Apparatus for Decoding a Signal Comprising Transients
using a Combining Unit and a Mixer
Specification
The present invention relates to the field of audio processing and audio decoding, in
particular to decoding a signal comprising transients.
Audio processing and/or decoding has advanced in many ways. In particular, spatial audio
applications have become more and more important. Audio signal processing is often used
to decorrelate or render signals. Moreover, decorrelation and rendering of signals is
employed in the process of mono-to-stereo-upmix, mono/stereo to multi-channel upmix,
artificial reverberation, stereo widening or user interactive mixing/rendering.
Several audio signal processing systems employ decorrelators. An important example is
the application of decorrelating systems in parametric spatial audio decoders to restore
specific decorrelation properties between two or more signals that are reconstructed from
one or several downmix signals. The application of decorrelators significantly improves
the perceptual quality of the output signal, e.g., when compared to intensity stereo.
Specifically, the use of decorrelators enables the proper synthesis of spatial sound with a
wide sound image, several concurrent sound objects and/or ambience. However,
decorrelators are also known to introduce artifacts like changes in temporal signal
structure, timbre, etc.
Other application examples of decorrelators in audio processing are, e.g., the generation of
artificial reverberation to change the spatial impression or the use of decorrelators in
multichannel acoustic echo cancellation systems to improve the convergence behavior.
A typical state of the art application of a decorrelator in a mono to stereo up-mixer, e.g.
applied in Parametric Stereo (PS), is illustrated in Fig. 1, where a mono input signal M (a
"dry" signal) is provided to a decorrelator 110. The decorrelator 110 decorrelates the mono
input signal M according to a decorrelation method to provide a decorrelated signal D (a
"wet" signal) at its output. The decorrelated signal D is fed into a mixer 120 as a first
mixer input signal along with the dry mono signal M as a second mixer input signal.
Furthermore an up-mix control unit 130 feeds up-mix control parameters into the mixer
1 0. The mixer 1 0 then generates two output channels L and R (L = left stereo output
channel; R = right stereo output channel) according to a mixing matrix H. The coefficients
of the mixing matrix can be fixed, signal dependent or controlled by a user.
Alternatively, the mixing matrix is controlled by side information that is transmitted along
with the downmix containing a parametric description on how to up-mix the signals of the
downmix to form the desired multi-channel output. This spatial side information is usually
generated during the mono downmix process in an accordant signal encoder.
This principle is widely applied in spatial audio coding, e.g. Parametric Stereo, see, for
example, J. Breebaart, S. van de Par, A. Kohlrausch, E. Schuijers, "High-Quality
Parametric Spatial Audio Coding at Low Bitrates" in Proceedings of the AES 116th
Convention, Berlin, Preprint 6072, May 2004.
A further typical state of the art structure of a parametric stereo decoder is illustrated in
Fig. 2, wherein a decorrelation process is performed in a transform domain. An analysis
filterbank 210 transforms a mono input signal into a transform domain, for example into a
frequency domain. Decorrelation of the transformed mono input signal M is then
performed by a decorrelator 220 which generates a decorrelated signal D. Both the
transformed mono input signal M and the decorrelated signal D are fed into a mixing
matrix 230. The mixing matrix 230 then generates two output signals L and R taking upmix
parameters into account, which are provided by parameter modification unit 240,
which is provided with spatial parameters and which is coupled to a parameter control unit
250. In Fig. 2, the spatial parameters can be modified by a user or additional tools, e.g.,
post-processing for binaural rendering/presentation. In this example, the up-mix
parameters are combined with the parameters from the binaural filters to form the input
parameters for the up-mix matrix. Finally, the output signals generated by the mixing
matrix 230 are fed into a synthesis filterbank 260, which determines the stereo output
signal.
The output L/R of the mixing matrix 230 is computed from the mono input signal M and
the decorrelated signal D according to a mixing rule, e.g. by applying the following
formula:
In the mixing matrix, the amount of decorrelated sound fed to the output is controlled on
the basis of transmitted parameters, e.g., Inter-Channel Correlation/Coherence (ICC)
and/or fixed or user-defined settings.
Conceptually, the output signal of the decorrelator output D replaces a residual signal that
would ideally allow for a perfect decoding of the original L/R signals. Utilizing the
decorrelator output D instead of a residual signal in the upmixer results in a saving of bit
rate that would otherwise have been required to transmit the residual signal. The aim of the
decorrelator is thus to generate a signal D from the mono signal M, which exhibits similar
properties as the residual signal that is replaced by D.
