Apparatus and Method for Processing an Audio Signal and for Providing a Higher
Temporal Granularity for a Combined Unified Speech and Audio Codec (I!SAC)
Specification
The present invention relates to audio processing and, in particular to an apparatus and
method for processing an audio signal and for providing a higher temporal granularity for a
Combined Unified Speech and Audio Codec (USAC).
USAC, as other audio codecs, exhibits a fixed frame size (USAC: 2048 samples/frame).
Although there is the possibility to switch to a limited set of shorter transform sizes within
one frame, the frame size still limits the temporal resolution of the complete system. To
increase the temporal granularity of the complete system, for traditional audio codecs the
sampling rate is increased, leader to a shorter duration of one frame in time (e.g.
milliseconds). However, this is not easily possible for the USAC codec:
The USAC codec comprises a combination of tools from traditional general audio codecs,
such as AAC (Advanced Audio Coding) transform coder, SBR (Spectral Band Replication)
and MPEG Surround (MPEG = Moving Picture Experts Group), plus tools from traditional
speech coders, such as ACELP (ACELP = Algebraic Code Excited Linear Prediction).
Both, ACELP and transform coder, run usually at the same time within the same
environment (i.e. frame size, sampling rate), and can be easily switched: usually, for clean
speech signals, the ACELP tool is used, and for music, mixed signals the transform coder
is used.
The ACELP tool is at the same time limited to work only at comparably low sampling
rates. For 24 kbit/s, a sampling rate of only 17075Hz is used. For higher sampling rates, the
ACELP tool starts to drop significantly in performance. The transform coder as well as
SBR and MPEG Surround however would benefit from a much higher sampling rate, for
example 22050 Hz for the transform coder and 44100 Hz for SBR and MPEG Surround.
So far, however, the ACELP tool limited the sampling rate of the complete system, leading
to a suboptimal system in particular for music signals.
The object of the present invention is to provide improved concepts for an apparatus and
method for processing an audio signal. The object of the present invention is solved by an
apparatus according to claim 1, a method according to claim 15, an apparatus according to
claim 16, a method according to claim 18 and a computer program according to claim 19.
The current USAC RM provides high coding performance over a large number of
operating points, tanging from very low bitrates such as 8 kbit/s up to transparent quality at
bitrates of 128 kbit/s and above. To reach this high quality over such a broad range of
bitrates, a combination of tools, such as MPEG Surround, SBR, ACELP and traditional
transform coders are used. Such a combination of tools of course requires a joint
optimization process of the tool interoperation and a common environment, where these
tools are placed.
It was found in this joint optimization process that some of the tools have deficiencies
reproducing signals, which expose a high temporal structure in the mid-bitrate range (24
kbit/s - 3 2 kbit/s). In particular the tools MPEG Surround. SBR and the FD transform
coders (FD, TCX) (FD = Frequency Domain: TCX = Transform Coded Excitation), i.e. all
tools, which operate in the frequency domain, can perform better when operated with
higher temporal granularity, which is identical to a shorter frame size in time domain.
Compared to state of the art HE-AACV2 encoder (High-Efficiency AAC v2 encoder) it
was found that the current USAC reference quality encoder operates at bitrates such as 2 4
kbit/s and 3 2 kbit/s at a significantly lower sampling rate, while using the same frame size
(in samples). This means the duration of the frames in milliseconds is significantly longer.
To compensate for these deficiencies, the temporal granularity needs to be increased. This
can be either reached by increasing the sampling frequency or shortening the frame sizes
(e.g. of systems using a fixed frame size).
Whereas increasing the sampling frequency is a reasonable way forward for SBR and
MPEG Surround to increase the performance for temporal dynamic signals, this will not
work for all core-coder tools: It is well known that a higher sampling frequency would be
beneficial to the transform coder, but at the same time drastically decreases the
performance of the ACELP tool.
An apparatus for processing an audio signal is provided. The apparatus comprises a signal
processor and a configurator. The signal processor is adapted to receive a first audio signal
frame having a first configurable number of samples of the audio signal. Moreover, the
signal processor is adapted to upsample the audio signal by a configurable upsampling
factor to obtain a processed audio signal. Furthermore, the signal processor is adapted to
output a second audio signal frame having a second configurable number of samples of the
processed audio signal.
The configurator is adapted to configure the signal processor based on configuration
information such that the configurable upsampling factor is equal to a first upsampling
value when a first ratio of the second configurable number of samples to the first
configurable number of samples has a first ratio value. Moreover, the configurator is
adapted to configure the signal processor such that the configurable upsampling factor is
equal to a different second upsampling value, when a different second ratio of the second
configurable number of samples to the first configurable number of samples has a different
second ratio value. The first or the second ratio value is not an integer value.
According to the above-described embodiment, a signal processor upsamples an audio
signal to obtain a processed upsamplcd audio signal. In the above embodiment, the
upsampling factor is configurable and can be a non-integer value. The configurability and
the fact that the upsampling factor can be a non-integer value increases the flexibility of
the apparatus. When a different second ratio of the second configurable number of samples
to the first configurable number of samples has a different second ratio value, then the
configurable upsampling factor has a different second upsampling value. Thus, the
apparatus is adapted to take a relationship between the upsampling factor and the ratio of
the frame length (i.e. the number of samples) of the second and the first audio signal frame
into account.
In an embodiment, the configurator is adapted to configure the signal processor such that
the different second upsampling value is greater than the first upsampling value, when the
second ratio of the second configurable number of samples to the first configurable number
of samples is greater than the first ratio of the second configurable number of samples to
the first configurable number of samples.
According to an embodiment, a new operating mode (in the following called "extra
setting") for the USAC codec is proposed, which enhances the performance of the system
for mid-data rates, such as 24 kbit/s and 32 kbit/s. It was found that for these operating
points, the temporal resolution of the current USAC reference codec is too low. It is
therefore proposed to a) increase this temporal resolution by shortening the core-coder
frame sizes without increasing the sampling rate for the core-coder, and further b) to
increase the sampling rate for SBR and MPEG Surround without changing the frame size
for these tools.
