Apparatus and Method for Audio Encoding and Decoding
Employing Sinusoidal Substitution
Description
The present invention relates to audio signal encoding, decoding and processing, and, in
particular, to audio encoding and decoding employing sinusoidal substitution.
Audio signal processing becomes more and more important. Challenges arise, as modern
perceptual audio codecs are required to deliver satisfactory audio quality at increasingly
low bit rates. Additionally, often the permissible latency is also very low, e.g. for bi¬
directional communication applications or distributed gaming etc.
Modern audio codecs, like e.g. USAC (Unified Speech and Audio Coding), often switch
between time domain predictive coding and transform domain coding, nevertheless music
content is still predominantly coded in the transform domain. At low bit rates, e.g. < 14
kbit/s, tonal components in music items often sound bad when coded through transform
coders, which makes the task of coding audio at sufficient quality even more challenging.
Additionally, low-delay constraints generally lead to a sub-optimal frequency response of
the transform coder's filter bank (due to low-delay optimized window shape and/or
transform length) and therefore further compromise the perceptual quality of such codecs.
According to the classic psychoacoustic model, pre-requisites for transparency with respect
to quantization noise are defined. At high bit rates, this relates to a perceptually adapted
optimal time/frequency distribution of quantization noise that obeys the human auditory
masking levels. At low bit rates, however, transparency cannot be reached. Therefore, a
masking level requirements reduction strategy may be employed at low bit rates.
Already, top-notch codecs have been provided for music content, in particular, transform
coders based on the Modified Discrete Cosine Transform (MDCT), which quantize and
transmit spectral coefficients in the frequency domain. However, at very low data rates,
only very few spectral lines of each time frame can be coded by the available bits for that
frame. As a consequence, temporal modulation artifacts and so-called warbling artifacts
are inevitably introduced into the coded signal.
Most prominently, these types of artifacts are perceived in quasi-stationary tonal
components. This happens especially if, due to delay constraints, a transform window
shape has to be chosen that induces significant crosstalk between adjacent spectral
coefficients (spectral broadening) due to the well-known leakage effect. However,
nonetheless usually only one or few of these adjacent spectral coefficients remain non-zero
after the coarse quantization by the low-bit rate coder.
As stated above, in the prior art, according to one approach, transform coders are
employed. Contemporary high compression ratio audio codecs that are well-suited for
coding of music content all rely on transform coding. Most prominent examples are
MPEG2/4 Advanced Audio Coding (AAC) and MPEG-D Unified Speech and Audio
Coding (USAC). USAC has a switched core consistent of an Algebraic Code Excited
Linear Prediction (ACELP) module plus a Transform Coded Excitation (TCX) module
(see [5]) intended mainly for speech coding and, alternatively, AAC mainly intended for
coding of music. Like AAC, also TCX is a transform based coding method. At low bit rate
settings, these coding schemes are prone to exhibit warbling artifacts, especially if the
underlying coding schemes are based on the Modified Discrete Cosine Transform (MDCT)
(see [l]).
For music reproduction, transform coders are the preferred technique for audio data
compression. However, at low bit rates, traditional transform coders exhibit strong
warbling and roughness artifacts. Most of the artifacts originate from too sparsely coded
tonal spectral components. This happens especially if these are spectrally smeared by a
suboptimal spectral transfer function (leakage effect) that is mainly designed to meet strict
delay constraints.
According to another approach in the prior art, the coding schemes are fully parametric for
transients, sinusoids and noise. In particular, for medium and low bit rates, fully parametric
audio codecs have been standardized, the most prominent of which are MPEG-4 Part 3,
Subpart 7 Harmonic and Individual Lines plus Noise (HILN) (see [2]) and MPEG-4 Part 3,
Subpart 8 SinuSoidal Coding (SSC) (see [3]). Parametric coders, however, suffer from an
unpleasantly artificial sound and, with increasing bit rate, do not scale well towards
perceptual transparency.
A further approach provides hybrid waveform and parametric coding. In [4], a hybrid of
transform based waveform coding and MPEG 4-SSC (sinusoidal part only) is proposed. In
an iterative process, sinusoids are extracted and subtracted from the signal to form a
residual signal to be coded by transform coding techniques. The extracted sinusoids are
coded by a set of parameters and transmitted alongside with the residual. In [6], a hybrid
coding approach is provided that codes sinusoids and residual separately. In [7], at the socalled
Constrained Energy Lapped Transform (CELT) codec/Ghost webpage, the idea of
utilizing a bank of oscillators for hybrid coding is depictured.
At medium or higher bit rates, transform coders are well-suited for coding of music due to
their natural sound. There, the transparency requirements of the underlying psychoacoustic
model are fully or almost fully met. However, at low bit rates, coders have to seriously
violate the requirements of the psychoacoustic model and in such a situation transform
coders are prone to warbling, roughness, and musical noise artifacts.
Although fully parametric audio codecs are most suited for lower bit rates, they are,
however, known to sound unpleasantly artificial. Moreover, these codecs do not seamlessly
scale to perceptual transparency, since a gradual refinement of the rather coarse parametric
model is not feasible.
Hybrid waveform and parametric coding could potentially overcome the limits of the
individual approaches and could potentially benefit from the mutual orthogonal properties
of both techniques. However, it is, in the current state of the art, hampered by a lack of
interplay between the transform coding part and the parametric part of the hybrid codec.
Problems relate to signal division between parametric and transform codec part, bit budget
steering between transform and parametric part, parameter signalling techniques and
seamless merging of parametric and transform codec output.
The object of the present invention is to provide improved concepts for hybrid audio
encoding and decoding. The object of the present invention is solved by an apparatus
according to claim 1, an apparatus according to claim 12, by a method according to claim
29, by a method according to claim 30, and by a computer program according to claim 31.
An apparatus for generating an audio output signal based on an encoded audio signal
spectrum is provided.
The apparatus comprises a processing unit for processing the encoded audio signal
spectrum to obtain a decoded audio signal spectrum. The decoded audio signal spectrum
comprises a plurality of spectral coefficients, wherein each of the spectral coefficients has
a spectral location within the encoded audio signal spectrum and a spectral value, wherein
the spectral coefficients are sequentially ordered according to their spectral location within
the encoded audio signal spectrum so that the spectral coefficients form a sequence of
spectral coefficients.
Moreover, the apparatus comprises a pseudo coefficients determiner for determining one or
more pseudo coefficients of the decoded audio signal spectrum, each of the pseudo
coefficients having a spectral location and a spectral value.
Furthermore, the apparatus comprises a spectrum modification unit for setting the one or
more pseudo coefficients to a predefined value to obtain a modified audio signal spectrum.
Moreover, the apparatus comprises a spectrum-time conversion unit for converting the
modified audio signal spectrum to a time-domain to obtain a time-domain conversion
signal.
Furthermore, the apparatus comprises a controllable oscillator for generating a timedomain
oscillator signal, the controllable oscillator being controlled by the spectral
location and the spectral value of at least one of the one or more pseudo coefficients.
Moreover, the apparatus comprises a mixer for mixing the time-domain conversion signal
and the time-domain oscillator signal to obtain the audio output signal.
The proposed concepts enhance the perceptual quality of conventional block based
transform codecs at low bit rates. It is proposed to substitute local tonal regions in audio
signal spectra, spanning neighbouring local minima, encompassing a local maximum, by
pseudo-lines (also referred to as pseudo coefficients) having, in some embodiments, a
similar energy or level as said regions to be substituted.
According to embodiments, low delay and low bit rate audio coding is provided. Some
embodiments are based on a new and inventive concept referred to as ToneFilling (TF).
The term ToneFilling denotes a coding technique, in which otherwise badly coded natural
tones are replaced by perceptually similar yet pure sine tones. Thereby, amplitude
modulation artifacts at a certain rate, dependent on spectral position of the sinusoid with
respect to the spectral location of the nearest MDCT bin, are avoided (known as
"warbling").
According to embodiments, a degree of annoyance of all conceivable artifacts is weighted.
This relates to perceptual aspects like e.g. pitch, harmonicity, modulation and to stationary
of artifacts. All aspects are evaluated in a Sound Perception Annoyance Model (SPAM).
Steered by such a model, ToneFilling provides significant advantages. A pitch and
modulation error that is introduced by replacing a natural tone with a pure sine tone, is
weighted versus an impact of additive noise and poor stationarity ("warbling") caused by a
sparsely quantized natural tone.
ToneFilling provides significant differences to sinusoids-plus-noise codecs. For example,
TF substitutes tones by sines, instead of a subtraction of sinusoids. Perceptually similar
tones have the same local Centers Of Gravity (COG) as the original sound component to
be substituted. According to embodiments, original tones are erased in the audio spectrum
(left to right foot of COG function). Typically, the frequency resolution of the sinusoid
used for substitution is as coarse as possible to minimize side information, while, at the
same time, accounting for perceptual requirements to avoid an out-of -tune sensation.
In some embodiments, ToneFilling may be conducted above a lower cut-off frequency due
to said perceptual requirements, but not below the lower cut-off frequency. When
conducting ToneFilling, tones are represented via spectral pseudo-lines within a transform
coder. However, in a ToneFilling equipped encoder, pseudo-lines are subjected to the
regular processing controlled by the classic psychoacoustic model. Therefore, when
conducting ToneFilling, there is no need for a-priori restrictions of the parametric part (at
bit rate x, y tonal components are substituted). Such, a tight integration into a transform
codec is achieved.
ToneFilling functionality may be employed at the encoder, by detecting local COGs
(smoothed estimates; peak quality measures), by removing tonal components, by
generating substituted pseudo-lines (e.g. pseudo coefficients) which carry a level
information via the amplitude of the pseudo-lines, a frequency information via the spectral
position of the pseudo-lines and a fine frequency information {half bin offset) via the sign
of the pseudo-lines. Pseudo coefficients (pseudo-lines) are handled by a subsequent
quantizer unit of the codec just like any regular spectral coefficient (spectral line).
ToneFilling may moreover be employed at the decoder by detecting isolated spectral lines,
wherein true pseudo coefficients (pseudo-lines) may be marked by flag array (e.g. a bit
field). The decoder may link pseudo-line information to build sinusoidal tracks. A
birth/continuation/death scheme may be employed to synthesize continuous tracks.
For decoding, pseudo coefficients (pseudo-lines) may be marked as such by a flag array
transmitted within the side information. A half-bin frequency resolution of the pseudo-lines
can be signalled by the sign of the pseudo coefficients (pseudo-lines). At the decoder, the
pseudo-lines may be erased from the spectrum before the inverse transform unit and
synthesized separately by a bank of oscillators. Over time, pairs of oscillators may be
linked and parameter interpolation is employed to ensure a smoothly evolving oscillator
output.
The on- and offsets of the parameter-driven oscillators may be shaped such that they
closely correspond to the temporal characteristics of the windowing operation of the
transform codec thus ensuring seamless transition between transform codec generated parts
and oscillator generated parts of the output signal.
The provided concepts integrate nicely and effortlessly into existing transform coding
schemes like AAC, TCX or similar configurations. Steering of the parameter quantization
precision may be implicitly performed by the codec's existing rate control.
According to an embodiment, each of the spectral coefficients may have at least one of an
immediate predecessor and an immediate successor, wherein the immediate predecessor of
said spectral coefficient may be one of the spectral coefficients that immediately precedes
said spectral coefficient within the sequence, wherein the immediate successor of said
spectral coefficient may be one of the spectral coefficients that immediately succeeds said
spectral coefficient within the sequence. The pseudo coefficients determiner may be
configured to determine the one or more pseudo coefficients of the decoded audio signal
spectrum by determining at least one spectral coefficient of the sequence which has a
spectral value which is different from the predefined value, which has an immediate
predecessor the spectral value of which is equal to the predefined value, and which has an
immediate successor the spectral value of which is equal to the predefined value.
In an embodiment, the predefined value may be zero.
According to an embodiment, the pseudo coefficients determiner may be configured to
determine the one or more pseudo coefficients of the decoded audio signal spectrum by
determining the at least one spectral coefficient of the sequence as a pseudo coefficient
candidate, which has an immediate predecessor, the spectral value of which is equal to the
predefined value, and which has an immediate successor, the spectral value of which is
equal to the predefined value. The pseudo coefficients determiner may be configured to
determine whether the pseudo coefficient candidate is a pseudo coefficient by determining
whether side information indicates that said pseudo coefficient candidate is a pseudo
coefficient.
