
frequency-domain noise shaper 136 into transform coefficient levels 60 for insertion into
the corresponding frames of data stream 114.
In accordance with the embodiments described above, an embodiment of the present
invention may be derived when modifying the USAC codec discussed in the introductory
portion of the specification of the present application by modifying the USAC encoder to
operate in different operating modes so as to refrain from choosing the ACELP mode in
case of a certain one of the operating modes. In order to enable the achievement of a lower
delay, the USAC codec may be further modified in the following way: for example,
independent from the operating mode, only TCX and ACELP frame coding modes may be
used. To achieve lower delay, the frame length may be reduced in order to reach the
framing of 20 milliseconds. In particular, in rendering a USAC codec more efficient in
accordance with the above embodiments, the operation modes of USAC, namely
narrowband (NB), wideband (WB) and super-wideband (SWB), may be amended such that
merely a proper subset of the overall available frame coding modes are available within the
individual operation modes in accordance with the subsequently explained table:
As the above table makes clear, in the embodiments described above, the decoder's
operation mode may not only be determined from an external signal or the data stream
exclusively, but based on a combination of both. For example, in the above table, the data
stream may indicate to the decoder a main mode, i.e. NB, WB, SWB, FB, by way of a
coarse operation mode syntax element which is present in the data stream in some rate
which may be lower than the frame rate. The encoder inserts this syntax element in
addition to syntax elements 38. The exact operation mode, however, may necessitate the
inspection of an additional external signal indicative of the available birate. In case of
SWB, for example, the exact mode depends on the available bitrate lying below 48kbps,
being equal to or greater than 48kbps, and being lower than 96kbps, or being equal to or
greater than 96kbps.
Regarding the above embodiments it should be noted that, although in accordance with
alternative embodiments, it is preferred if the set of all plurality of frame coding modes
with which the frames/time portions of the information signal are associatable, exclusively
consists of time-domain or frequency-domain frame coding modes, this may be different,
so that there may also be one or more than one frame coding mode which is neither timedomain
nor frequency-domain coding mode.
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. Some or all of the method steps may
be executed by (or using) a hardware apparatus, like for example, a microprocessor, a
programmable computer or an electronic circuit. In some embodiments, some one or more
of the most important method steps may be executed by such an apparatus.
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 Blu-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. The data carrier,
the digital storage medium or the recorded medium are typically tangible and/or nontransitionary.
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.
A further embodiment according to the invention comprises an apparatus or a system
configured to transfer (for example, electronically or optically) a computer program for
performing one of the methods described herein to a receiver. The receiver may, for
example, be a computer, a mobile device, a memory device or the like. The apparatus or
system may, for example, comprise a file server for transferring the computer program to
the receiver .
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 enibodiments 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.
Literature:
[1]: 3GPP, "Audio codec processing functions; Extended Adaptive Multi-Rate - Wideband
(AMR-WB+) codec; Transcoding functions", 2009, 3GPP TS 26.290.
[2]: USAC codec (Unified Speech and Audio Codec), ISO/IEC CD 23003-3 dated
September 24, 2010
Claims
Audio decoder comprising
a time-domain decoder (12);
a frequency-domain decoder (14);
an associator (16) configured to associate each of consecutive frames (18a-c) of a
data stream (20), each of which represents a corresponding one of consecutive
portions (24a-24c) of an audio signal, with one out of a mode dependent set of a
plurality (22) of frame coding modes,
wherein the time-domain decoder (12) is configured to decode frames (18a-c)
having one of a first subset (30) of one or more of the plurality (22) of frame coding
modes associated therewith, and the frequency-domain decoder (14) is configured
to decode frames (18a-c) having one of a second subset (32) of one or more of the
plurality (22) of frame coding modes associated therewith, the first and second
subsets being disjoint to each other;
wherein the associator (16) is configured to perform the association dependent on a
frame mode syntax element (38) associated with the frames (18a-c) in the data
stream (20), and operate in an active one of a plurality of operating modes with
selecting the active operating mode out of the plurality of operating modes
depending on the data stream and/or an external control signal, and changing the
dependency of the performance of the association depending on the active
operating mode.