The proposed extra setting greatly improves the flexibility of the system, since it allows the
system including the ACLLP tool to be operated at higher sampling rates, such as 44. 1 and
48 kHz. Since these sampling rates are typically requested in the marketplace, it is
expected that this would help for the acceptance of the USAC codec.
The new operating mode for the current MPEG Unified Speech and Audio Coding
(USAC) work item increases the temporal flexibility of the whole codec, by increasing the
temporal granularity of the complete audio codec. If (assuming that the second number of
samples remained the same) the second ratio is greater than the first ratio, then the first
configurable number of samples has been reduced, i.e. the frame size of the first audio
signal frame has been shortened. This results in a higher temporal granularity, and all tools
which operate in the frequency domain and which process the first audio signal frame can
perform better. In such a high efficient operating mode, however, it is also desirable to
increase the performance of tools which process the second audio signal frame comprising
the upsampled audio signal. Such an increase in performance of these tools can be realized
by a higher sampling rate of the upsampled audio signal, i.e. by increasing the upsampling
factor for such an operating mode. Moreover, tools exist, such as the ACELP decoder in
USAC, which do not operate in the frequency domain, which process the first audio signal
frame and which operate best when the sampling rate of the (original) audio signal is
relatively low. These tools benefit from a high upsampling factor, as this means that the
sampling rate of the (original) audio signal is relatively low compared to the sampling rate
of the upsampled audio signal, The above described embodiment provides an apparatus
adapted for providing a configuration mode for an efficient operation mode for such an
environment.
The new operating mode increases the temporal flexibility of the whole codec, by
increasing the temporal granularity of the complete audio codec.
In an embodiment, the configurator is adapted to configure the signal processor such that
the configurable upsampling factor is equal to the first ratio value when the first ratio of the
second configurable number of samples to the first configurable number of samples has the
first ratio value, and wherein the configurator is adapted to configure the signal processor
such that the configurable upsampling factor is equal to the different second ratio value
when the second ratio of the second configurable number of samples to the first
configurable number of samples has the different second ratio value.
In an embodiment, the configurator is adapted to configure the signal processor such that
the configurable upsampling factor is equal to 2 when the first ratio has the first ratio
value, and wherein the configurator is adapted to configure the signal processor such that
the configurable upsampling factor is equal to 8/3 when the second ratio has the different
second ratio value.
According to a further embodiment, the configurator is adapted to configure the signal
processor such that the first configurable number of samples is equal to 1024 and the
second configurable number of samples is equal to 2048 when the first ratio has the first
ratio value, and wherein the configurator is adapted to configure the signal processor such
that that the first configurable number of samples is equal to 768 and the second
configurable number of samples is equal to 2048 when the second ratio has the different
second ratio value.
In an embodiment, it is proposed to introduce an additional setting of the USAC coder,
where the core-coder is operated at a shorter frame size (768 instead of 1024 samples).
Furthermore, it is proposed to modify in this context the resampling inside the SBR
decoder from 2:1 to 8:3, to allow SBR and MPEG Surround being operated at a higher
sampling rate.
Furthermore, according to an embodiment, the temporal granularity of the core-coder is
increased by shrinking the core-coder frame size from 1024 to 768 samples. By this step,
the temporal granularity of the core coder is increased by 4/3 while leaving the sampling
rate constant: This allows the ACELP to run at an appropriate sampling frequency (Fs).
Moreover, at the SBR tool, a resampling of ratio 8/3 (so far: ratio 2) is applied, converting
a core-coder frame of size 768 at 3/8 Fs to a output frame of size 2048 at Fs. This allows
the SBR tool and an MPEG Surround Tool to be run at a traditionally high sampling rate
(e.g. 44100 Hz). Thus, good quality for speech and music signals is provided, as all tools
are to be run in their optimal operating point.
In an embodiment, the signal processor comprises a core decoder module for decoding the
audio signal to obtain a preprocessed audio signal, an analysis filter bank having a number
of analysis filter bank channels for transforming the first preprocessed audio signal from a
time domain into a frequency domain to obtain a frequency-domain preprocessed audio
signal comprising a plurality of subband signals, a subband generator for creating and
adding additional subband signals for the frequency-domain preprocessed audio signal, and
a synthesis filter bank having a number of synthesis filter bank channels for transforming
the first preprocessed audio signal from the frequency domain into the time domain to
obtain the processed audio signal. The configurator may be adapted to configure the signal
processor by configuring the number of synthesis filter bank channels or the number of
analysis filter bank channels such that the configurable upsainpling factor is equal to a
third ratio of the number of synthesis filter bank channels to the number of analysis filter
bank channels. The subband generator may be a Spectral Band Replicator being adapted to
replicate subband signals of the preprocessed audio signal generator for creating the
additional subband signals for the frequency-domain preprocessed audio signal. The signal
processor may furthermore comprise an MPEG Surround decoder for decoding the
preprocessed audio signal to obtain a preprocessed audio signal comprising stereo or
surround channels. Moreover, the subband generator may be adapted to feed the
frequency-domain preprocessed audio signal into the MPEG Surround decoder after the
additional subband signals for the frequency-domain preprocessed audio signal have been
created and added to the frequency-domain preprocessed audio signal.
The core decoder module may comprise a first core decoder and a second core decoder,
wherein the first core decoder may be adapted to operate in a time domain and wherein the
second core decoder may be adapted to operate in a frequency domain. The first core
decoder may be an ACELP decoder and the second core decoder may be a FD transfom
decoder or a TCX transform decoder.
In an embodiment, the super-frame size for the ACELP codec is reduced from 1024 to 768
samples. This could be done by combining 4 ACELP frames of size 192 (3 sub-frames of
size 64) to one core-coder frame of size 768 (previously: 4 ACELP frames of size 256
were combined to a core-coder frame of size 1024). Another solution for reaching a corecoder
frame size of 768 samples would be for example to combine 3 ACELP frames of
size 256 (4 sub-frames of size 64).
According to a further embodiment, the configurator is adapted to configure the signal
processor based on the configuration information indicating at least one of the first
configurable number of samples of the audio signal or the second configurable number of
samples of the processed audio signal.