In an embodiment, the controllable oscillator may be configured to generate the timedomain
oscillator signal having a oscillator signal frequency so that the oscillator signal
frequency of the oscillator signal depends on the spectral location of one of the one or
more pseudo coefficients.
In some embodiments, the signal frequency of the oscillator signal is generated by
conducting an interpolation between the spectral location of two or more temporally
consecutive pseudo coefficients.
According to an embodiment, the pseudo coefficients are signed values, each comprising a
sign component. The controllable oscillator may be configured to generate the timedomain
oscillator signal so that the oscillator signal frequency of the oscillator signal
furthermore depends on the sign component of one of the one or more pseudo coefficients
so that the oscillator signal frequency has a first frequency value, when the sign component
has a first sign value, and so that the oscillator signal frequency has a different second
frequency value, when the sign component has a different second value.
In an embodiment, the controllable oscillator may be configured to generate the timedomain
oscillator signal wherein the amplitude of the oscillator signal may depend on the
spectral value of one of the one or more pseudo coefficients, so that the amplitude of the
oscillator signal has a first amplitude value when the spectral value has a third value, and
so that the amplitude of the oscillator signal has a different second amplitude value when
the spectral value has a different fourth value, the second amplitude value being greater
than the first amplitude value, when the fourth value is greater than the third value.
According to some embodiments, the amplitude value of the oscillator signal is generated
by conducting an interpolation between the spectral values of two or more temporally
consecutive pseudo coefficients. E.g. in some embodiments, the amplitude of the oscillator
signal is generated by conducting an interpolation between the points in time for which a
value is transmitted.
In an embodiment, the controllable oscillator may also be additionally controlled through
extrapolated parameters derived from the pseudo coefficient of the preceding frame in
order to e.g. conceal a data frame loss during transmission, or to smooth an unstable
behaviour of the oscillator control.
According to some embodiments, the amplitude value of the oscillator signal is generated
by conducting an interpolation between the spectral values of two or more pseudo
coefficients. E.g. in some embodiments, the amplitude of the oscillator signal is generated
by conducting an interpolation between the points in time for which a value is transmitted.
According to an embodiment, the modified audio signal spectrum may be an MDCT
spectrum, comprising MDCT coefficients. The spectrum-time conversion unit may be
configured to convert the MDCT spectrum from an MDCT domain to the time domain by
converting at least some of the coefficients of the decoded audio signal spectrum to the
time domain.
In an embodiment, the mixer may be configured to mix the time-domain conversion signal
and the time-domain oscillator signal by adding the time-domain conversion signal to the
time-domain oscillator signal in the time-domain.
Moreover, an apparatus for encoding an audio signal input spectrum is provided. The audio
signal input spectrum comprises a plurality of spectral coefficients, wherein each of the
spectral coefficients has a spectral location within the audio signal input spectrum and a
spectral value. The spectral coefficients are sequentially ordered according to their spectral
location within the audio signal input spectrum so that the spectral coefficients form a
sequence of spectral coefficients. Each of the spectral coefficients has at least one of has at
least one of one or more predecessors and has at least one of one or more successors,
wherein each one of the predecessors of said spectral coefficient is one of the spectral
coefficients that precedes said spectral coefficient within the sequence. Each one of the
successors of said spectral coefficient is one of the spectral coefficients that succeeds said
spectral coefficient within the sequence.
The apparatus comprises an extrema determiner for determining one extremum or more
extrema, preferably in a higher spectral resolution as provided by the underlying timefrequency
transform.
For example the audio signal input spectrum may be an MDCT spectrum having a plurality
of MDCT coefficients.
The extrema determiner may determine the extremum or the extrema on a comparison
spectrum, wherein a comparison value of a coefficient of the comparison spectrum is
assigned to each of the MDCT coefficients of the MDCT spectrum. However, the
comparison spectrum may have a higher spectral resolution than the audio signal input
spectrum. For example, the comparison spectrum may be a Discrete Fourier Transform
(DFT) spectrum (evenly or oddly stacked DFT) having twice the spectral resolution than
the MDCT audio signal input spectrum. By this, only every second spectral value of the
DFT spectrum is then assigned to a spectral value of the MDCT spectrum. However, the
other coefficients of the comparison spectrum may be taken into account when the
extremum or the extrema of the comparison spectrum are determined. By this, a coefficient
of the comparison spectrum may be determined as an extremum which is not assigned to a
spectral coefficient of the audio signal input spectrum, but which has an immediate
predecessor and an immediate successor, which are assigned to a spectral coefficient of the
audio signal input spectrum and to the immediate successor of that spectral coefficient of
the audio signal input spectrum, respectively. Thus, it can be considered that said
extremum of the comparison spectrum (e.g. of the high-resolution DFT spectrum) is
assigned to a spectral location within the (MDCT) audio signal input spectrum which is
located between said spectral coefficient of the (MDCT) audio signal input spectrum and
said immediate successor of said spectral coefficient of the (MDCT) audio signal input
spectrum. Such a situation may be encoded by choosing an appropriate sign value of the
pseudo coefficient as explained later on. By this, sub-bin resolution is achieved.
Moreover, the apparatus comprises a spectrum modifier for modifying the audio signal
input spectrum to obtain a modified audio signal spectrum by setting the spectral value of
at least one of the predecessors or the at least one of the successors of at least one of the
extremum coefficients to a predefined value. Moreover, the spectrum modifier is
configured to not set the spectral values of the one or more extremum coefficients to the
predefined value, or is configured to replace at least one of the one or more extremum
coefficients by a pseudo coefficient, wherein the spectral value of the pseudo coefficient is
different from the predefined value.
Furthermore, the apparatus comprises a processing unit for processing the modified audio
signal spectrum to obtain an encoded audio signal spectrum.
Moreover, the apparatus comprises a side information generator for generating and
transmitting side information, wherein the side information generator is configured to
locate one or more pseudo coefficient candidates within the modified audio signal input
spectrum generated by the spectrum modifier, wherein the side information generator is
configured to select at least one of the pseudo coefficient candidates as selected candidates,
and wherein the side information generator is configured to generate the side information
so that the side information indicates the selected candidates as the pseudo coefficients.
The extrema determiner is configured to determine the one or more extremum coefficients,
preferably in a higher spectral resolution as provided by the underlying time-frequency
transform, so that each of the extremum coefficients is one of the spectral coefficients the
spectral value of which is greater than the spectral value of at least one of its predecessors
and the spectral value of which is greater than the spectral value of at least one of its
successors. Or, each of the spectral coefficients has a comparison value associated with
said spectral coefficient, and the extrema determiner is configured to determine the one or
more extremum coefficients, so that each of the extremum coefficients is one of the
spectral coefficients the comparison value of which is greater than the comparison value of
at least one of its predecessors and the comparison value of which is greater than the
comparison value of at least one of its successors.
According to embodiments, the side information generated by the side information
generator can be of a static, predefined size or its size can be estimated iteratively in a
signal-adaptive manner. In this case, the actual size of the side information is transmitted to
the decoder as well. So, according to an embodiment, the side information generator 440 is
configured to transmit the size of the side information.
In an embodiment, the spectrum modifier is configured to modify the audio signal input
spectrum so that the spectral values of at least some of the spectral coefficients of the audio
signal input spectrum are left unmodified in the modified audio signal spectrum.
According to an embodiment, each of the spectral coefficients has at least one of an
immediate predecessor as one of its predecessors and an immediate successor as one of its
successors, wherein the immediate predecessor of said spectral coefficient is one of the
spectral coefficients that immediately precedes said spectral coefficient within the
sequence, wherein the immediate successor of said spectral coefficient is one of the
spectral coefficients that immediately succeeds said spectral coefficient within the
sequence.
The spectrum modifier may be configured to modify the audio signal input spectrum to
obtain the modified audio signal spectrum by setting the spectral value of the immediate
predecessor or the immediate successor of at least one of the extremum coefficients to the
predefined value, wherein the spectrum modifier may be configured to not set the spectral
values of the one or more extremum coefficients to the predefined value, or may be
configured to replace at least one of the one or more extremum coefficients by a pseudo
coefficient, wherein the spectral value of the pseudo coefficient is different from the
predefined value. It should be noted, that, when the extrema determiner determines the
extremum coefficients based on a comparison spectrum (e.g. a power spectrum), the
spectral coefficients, which may, for example, be a local maximum of the comparison
spectrum (e.g. the power spectrum) do not have to be a local maximum of the audio signal
input spectrum (e.g. the MDCT spectrum).
The extrema determiner may be configured to determine the one or more extremum
coefficients, so that each of the extremum coefficients is one of the spectral coefficients the
spectral value of which is greater than the spectral value of its immediate predecessor and
the spectral value of which is greater than the spectral value of its immediate successor. Or
each of the spectral coefficients has a comparison value associated with said spectral
coefficient, and the extrema determiner may be configured to determine the one or more
extremum coefficients, so that each of the extremum coefficients is one of the spectral
coefficients the comparison value of which is greater than the comparison value of its
immediate predecessor and the comparison value of which is greater than the comparison
value of its immediate successor.
According to an embodiment, the extrema determiner may be configured to determine one
or more minimum coefficients, so that each of the one or more minimum coefficients is
one of the spectral coefficients the spectral value of which is smaller than the spectral
value of one of its predecessors and the spectral value of which is smaller than the spectral
value of one of its successors, or wherein each of the spectral coefficients has a comparison
value associated with said spectral coefficient, wherein the extrema determiner is
configured to determine the one or more minimum coefficients, so that each of the
minimum coefficients is one of the spectral coefficients the comparison value of which is
smaller than the comparison value of one of its predecessors and the comparison value of
which is smaller than the comparison value of one of its successors. In such an
embodiment, the spectrum modifier may be configured to determine a representation value
based on the spectral values or comparison values of one or more of the extremum
coefficients and one or more of the minimum coefficients, so that the representation value
is different from the predefined value. Furthermore, the spectrum modifier may be
configured to change the spectral value of one of the coefficients of the audio signal input
sequence by setting said spectral value to the representation value.
According to an embodiment, the spectrum modifier may be configured to determine
whether a value difference between one of the comparison value or the spectral value of
one of the extremum coefficients is smaller than a threshold value. Moreover, the spectrum
modifier may be configured to modify the audio signal input spectrum so that the spectral
values of at least some of the spectral coefficients of the audio signal input spectrum are
left unmodified in the modified audio signal spectrum depending on whether the value
difference is smaller than the threshold value.
In an embodiment, the extrema determiner may be configured to determine one or more
sub-sequences of the sequence of spectral values, so that each one of the sub-sequences
comprises a plurality of subsequent spectral coefficients the audio signal input spectrum.
The subsequent spectral coefficients may be sequentially ordered within the sub-sequence
according to their spectral position. Each of the sub-sequences may have a first element
being first in said sequentially-ordered sub-sequence and a last element being last in said
sequentially-ordered sub-sequence. Moreover, each of the sub-sequences may comprise
exactly two of the minimum coefficients and exactly one of the extremum coefficients, one
of the minimum coefficients being the first element of the sub-sequence, the other one of
the minimum coefficients being the last element of the sub-sequence. In such an
embodiment, the spectrum modifier may be configured to determine the representation
value based on the spectral values or the comparison values of the coefficients of one of
the sub-sequences. The spectrum modifier may be configured to change the spectral value
of one of the coefficients of said sub-sequence by setting said spectral value to the
representation value.
According to an embodiment, the extrema determiner may be configured to determine a
center-of-gravity coefficient by determining the product of the comparison value and the
location value for each spectral coefficient of the sub-sequence to obtain a plurality of
weighted coefficients, by summing up the weighted coefficients to obtain a first sum,
summing up the comparison values of all spectral coefficients of the sub-sequence to
obtain a second sum; by dividing the first sum by the second sum to obtain an intermediate
result; and by applying round-to-nearest rounding on the intermediate result to obtain the
center-of-gravity coefficient, and wherein the spectrum modifier is configured to set the
spectral values of all spectral coefficients of the sub-sequence, which are not the center-ofgravity
coefficient to the predefined value. Or, the extrema determiner may be configured
to determine a center-of-gravity coefficient by determining the product of the spectral
value and the location value for each spectral coefficient of the sub-sequence to obtain a
plurality of weighted coefficients, by summing up the weighted coefficients to obtain a
first sum, summing up the spectral values of all spectral coefficients of the sub-sequence to
obtain a second sum; by dividing the first sum by the second sum to obtain an intermediate
result; and by applying round-to-nearest rounding on the intermediate result to obtain the
center-of-gravity coefficient, and wherein the spectrum modifier is configured to set the
spectral values of all spectral coefficients of the sub-sequence, which are not the center-ofgravity
coefficient to the predefined value.