Audio decoder according to claim 1, wherein the associator (16) is configured such
that if the active operating mode is a first operating mode, the mode dependent set
(40) of the plurality of frame coding modes is disjoint to the first subset (30) and
overlaps with the second subset (32), and
if the active operating mode is a second operating mode, the mode dependent set
(42) of the plurality of frame coding modes overlaps with the first and second
subsets (30, 32).
3. Audio decoder according to claim 1 or 2, wherein the frame mode syntax element is
coded into the data stream (20) so that a number of differentiable possible values
for the frame mode syntax element (38) relating to each frame is independent from
the active operating mode being the first or second operating mode.
4. Audio decoder according to claim 3, wherein the number of differentiable possible
values is two and the associator (16) is configured such that, if the active operating
mode is the first operating mode, the mode dependent set (40) comprises a first and
a second frame coding mode of the second subset (32) of one or more frame coding
modes, and the frequency-domain decoder (14) is configured to use different timefrequency
resolutions in decoding frames having the first and second frame coding
mode associated therewith.
5. Audio decoder according to any of the previous claims, wherein the time-domain
decoder (12) is a code-excited linear-prediction decoder.
6. Audio decoder according to any of the previous claims, wherein the frequencydomain
decoder is a transform decoder configured to decode the frames having one
of the second subset (32) of one or more of the frame coding modes associated
therewith, based on transform coefficient levels encoded therein.
7. Audio decoder according to any of the previous claims, wherein the time-domain
decoder (12) and the frequency-domain decoder are LP based decoders configured
to obtain linear prediction filter coefficients for each frame from the data stream,
wherein the time-domain decoder (12) is configured to reconstruct the portions of
the audio signal (26) corresponding to the frames having one of the first subset of
one or more of the frame coding modes associated therewith by applying an LP
synthesis filter depending on the LPC filter coefficients for the frames having one
of the first subset of one or more of the plurality of frame coding modes associated
therewith, onto an excitation signal constructed using codebook indices in the
frames having one of the first subset of one or more of the plurality of frame coding
modes associated therewith, and the frequency-domain decoder (14) is configured
to reconstruct the portions of the audio signal corresponding to the frames having
one of the second subset of one or more of the frame coding modes associated
therewith by shaping an excitation spectrum defined by transform coefficient levels
in the frames having one of the second subset associated therewith, in accordance
with the LPC filter coefficients for the frames having one of the second subset
associated therewith, and retransforming the shaped excitation spectrum.
8. Audio encoder comprising
a time-domain encoder (104);
a frequency-domain encoder (106); and
an associator (102) configured to associate each of consecutive portions ( 1 16a-c) of
an audio signal ( 1 12) with one out of a mode dependent set (40, 42) of a plurality
(22) of frame coding modes,
wherein the time-domain encoder (104) is configured to encode portions having
one of a first subset (30) of one or more of the plurality (22) of frame coding modes
associated wherewith, into a corresponding frame ( 118a-c) of a data stream ( 114),
and wherein the frequency-domain encoder (106) is configured to encode portions
having one of a second subset (32) of one or more of the plurality of encoding
modes associated therewith, into a corresponding frame of the data stream,
wherein the associator (102) is configured to operate in an active one of a plurality
of operating modes such that, if the active operating mode is a first operating mode,
the mode dependent set (40) of the plurality of frame coding modes is disjoint to
the first subset (30) and overlaps with the second subset (32) and if the active
operating mode is a second operating mode, the mode dependent set of the plurality
of encoding modes overlaps with the first and second subset (30, 32).
9. Audio encoder according to claim 8, wherein the associator (102) is configured to
encode a frame mode syntax element (122) into the data stream ( 114) so as to
indicate, for each portion, as to which frame coding mode of the plurality of frame
coding modes the respective portion is associated with.
10. Audio encoder according to claim 9, wherein the associator (102) is configured to
encode the frame mode syntax element (122) into the data stream ( 114) using a
bijective mapping between a set of possible values of the frame mode syntax
element associated with a respective portion on the one hand, and the mode
dependent set of the frame coding modes on the other hand, which bijective
mapping (52) changes depending on the active operating mode.