In another embodiment, the configurator is adapted to configure the signal processor based
on the configuration information, wherein the configuration information indicates the first
configurable number of samples of the audio signal and the second configurable number of
samples of the processed audio signal, wherein the configuration information is a
configuration index.
Moreover, an apparatus for processing an audio signal is provided. The apparatus
comprises a signal processor and a configurator. The signal processor is adapted to receive
a first audio signal frame having a first configurable number of samples of the audio signal.
Moreover, the signal processor is adapted to downsample the audio signal by a
configurable downsampling factor to obtain a processed audio signal. Furthermore, the
signal processor is adapted to output a second audio signal frame having a second
configurable number of samples of the processed audio signal.
The configurator may be adapted to configure the signal processor based on configuration
information such that the configurable downsampling factor is equal to a first
downsampling value when a first ratio of the second configurable number of samples to the
first configurable number of samples has a first ratio value. Moreover, the configurator is
adapted to configure the signal processor such that the configurable downsampling factor
is equal to a different second downsampling value, when a different second ratio of the
second configurable number of samples to the first configurable number of samples has a
different second ratio value. The first or the second ratio value is not an integer value.
Preferred embodiments of the present invention are subsequently discussed with respect to
the accompanying figures, in which:
Fig. 1 illustrates an apparatus for processing an audio signal according to an
embodiment,
Fig. 2 illustrates an apparatus for processing an audio signal according to another
embodiment,
Fig. 3 illustrates an upsampling process conducted by an apparatus according to an
embodiment,
Fig. 4 illustrates an apparatus for processing an audio signal according to a further
embodiment,
Fig. 5a illustrates a core decoder module according to an embodiment,
Fig. 5b illustrates an apparatus for processing an audio signal according to the
embodiment of Fig. 4 with a core decoder module according to Fig. 5a,
Fig. 6a illustrates an ACELP super frame comprising 4 ACHLP frames,
Fig. 6b illustrates an ACELP super frame comprising 3 ACELP frames,
Fig. 7a illustrates the default setting of USAC,
Fig. 7b illustrates an extra setting for USAC according to an embodiment,
Fig. 8a, 8b illustrate the results of a listening test according to MUSHRA methodology.
and
Fig. 9 illustrates an apparatus for processing an audio signal according to an
alternative embodiment.
Fig. 1 illustrates an apparatus for processing an audio signal according to an embodiment.
The apparatus comprises a signal processor 110 and a configurator 120. The signal
processor 110 is adapted to receive a first audio signal frame 140 having a first
configurable number of samples 145 of the audio signal Moreover, the signal processor
110 is adapted to upsample the audio signal by a configurable upsampling factor to obtain
a processed audio signal. Furthermore, the signal processor is adapted to output a second
audio signal frame 150 having a second configurable number of samples 155 of the
processed audio signal.
The configurator 120 is adapted to configure the signal processor 110 based on
configuration information ci such that the configurable upsampling factor is equal to a first
upsampling value when a first ratio of the second configurable number of samples to the
first configurable number of samples has a first ratio value. Moreover, the configurator 120
is adapted to configure the signal processor 110 such that the configurable upsampling
factor is equal to a different second upsampling value, when a different second ratio of the
second configurable number of samples to the first configurable number of samples has a
different second ratio value. The first or the second ratio value is not an integer value.
An apparatus according to Fig. 1may for example be employed in the process of decoding.
According to an embodiment, the configurator 120 may be adapted to configure the signal
processor 110 such that the different second upsampling value is greater than the first
different upsampling value, when the second ratio of the second configurable number of
samples to the first configurable number of samples is greater than the first ratio of the
second configurable number of samples to the first configurable number of samples. In a
further embodiment, the configurator 120 is adapted to configure the signal processor 110
such that the configurable upsampling factor is equal to the first ratio value when the first
ratio of the second configurable number of samples to the first configurable number of
samples has the first ratio value, and wherein the configurator 120 is adapted to configure
the signal processor 110 such that the configurable upsampling factor is equal to the
different second ratio value when the second ratio of the second configurable number of
samples to the first configurable number of samples has the different second ratio value.
In another embodiment, the configurator 120 is adapted to configure the signal processor
110 such that the configurable upsampling factor is equal to 2 when the first ratio has the
first ratio value, and wherein the configurator 120 is adapted to configure the signal
processor 110 such that the configurable upsampling factor is equal to 8/3 when the second
ratio has the different second ratio value. According to a further embodiment, the
configurator 120 is adapted to configure the signal processor 110 such that the first
configurable number of samples is equal to 1024 and the second configurable number of
samples is equal to 2048 when the first ratio has the first ratio value, and wherein the
configurator 120 is adapted to configure the signal processor 110 such that that the first
configurable number of samples is equal to 768 and the second configurable number of
samples is equal to 2048 when the second ratio has the different second ratio value.
In an embodiment, the configurator 120 is adapted to configure the signal processor 110
based on the configuration information ci, wherein the configuration information ci
indicates the upsampling factor, the first configurable number of samples of the audio
signal and the second configurable number of samples of the processed audio signal,
wherein the configuration information is a configuration index.
The following table illustrates an example for a configuration index as configuration
information:
wherein "Index" indicates the configuration index, wherein "coreCoderFrameLength"
indicates the first configurable number of samples of the audio signal, wherein "sbrRatio"
indicates the upsampling factor and wherein "outputFrameLength" indicates the second
configurable number of samples of the processed audio signal.
Fig. 2 illustrates an apparatus according to another embodiment. The apparatus comprises a
signal processor 205 and a configurator 208. The signal processor 205 comprises a core
decoder module 210, an analysis filter bank 220, a subband generator 230 and a synthesis
filter bank 240.
The core decoder module 210 is adapted to receive an audio signal asl . After receiving the
audio signal asl, the core decoder module 210 decodes the audio signal to obtain a
preprocessed audio signal as2. Then, the core decoder module 210 feeds the preprocessed
5 audio signal as2, being represented in a time domain, into the analysis filter bank 220.