In an embodiment, the predefined value is zero.
According to an embodiment, the comparison value of each spectral coefficient is a square
value of a further coefficient of a further spectrum resulting from an energy preserving
transformation of the audio signal.
In an embodiment, wherein the comparison value of each spectral coefficient is an
amplitude value of a further coefficient of a further spectrum resulting from an energy
preserving transformation of the audio signal.
According to an embodiment, the further spectrum is a Discrete Fourier Transform (DFT)
spectrum and wherein the energy preserving transformation is a Discrete Fourier
Transform (evenly or oddly stacked DFT).
According to another embodiment, the further spectrum is a Complex Modified Discrete
Cosine Transform (CMDCT) spectrum and wherein the energy preserving transformation
is a CMDCT.
According to an embodiment, the spectrum modifier may be configured to receive finetuning
information. The coefficients of the audio signal input spectrum may be signed
values, each comprising a sign component. The spectrum modifier may be configured to
set the sign component one of the one or more extremum coefficients or of the pseudo
coefficient to a first sign value, when the fine-tuning information is in a first fine-tuning
state. And the spectrum modifier may be configured to set the sign component one of the
one or more extremum coefficients or of the pseudo coefficient to a different second sign
value, when the fine-tuning information is in a different second fine-tuning state.
In an embodiment, the audio signal input spectrum may be an MDCT spectrum comprising
MDCT coefficients.
According to an embodiment, the processing unit may be configured to quantize the
modified audio signal spectrum to obtain a quantized audio signal spectrum. The
processing unit may furthermore be configured to process the quantized audio signal
spectrum to obtain an encoded audio signal spectrum. Moreover, the processing unit may
furthermore be configured to generate side information indicating only for those spectral
coefficients of the quantized audio signal spectrum which have an immediate predecessor
the spectral value of which is equal to the predefined value and an immediate successor,
the spectral value of which is equal to the predefined value, whether a said coefficient is
one of the extremum coefficients. The immediate predecessor of said spectral coefficient is
another spectral coefficient which immediately precedes said spectral coefficient within the
quantized audio signal spectrum, and wherein the immediate successor of said spectral
coefficient is another spectral coefficient which immediately succeeds said spectral
coefficient within the quantized audio signal spectrum.
Moreover, a method for generating an audio output signal based on an encoded audio
signal spectrum is provided. Each of the spectral coefficients has a spectral location within
the encoded audio signal spectrum and a spectral value. The spectral coefficients are
sequentially ordered according to their spectral location within the encoded audio signal
spectrum so that the spectral coefficients form a sequence of spectral coefficients. The
method for generating an audio output signal comprises:
Processing the encoded audio signal spectrum to obtain a decoded audio signal
spectrum the decoded audio signal spectrum comprising a plurality of spectral
coefficients.
Determining one or more pseudo coefficients of the decoded audio signal spectrum,
each of the pseudo coefficients having a spectral location and a spectral value.
- Setting the one or more pseudo coefficients to a predefined value to obtain a
modified audio signal spectrum.
Converting the modified audio signal spectrum to a time-domain to obtain a timedomain
conversion signal.
Generating a time-domain oscillator signal by a controllable oscillator being
controlled by the spectral location and the spectral value of at least one of the one
or more pseudo coefficients. And:
- Mixing the time-domain conversion signal and the time-domain oscillator signal to
obtain the audio output signal.
Furthermore, a method for encoding an audio signal input spectrum is provided. The audio
signal input spectrum comprises a plurality of spectral coefficients. Each of the spectral
coefficients has a spectral location within the audio signal input spectrum and a spectral
value. The spectral coefficients are sequentially ordered according to their spectral location
within the audio signal input spectrum so that the spectral coefficients form a sequence of
spectral coefficients. Each of the spectral coefficients has at least one of has at least one of
one or more predecessors and has at least one of one or more successors. Each predecessor
of said spectral coefficient is one of the spectral coefficients that precedes said spectral
coefficient within the sequence. Each successor of said spectral coefficient is one of the
spectral coefficients that succeeds said spectral coefficient within the sequence. The
method for encoding an audio signal input spectrum comprises:
Determining one or more extremum coefficients.
Modifying the audio signal input spectrum to obtain a modified audio signal
spectrum by setting the spectral value of at least one of the predecessors or at least
one of the successors of at least one of the extremum coefficients to a predefined
value, wherein modifying the audio signal input spectrum is conducted by not
setting the spectral values of the one or more extremum coefficients to the
predefined value, or by replacing at least one of the one or more extremum
coefficients by a pseudo coefficient, wherein the spectral value of the pseudo
coefficient is different from the predefined value.
Processing the modified audio signal spectrum to obtain an encoded audio signal
spectrum. And:
Generating and transmitting side information, wherein the side information is
generated by locating one or more pseudo coefficient candidates within the
modified audio signal input spectrum, wherein the side information is generated by
selecting at least one of the pseudo coefficient candidates as selected candidates,
and wherein the side information is generated so that the side information indicates
the selected candidates as the pseudo coefficients.
The one or more extremum coefficients are determined, so that each of the extremum
coefficients is one of the spectral coefficients the spectral value of which is greater than the
spectral value of one of its predecessors and the spectral value of which is greater than the
spectral value of one of its successors. Or, each of the spectral coefficients has a
comparison value associated with said spectral coefficient, wherein the one or more
extremum coefficients are determined, so that each of the extremum coefficients is one of
the spectral coefficients the comparison value of which is greater than the comparison
value of at least one of its predecessors and the comparison value of which is greater than
the comparison value of at least one of its successors.
Moreover, a computer program for implementing the above-described methods when being
executed on a computer or signal processor is provided.
An audio encoder, audio decoder, related methods and programs or encoded audio signal
are provided. Moreover, concepts for sinusoidal substitution for waveform coders are
provided.
At low bit rates, the present invention provides concepts how to tightly integrate waveform
coding and parametric coding to obtain an improved perceptual quality and an improved
scaling of perceptual quality versus bit rate over the single techniques.
In some embodiments, peaky areas (spanning neighbouring local minima, encompassing a
local maximum) of spectra may be fully substituted by a single sinusoid each; as opposed
to sinusoidal coders which iteratively subtract synthesized sinusoids from the residual.
Suitable peaky areas are extracted on a smoothed and slightly whitened spectral
representation and are selected with respect to certain features (peak height, peak shape).
According to some embodiments, these substitution sinusoids may be represented as
pseudo-lines (pseudo coefficients) within the spectrum to be coded and reflect the full
amplitude or energy of the sinusoid (as opposed, e.g. regular MDCT lines correspond to
the real projection of the true value).
In some embodiments, pseudo-lines (pseudo coefficients) may be handled by the codecs
existing quantizer just like any regular spectral line; as opposed to separate signalling of
sinusoidal parameters.
According to some embodiments, pseudo-lines (pseudo coefficients) may be marked as
such by side info flag array.
In some embodiments, the choice of sign of the pseudo-lines may denote semi subband
frequency resolution.
According to some embodiments, a lower cut-off frequency for sinusoidal substitution may
be advisable due to the limited frequency resolution (e.g. semi-subband).
In some embodiments, in the decoder, pseudo-lines may be deleted from the regular
spectrum; pseudo-line synthesis is accomplished by a bank of interpolating oscillators.
In some embodiments, an optionally measured start phase of a sinusoidal track obtained
from extrapolation of preceding spectra may be employed.
According to some embodiments, an optional Time Domain Alias Cancellation (TDAC)
technique may be employed by modelling of the alias at on-/off-set of a sinusoidal track.
According to some embodiments, an optional TDAC alias cancellation by modelling of
alias at on-/off-set may be employed.
In the following, embodiments of the present invention are described in more detail with
reference to the figures, in which:
Fig. 1 illustrates an apparatus for generating an audio output signal based on an
encoded audio signal spectrum according to an embodiment,
Fig. 2 depicts an apparatus for generating an audio output signal based
encoded audio signal spectrum according to another embodiment,
Fig. 3 shows two diagrams comparing original sinusoids and sinusoids after
processed by an MDCT / inverse MDCT chain,
Fig. 4 illustrates an apparatus for encoding an audio signal input spectrum
according to an embodiment,
Fig. 5 depicts an audio signal input spectrum, a corresponding power spectrum and
a modified (substituted) audio signal spectrum, and
Fig. 6 illustrates another power spectrum, another modified (substituted) audio
signal spectrum, and a quantized audio signal spectrum, wherein the
quantized audio signal spectrum generated at an encoder side, may, in some
embodiments, correspond to the decoded audio signal spectrum decoded at a
decoding side.
Fig. 4 illustrates an apparatus for encoding an audio signal input spectrum according to an
embodiment. The apparatus for encoding comprises an extrema determiner 410, a
spectrum modifier 420, a processing unit 430 and a side information generator 440.
Before considering the apparatus of Fig. 4 in more detail, the audio signal input spectrum
that is encoded by the apparatus of Fig. 4 is considered in more detail.
In principle any kind of audio signal spectrum can be encoded by the apparatus of Fig. 4.
The audio signal input spectrum may, for example, be an MDCT (Modified Discrete
Cosine Transform) spectrum, a DFT (Discrete Fourier Transform) magnitude spectrum or
an MDST (Modified Discrete Sine Transform) spectrum.
Fig. 5 illustrates an example of an audio signal input spectrum 510. In Fig. 5, the audio
signal input spectrum 510 is an MDCT spectrum.
The audio signal input spectrum comprises a plurality of spectral coefficients. Each of the
spectral coefficients has a spectral location within the audio signal input spectrum and a
spectral value.
Considering the example of Fig. 5, where the audio signal input spectrum results from an
MDCT transform of the audio signal, e.g., a filter bank that has transformed the audio
signal to obtain the audio signal input spectrum, may, for example, use 1024 channels.
Then, each of the spectral coefficients is associated with one of the 024 channels and the
channel number (for example, a number between 0 and 1023) may be considered as the
spectral location of said spectral coefficients. In Fig. 5, the abscissa 5 11 refers to the
spectral location of the spectral coefficients. For better illustration, only the coefficients
with spectral locations between 52 and 148 are illustrated by Fig. 5.
In Fig. 5, the ordinate 512 helps to determine the spectral value of the spectral coefficients.
In the example of Fig. 5 which depicts an MDCT spectrum, there, the spectral values of the
spectral coefficients of the audio signal input spectrum, the abscissa 512 refers to the
spectral values of the spectral coefficients. It should be noted that spectral coefficients of
an MDCT audio signal input spectrum can have positive as well as negative real numbers
as spectral values.
Other audio signal input spectra, however, may only have spectral coefficients with
spectral values that are positive or zero. For example, the audio signal input spectrum may
be a DFT magnitude spectrum, with spectral coefficients having spectral values that
represent the magnitudes of the coefficients resulting from the Discrete Fourier Transform.
Those spectral values can only be positive or zero.
In further embodiments, the audio signal input spectrum comprises spectral coefficients
with spectral values that are complex numbers. For example, a DFT spectrum indicating
magnitude and phase information may comprise spectral coefficients having spectral
values which are complex numbers.
As exemplarily shown in Fig. 5, the spectral coefficients are sequentially ordered
according to their spectral location within the audio signal input spectrum so that the
spectral coefficients form a sequence of spectral coefficients. Each of the spectral
coefficients has at least one of one or more predecessors and one or more successors,
wherein each predecessor of said spectral coefficient is one of the spectral coefficients that
precedes said spectral coefficient within the sequence. Each successor of said spectral
coefficient is one of the spectral coefficients that succeeds said spectral coefficient within
the sequence. For example, in Fig. 5, a spectral coefficient having the spectral location 81,
82 or 83 (and so on) is a successor for the spectral coefficient with the spectral location 80.