Correspondingly, on the encoder side, two types of spatial parameters are extracted: A first
group of parameters comprises correlation/coherence parameters (e.g., ICCs = Inter-
Channel Correlation/Coherence parameters) representing the coherence or cross correlation
between two input channels that shall be encoded. A second group of parameters
comprises level difference parameters (e.g., ILDs = Inter Channel Level Difference
parameters) representing the level difference between the two input channels.
Furthermore, a downmix signal is generated by downmixing the two input channels.
Moreover a residual signal is generated. Residual signals are signals which can be used to
regenerate the original signals by additionally employing the downmix signal and an
upmix matrix. When, for example, N signals are downmixed to 1 signal, the downmix is
typically 1 of the N components which result from the mapping of the N input signals. The
remaining components resulting from the mapping (e.g., N-l components) are the residual
signals and allow reconstructing the original N signals by an inverse mapping. The
mapping may, for example, be a rotation. The mapping shall be conducted such that the
downmix signal is maximized and the residual signals are minimized, e.g., similar as a
principal axis transformation. E.g., the energy of the downmix signal shall be maximized
and the energies of the residual signals shall be minimized. When downmixing 2 signals to
1 signal, the downmix is normally one of the two components which result from the
mapping of the 2 input signals. The remaining component resulting from the mapping is
the residual signal and allows reconstructing the original 2 signals by an inverse mapping.
In some cases, the residual signal may represent an error associated with representing the
two signals by their downmix and associated parameters. For example, the residual signal
may be an error signal which represents the error between original channels L, R and
channels L R', resulting from upmixing the downmix signal that was generated based on
the original channels L and R.
In other words, a residual signal can be considered as a signal in the time domain or a
frequency domain or a subband domain, which together with the downmix signal alone or
with the downmix signal and parametric information allows a correct or nearly correct
reconstruction of an original channel. Nearly correct has to be understood that the
reconstruction with the residual signal having an energy greater than zero is closer to the
original channel compared to a reconstruction using the downmix without the residual
signal or using the downmix and the parametric information without the residual signal.
Considering MPEG Surround (MPS), structures similar to PS termed One-To-Two boxes
(OTT boxes) are employed in spatial audio decoding trees. This can be seen as a
generalization of the concept of mono-to-stereo upmix to multichannel spatial audio
coding/decoding schemes. In MPS, two-to-three upmix systems (TTT boxes) also exist
that may apply decorrelators depending on the TTT mode of operation. Details are
described in J . Herre, K. Kjorling, J. Breebaart, et al, "MPEG surround—the ISO/MPEG
standard for efficient and compatible multi-channel audio coding," in Proceedings of the
122th AES Convention, Vienna, Austria, May 2007.
Regarding Directional Audio Coding (DirAC), DirAC relates to a parametric sound field
coding scheme that is not bound to a fixed number of audio output channels with fixed
loudspeaker positions. DirAC applies decorrelators in the DirAC Tenderer, i.e., in the
spatial audio decoder to synthesize non-coherent components of sound fields. More
information relating to directional audio coding can be found in Pulkki, Ville: "Spatial
Sound Reproduction with Directional Audio Coding," in J. Audio Eng. Soc, Vol. 55, No.
6, 2007.
Regarding state of the art decorrelators in spatial audio decoders, reference is made to
ISO/IEC International Standard "Information Technology- MPEG audio technologies -
Parti: MPEG Surround", ISO/IEC 23003-1:2007 and also to J. Engdegard, H. Purnhagen,
J . Roden, L.Liljeryd, "Synthetic Ambience in Parametric Stereo Coding" in Proceedings of
the AES 116th Convention, Berlin, Preprint, May 2004. IIR lattice allpass structures are
used as decorrelators in spatial audio decoders like MPS as described in J. Herre, K.
Kjorling, J. Breebaart, et al., "MPEG surround—the ISO/MPEG standard for efficient and
compatible multi-channel audio coding," in Proceedings of the 122th AES Convention,
Vienna, Austria, May 2007, and as described in ISO/IEC International Standard
"Information Technology- MPEG audio technologies - Parti: MPEG Surround", ISO/IEC
23003-1:2007. Other state of the art decorrelators apply (potentially frequency dependent)
delays to decorrelate signals or convolve the input signals, e.g., with exponentially
decaying noise bursts. For an overview of state of the art decorrelators for spatial audio
upmix systems, see "Synthetic Ambience in Parametric Stereo Coding" in Proceedings of
the AES 116th Convention, Berlin, Preprint, May 2004.