The analysis filter bank 220 is adapted to transform the preprocessed audio signal as2 from
a time domain into a frequency domain to obtain a frequency-domain preprocessed audio
signal as3 comprising a plurality of subband signals. The analysis filter bank 220 has a
10 configurable number of analysis filter bank channels (analysis filter bank bands). The
number of analysis filter bank channels determines the number of subband signals that are
generated from the time-domain preprocessed audio signal as2. In an embodiment, the
number of analysis filter bank channels may be set by setting the value of a configurable
parameter cl . For example, the analysis filter bank 220 may be configured to have 32 or 24
5 analysis filter bank channels. In the embodiment of Fig. 2, the number of analysis filter
bank channels may be set according to configuration information ci of a configurator 208.
After transforming the preprocessed audio signal as2 into the frequency domain, the
analysis filter bank 220 feeds the frequency-domain preprocessed audio signal as3 into the
subband generator 230.
20
The subband generator 230 is adapted to create additional subband signals for the
frequency-domain audio signal as3. Moreover, the subband generator 230 is adapted to
modify the preprocessed frequency-domain audio signal as3 to obtain a modified
frequency-domain audio signal as4 which comprises the subband signals of the
25 preprocessed frequency-domain audio signal as3 and the created additional subband
signals created by the subband generator 230. The number of additional subband signals
that are generated by the subband generator 230 is configurable. In an embodiment, the
subband generator is a Spectral Band Replicator (SBR). The subband generator 230 then
feeds the modified frequency-domain preprocessed audio signal as4 into the synthesis filter
M) bank.
The synthesis filter bank 240 is adapted to transfonn the modified frequency-domain
preprocessed audio signal as4 from a frequency domain into a time domain to obtain a
time-domain processed audio signal as5. The synthesis filter bank 240 has a configurable
35 number of synthesis filter bank channels (synthesis filter bank bands). The number of
synthesis filter bank channels is confi gurable in an embodiment, the number of synthesis
filter bank channels may be set by setting the value of a configurable parameter c2. For
example, the synthesis filter bank 240 may be configured to have 64 synthesis filter bank
channels. In the embodiment of Fig. 2, the configuration information ci of the configurator
208 may set the number of analysis filter bank channels. By transforming the modified
frequency-domain preprocessed audio signal as4 into the time domain, the processed audio
signal asS is obtained.
In an embodiment, the number of subband channels of the modified frequency-domain
preprocessed audio signal as4 is equal to the number of synthesis filter bank channels. In
such an embodiment, the configurator 208 is adapted to configure the number of additional
subband channels that are created by the subband generator 230. The configurator 208 may
be adapted to configure the number of additional subband channels that are created by the
subband generator 230 such that the number of synthesis filter bank channels c2,
configured by the configurator 208, is equal to the number of subband channels of the
preprocessed frequency-domain audio signal as3 plus the number of additional subband
signals created by the subband generator 230. By this, the number of synthesis filter bank
channels is equal to the number of subband signals of the modified preprocessed
frequency-domain audio signal as4.
Assuming that the audio signal asl has a sampling rate srl, and assuming that the analysis
filter bank 220 has c l analysis filter bank channels and that the synthesis filter bank 240
has c2 synthesis filter bank channels, the processed audio signal as5 has a sampling rate
sr-5:
sr5= (c2/cl) - srl .
c2/c 1 determines the upsampling factor u:
u = c2/cl .
In the embodiment of Fig. 2, the upsampling factor u can be set to a number that is not an
integer value. For example, the upsampling factor u may be set to the value 8/3, by setting
the number of analysis filter bank channels: c l = 24 and by setting the number of synthesis
filter bank channels: c2 = 64, such that:
u = 8/3 = 64/24.
Assuming that the subband generator 230 is a Spectral Band Replicator, a Spectral Band
Replicator according to an embodiment is capable to generate an arbitrary number of
additional subbands from the original subbands, wherein the ratio of the number of
generated additional subbands to the number of already available subbands does not have
to be an integer. For example, a Spectral Band Replicator according to an embodiment may
conduct the following steps:
In a first step, the Spectral Band Replicator replicates the number of subband signals by
generating a number of additional subbands, wherein the number of generated additional
subbands may be an integer multiple of the number of the already available subbands. For
example, 24 (or, for example, 48) additional subband signals may be generated from 24
original subband signals of an audio signal (e.g. the total number of subband signals may
be doubled or tripled).
In a second step, assuming that the desired number of subband signals is cl2 and the
number of actual available subband signals is cl 1, three different situations can be
distinguished:
If el l is equal to e l 2, then the number c l 1 of available subband signals is equal to the
number cl2 of subband signals needed. No subband adjustment is required.
If cl2 is smaller than cl 1, then the number cl 1 of available subband signals is greater than
the number cl2 of subband signals needed. According to an embodiment, the highest
frequency subband signals might be deleted. For example, if 64 subband signals are
available and if only 6 1 subband signals are needed, the three subband signals with the
highest frequency might be discarded.
If cl 2 is greater than e l l , then the number cl 1 of available subband signals is smaller than
the number e l2 of subband signals needed.
According to an embodiment, additional subband signals might be generated by adding
zero signals as additional subband signals, i.e. signals where the amplitude values of each
subband sample are equal to zero. According to another embodiment, additional subband
signals might be generated by adding pseudorandom subband signals as additional subband
signals, i.e. subband signals where the values of each subband sample comprise
pseudorandom data. In another embodiment, additional subband signals might be
generated by copying the sample values of the highest subband signal, or the highest
suband signals, and to use them as sample values of the additional subband signals (copied
subband signals).
In a Spectral Band Replicator according to an embodiment, available baseband subbands
may be copied and employed as highest subbands such that all subbands are filled. The
same baseband subband may be copied twice or a plurality of times such that all missing
subbands can be filled with values.