Audio encoder according to claim 9, wherein the associator (102) is configured
such that if the active operating mode is the first operating mode, the mode
dependent set of the plurality of frame coding modes is disjoint to the first subset
(30) and overlaps with the second subset (32), and
if the active operating mode is a second operating mode, the mode dependent set of
the plurality of frame coding modes overlaps with the first and second subsets.
Audio decoder according to claim 11, wherein a number of possible values in the
set of possible values is two and the associator (102) is configured such that, if the
active operating mode is the first operating mode, the mode dependent set
comprises a first and a second frame coding mode of the second set of one or more
frame coding modes, and the frequency-domain encoder is configured to use
different time-frequency resolutions in encoding portions having the first and
second frame coding mode associated therewith.
Audio encoder according to any of claims 8 to 12, wherein the time-domain
encoder (104) is a code-excited linear-prediction encoder.
Audio encoder according to any of claims 8 to 13, wherein the frequency-domain
encoder (106) is a transform encoder configured to encode the portions having one
of the second subset of one or more of the frame coding modes associated
therewith, using transform coefficient levels and encode same into the
corresponding frames of the data stream.
Audio encoder according to any of claims 8 to 14, wherein the time-domain
decoder and the frequency-domain decoder are LP based encoders configured to
signal LPC-filter coefficients for each portion of the audio signal ( 112), wherein the
time-domain encoder (104) is configured to apply an LP analysis filter depending
on the LPC filter coefficients onto the portions of the audio signal ( 112) having one
of the first subset of one or more of the frame coding modes associated therewith so
as to obtain an excitation signal (150), and to approximate the excitation signal by
use of codebook indices and insert same into the corresponding frames, wherein the
frequency-domain encoder (106) is configured to transform the portions of the
audio signal having one of the second subset of one or more of the frame coding
modes associated therewith, so as to obtain a spectrum, and shaping the spectrum in
accordance with the LPC filter coefficients for the portions having one of the
second subset associated therewith, so as to obtain an excitation spectrum, quantize
the excitation spectrum into transform coefficient levels in the frames having one of
the second subset associated therewith, and insert the quantized excitation spectrum
into the corresponding frames.
16. Audio decoding method using a time-domain decoder (12), and a frequencydomain
decoder (14), the method comprising:
associating each of consecutive frames (18a-c) of a data stream (20), each of which
represents a corresponding one of consecutive portions (24a-24c) of an audio
signal, with one out of a mode dependent set of a plurality (22) of frame coding
modes,
decoding frames (18a-c) having one of a first subset (30) of one or more of the
plurality (22) of frame coding modes associated therewith, by the time-domain
decoder (12),
decoding frames (18a-c) having one of a second subset (32) of one or more of the
plurality (22) of frame coding modes associated therewith, by the frequencydomain
decoder (14), the first and second subsets being disjoint to each other;
wherein the association is dependent on a frame mode syntax element (38)
associated with the frames (18a-c) in the data stream (20),
and wherein the association is performed in an active one of a plurality of operating
modes with selecting the active operating mode out of the plurality of operating
modes depending on the data stream and/or an an external control signal, such that
the dependency of the performance of the association changes depending on the
active operating mode.
17. Audio encoding method using a time-domain encoder (104) and a frequencydomain
encoder (106), the method comprising
associating each of consecutive portions ( 116a-c) of an audio signal ( 1 12) with one
out of a mode dependent set (40, 42) of a plurality (22) of frame coding modes;
encoding portions having one of a first subset (30) of one or more of the plurality
(22) of frame coding modes associated wherewith, into a corresponding frame
( 118a-c) of a data stream ( 114) by the time-domain encoder (104);
encoding portions having one of a second subset (32) of one or more of the
plurality of encoding modes associated therewith, into a corresponding frame of the
data stream by the frequency-domain encoder (106),
wherein the association is performed in an active one of a plurality of operating
modes such that, if the active operating mode is a first operating mode, the mode
dependent set (40) of the plurality of frame coding modes is disjoint to the first
subset (30) and overlaps with the second subset (32) and if the active operating
mode is a second operating mode, the mode dependent set of the plurality of
encoding modes overlaps with the first and second subset (30, 32).
18. Computer program having a program code for performing, when running on a
computer, a method according to claim 16 or 17.