A spectral coefficient having the spectral location 79, 78 or 77 (and so on) is a predecessor
for the spectral coefficient with the spectral location 80. For the example of an MDCT
spectrum, the spectral location of a spectral coefficient may be the channel of the MDCT
transform, the spectral coefficient relates to (for example, a channel number between, e.g.
0 and 1023). Again it should be noted that, for illustrative purposes, the MDCT spectrum
510 of Fig. 5 only illustrates spectral coefficients with spectral locations between 52 and
148.
Returning to Fig. 4, the extrema determiner 410 is now described in more detail. The
extrema determiner 410 is configured to determine one or more extremum coefficients.
In general, the extrema determiner 410 examines the audio signal input spectra or a
spectrum that is related to the audio signal input spectrum for extremum coefficients. The
purpose of determining extremum coefficients is, that later on, one or more local tonal
regions shall be substituted in the audio signal spectrum by pseudo coefficients, for
example, by a single pseudo coefficient for each tonal region.
In general, peaky areas in a power spectrum of the audio signal, the audio signal input
spectrum relates to, indicate tonal regions. It may therefore be preferred to identify peaky
areas in a power spectrum of the audio signal to which the audio signal input spectrum
relates. The extrema determiner 410 may, for example, examine a power spectrum,
comprising coefficients, which may be referred to as comparison coefficients (as their
spectral values are pairwise compared by the extrema determiner), so that each of the
spectral coefficients of the audio signal input spectrum has a comparison value associated
to it.
In Fig. 5, a power spectrum 520 is illustrated. The power spectrum 520 and the MDCT
audio signal input spectrum 510 relate to the same audio signal. The power spectrum 520
comprises coefficients referred to as comparison coefficients. Each spectral coefficient
comprises a spectral location which relates to abscissa 521 and a comparison value. Each
spectral coefficient of the audio signal input spectrum has a comparison coefficient
associated with it and thus, moreover has the comparison value of its comparison
coefficient associated with it. For example, the comparison value associated with a spectral
value of the audio signal input spectrum may be the comparison value of the comparison
coefficient with the same spectral position as the considered spectral coefficient of the
audio signal input spectrum. The association between three of the spectral coefficients of
the audio signal input spectrum 510 and three of the comparison coefficients (and thus the
association with the comparison values of these comparison coefficients) of the power
spectrum 520 is indicated by the dashed lines 513, 514, 515 indicating an association of the
respective comparison coefficients (or their comparison values) and the respective spectral
coefficients of the audio signal input spectrum 510.
The extrema determiner 410 may be configured to determine one or more extremum
coefficients, so that each of the extremum coefficients is one of the spectral coefficients the
comparison value of which is greater than the comparison value of one of its predecessors
and the comparison value of which is greater than the comparison value of one of its
successors.
For example, the extrema determiner 410 may determine the local maxima values of the
power spectrum. In other words, the extrema determiner 410 may be configured to
determine the one or more extremum coefficients, so that each of the extremum
coefficients is one of the spectral coefficients the comparison value of which is greater than
the comparison value of its immediate predecessor and the comparison value of which is
greater than the comparison value of its immediate successor. Here, the immediate
predecessor of a spectral coefficient is the one of the spectral coefficients that immediately
precedes said spectral coefficient in the power spectrum. The immediate successor of said
spectral coefficient is one of the spectral coefficients that immediately succeeds said
spectral coefficient in the power spectrum.
However, other embodiments do not require that the extrema determiner 410 determines
all local maxima. For example, in some embodiments, the extrema determiner may only
examine certain portions of the power spectrum, for example, relating to a certain
frequency range, only.
In other embodiments, the extrema determiner 410 is configured to only those coefficients
as extremum coefficients, where a difference between the comparison value of the
considered local maximum and the comparison value of the subsequent local minimum
and/or preceding local minimum is greater than a threshold value.
The extrema determiner 410 may determine the extremum or the extrema on a comparison
spectrum, wherein a comparison value of a coefficient of the comparison spectrum is
assigned to each of the MDCT coefficients of the MDCT spectrum. However, the
comparison spectrum may have a higher spectral resolution than the audio signal input
spectrum. For example, the comparison spectrum may be a DFT spectrum having twice the
spectral resolution than the MDCT audio signal input spectrum. By this, only every second
spectral value of the DFT spectrum is then assigned to a spectral value of the MDCT
spectrum. However, the other coefficients of the comparison spectrum may be taken into
account when the extremum or the extrema of the comparison spectrum are determined. By
this, a coefficient of the comparison spectrum may be determined as an extremum which is
not assigned to a spectral coefficient of the audio signal input spectrum, but which has an
immediate predecessor and an immediate successor, which are assigned to a spectral
coefficient of the audio signal input spectrum and to the immediate successor of that
spectral coefficient of the audio signal input spectrum, respectively. Thus, it can be
considered that said extremum of the comparison spectrum (e.g. of the high-resolution
DFT spectrum) is assigned to a spectral location within the (MDCT) audio signal input
spectrum which is located between said spectral coefficient of the (MDCT) audio signal
input spectrum and said immediate successor of said spectral coefficient of the (MDCT)
audio signal input spectrum. Such a situation may be encoded by choosing an appropriate
sign value of the pseudo coefficient as explained later on. By this, sub-bin resolution is
achieved.
It should be noted that in some embodiments, an extremum coefficient does not have to
fulfil the requirement that its comparison value is greater than the comparison value of its
immediate predecessor and the comparison value of its immediate successor. Instead, in
those embodiments, it might be sufficient that the comparison value of the extremum
coefficient is greater than one of its predecessors and one of its successors. Consider for
example the situation, where:
Spectral Location 212 213 214 215 216
Comparison Value 0.02 0.84 0.83 0.85 0.01
Table 1
In the situation described by Table 1, the extrema determiner 410 may reasonably consider
the spectral coefficient at spectral location 214 as an extremum coefficient. The
comparison value of spectral coefficient 214 is not greater than that of its immediate
predecessor 213 (0.83 < 0.84) and not greater than that of its immediate successor 215
(0.83 < 0.85), but it is (significantly) greater than the comparison value of another one of
its predecessors, predecessor 212 (0.83 > 0.02), and it is (significantly) greater than the
comparison value of another one of its successors, successor 216 (0.83 > 0.01). It appears
moreover reasonable to consider spectral coefficient 214 as the extremum of this "peaky
area", as spectral coefficient is located in the middle of the three coefficients 213, 214, 215
which have relatively big comparison values compared to the comparison values of
coefficients 212 and 216.
For example, the extrema determiner 410 may be configured to determine form some or all
of the comparison coefficients, whether the comparison value of said comparison
coefficient is greater than at least one of the comparison values of the three predecessors
being closest to the spectral location of said comparison coefficient. And/or, the extrema
determiner 410 may be configured to determine form some or all of the comparison
coefficients, whether the comparison value of said comparison coefficient is greater than at
least one of the comparison values of the three successors being closest to the spectral
location of said comparison coefficient. The extrema determiner 410 may then decide
whether to select said comparison coefficient depending on the result of said
determinations.
In some embodiments, the comparison value of each spectral coefficient is a square value
of a further coefficient of a further spectrum (a comparison spectrum) resulting from an
energy preserving transformation of the audio signal.
In further embodiments, the comparison value of each spectral coefficient is an amplitude
value of a further coefficient of a further spectrum resulting from an energy preserving
transformation of the audio signal.
According to an embodiment, the further spectrum is a Discrete Fourier Transform
spectrum and wherein the energy preserving transformation is a Discrete Fourier
Transform.
According to a further embodiment, the further spectrum is a Complex Modified Discrete
Cosine Transform (CMDCT) spectrum, and wherein the energy preserving transformation
is a CMDCT.
In another embodiment, the extrema determiner 410 may not examine a comparison
spectrum, but instead, may examine the audio signal input spectrum itself. This may, for
example, be reasonable, when the audio signal input spectrum itself results from an energy
preserving transformation, for example, when the audio signal input spectrum is a Discrete
Fourier Transform magnitude spectrum.
For example, the extrema determiner 410 may be configured to determine the one or more
extremum coefficients, so that each of the extremum coefficients is one of the spectral
coefficients the spectral value of which is greater than the spectral value of one of its
predecessors and the spectral value of which is greater than the spectral value of one of its
successors.
In an embodiment, the extrema determiner 410 may be configured to determine the one or
more extremum coefficients, so that each of the extremum coefficients is one of the
spectral coefficients the spectral value of which is greater than the spectral value of its
immediate predecessor and the spectral value of which is greater than the spectral value of
its immediate successor.
Moreover, the apparatus comprises a spectrum modifier 420 for modifying the audio signal
input spectrum to obtain a modified audio signal spectrum by setting the spectral value of
the predecessor or the successor of at least one of the extremum coefficients to a
predefined value. The spectrum modifier 420 is configured to not set the spectral values of
the one or more extremum coefficients to the predefined value, or is configured to replace
at least one of the one or more extremum coefficients by a pseudo coefficient, wherein the
spectral value of the pseudo coefficient is different from the predefined value.
Preferably, the predefined value may be zero. For example, in the modified (substituted)
audio signal spectrum 530 of Fig. 5, the spectral values of a lot of spectral coefficients
have been set to zero by the spectrum modifier 420.
In other words, to obtain the modified audio signal spectrum, the spectrum modifier 420
will set at least the spectral value of a predecessor or a successor of one of the extremum
coefficients to a predefined value. The predefined value may e.g. be zero. The comparison
value of such a predecessor or successor is smaller than the comparison value of said
extremum value.
Moreover, regarding the extremum coefficients themselves, the spectrum modifier 420 will
proceed as follows:
The spectrum modifier 420 will not set the extremum coefficients to the predefined
value, or:
- The spectrum modifier 420 will replace at least one of the extremum coefficients by
a pseudo coefficient, wherein the spectral value of the pseudo coefficient is
different from the predefined value. This means that the spectral value of at least
one of the extremum coefficients is set to the predefined value, and the spectral
value of another one of the spectral coefficients is set to a value which is different
from the predefined value. Such a value may, for example, be derived from the
spectral value of said extremum coefficient, of one of the predecessors of said
extremum coefficient or of one of the successors of said extremum coefficient. Or,
such a value may, for example, be derived from the comparison value of said
extremum coefficient, of one of the predecessors of said extremum coefficient or of
one of the successors of said extremum coefficient
The spectrum modifier 420 may, for example, be configured to replace one of the
extremum coefficients by a pseudo coefficient having a spectral value derived from the
spectral value or the comparison value of said extremum coefficient, from the spectral
value or the comparison value of one of the predecessors of said extremum coefficient or
from the spectral value or the comparison value of one of the successors of said extremum
coefficient.
Furthermore, the apparatus comprises a processing unit 430 for processing the modified
audio signal spectrum to obtain an encoded audio signal spectrum.
For example, the processing unit 430 may be any kind of audio encoder, for example, an
MP3 (MPEG-1 Audio Layer III or MPEG-2 Audio Layer III; MPEG = Moving Picture
Experts Group) audio encoder, an audio encoder for WMA (Windows Media Audio), an
audio encoder for WAVE-files or an MPEG-2/4 AAC (Advanced Audio Coding) audio
encoder or an MPEG-D USAC (Unified Speed and Audio Coding) coder.
The processing unit 430 may, for example, be an audio encoder as described in [8]
(ISO/IEC 14496-3:2005 - Information technology - Coding of audio-visual objects - Part
3: Audio, Subpart 4) or as described in [9] (ISO/IEC 14496-3:2005 - Information
technology - Coding of audio-visual objects - Part 3 : Audio, Subpart 4). For example, the
processing unit 430 may comprise a quantizer, and/or a temporal noise shaping tool, as, for
example, described in [8] and/or the processing unit 430 may comprise a perceptual noise
substitution tool, as, for example, described in [8].
Moreover, the apparatus comprises a side information generator 440 for generating and
transmitting side information. The side information generator 440 is configured to locate
one or more pseudo coefficient candidates within the modified audio signal input spectrum
generated by the spectrum modifier 420. Furthermore, the side information generator 440
is configured to select at least one of the pseudo coefficient candidates as selected
candidates. Moreover, the side information generator 440 is configured to generate the side
information so that the side information indicates the selected candidates as the pseudo
coefficients.