Fig. 3 illustrates an upsampling process conducted by an apparatus according to an
embodiment. A time domain audio signal 310 and some samples 315 of the audio signal
310 are illustrated. The audio signal is transformed in a frequency domain, e.g. a timefrequency
domain to obtain a frequency-domain audio signal 320 comprising three
subband signals 330. (In this simplifying example, it is assumed that the analysis filter
bank comprises 3 channels.) The subband signals of the frequency domain audio signal
330 may then be replicated to obtain three additional subband signals 335 such that the
frequency domain audio signal 320 comprisies the original three subband signals 330 and
the generated three additional subband signals 335. Then, two further additional subband
signals 338 are generated, e.g. zero signals, pseudorandom subband signals or copied
subband signals. The frequency domain audio signal is then transformed back into the time
domain resulting in a time-domain audio signal 350 having a sampling rate that is 8/3 time
the sampling rate of the original time-domain audio signal 310.
Fig. 4 illustrates an apparatus according to a further embodiment. The apparatus comprises
a signal processor 405 and a configurator 408. The signal processor 405 comprises a core
decoder module 210, an analysis filter bank 220, a subband generator 230 and a synthesis
filter bank 240, which correspond to the respective units in the embodiment of Fig. 2. The
signal processor 405 furthermore comprises an MPEG Surround decoder 410 (MPS
decoder) for decoding the preprocessed audio signal to obtain a preprocessed audio signal
with stereo or surround channels. The subband generator 230 is adapted to feed the
frequency-domain preprocessed audio signal into the MPEG Surround decoder 410 after
the additional subband signals for the frequency-domain preprocessed audio signal have
been created and added to the frequency-domain preprocessed audio signal.
Fig. 5a illustrates a core decoder module according to an embodiment. The core decoder
module comprises a first core decoder 510 and a second core decoder 520. The first core
decoder 510 is adapted to operate in a time domain and wherein the second core decoder
520 is adapted to operate in a frequency domain. In Fig. 5a, the first core decoder 510 is an
ACCLP decoder and the second core decoder 520 is an FD transfom decoder, e.g. an AAC
transform decoder. In an alternative embodiment, the second core decoder 520 is a TCX
transform decoder. Depending on whether an arriving audio signal portion asp contains
speech data or other audio data, the arriving audio signal portion asp is either processed by
the ACELP decoder 510 or by the FD transform decoder 520. The output of the core
decoder module is a prepvocessed portion of the audio signal pp-asp.
Fig. 5b illustrates an apparatus for processing an audio signal according to the embodiment
of Fig. 4 with a core decoder module according to Fig. 5a.
In an embodiment, the super-frame size for the ACELP codec is reduced from 1024 to 768
samples. This could be done by combining 4 ACELP frames of size 192 (3 sub-frames of
size 64) to one core-coder frame of size 768 (previously: 4 ACELP frames of size 256
were combined to a core-coder frame of size 1024). Fig. 6a illustrates an ACELP super
frame 605 comprising 4 ACELP frames 6 10. Each one of the ACELP frames 610
comprises 3 sub-frames 615.
Another solution for reaching a core-coder frame size of 768 samples would be for
example to combine 3 ACELP frames of size 256 (4 sub-frames of size 64). Fig. 6b
illustrates an ACELP super frame 625 comprising 3 ACELP frames 630. Each one of the
ACELP frames 630 comprises 4 sub-frames 635.
Fig. 7b outlines the proposed additional setting from a decoder perspective and compares it
to the traditional USAC setting. Fig. 7a and 7b outline the decoder stractiire as typically
used at operating points as 24 kbit/s or 32 kbit/s.
In Fig. 7a, illustrating USAC RM9 (USAC reference model 9), default setting, an audio
signal frame is inputted a QMF analysis filter bank 710. The QMF analysis filter bank 710
has 32 channels. The QMF analysis filter bank 710 is adapted to transform a time domain
audio signal into a frequency domain, wherein the frequency domain audio signal
comprises 32 subbands. The frequency domain audio signal is then inputted into an
upsampler 720. The upsampler 720 is adapted to upsample the frequency domain audio
signal by an upsampling factor 2. Thus, a frequency domain upsampler output signal
comprising 64 subbands is generated by the upsampler. The upsampler 720 is an SBR
(Spectral Band Replication) upsampler. As already mentioned, Spectral Band Replication
is employed to generate higher frequency subbands from lower frequency subbands being
inputted into the spectral band replicator.
The upsampled frequency domain audio signal is then fed into an MPEG Surround (MPS)
decoder 730. The MPS decoder 730 is adapted to decode a downmixed surround signal to
derive frequency domain channels of a surround signal. For example, the MPS decoder
730 may be adapted to generate 2 upmixed frequency domain surround channels of a
frequency domain surround signal. In another embodiment, the MPS decoder 730 may be
adapted to generate 5 upmixcd frequency domain surround channels of a frequency domain
surround signal. The channels of the frequency domain surround signal are then fed into
the QMF synthesis filter bank 740. The QMF synthesis filter bank 740 is adapted to
transform the channels of the frequency domain surround signal into a time domain to
obtain time domain channels of the surround signal.
As can be seen, the USAC decoder operates in its default setting as a 2:1 system. The corecodec
operates in the granularity of 1024 samples/frame at half of output sampling rate fout .
The upsampling by a factor of 2 is implicitly performed inside the SBR tool, by combining
a 32 band analysis QMF filter bank with a 64 band synthesis QMF bank running at the
same rate. The SBR tool outputs frames of size 2048 at f0l„.
Fig. 7b illustrates the proposed extra setting for USAC. An QMF analysis filter bank 750,
an upsampler 760, an MPS decoder 770 and a synthesis filter bank 780 are illustrated.
In contrast to the default setting, the USAC codec operates in the proposed extra setting as
an 8/3 system. The core-coder mns at 3/8lh of the output sampling rate f„ut. In the same
context, the core-coder frame size was scaled down by a factor of ¾. By combination of a
24 band analysis QMF filter bank and a 64 band synthesis filter bank inside the SBR tool,
an output sampling rate of f0lil at a frame length of 2048 samples can be achieved.
This setting allows for a very much increased temporal granularity for both, core-coder and
additional tools: Whereas tools such as SBR and MPEG Surround can be operated at a
higher sampling rate, the core-coder sampling rate is reduced and instead the frame length
shortened. By this way, all components can work in their optimal environment.