Another technique of processing signals is "semantic upmix processing". Semantic upmix
processing is a technique to decompose signals into components with different semantic
properties (i.e., signal classes) and apply different upmix strategies to the different signal
components. The different upmix algorithms can be optimized according to the different
semantic properties in order to improve the overall signal processing scheme. This concept
is described in WO/2010/0 17967, An apparatus for determining a spatial output
multichannel-channel audio signal, International patent application, PCT/EP2009/005828,
11.8.2009, 11.6.2010 (FH090802PCT).
A further spatial audio coding scheme is the "temporal permutation method", as described
in Hotho, G., van de Par, S., and Breebaart, J.: "Multichannel coding of applause signals",
EURASIP Journal on Advances in Signal Processing, Jan. 2008, art. 10.
DOI=http://dx.doi.org/l 0.1 155/2008/. In this document, a spatial audio coding scheme is
proposed that is tailored to the coding/decoding of applause-like signals. This scheme
relies on the perceptual similarity of segments of a monophohic audio signal, esp. a
downmix signal of a spatial audio coder. The monophonic audio signal is segmented into
overlapping time segments. These segments are temporarily permuted pseudo randomly
(mutually independent for n output channels) within a "super"-block to form the
decorrelated output channels.
A further spatial audio coding technique is the "temporal delay and swapping method". In
DE 10 2007 018032 A: 20070417, Erzeugung dekorrelierter Signale, 17.4.2007,
23.10.2008 (FH070414PDE), a scheme is proposed that is also tailored to the
coding/decoding of applause-like signals for binaural presentation. This scheme also relies
on the perceptual similarity of segments of a monophonic audio signal and delays on
output channels with respect to the other one. In order to avoid a localization bias towards
the leading channel, leading and lagging channel are swapped periodically.
In general, stereo or multichannel applause-like signals coded/decoded in parametric
spatial audio coders are known to result in reduced signal quality (see, for example, Hotho,
G., van de Par, S., and Breebaart, J.: "Multichannel coding of applause signals", EURASIP
Journal on Advances in Signal Processing, Jan. 2008, art. 10.
DOI=http://dx.doi.org/10.1 155/2008/531693, see also DE 10 2007 018032 A). Applauselike
signals are characterized by containing temporarily dense mixtures of transients from
different directions. Examples for such signals are applause, the sound of rain, galloping
horses, etc. Applause-like signals often also contain sound components from distant sound
sources, that are perceptually fused into a noise-like, smooth, background sound field.
State of the art decorrelation techniques employed in spatial audio decoders like MPEG
Surround contain lattice allpass structures. These act as artificial reverb generators and are
consequently well suited for generating homogeneous, smooth, noise-like, immersive
sounds (like room reverberation tails). However, there are examples of sound fields with a
non-homogeneous spatio-temporal structure that are still immersing the listener: one
prominent example are applause-like sound fields that create listener-envelopment not only
by homogeneous noise-like fields, but also by rather dense sequences of single claps from
different directions. Hence, the non-homogeneous component of applause sound fields
may be characterized by a spatially distributed mixture of transients. Obviously, these
distinct claps are not homogeneous, smooth and noise-like at all.
Due to their reverb-like behavior, lattice allpass decorrelators are incapable of generating
immersive sound field with the characteristics, e.g., of applause. Instead, when applied to
applause-like signals, they tend to temporarily smear the transients in the signals. The
undesired result is a noise-like immersive sound field without the distinctive spatiotemporal
structure of applause-like sound fields. Further, transient events like a single
handclap might evoke ringing artifacts of the decorrelator filters.
A system according to Hotho, G., van de Par, S., and Breebaart, J.: "Multichannel coding
of applause signals", EURASIP Journal on Advances in Signal Processing, Jan. 2008, art.