In the embodiment illustrated by Fig. 4, the side information generator 440 is configured to
receive the positions of the pseudo coefficients (e.g. the position of each of the pseudo
coefficients) by the spectrum modifier 420. Moreover, in the embodiment of Fig. 4, the
side information generator 440 is configured to receive the positions of the pseudo
coefficient candidates (e.g. the position of each of the pseudo coefficient candidates).
For example, in some embodiments, the processing unit 430 may be configured to
determine the pseudo coefficient candidates based on a quantized audio signal spectrum. In
an embodiment, the processing unit 430 may have generated the quantized audio signal
spectrum by quantizing the modified audio signal spectrum. For example, the processing
unit 430 may determine the at least one spectral coefficient of the quantized audio signal
spectrum as a pseudo coefficient candidate, which has an immediate predecessor, the
spectral value of which is equal to the predefined value (e.g. equal to 0), and which has an
immediate successor, the spectral value of which is equal to the predefined value.
Alternatively, in other embodiments, the processing unit 430 may pass the quantized audio
signal spectrum to the side information generator 440 and the side information generator
440 may itself determine the pseudo coefficient candidates based on the quantized audio
signal spectrum. According to other embodiments, the pseudo coefficient candidates are
determined in an alternative way based on the modified audio signal spectrum.
The side information generated by the side information generator can be of a static,
predefined size or its size can be estimated iteratively in a signal-adaptive manner. In this
case, the actual size of the side information is transmitted to the decoder as well. So,
according to an embodiment, the side information generator 440 is configured to transmit
the size of the side information.
According to an embodiment, the extrema determiner 410 is configured to examine the
comparison coefficients, for example, the coefficients of the power spectrum 520 in Fig. 5,
and is configured to determine the one or more minimum coefficients, so that each of the
minimum coefficients is one of the spectral coefficients the comparison value of which is
smaller than the comparison value of one of its predecessors and the comparison value of
which is smaller than the comparison value of one of its successors. In such an
embodiment, the spectrum modifier 420 may be configured to determine a representation
value based on the comparison values of one or more of the extremum coefficients and of
one or more of the minimum coefficients, so that the representation value is different from
the predefined value. Furthermore, the spectrum modifier 420 may be configured to change
the spectral value of one of the coefficients of the audio signal input spectrum by setting
said spectral value to the representation value.
In a specific embodiment, the extrema determiner is configured to examine the comparison
coefficients, for example, the coefficients of the power spectrum 520 in Fig. 5, and is
configured to determine the one or more minimum coefficients, so that each of the
minimum coefficients is one of the spectral coefficients the comparison value of which is
smaller than the comparison value of its immediate predecessor and the comparison value
of which is smaller than the comparison value of its immediate successor.
Alternatively, the extrema determiner 410 is configured to examine the audio signal input
spectrum 510 itself and is configured to determine one or more minimum coefficients, so
that each of the one or more minimum coefficients is one of the spectral coefficients the
spectral value of which is smaller than the spectral value of one of its predecessors and the
spectral value of which is smaller than the spectral value of one of its successors. In such
an embodiment, the spectrum modifier 420 may be configured to determine a
representation value based on the spectral values of one or more of the extremum
coefficients and of one or more of the minimum coefficients, so that the representation
value is different from the predefined value. Moreover, the spectrum modifier 420 may be
configured to change the spectral value of one of the coefficients of the audio signal input
spectrum by setting said spectral value to the representation value.
In a specific embodiment, the extrema determiner 410 is configured to examine the audio
signal input spectrum 510 itself and is configured to determine one or more minimum
coefficients, so that each of the one or more minimum coefficients is one of the spectral
coefficients the spectral value of which is smaller than the spectral value of its immediate
predecessor and the spectral value of which is smaller than the spectral value of its
immediate successor
In both embodiments, the spectrum modifier 420 takes the extremum coefficient and one
or more of the minimum coefficients into account, in particular their associated
comparison values or their spectral values, to determine the representation value. Then, the
spectral value of one of the spectral coefficients of the audio signal input spectrum is set to
the representation value. For, the spectral coefficient, the spectral value of which is set to
the representation value may, for example, be the extremum coefficient itself, or the
spectral coefficient, the spectral value of which is set to the representation value may be
the pseudo coefficient which replaces the extremum coefficient.
In an embodiment, the extrema determiner 410 may be configured to determine one or
more sub-sequences of the sequence of spectral values, so that each one of the sub
sequences comprises a plurality of subsequent spectral coefficients of the audio signal
input spectrum. The subsequent spectral coefficients are sequentially ordered within the
sub-sequence according to their spectral position. Each of the sub-sequences has a first
element being first in said sequentially-ordered sub-sequence and a last element being last
in said sequentially-ordered sub-sequence.
In a specific embodiment, each of the sub-sequences may, for example, comprise exactly
two of the minimum coefficients and exactly one of the extremum coefficients, one of the
minimum coefficients being the first element of the sub-sequence, the other one of the
minimum coefficients being the last element of the sub-sequence.
In an embodiment, the spectrum modifier 420 may be configured to determine the
representation value based on the spectral values or the comparison values of the
coefficients of one of the sub-sequences. For example, if the extrema determiner 410 has
examined the comparison coefficients of the comparison spectrum, e.g. of the power
spectrum 520, the spectrum modifier 420 may be configured to determine the
representation value based on the comparison values of the coefficients of one of the sub¬
sequences. If, however, the extrema determiner 410 has examined the spectral coefficients
of the audio signal input spectrum 510, the spectrum modifier 420 may be configured to
determine the representation value based on the spectral values of the coefficients of one of
the sub-sequences.
The spectrum modifier 420 is configured to change the spectral value of one of the
coefficients of said sub-sequence by setting said spectral value to the representation value.
Table 2 provides an example with five spectral coefficients at the spectral locations 252 to
258.
Table 2
The extrema determiner 410 may determine that the spectral coefficient 255 (the spectral
coefficient with the spectral location 255) is an extremum coefficient, as its comparison
value (0.73) is greater than the comparison value (0.48) of its (here: immediate)
predecessor 254, and as its comparison value (0.73) is greater than the comparison value
(0.45) of its (here: immediate) successor 256.
Moreover, the extrema determiner 410 may determine that the spectral coefficient 253 (the
is a minimum coefficient, as its comparison value (0.05) is smaller than the comparison
value (0.12) of its (here: immediate) predecessor 252, and as its comparison value (0.05) is
smaller than the comparison value (0.48) of its (here: immediate) successor 254.
Furthermore, the extrema determiner 410 may determine that the spectral coefficient 257 is
a minimum coefficient as its comparison value (0.03) is smaller than the comparison value
(0.45) of its (here: immediate) predecessor 256 and as its comparison value (0.03) is
smaller than the comparison value (0.18) of its (here: immediate) successor 258.
The extrema determiner 410 may thus determine a sub-sequence comprising the spectral
coefficients 253 to 257, by determining that spectral coefficient 255 is an extremum
coefficient, by determining spectral coefficient 253 as the minimum coefficient being the
closest preceding minimum coefficient to the extremum coefficient 255, and by
determining spectral coefficient 257 as the minimum coefficient being the closest
succeeding minimum coefficient to the extremum coefficient 255.
The spectrum modifier 420 may now determine a representation value for the subsequence
253 - 257 based on the comparison values of all the spectral coefficients 253 -
257.
For example, the spectrum modifier 420 may be configured to sum up the comparison
values of all the spectral coefficients of the sub-sequence. (For example, for Table 2, the
representation value for sub-sequence 253 - 257 then sums up to: 0.05 + 0.48 + 0.73 +
0.45 + 0.03 = 1.74).
Or, e.g., the spectrum modifier 420 may be configured to sum up the squares of the
comparison values of all the spectral coefficients of the sub-sequence. (For example, for
Table 2, the representation value for sub-sequence 253 - 257 then sums up to: (0.05) +
(0.48) 2 + (0.73) 2 + (0.45) 2 + (0.03) 2 = 0.9692).
Or, for example, the spectrum modifier 420 may be configured to square root the sum of
the squares of the comparison values of all the spectral coefficients of the sub-sequence
253 - 257. (For example, for Table 2, the representation value is then 0.98448).
According to some embodiments, the spectrum modifier 420 will set the spectral value of
the extremum coefficient (in Table to, the spectral value of spectral coefficient 253) to the
predefined value.
Other embodiments, however, use a center-of-gravity approach. Table 3 illustrates a sub
sequence comprising the spectral coefficients 282 - 288:
Table 3
Although the extremum coefficient is located at spectral location 285, according to the
center of gravity approach, the center-of-gravity is located at a different spectral location.
To determine the spectral location of the center-of-gravity, the extrema determiner 410
sums up weighted spectral locations of all spectral coefficients of the sub-sequence and
divides the result by the sum of the comparison values of the spectral coefficients of the
sub-sequence. Commercial rounding may then be employed on the result of the division to
determine the center-of-gravity. The weighted spectral location of a spectral coefficient is
the product of its spectral location and its comparison values.
In short: The extrema determiner may obtain the center-of-gravity by:
1) Determining the product of the comparison value and spectral location for each
spectral coefficient of the sub-sequence.
2) Summing up the products determined in 1) to obtain a first sum
3) Summing up the comparison values of all spectral coefficients of the sub-sequence
to obtain a second sum
4) Dividing the first sum by the second sum to generate an intermediate result; and
5) Apply round-to-nearest rounding on the intermediate result to obtain the center-ofgravity
(round-to-nearest rounding: 8.49 is rounded to 8; 8.5 is rounded to 9)
Thus, for the example of Table 3, the center-of-gravity is obtained by:
(0.04 · 282 + 0.10 · 283 + 0.20 · 284 + 0.93 · 285 + 0.92 · 286 + 0.90 · 287 + 0.05 · 288) /
/ (0.04 + 0.10 + 0.20 + 0.93 + 0.92 + 0.90 + 0.05) = 897.25 / 3.14 = 285.75 = 286.
Thus, in the example of Table 3, the extrema determiner 410 would be configured to
determine the spectral location 286 as the center-of-gravity.
In some embodiments, the extrema determiner 410 does not examine the complete
comparison spectrum (e.g. the power spectrum 520) or does not examine the complete
audio signal input spectrum. Instead, the extrema determiner 410 may only partially
examine the comparison spectrum or the audio signal input spectrum.
Fig. 6 illustrates such an example. There, the power spectrum 620 (as a comparison
spectrum) has been examined by an extrema determiner 410 starting at coefficient 55. The
coefficients at spectral locations smaller than 55 have not been examined. Therefore,
spectral coefficients at spectral locations smaller than 55 remain unmodified in the
substituted MDCT spectrum 630. In contrast Fig. 5 illustrates a substituted MDCT
spectrum 530 where all MDCT spectral lines have been modified by the spectrum modifier
420.
Thus, the spectrum modifier 420 may be configured to modify the audio signal input
spectrum so that the spectral values of at least some of the spectral coefficients of the audio
signal input spectrum are left unmodified.
In some embodiments, the spectrum modifier 420 is configured to determine, whether a
value difference between one of the comparison value or the spectral value of one of the
extremum coefficients is smaller than a threshold value. In such embodiments, the
spectrum modifier 420 is configured to modify the audio signal input spectrum so that the
spectral values of at least some of the spectral coefficients of the audio signal input
spectrum are left unmodified in the modified audios signal spectrum depending on whether
the value difference is smaller than the threshold value.
For example, in an embodiment, the spectrum modifier 420 may be configured not to
modify or replace all, but instead modify or replace only some of the extremum
coefficients. For example, when the difference between the comparison value of the
extremum coefficient (e.g. a local maximum) and the comparison value of the subsequent
and/or preceding minimum value is smaller than a threshold value, the spectrum modifier
may be determined not to modify these spectral values (and e.g. the spectral values of
spectral coefficients between them), but instead leave these spectral values unmodified in
the modified (substituted) MDCT spectrum 630. In the modified MDCT spectrum 630 of
Fig. 6, the spectral values of the spectral coefficients 100 to 112 and the spectral values of
the spectral coefficients 124 to 136 have been left unmodified by the spectral modifier in
the unmodified (substituted) spectrum 630.
The processing unit may furthermore be configured to quantize coefficients of the
modified (substituted) MDCT spectrum 630 to obtain a quantized MDCT spectrum 635.