In an embodiment, an AAC coder employed as core coder may still determine scalefactors
based on an ½ fout sampling rate, even if the AAC coder operates at 3/8th of the output
sampling rate ί oui·
The table below provides detailed numbers on sampling rates and frame duration for the
USAC as used in the USAC reference quality encoder. As can be seen, the frame duration
in the proposed new setting can be reduced by nearly 25%, which leads to positive effects
for all non-stationary signals, since the spreading of coding noise can also be reduced by
the same ratio. This reduction can be achieved without increasing the core-coder sampling
frequency, which would have moved the ACELP tool out of its optimized operation range.
The table illustrates sampling rates and frame duration for default and proposed new
setting as used in the reference quality encoder at 24 kbit's.
In the following, the necessary modifications to the USAC decoder, to implement the
proposed new setting are described in more detail.
With respect to the transform coder, the shorter frame sizes can be easily achieved by
scaling the transform and window sizes by a factor of ¾. Whereas the FD coder in the
standard mode operates with transform sizes of 1024 and 128, additional transforms of size
768 and 96 are introduced by the new setting. For the TCX, additional transfomis of size of
768, 384 and 192 are needed. Apart from specifying new transform sizes according
window coefficients, the transform coder can remain unchanged.
Regarding the ACELP tool, the total frame size needs to be adapted to 768 samples. One
way to achieve this goal is to leave the overall structure of the frame is unchanged with 4
ACELP frames of 192 samples fitting within each frame of 768 samples. The adaptation to
the reduced frame size is achieved by decreasing the number of subframes per frame from
4 to 3. The ACELP subframe length is unchanged at 64 samples. In order to allow for the
reduced number of subframes, the pitch information is encoded using a slightly different
scheme: three pitch values are encoded using an absolute-relative-relative scheme using 9,
6 and 6 bits respectively instead of an absolute-relative-absolute-relative scheme using 9,
6, 9 and 6 bits in the standard model. However, other ways of coding the pitch information
is possible. The other elements of the ACELP codec, such as the ACELP codebooks as
well as the various quantizers (I PC filters, gains, etc.), are left unchanged.
Another way of achieving a total frame size of 768 samples would be to combine three
ACELP frames of size 256 for one core-coder frame of size 768.
The functionality of the SBR tool remains unchanged. However, the additional to the 32
band analysis band QMF, a 24 band analysis QMF is needed, to allow for an upsampling
of factor 8/3.
In the following, the impact of the proposed extra operating point on the computational
complexity is explained. This is at first done on a per codec-tool base and summarized at
the end. The complexity is compared against the default low sampling rate mode and
against a higher sampling rate mode, as used by the USAC reference quality encoder at
higher bitrates which is comparable to the corresponding HE-AACV2 setting for these
operating points.
Regarding the Transform coder, the complexity of the transform coder parts scales with
sampling rate and transform length. The proposed core-coder sampling rates stay roughly
the same. The transform sizes are reduced by a factor of ¾. By this, the computational
complexity is reduced by nearly the same factor, assuming a mixed radix approach for the
underlying FFTs. Overall, the complexity of the transform based decoder is expected to be
slightly reduced compared to the cuixent USAC operating point and reduced by a factor of
¾ compared against a high-sampling operating mode.
With respect to ACELP, the complexity of the ACELP tools mainly assembles of the
following operations:
Decoding of the excitation: the complexity of that operation is proportional to the number
of siibframes per second, which in turn is directly proportional to the core-coder sampling
frequency (the subframe size being unchanged at 64 samples). It is therefore nearly the
same with the new setting.
LPC filtering and other synthesis operations, including the bass-postfilter: the complexity
of this operation is directly proportional to the core-coder sampling frequency and is
therefore nearly the same.
Overall, the expected complexity of the ACELP decoder is expected to be unchanged
compared to the current USAC operating point and reduced by a factor of ¾ compared
against a high-sampling operating mode.
Regarding SBR, the main contributors to the SBR complexity are the QMF filterbanks.
The complexity here scales with sampling rate and transform size. In particular the
complexity of the analysis tiltevbank is reduced by roughly a factor of ¾.
With respect to MPEG Surround, the complexity of the MPEG Surround part scales with
the sampling rate. The proposed extra operation mode has no direct impact on the
complexity of the MPEG Surround tool.
In total, the complexity of the proposed new operating mode was found to be slightly more
complex compared to the low sampling rate mode, but below the complexity of the USAC
decoder, when run at a higher sampling rate mode (USAC RM9, high SR : 13.4 MOPS,
proposed new operating point: 12.8 MOPS).
For the tested operating point, the complexity evaluates as follows:
USAC RM9, operated at 34.15kHz: approx. 4.6 WMOPS;
USAC RM9, operated at 44.1 kHz: approx. 5.6 WMOPS;
proposed new operating point: approx. 5.0 WMOPS.
Since it is expected that a USAC decoder needs to be capable of handling sampling rates
up to 48 kHz in its default configuration, no drawback is expected by this proposed new
operating point.
With respect to the memory demand, the proposed extra operating mode requires the
storage of additional MDCT window prototypes, which sum up in total to below 900
words (32 bit) additional ROM demand. In light of the total decoder ROM demand, which
is roughly 25 kWord, this seems to be negligible.
Listening test results show a significant improvement for music and mixed test items,
without degrading the quality for speech items. This extra setting is intended as an
additional operating mode of the USAC codec.
A listening test according to MUSHRA methodology was conducted to evaluate the
performance of the proposed new setting at 24 kbit/s mono. The following conditions were
contained in the test: Hidden reference; 3.5 kHz low-pass anchor; USAC WD7 reference
quality (WD7@34. 15kHz); USAC WD7 operated at high sampling rate (WD7@44.lkHz);
and USAC WD7 reference quality, proposed new setting (WD7_CE@44.lkH2).
The test covered the 12 test items from the USAC test set, and the following additional
items: si02: castanets; velvet: electronic music; and xylophone: music box.
Fig. 8a and 8b illustrate the results of the test. 22 subjects participated in the listening test.
A Student-t probability distribution was used for the evaluation.