10. DOI=http://dx.doi.org/10.1 155/2008/531693, will exhibit perceivable degradation of
the output sound due to a certain repetitive quality in the output audio signal. This is
because of the fact that one and the same segment of the input signal appears unaltered in
every output channel (though at a different point in time). Furthermore, to avoid increased
applause density, some original channels have to be dropped in the upmix and thus some
important auditory event might be missed in the resulting upmix. The method is only
applicable if it is possible to find signal segments that share the same perceptual properties,
i.e.: signal segments that sound similar. The method in general heavily changes the
temporal structure of the signals, which might be acceptable only for very few signals. In
the case of applying the scheme to non-applause-like signals (e.g., due to signal
misclassification), the temporal permutation will most often lead to unacceptable results.
The temporal permutation further limits the applicability to cases where several signal
segments may be mixed together without artifacts like echoes or comb-filtering. Similar
drawbacks apply to the method described in DE 10 2007 018032 A.
The semantic upmix processing described in WO/2010/017967 separates the transient
components of signals prior to the application of decorrelators. The remaining (transientfree)
signal is fed to the conventional decorrelation and upmix processor, whereas the
transient signals are handled differently: the latter are (e.g., randomly) distributed to
different channels of the stereo or multichannel output signal by application of amplitude
panning techniques. The amplitude panning shows several disadvantages:
Amplitude panning does not necessarily produce an output signal that is close to the
original. The output signal may be only close to the original if the distribution of the
transients in the original signal can be described by amplitude panning laws. I.e.: The
amplitude panning can only reproduce purely amplitude panned events correctly, but no
phase or time differences between the transient components in different output channels.
Moreover, application of the amplitude panning approach in MPS would require bypassing
not only the decorrelator but also the upmix matrix. Since the upmix matrix reflects the
spatial parameters (inter channel correlations: ICCs, inter channel level differences: ILDs)
that are necessary to synthesize an upmix output that shows the correct spatial properties,
the panning system itself has to apply some rule to synthesize output signals with the
correct spatial properties. A generic rule for doing so is not known. Further, this structure
adds complexity since the spatial parameters have to be taken care of twice: once, for the
non-transient part of the signal and, second, for the amplitude-panned transient part of the
signal.
It is therefore an object of the present invention to provide an improved concept for
generating a decorrelated signal for decoding a signal. The object of the present invention
is solved by an apparatus for generating for decoding a signal according to claim 1, by a
method for decoding a signal according to claim 3 and by a computer program according
to claim 14.
An apparatus according to an embodiment comprises a transient separator for separating an
input signal into a first signal component and into a second signal component such that the
first signal component comprises transient signal portions of the input signal and such that
the second signal component comprises non-transient signal portions of the input signal.
The transient separator may separate the different signal components from each other to
allow that signal components which comprise transients may be processed differently than
signal components which do not comprise transients.
The apparatus furthermore comprises a transient decorrelator for decorrelating signal
components comprising transients according to a decorrelation method which is
particularly suited for decorrelating signal components comprising transients. Moreover,
the apparatus comprises a second decorrelator for decorrelating signal components which
do not comprise transients.
Thus, the apparatus is capable to either process signal components using a standard
decorrelator or alternatively process signal components using the transient decorrelator
particularly suited for processing transient signal components. In an embodiment, the
transient separator decides whether a signal component is either fed into the standard
decorrelator or into the transient decorrelator.
Furthermore, the apparatus may be adapted to separate a signal component such that the
signal component is partially fed into the transient decorrelator and partially fed into the
second decorrelator.
Moreover, the apparatus comprises a combining unit for combining the signal components
outputted by the standard decorrelator and the transient decorrelator to generate a
decorrelated combination signal.
In an embodiment, the apparatus comprises a mixer being adapted to receive input signals
and moreover being adapted to generate output signals based on the input signals and on a
mixing rule. An apparatus input signal is fed into a transient separator and afterwards
decorrelated by a transient separator and/or a second decorrelator as described above. The
combination unit and the mixer may be arranged so that the decorrelated combination
signal is fed into the mixer as a first mixer input signal. A second mixer input signal may
be the apparatus input signal or a signal derived from the apparatus input signal. As the
decorrelation process is already completed when the decorrelated combination signal is fed
into the mixer, transient decorrelation does not have to be taken into account by the mixer.
Therefore, a conventional mixer may be employed.