According to an embodiment, the spectrum modifier 420 may be configured to receive
fine-tuning information. The spectral values of the spectral coefficients of the audio signal
input spectrum may be signed values, each comprising a sign component. The spectrum
modifier may be configured to set the sign component of one of the one or more extremum
coefficients or of the pseudo coefficient to a first sign value, when the fine-tuning
information is in a first fine-tuning state. And the spectrum modifier may be configured to
set the sign component of the spectral value of one of the one or more extremum
coefficients or of the pseudo coefficient to a different second sign value, when the finetuning
information is in a different second fine-tuning state.
For example, in Table 4,
Table 4
the spectral values of the spectral coefficients indicate that spectral coefficient 291 is in a
first fine-tuning state, spectral coefficient 301 is in a second fine-tuning state, spectral
coefficient 321 is in the first fine-tuning state, etc.
For example, returning to the center-of-gravity determination explained above, if the center
of gravity is (e.g. approximately in the middle) between two spectral locations, the spectral
modifier may set the sign so that the second fine-tuning state is indicated.
According to an embodiment, the processing unit 430 may be configured to quantize the
modified audio signal spectrum to obtain a quantized audio signal spectrum. The
processing unit 430 may furthermore be configured to process the quantized audio signal
spectrum to obtain an encoded audio signal spectrum.
Moreover, the processing unit 430 may furthermore be configured to generate side
information indicating only for those spectral coefficients of the quantized audio signal
spectrum which have an immediate predecessor the spectral value of which is equal to the
predefined value and an immediate successor, the spectral value of which is equal to the
predefined value, whether a said coefficient is one of the extremum coefficients.
Such information can be provided by the extrema determiner 410 to the processing unit
430.
For example, such an information may be stored by the processing unit 430 in a bit field,
indicating for each of the spectral coefficients of the quantized audio signal spectrum
which has an immediate predecessor the spectral value of which is equal to the predefined
value and an immediate successor, the spectral value of which is equal to the predefined
value, whether said coefficient is one of the extremum coefficients (e.g. by a bit value 1) or
whether said coefficient is not one of the extremum coefficients (e.g. by a bit value 0). In
an embodiment, a decoder can later on use this information for restoring the audio signal
input spectrum. The bit field may have a fixed length or a signal adaptively chosen length.
In the latter case, the length of the bit field might be additionally conveyed to the decoder.
For example, a bit field [0001 11 11] generated by the processing unit 430 might indicate,
that the first three "stand-alone" coefficients (their spectral value is not equal to the
predefined value, but the spectral values of their predecessor and of their successor are
equal to the predefined value) that appear in the (sequentially ordered) (quantized) audio
signal spectrum are not extremum coefficients, but the next six "stand-alone" coefficients
are extremum coefficients. This bit field describes the situation that can be seen in the
quantized MDCT spectrum 635 in Fig. 6, where the first three "stand-alone" coefficients 5,
8, 25 are not extremum coefficients, but where the next six "stand-alone" coefficients 59,
71, 83, 94, 116, 141 are extremum coefficients.
Again, the immediate predecessor of said spectral coefficient is another spectral coefficient
which immediately precedes said spectral coefficient within the quantized audio signal
spectrum, and the immediate successor of said spectral coefficient is another spectral
coefficient which immediately succeeds said spectral coefficient within the quantized
audio signal spectrum.
In the following, an apparatus for generating an audio output signal based on an encoded
audio signal spectrum according to an embodiment is described.
Fig. 1 illustrates such an apparatus for generating an audio output signal based on an
encoded audio signal spectrum according to an embodiment.
The apparatus comprises a processing unit 110 for processing the encoded audio signal
spectrum to obtain a decoded audio signal spectrum. The decoded audio signal spectrum
comprises a plurality of spectral coefficients, wherein each of the spectral coefficients has
a spectral location within the encoded audio signal spectrum and a spectral value, wherein
the spectral coefficients are sequentially ordered according to their spectral location within
the encoded audio signal spectrum so that the spectral coefficients form a sequence of
spectral coefficients.
Moreover, the apparatus comprises a pseudo coefficients determiner 120 for determining
one or more pseudo coefficients of the decoded audio signal spectrum using side
information (side info), each of the pseudo coefficients having a spectral location and a
spectral value.
Furthermore, the apparatus comprises a spectrum modification unit 130 for setting the one
or more pseudo coefficients to a predefined value to obtain a modified audio signal
spectrum.
Moreover, the apparatus comprises a spectrum-time conversion unit 140 for converting the
modified audio signal spectrum to a time-domain to obtain a time-domain conversion
signal.
Furthermore, the apparatus comprises a controllable oscillator 150 for generating a timedomain
oscillator signal, the controllable oscillator being controlled by the spectral
location and the spectral value of at least one of the one or more pseudo coefficients.
Moreover, the apparatus comprises a mixer 160 for mixing the time-domain conversion
signal and the time-domain oscillator signal to obtain the audio output signal.
In an embodiment, the mixer may be configured to mix the time-domain conversion signal
and the time-domain oscillator signal by adding the time-domain conversion signal to the
time-domain oscillator signal in the time-domain.
The processing unit 110 may, for example, be any kind of audio decoder, for example, an
MP3 audio decoder, an audio decoder for WMA, an audio decoder for WAVE-files, an
AAC audio decoder or an USAC audio decoder.
The processing unit 110 may, for example, be an audio decoder as described in [8]
(ISO/IEC 14496-3:2005 - Information technology - Coding of audio-visual objects - Part
3 : Audio, Subpart 4) or as described in [9] (ISO/IEC 14496-3:2005 - Information
technology - Coding of audio-visual objects - Part 3 : Audio, Subpart 4). For example, the
processing unit 430 may comprise a rescaling of quantized values ("de-quantization"),
and/or a temporal noise shaping tool, as, for example, described in [8] and/or the
processing unit 430 may comprise a perceptual noise substitution tool, as, for example,
described in [8].
According to an embodiment, each of the spectral coefficients may have at least one of an
immediate predecessor and an immediate successor, wherein the immediate predecessor of
said spectral coefficient may be one of the spectral coefficients that immediately precedes
said spectral coefficient within the sequence, wherein the immediate successor of said
spectral coefficient may be one of the spectral coefficients that immediately succeeds said
spectral coefficient within the sequence.
The pseudo coefficients determiner 1 0 may be configured to determine the one or more
pseudo coefficients of the decoded audio signal spectrum by determining at least one
spectral coefficient of the sequence, which has a spectral value which is different from the
predefined value, which has an immediate predecessor the spectral value of which is equal
to the predefined value, and which has an immediate successor the spectral value of which
is equal to the predefined value. In an embodiment, the predefined value may be zero and
the predefined value may be zero.
In other words: The pseudo coefficients determiner 120 determines for some or all of the
coefficients of the decoded audio signal spectrum whether the respectively considered
coefficient is different from the predefined value (preferably: different from 0), whether
the spectral value of the preceding coefficient is equal to the predefined value (preferably:
equal to 0) and whether the spectral value of the succeeding coefficient is equal to the
predefined value (preferably: equal to 0).
In some embodiments, such a determined coefficient is (always) a pseudo coefficient.
In other embodiments, however, such a determined coefficient is (only) a pseudo
coefficient candidate and may or may not be a pseudo coefficient. In those embodiments,
the pseudo coefficients determiner 120 is configured to determine the at least one pseudo
coefficient candidate, which has a spectral value which is different from the predefined
value, which has an immediate predecessor, the spectral value of which is equal to the
predefined value, and which may have an immediate successor, the spectral value of which
is equal to the predefined value.
The pseudo coefficients determiner 120 is then configured to determine whether the
pseudo coefficient candidate is a pseudo coefficient by determining whether side
information indicates that said pseudo coefficient candidate is a pseudo coefficient.
For example, such side information may be received by the pseudo coefficients determiner
120 in a bit field, which indicates for each of the spectral coefficients of the quantized
audio signal spectrum which has an immediate predecessor the spectral value of which is
equal to the predefined value and an immediate successor, the spectral value of which is
equal to the predefined value, whether said coefficient is one of the extremum coefficients
(e.g. by a bit value 1) or whether said coefficient is not one of the extremum coefficients
(e.g. by a bit value 0).
E.g., a bit field [0001 111 1] might indicate, that the first three "stand-alone" coefficients
(their spectral value is not equal to the predefined value, but the spectral values of their
predecessor and of their successor are equal to the predefined value) that appear in the
(sequentially ordered) (quantized) audio signal spectrum are not extremum coefficients,
but the next six "stand-alone" coefficients are extremum coefficients. This bit field
describes the situation that can be seen in the quantized MDCT spectrum 635 in Fig. 6,
where the first three "stand-alone" coefficients 5, 8, 25 are not extremum coefficients, but
where the next six "stand-alone" coefficients 59, 71, 83, 94, 16, 141 are extremum
coefficients.
The spectrum modification unit 130 may be configured to "delete" the pseudo coefficients
from the decoded audio signal spectrum. In fact, the spectrum modification unit sets the
spectral value of the pseudo coefficients of the decoded audio signal spectrum to the
predefined value (preferably to 0). This is reasonable, as the (at least one) pseudo
coefficients will only be needed to control the (at least one) controllable oscillator 150.
Thus, consider, for example, the quantized MDCT spectrum 635 in Fig. 6. If the spectrum
635 is considered as the decoded audio signal spectrum, the spectrum modification unit
130 would set the spectral values of the extremum coefficients 59, 71, 83, 94, 116 and 141
to obtain the modified audio signal spectrum and would leave the other coefficients of the
spectrum unmodified.
The spectrum-time conversion unit 140 converts the modified audio signal spectrum from
a spectral domain to a time-domain. For example, the modified audio signal spectrum may
be an MDCT spectrum, and the spectrum-time conversion unit 140 may be an Inverse
Modified Discrete Cosine Transform (IMDCT) filter bank. In other embodiments, the
spectrum may be an MDST spectrum and the spectrum-time conversion unit 140 may be
an Inverse Modified Discrete Sine Transform (IMDST) filter bank. Or, in further
embodiments, the spectrum may be a DFT spectrum and the spectrum-time conversion unit
140 may be an Inverse Discrete Fourier Transform (IDFT) filter bank.
The controllable oscillator 150 may be configured to generate the time-domain oscillator
signal having a oscillator signal frequency so that the oscillator signal frequency of the
oscillator signal may depend on the spectral location of one of the one or more pseudo
coefficients. The oscillator signal generated by the oscillator may be a time-domain sine
signal. The controllable oscillator 150 may be configured to control the amplitude of the
time-domain sine signal depending on the spectral value of one of the one or more pseudo
coefficients.
According to an embodiment, the pseudo coefficients are signed values, each comprising a
sign component. The controllable oscillator 150 may be configured to generate the timedomain
oscillator signal so that the oscillator signal frequency of the oscillator signal
furthermore may depend on the sign component of one of the one or more pseudo
coefficients so that the oscillator signal frequency may have a first frequency value, when
the sign component has a first sign value, and so that the oscillator signal frequency may
have a different second frequency value, when the sign component has a different second
value.
For example, consider the pseudo coefficient at spectral location 59 in the MDCT spectrum
635 of Fig. 6. If frequency 8200 Hz would be assigned to spectral location 59 and if
frequency 8400 Hz would be assigned to spectral location 60, then, the controllable
oscillator may, for example, be configured set the oscillator frequency to 8200 Hz, if the
sign of the of the spectral value of the pseudo coefficient is positive, and may, for example,
be configured set the oscillator frequency to 8300 Hz, if the sign of the spectral value of
the pseudo coefficient is negative.
Thus, the sign of the spectral value of the pseudo coefficient can be used to control,
whether the controllable oscillator sets the oscillator frequency to a frequency (e.g.
8200 Hz) assigned to the spectral location of the pseudo coefficient (e.g. spectral location
59) or to a frequency (e.g. 8300Hz) between the frequency (e.g. 8200 Hz) assigned to the
spectral location of the pseudo coefficient (e.g. spectral location 59) and the frequency
(e.g. 8400 Hz) assigned to the spectral location that immediately follows the spectral
location of the pseudo coefficient (e.g. spectral location 60).