For the evaluation of the average scores (95% level of significance) it can be observed that
WD7 operated at a higher sampling rate of 44.1 kHz performs significantly worse than
WD7 for two items (esOl, HarryPotter). Between WD7 and the WD7 featuring the
technology, no significant difference can be observed.
For the evaluation of the differential scores it can be observed that WD7 operated at 44,1
kHz performs worse than WD7 for 6 items (esOl, louis_raquin, tel, WeddingSpeech,
HarryPotter, SpcechOverMusic_4) and averaged over all items. The items it performs
worse for include all pure speech items and two of (he mixed speech/music items. Further
on can be observed that WD7 operated at 44.1 kHz performs significantly better than WD7
for four items (twinkle, salvation, si()2, velvet). All of these items contain significant
portions of music signals or are classified as music.
For the technology under test can be observed that it performs better than WD7 for five
items (twinkle, salvation, tel 5, si02, velvet), and additionally when averaged over all
items. All of the items it performs better for contain significant portions of music signals or
are classified as music. No degradation could be observed.
By the above-described embodiments, a new setting for mid USAC bitrates is provided.
This new setting enables the USAC codec to increase its temporal granularity for all
relevant tools, such as transform coders, SBR and MPEG Suiround, without sacrificing the
quality of the ACELP tool. By this, the quality for the mid bitrate range can be improved,
in particular for music and mixed signals exhibiting a high temporal structure. Further on,
the USAC systems gains at flexibility, since the USAC codec including the ACELP tool
can now be used at a wider range of sampling rates, such as 44.1 kHz.
Fig. 9 illustrates an apparatus for processing an audio signal. The apparatus comprises a
signal processor 910 and a configurator 920. The signal processor 910 is adapted to receive
a first audio signal frame 940 having a first configurable number of samples 945 of the
audio signal. Moreover, the signal processor 910 is adapted to downsampie the audio
signal by a configurable downsampling factor to obtain a processed audio signal.
Furthermore, the signal processor is adapted to output a second audio signal frame 950
having a second configurable number of samples 955 of the processed audio signal.
The configurator 920 is adapted to configure the signal processor 910 based on
configuration information ci2 such that the configurable downsampling factor is equal to a
first downsampling value when a first ratio of the second configurable number of samples
to the first configurable number of samples has a first ratio value. Moreover, the
configurator 920 is adapted to configure the signal processor 910 such that the
configurable downsampling factor is equal to a different second downsampling value,
when a different second ratio of the second configurable number of samples to the first
configurable number of samples has a different second ratio value. The first or the second
ratio value is not an integer value.
An apparatus according to Fig. 9 may for example be employed in the process of encoding.
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 perfomiing
one of the methods when the computer program product inns 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.
5 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
10 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
. 5 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
20 described herein.
A further embodiment comprises a computer having installed thereon the computer
program for performing one of the methods described herein.
25 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.
J Q
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
35 specific details presented by way of description and explanation of the embodiments
herein.
Claims
1. An apparatus for processing an audio signal, comprising:
a signal processor ( 110; 205; 405) being adapted to receive a first audio signal
frame having a first configurable number of samples of the audio signal, being
adapted to upsampie the audio signal by a configurable upsampling factor to obtain
a processed audio signal, and being adapted to output a second audio signal frame
having a second configurable number of samples of the processed audio signal; and
a configurator (120; 208; 408) being adapted to configure the signal processor ( 1 10;
205; 405),
wherein the configurator (120; 208; 408) is adapted to configure the signal
processor ( 1 10; 205; 405) based on configuration information such that the
configurable upsampling factor is equal to a first upsampling value when a first
ratio of the second configurable number of samples to the first configurable number
of samples has a first ratio value, and wherein the configurator (120; 208; 408) is
adapted to configure the signal processor ( 1 10; 205; 405) such that the configurable
upsampling factor is equal to a different second upsampling value, when a different
second ratio of the second configurable number of samples to the first configurable
number of samples has a different second ratio value, and wherein the first or the
second ratio value is not an integer value.
2. An apparatus according to claim 1, wherein the configurator (120; 208; 408) is
adapted to configure the signal processor ( 1 10; 205; 405) such that the different
second upsampling value is greater than the first upsampling value, when the
second ratio of the second configurable number of samples to the first configurable
number of samples is greater than the first ratio of the second configurable number
of samples to the first configurable number of samples.
3 . An apparatus according to claim 1 or 2, wherein the configurator (120; 208; 408) is
adapted to configure the signal processor {1 10; 205; 405) such that the configurable
upsampling factor is equal to the first ratio value when the first ratio of the second
configurable number of samples to the first configurable number of samples has the
first ratio value, and wherein the configurator (120; 208; 408) is adapted to
configure the signal processor ( 110; 205; 405) such that the configurable
upsampling factor is equal to the different second ratio value when the second ratio
of the second configurable number of samples to the first configurable number of
samples has the different second ratio value.
4 . An apparatus according to one of the preceding claims, wherein the configurator
(120; 208; 408) is adapted to configure the signal processor ( 1 10; 205; 405) such
that the configurable upsampling factor is equal to 2 when the first ratio has the first
ratio value, and wherein the conf igurator (120; 208; 408) is adapted to configure the
signal processor ( 1 10; 205; 405) such that the configurable upsampling factor is
equal to 8/3 when the second ratio has the different second ratio value.
5 . An apparatus according to one of the preceding claims, wherein the configurator
(120; 208; 408) is adapted to configure the signal processor ( 110; 205; 405) such
that the first configurable number of samples is equal to 1024 and the second
configurable number of samples is equal to 2048 when the first ratio has the first
ratio value, and wherein the configurator (120; 208; 408) is adapted to configure the
signal processor ( 110; 205; 405) such that that the first configurable number of
samples is equal to 768 and the second configurable number of samples is equal to
2048 when the second ratio has the different second ratio value.