In an embodiment, the controllable oscillator 150 is additionally controlled by one or more
extrapolated parameters derived from a pseudo coefficient of a preceding frame. For
example, the controllable oscillator 150 may also be additionally controlled through
extrapolated parameters derived from the pseudo coefficient of the preceding frame in
order to e.g. conceal a data frame loss during transmission, or to smooth an unstable
behaviour of the oscillator control. An extrapolated parameters may, for example, be a
spectral location or a spectral value. For example, when spectral coefficients of a timefrequency
domain are considered, the spectral coefficients relating to time-instant t-1 may
be comprised by a first frame, and the spectral coefficients relating to time-instant t may be
assigned to a second frame. E.g. the spectral value and/or the spectral location of a pseudo
coefficient relating to time-instant t-1 may be copied to obtain an extrapolated parameter
for a current frame relating to time-instant t .
Fig. 2 illustrates an embodiment, wherein the apparatus comprises further controllable
oscillators 252, 254, 256 for generating further time-domain oscillator signals controlled
by the spectral locations and the spectral values of further pseudo coefficients of the one or
more pseudo coefficients.
The further controllable oscillators 252, 254, 256 each generate one of the further timedomain
oscillator signals. Each of the controllable oscillators 252, 254, 256 is configured
to steer the oscillator signal frequency based on the spectral location of one of the pseudo
coefficients. And/or each of the controllable oscillators 252, 254, 256 is configured to steer
the amplitude of the oscillator signal based on the spectral value of one of the pseudo
coefficients.
The mixer 160 of Fig. 1 and Fig. 2 is configured to mix the time-domain conversion signal
generated by the spectrum-time conversion unit 140 and the one or more time-domain
oscillator signal generated by the one or more controllable oscillators 150, 252, 254, 256 to
obtain the audio output signal. The mixer 160 may generate the audio output signal by a
superposition of the time-domain conversion signal and the one or more time-domain
oscillator signals.
Fig. 3 illustrates two diagrams comparing original sinusoids (left) and sinusoids after
processed by an MDCT/IMDCT chain (right). After being processed by the MDCT/
IMDCT chain, the sinusoid comprises warbling artifacts. The concepts provided above
avoid that sinusoids are processed by the MDCT/IMDCT chain, but instead, sinusoidal
information is encoded by a pseudo coefficient and/or the sinusoid is reproduced by a
controllable oscillator.
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 decomposed 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 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.
Some embodiments according to the invention comprise a non-transitory 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.
References
[1] Daudet, L.; Sandler, M.; , "MDCT analysis of sinusoids: exact results and
applications to coding artifacts reduction," Speech and Audio Processing, IEEE
Transactions on, vol.12, no.3, pp. 302-312, May 2004
[2] Purnhagen, H.; Meine, N.;, "HILN-the MPEG-4 parametric audio coding tools,"
Circuits and Systems, 2000. Proceedings. ISCAS 2000 Geneva. The 2000 IEEE
International Symposium an, vol.3, no., pp.201-204 vol.3, 2000
[3] Oomen, Werner; Schuijers, Erik; den Brinker, Bert; Breebaart, Jeroen:," Advances
in Parametrie Coding for High-Quality Audio," Audio Engineering Society
Convention 114, preprint, Amsterdam/NL, March 2003
[4] van Schijndel, N.H. ; van de Par, S.; , "Rate-distortion optimized hybrid sound
coding," Applications of Signal Processing to Audio and Acoustics, 2005. IEEE
Workshop on, vol., no., pp. 235-238, 16-19 Oct. 2005
[5] Bessette, 8.; Lefebvre, R.; Salami, R. ; , "Universal speech/audio coding using
hybrid ACELP/TCX techniques," Acoustics, Speech, and Signal Processing, 2005.
Proceedings. (ICASSP Ό5). IEEE International Conference on, vol.3, no., pp.
iii/301- iii/304 Val. 3, 18-23 March 2005
[6] Ferreira, A.J.S. "Combined spectral envelope normalization and subtraction of
sinusoidal components in the ODFT and MDCT frequency domains," Applications
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no., pp.5 1-54, 2001
[7] http://people.xiph.org/~xiphmont/demo/ghost/demo.html
The corresponding archive.org-website is stored at:
http://web.archive.org/web/201 10121 141 149/http://people.xiph.org/~xiphmont
/demo/gho st/demo .html
[8] ISO/IEC 14496-3 :2005(E) - Information technology - Coding of audio-visual
objects - Part 3: Audio, Subpart 4
[9] ISO/IEC 4496-3 :2009(E) - Information technology - Coding of audio-visual
objects - Part 3: Audio, Subpart 4
Claims
1. An apparatus for generating an audio output signal based on an encoded audio
signal spectrum, wherein the apparatus comprises:
a processing unit ( 1 10) for processing the encoded audio signal spectrum to obtain
a decoded audio signal spectrum the decoded audio signal spectrum comprising a
plurality of spectral coefficients, wherein each of the spectral coefficients has a
spectral location within the encoded audio signal spectrum and a spectral value,
wherein the spectral coefficients are sequentially ordered according to their spectral
location within the encoded audio signal spectrum so that the spectral coefficients
form a sequence of spectral coefficients,
a pseudo coefficients determiner (120) for determining one or more pseudo
coefficients of the decoded audio signal spectrum, each of the pseudo coefficients
having a spectral location and a spectral value,
a spectrum modification unit (130) for setting the one or more pseudo coefficients
to a predefined value to obtain a modified audio signal spectrum,
a spectrum-time conversion unit (140) for converting the modified audio signal
spectrum to a time-domain to obtain a time-domain conversion signal,
a controllable oscillator (150) for generating a time-domain oscillator signal, the
controllable oscillator (150) being controlled by the spectral location and the
spectral value of at least one of the one or more pseudo coefficients, and
a mixer (160) for mixing the time-domain conversion signal and the time-domain
oscillator signal to obtain the audio output signal.
2. An apparatus according to claim 1,
wherein each of the spectral coefficients has at least one of an immediate
predecessor and an immediate successor, wherein the immediate predecessor of
said spectral coefficient is one of the spectral coefficients that immediately
precedes said spectral coefficient within the sequence of spectral coefficients,
wherein the immediate successor of said spectral coefficient is one of the spectral
coefficients that immediately succeeds said spectral coefficient within the sequence,
wherein the pseudo coefficients determiner (120) is configured to determine the one
or more pseudo coefficients of the decoded audio signal spectrum by determining at
least one spectral coefficient of the sequence which has a spectral value which is
different from the predefined value, which has an immediate predecessor the
spectral value of which is equal to the predefined value, and which has an
immediate successor the spectral value of which is equal to the predefined value.
3. An apparatus according to claim 2, wherein the predefined value is zero.
4. An apparatus according to claim 2 or 3,
wherein the pseudo coefficients determiner (120) is configured to determine the one
or more pseudo coefficients of the decoded audio signal spectrum by determining
the at least one spectral coefficient of the sequence as a pseudo coefficient
candidate, which has an immediate predecessor, the spectral value of which is equal
to the predefined value, and which has an immediate successor, the spectral value
of which is equal to the predefined value, and
wherein the pseudo coefficients determiner (120) is configured to determine
whether the pseudo coefficient candidate is a pseudo coefficient by determining
whether side information indicates that said pseudo coefficient candidate is a
pseudo coefficient,.
5. An apparatus according to one of the preceding claims, wherein the controllable
oscillator (150) is configured to generate the time-domain oscillator signal having a
oscillator signal frequency so that the oscillator signal frequency of the oscillator
signal depends on the spectral location of one of the one or more pseudo
coefficients.
6. An apparatus according to claim 5,
wherein the pseudo coefficients are signed values, each comprising a sign
component, and
wherein the controllable oscillator (150) is configured to generate the time-domain
oscillator signal so that the oscillator signal frequency of the oscillator signal
furthermore depends on the sign component of one of the one or more pseudo
coefficients so that the oscillator signal frequency has a first frequency value, when
the sign component has a first sign value, and so that the oscillator signal frequency
has a different second frequency value, when the sign component has a different
second value.
7. An apparatus according to one of the preceding claims, wherein the controllable
oscillator (150) is configured to generate the time-domain oscillator signal, wherein
the amplitude of the oscillator signal depends on the spectral value of one of the
one or more pseudo coefficients, so that the amplitude of the oscillator signal has a
first amplitude value when the spectral value has a third value, and so that that the
amplitude of the oscillator signal has a different second amplitude value when the
spectral value has a different fourth value, the second amplitude value being greater
than the first amplitude value, when the fourth value is greater than the third value.
8. An apparatus according to one of the preceding claims, wherein the controllable
oscillator (150) is additionally controlled by one or more extrapolated parameters
derived from a pseudo coefficient of a preceding frame.
9. An apparatus according to one of the preceding claims,
wherein the modified audio signal spectrum is an MDCT spectrum, comprising
MDCT coefficients, and
wherein the spectrum-time conversion unit (140) is configured to convert the
MDCT spectrum from an MDCT domain to the time domain by converting at least
some of the coefficients of the decoded audio signal spectrum to the time domain.
10. An apparatus according to one of the preceding claims, wherein the mixer (160) is
configured to mix the time-domain conversion signal and the time-domain
oscillator signal by adding the time-domain conversion signal to the time-domain
oscillator signal in the time-domain.
11. An apparatus according to one of the preceding claims,
wherein the time-domain oscillator signal generated by the controllable oscillator
(150) is a first time-domain oscillator signal,
wherein the apparatus furthermore comprises one or more further controllable
oscillators (252, 254, 256) for generating one or more further time-domain
oscillator signals, wherein each of the one or more further controllable oscillators
(252, 254, 256) is configured to generate one of the one or more further timedomain
oscillator signals, wherein each of the further controllable oscillators (252,
254, 256) is controlled by the spectral location and the spectral value of at least one
of the one or more pseudo coefficients, and
wherein the mixer (160) is configured to mix the first time-domain oscillator signal,
the one or more further time-domain oscillator signals, and the time-domain
conversion signal to obtain the audio output signal.
An apparatus for encoding an audio signal input spectrum of an audio signal, the
audio signal input spectrum comprising a plurality of spectral coefficients, wherein
each of the spectral coefficients has a spectral location within the audio signal input
spectrum, a spectral value, wherein the spectral coefficients are sequentially
ordered according to their spectral location within the audio signal input spectrum
so that the spectral coefficients form a sequence of spectral coefficients, wherein
each of the spectral coefficients has at least one of one or more predecessors and
one or more successors, wherein the each of the predecessors of said spectral
coefficient is one of the spectral coefficients that precedes said spectral coefficient
within the sequence, wherein each of the successors of said spectral coefficient is
one of the spectral coefficients that succeeds said spectral coefficient within the
sequence, and wherein the apparatus comprises:
an extrema determiner (410) for determining one or more extremum coefficients,
a spectrum modifier (420) for modifying the audio signal input spectrum to obtain a
modified audio signal spectrum by setting the spectral value of at least one of the
predecessors or at least one of the successors of at least one of the extremum
coefficients to a predefined value, wherein the spectrum modifier (420) is
configured to not set the spectral values of the one or more extremum coefficients
to the predefined value, or is configured to replace at least one of the one or more
extremum coefficients by a pseudo coefficient, wherein the spectral value of the
pseudo coefficient is different from the predefined value,
a processing unit (430) for processing the modified audio signal spectrum to obtain
an encoded audio signal spectrum, and
a side information generator (440) for generating and transmitting side information,
wherein the side information generator (440) is configured to locate one or more
pseudo coefficient candidates within the modified audio signal input spectrum
generated by the spectrum modifier (420), wherein the side information generator
(440) is configured to select at least one of the pseudo coefficient candidates as
selected candidates, and wherein the side information generator (440) is configured
to generate the side information so that the side information indicates the selected
candidates as the pseudo coefficients,
wherein the extrema determiner (410) is configured to determine the one or more
extremum coefficients, so that each of the extremum coefficients is one of the
spectral coefficients the spectral value of which is greater than the spectral value of
at least one of its predecessors and the spectral value of which is greater than the
spectral value of at least one of its successors, or
wherein each of the spectral coefficients has a comparison value associated with
said spectral coefficient, wherein the extrema determiner (410) is configured to
determine the one or more extremum coefficients, so that each of the extremum
coefficients is one of the spectral coefficients the comparison value of which is
greater than the comparison value of at least one of its predecessors and the
comparison value of which is greater than the comparison value of at least one of
its successors.
13. An apparatus according to claim 1 , wherein the side information generator (440) is
configured to transmit the size of the side information.
14. An apparatus according to claim 1 or 13, wherein the spectrum modifier (420) is
configured to modify the audio signal input spectrum so that the spectral values of
at least some of the spectral coefficients of the audio signal input spectrum are left
unmodified in the modified audio signal spectrum.
15. An apparatus according to one of claims 12 to 14,
wherein each of the spectral coefficients has at least one of an immediate
predecessor as one of its predecessors and an immediate successor as one of its
successors, wherein the immediate predecessor of said spectral coefficient is one of
the spectral coefficients that immediately precedes said spectral coefficient within
the sequence, wherein the immediate successor of said spectral coefficient is one of
the spectral coefficients that immediately succeeds said spectral coefficient within
the sequence,
wherein the spectrum modifier (420) is configured to modify the audio signal input
spectrum to obtain the modified audio signal spectrum by setting the spectral value
of the immediate predecessor or the immediate successor of at least one of the
extremum coefficients to the predefined value, wherein the spectrum modifier (420)
is configured to not set the spectral values of the one or more extremum coefficients
to the predefined value, or is configured to replace at least one of the one or more
extremum coefficients by a pseudo coefficient, wherein the spectral value of the
pseudo coefficient is different from the predefined value, and
wherein the extrema determiner (410) is configured to determine the one or more
extremum coefficients, so that each of the extremum coefficients is one of the
spectral coefficients the spectral value of which is greater than the spectral value of
its immediate predecessor and the spectral value of which is greater than the
spectral value of its immediate successor, or wherein each of the spectral
coefficients has a comparison value associated with said spectral coefficient,
wherein the extrema determiner (410) is configured to determine the one or more
extremum coefficients, so that each of the extremum coefficients is one of the
spectral coefficients the comparison value of which is greater than the comparison
value of its immediate predecessor and the comparison value of which is greater
than the comparison value of its immediate successor.
16. An apparatus according to claim 15,
wherein the extrema determiner (410) is configured to determine one or more
minimum coefficients, so that each of the one or more minimum coefficients is one
of the spectral coefficients the spectral value of which is smaller than the spectral
value of one of its predecessors and the spectral value of which is smaller than the
spectral value of one of its successors, or wherein each of the spectral coefficients
has a comparison value associated with said spectral coefficient, wherein the
extrema determiner (410) is configured to determine the one or more minimum
coefficients, so that each of the minimum coefficients is one of the spectral
coefficients the comparison value of which is smaller than the comparison value of
one of its predecessors and the comparison value of which is smaller than the
comparison value of one of its successors, and
wherein the spectrum modifier (420) is configured to determine a representation
value based on the spectral values or the comparison values of one or more of the
extremum coefficients and one or more of the minimum coefficients, so that the
representation value is different from the predefined value, and wherein the
spectrum modifier (420) is configured to change the spectral value of one of the
coefficients of the audio signal input spectrum by setting said spectral value to the
representation value.
An apparatus according to claim 16,
wherein spectrum modifier (420) is configured to determine, whether a value
difference between one of the comparison value or the spectral value of one of the
extremum coefficients is smaller than a threshold value, and
wherein the spectrum modifier (420) is configured to modify the audio signal input
spectrum so that the spectral values of at least some of the spectral coefficients of
the audio signal input spectrum are left unmodified in the modified audio signal
spectrum depending on whether the value difference is smaller than the threshold
value.
An apparatus according to claim 16 or 17,
wherein the extrema determiner (410) is configured to determine one or more sub¬
sequences of the sequence of spectral values, so that each one of the sub-sequences
comprises a plurality of subsequent spectral coefficients the audio signal input
spectrum, the subsequent spectral coefficients being sequentially ordered within the
sub-sequence according to their spectral position, wherein each of the sub
sequences has a first element being first in said sequentially-ordered sub-sequence
and a last element being last in said sequentially-ordered sub-sequence, wherein
each of the sub-sequences comprises exactly two of the minimum coefficients and
exactly one of the extremum coefficients, one of the minimum coefficients being
the first element of the sub-sequence, the other one of the minimum coefficients
being the last element of the sub-sequence, and
wherein the spectrum modifier (420) is configured to determine the representation
value based on the spectral values or the comparison values of the coefficients of
one of the sub-sequences, and wherein the spectrum modifier (420) is configured to
change the spectral value of one of the coefficients of said sub-sequence by setting
said spectral value to the representation value.
19. An apparatus according to claim 18, wherein the spectrum modifier (420) is
configured to determine the representation value by determining a sum of the
squares of the comparison values of the coefficients of said one of the sub
sequences.
20. An apparatus according to claim 18 or 19,
wherein the extrema determiner (410) is configured to determine a center-of-gravity
coefficient by determining the product of the comparison value and the location
value for each spectral coefficient of the sub-sequence to obtain a plurality of
weighted coefficients, by summing up the weighted coefficients to obtain a first
sum, summing up the comparison values of all spectral coefficients of the sub¬
sequence to obtain a second sum; by dividing the first sum by the second sum to
obtain an intermediate result; and by applying round-to-nearest rounding on the
intermediate result to obtain the center-of-gravity coefficient, and wherein the
spectrum modifier (420) is configured to set the spectral values of all spectral
coefficients of the sub-sequence, which are not the center-of-gravity coefficient to
the predefined value, or
wherein the extrema determiner (410) is configured to determine a center-of-gravity
coefficient by determining the product of the spectral value and the location value
for each spectral coefficient of the sub-sequence to obtain a plurality of weighted
coefficients, by summing up the weighted coefficients to obtain a first sum,
summing up the spectral values of all spectral coefficients of the sub-sequence to
obtain a second sum; by dividing the first sum by the second sum to obtain an
intermediate result; and by applying round-to-nearest rounding on the intermediate
result to obtain the center-of-gravity coefficient, and wherein the spectrum modifier
(420) is configured to set the spectral values of all spectral coefficients of the sub¬
sequence, which are not the center-of-gravity coefficient to the predefined value.
21. An apparatus according to one of claims 12 to 20, wherein the predefined value is
zero.
22. An apparatus according to one of claims 12 to 21, wherein the comparison value of
each spectral coefficient is a square value of a further coefficient of a further
spectrum resulting from an energy preserving transformation of the audio signal.
23. An apparatus according to one of claims 2 to 22, wherein the comparison value of
each spectral coefficient is an amplitude value of a further coefficient of a further
spectrum resulting from an energy preserving transformation of the audio signal.
24. An apparatus according to one of claims 12 to 23, wherein the further spectrum is a
Complex Modified Discrete Cosine Transform spectrum, and wherein the energy
preserving transformation is a Complex Modified Discrete Cosine Transform.
25. An apparatus according to one of claims 12 to 24,
wherein the spectrum modifier (420) is configured to receive fine-tuning
information,
wherein the spectral coefficients of the audio signal input spectrum are signed
values, each comprising a sign component,
wherein the spectrum modifier (420) is configured to set the sign component of the
spectral value of one of the one or more extremum coefficients or of the pseudo
coefficient to a first sign value, when the fine-tuning information is in a first finetuning
state to obtain the modified audio signal spectrum, and
wherein the spectrum modifier (420) is configured to set the sign component of the
spectral value of one of the one or more extremum coefficients or of the pseudo
coefficient to a different second sign value, when the fine-tuning information is in a
different second fine-tuning state to obtain the modified audio signal spectrum.
26. An apparatus according to one of claims 12 to 25, wherein the audio signal input
spectrum is an MDCT spectrum comprising MDCT coefficients.
27. An apparatus according to one of claims 12 to 26,
wherein the processing unit (430) is configured to quantize the modified audio
signal spectrum to obtain a quantized audio signal spectrum,
wherein the processing unit (430) is furthermore configured to process the
quantized audio signal spectrum to obtain an encoded audio signal spectrum,
wherein the processing unit (430) is furthermore configured to generate side
information indicating only for those spectral coefficients of the quantized audio
signal spectrum which have an immediate predecessor the spectral value of which
is equal to the predefined value and an immediate successor, the spectral value of
which is equal to the predefined value, whether said coefficient is one of the
extremum coefficients,
wherein the immediate predecessor of said spectral coefficient is another spectral
coefficient which immediately precedes said spectral coefficient within the
quantized audio signal spectrum, and wherein the immediate successor of said
spectral coefficient is another spectral coefficient which immediately succeeds said
spectral coefficient within the quantized audio signal spectrum.
An apparatus according to one of claims 1 to 27, wherein the spectrum modifier
(420) is configured to replace one of the extremum coefficients by a pseudo
coefficient having a spectral value derived from the spectral value or the
comparison value of said extremum coefficient, from the spectral value or the
comparison value of said extremum coefficient of one of the predecessors of said
extremum coefficient or from the spectral value or the comparison value of said
extremum coefficient of one of the successors of said extremum coefficient.
29. A method for generating an audio output signal based on an encoded audio signal
spectrum, wherein each of the spectral coefficients has a spectral location within
the encoded audio signal spectrum and a spectral value, wherein the spectral
coefficients are sequentially ordered according to their spectral location within the
encoded audio signal spectrum so that the spectral coefficients form a sequence of
spectral coefficients, and wherein the method comprises:
processing the encoded audio signal spectrum to obtain a decoded audio signal
spectrum the decoded audio signal spectrum comprising a plurality of spectral
coefficients,
determining one or more pseudo coefficients of the decoded audio signal spectrum,
each of the pseudo coefficients having a spectral location and a spectral value,
setting the one or more pseudo coefficients to a predefined value to obtain a
modified audio signal spectrum,
converting the modified audio signal spectrum to a time-domain to obtain a timedomain
conversion signal,
generating a time-domain oscillator signal by a controllable oscillator being
controlled by the spectral location and the spectral value of at least one of the one
or more pseudo coefficients, and
mixing the time-domain conversion signal and the time-domain oscillator signal to
obtain the audio output signal.
A method for encoding an audio signal input spectrum, the audio signal input
spectrum comprising a plurality of spectral coefficients, wherein each of the
spectral coefficients has a spectral location within the audio signal input spectrum,
a spectral value and a comparison value, wherein the spectral coefficients are
sequentially ordered according to their spectral location within the audio signal
input spectrum so that the spectral coefficients form a sequence of spectral
coefficients, wherein each of the spectral coefficients has at least one of one or
more predecessors and one or more successors, wherein each one of the
predecessors of said spectral coefficient is one of the spectral coefficients that
precedes said spectral coefficient within the sequence, wherein each one of the
successors of said spectral coefficient is one of the spectral coefficients that
succeeds said spectral coefficient within the sequence, and wherein the method
comprises:
determining one or more extremum coefficients,
modifying the audio signal input spectrum to obtain a modified audio signal
spectrum by setting the spectral value of at least one of the predecessors or at least
one of the successors of at least one of the extremum coefficients to a predefined
value, wherein modifying the audio signal input spectrum is conducted by not
setting the spectral values of the one or more extremum coefficients to the
predefined value, or by replacing at least one of the one or more extremum
coefficients by a pseudo coefficient, wherein the spectral value of the pseudo
coefficient is different from the predefined value,
processing the modified audio signal spectrum to obtain an encoded audio signal
spectrum, and
generating and transmitting side information, wherein the side information is
generated by locating one or more pseudo coefficient candidates within the
modified audio signal input spectrum, wherein the side information is generated by
selecting at least one of the pseudo coefficient candidates as selected candidates,
and wherein the side information is generated so that the side information indicates
the selected candidates as the pseudo coefficients,
wherein the one or more extremum coefficients are determined, so that each of the
extremum coefficients is one of the spectral coefficients the spectral value of which
is greater than the spectral value of at least one of its predecessors and the spectral
value of which is greater than the spectral value of at least one of its successors, or
wherein each of the spectral coefficients has a comparison value associated with
said spectral coefficient, wherein the one or more extremum coefficients are
determined, so that each of the extremum coefficients is one of the spectral
coefficients the comparison value of which is greater than the comparison value of
at least one of its predecessors and the comparison value of which is greater than
the comparison value of at least one of its successors.
31. A computer program for implementing the method of claim 29 or 30 when being
executed on a computer or signal processor.