6 . An apparatus according to one of the preceding claims, wherein the signal
processor ( 1 10; 205; 405) comprises:
a core decoder module (210) for decoding the audio signal to obtain a preprocessed
audio signal,
an analysis filter bank (220) having a number of analysis filter bank channels for
transforming the first preprocessed audio signal from a time domain into a
frequency domain to obtain a frequency-domain preprocessed audio signal
comprising a plurality of subband signals,
a subband generator (230) for creating and adding additional subband signals for
the frequency-domain preprocessed audio signal, and
a synthesis filter bank (240) having a number of synthesis filter bank channels for
transforming the first preprocessed audio signal from the frequency domain into the
time domain to obtain the processed audio signal.
wherein the configurator (120; 208; 408) is adapted to configure the signal
processor ( 1 10; 205; 405) by configuring the number of number of synthesis filter
bank channels or the number of analysis filter bank channels such that the
configurable upsampling factor is equal to a third ratio of the number of synthesis
filter bank channels to the number of analysis filter bank channels.
An apparatus according claim 6, wherein the subband generator (230) is a spectral
7 .
band replicator being adapted to replicate subband signals of the preprocessed
audio signal generator for creating the additional subband signals for the frequencydomain
preprocessed audio signal.
8 . An apparatus according to claim 6 or 7, wherein the signal processor ( 1 10; 205;
405) further comprises an MPEG Surround decoder (410) for decoding the
preprocessed audio signal to obtain a preprocessed audio signals comprising stereo
or surround channels,
wherein the subband generator (230) is adapted to feed the frequency-domain
preprocessed audio signal into the MPEG Surround decoder (410) after the
additional subband signals for the frequency-domain preprocessed audio signal
have been created and added to the frequency-domain preprocessed audio signal.
9 . An apparatus according to one of claims 6 to 8, wherein the core decoder module
(210) comprises a first core decoder (510) and a second core decoder (520),
wherein the first core decoder (510) is adapted to operate in a time domain and
wherein the second core decoder (520) is adapted to operate in a frequency domain.
10. An apparatus according to claim 9, wherein the first core decoder (510) is an
ACELP decoder and wherein the second core decoder (520) is a FD transfom
decoder or a TCX transform decoder.
11. An apparatus according to claim 10, wherein the ACELP decoder (510) is adapted
to process the first audio signal frame, wherein the first audio signal frame has 4
ACELP frames, and wherein each one of the ACELP frames has 192 audio signal
samples, when the first configurable number of samples of the first audio signal
frame is equal to 768.
1 2 . An apparatus according to claim 10, wherein the ACELP decoder (5 10) is adapted
to process the first audio signal frame, wherein the first audio signal frame has 3
ACELP frames, and wherein each one of the ACELP frames has 256 audio signal
samples, when the first configurable number of samples of the first audio signal
frame is equal to 768.
13. An apparatus according to one of the preceding claims, wherein configurator (120;
208; 408) is adapted to configure the signal processor ( 1 10; 205; 405) based on the
configuration information indicating at least one of the first configurable number of
samples of the audio signal or the second configurable number of samples of the
processed audio signal.
14. An apparatus according to one of the preceding claims, wherein configurator (120;
208; 408) is adapted to configure the signal processor ( 1 10; 205; 405) based on the
configuration information, wherein the configuration information indicates the first
configurable number of samples of the audio signal and the second configurable
number of samples of the processed audio signal, wherein the configuration
information is a configuration index.
15. Method for processing an audio signal, comprising:
configuring a configurable upsampling factor,
receiving a first audio signal frame having a first configurable number of samples
of the audio signal, and
upsampling the audio signal by the configurable upsampling factor to obtain a
processed audio signal, and being adapted to output a second audio frame having a
second configurable number of samples of the processed audio signal; and
wherein the configurable upsampling factor is configured based on configuration
information such that the configurable upsampling factor is equal to a first
upsampling value when a first ratio of the second configurable number of samples
to the first configurable number of samples has a first ratio value, and wherein the
configurable upsampling factor is configured such that the configurable upsampling
factor is equal to a different second upsampling value, when a different second ratio
of the second configurable number of samples to the first configurable number of
samples lias a different second ratio value, and wherein the first or the second ratio
value is not an integer value.
16. An apparatus for processing an audio signal, comprising:
a signal processor (910) being adapted to receive a first audio signal frame having a
first configurable number of samples of the audio signal, being adapted to
downsample the audio signal by a configurable downsampling factor to obtain a
processed audio signal, and being adapted to output a second audio frame having a
second configurable number of samples of the processed audio signal; and
a configurator (920) being adapted to configure the signal processor,
wherein the configurator (920) is adapted to configure the signal processor (910)
based on configuration information such that the configurable downsampling factor
is equal to a first downsampling value when a first ratio of the second configurable
number of samples to the first configurable number of samples has a first ratio
value, and wherein the configurator (920) is adapted to configure the signal
processor (910) such that the configurable downsampling factor is equal to a
different second downsampling value, when a different second ratio of the second
configurable number of samples to the first configurable number of samples has a
different second ratio value, and wherein the first or the second ratio value is not an
integer value.
17. An apparatus according to claim 16, wherein the configurator is adapted to
configure the signal processor (910) such that the first downsampling value is
smaller than the different second downsampling value, when the first ratio of the
second configurable number of samples to the first configurable number of samples
is smaller than the second ratio of the second configurable number of samples to the
first configurable number of samples.
18. Method for processing an audio signal, comprising:
configuring a configurable downsampling factor,
receiving a first audio signal frame having a first configurable number of samples
of the audio signal, and
downsampling the audio signal by the configurable downsampling factor to obtain
a processed audio signal, and being adapted to output a second audio frame having
a second configurable number of samples of the processed audio signal; and
wherein the configurable downsampiing factor is configured based on configuration
information such that the configurable downsampiing factor is equal to a first
downsampiing value when a first ratio of the second configurable number of
samples to the first configurable number of samples has a first ratio value, and
wherein the configurable downsampiing factor is configured such that the
configurable downsampling factor is equal to a different second downsampiing
value, when a different second ratio of the second configurable number of samples
to the first configurable number of samples has a different second ratio value, and
wherein the first or the second ratio value is not an integer value.
19. Computer program for perfonniiig the method of claim 15 or 18, when the
computer program is executed by a computer or processor.