In a further embodiment, the mixer is adapted to receive correlation/coherence parameter
data indicating a correlation or coherence between two signals and is adapted to generate
the output signals based on the correlation/coherence parameter data. In another
embodiment, the mixer is adapted to receive level difference parameter data indicating an
energy difference between two signals and is adapted to generate the output signals based
on the level difference parameter data. In such an embodiment, the transient decorrelator,
the second decorrelator and the combining unit do not have to be adapted to process such
parameter data, as the mixer will take care of processing corresponding data. On the other
hand, a conventional mixer with conventional correlation/coherence and level difference
parameter processing may be employed in such an embodiment.
In an embodiment, the transient separator is adapted to either feed a considered signal
portion of an apparatus input signal into the transient decorrelator or to feed the considered
signal portion into the second decorrelator depending on transient separation information
which either indicates that the considered signal portion comprises a transient or which
indicates that the considered signal portion does not comprise a transient. Such an
embodiment allows easy processing of transient separation information.
In another embodiment, the transient separator is adapted to partially feed a considered
signal portion of an apparatus input signal into the transient decorrelator and to partially
feed the considered signal portion into the second decorrelator. The amount of the
considered signal portion that is fed into the transient separator and the amount of the
considered signal portion that is fed into the second decorrelator depend on transient
separation information. By this, the strength of a transient may be taken into account.
In a further embodiment, the transient separator is adapted to separate an apparatus input
signal which is represented in a frequency domain. This allows frequency dependent
transient processing (separation and decorrelation). Thus, certain signal components of a
first frequency band may be processed according to a transient decorrelation method, while
signal components of another frequency band may be processed according to another, e.g.,
conventional decorrelation method. Accordingly, in an embodiment the transient separator
is adapted to separate an apparatus input signal based on frequency dependent transient
separation information. However, in an alternative embodiment, the transient separator is
adapted to separate an apparatus input signal based on frequency independent separation
information. This allows more efficient transient signal processing.
In another embodiment, the transient separator may be adapted to separate an apparatus
input signal which is represented in a frequency domain such that all signal portions of the
apparatus input signal within a first frequency range are fed into the second decorrelator.
An corresponding apparatus is therefore adapted to restrict transient signal processing to
signal components with signal frequencies in a second frequency range, while no signal
components with signal frequencies in the first frequency range are fed into the transient
decorrelator (but instead into the second decorrelator).
In a further embodiment, the transient decorrelator may be adapted to decorrelate the first
signal component by applying phase information representing a phase difference between a
residual signal and a downmix signal. On the encoder side, a "reverse" mixing matrix may
be employed to create a downmix signal and a residual signal, e.g., from the two channels
of a stereo signal, as has been explained above. While the downmix signal may be
transmitted to the decoder, the residual signal may be discarded. According to an
embodiment, the phase difference employed by the transient decorrelator may be the phase
difference between the residual signal and the downmix signal. It may thus be possible to
reconstruct an "artificial" residual signal, by applying the original phase of the residual on
the downmix. In an embodiment, the phase difference may relate to a certain frequency
band, i.e., may be frequency dependent. Alternatively, a phase difference does not relate to
certain frequency bands but may be applied as a frequency independent broadband
parameter.
In an embodiment, the apparatus comprises a receiving unit for receiving phase
information, wherein the transient decorrelator is adapted to apply the phase information to
the first signal component. The phase information might be generated by a suitable
encoder.
In a further embodiment a phase term might be applied to the first signal component by
multiplying the phase term with the first signal component.
In a further embodiment, the second decorrelator may be a conventional decorrelator, e.g.,
a lattice IIR decorrelator.
Embodiments are now explained in more detail with respect to the figures, wherein:
Fig. 1 illustrates a state of the art application of a decorrelator in a mono to stereo
up-mixer;
Fig. 2 depicts a further state of the art application of a decorrelator in a mono to
stereo up-mixer;
Fig. 3 illustrates an apparatus for generating a decorrelated signal according to an
embodiment;
Fig. 4 illustrates an apparatus for decoding a signal according to an embodiment;
Fig. 5 is a one-to-two (OTT) system overview according to an embodiment;
Fig. 6 illustrates an apparatus for generating a decorrelated signal comprising a
receiving unit according to a further embodiment;
Fig. 7 is a one-to-two system overview according to another further embodiment;
Fig. 8 illustrates exemplary mappings from phase consistency measures
transient separation strength;
Fig. 9 is a one-to-two system overview according to another further embodiment;
Fig. 10 illustrates an apparatus for encoding an audio signal having a plurality of
channels according to an embodiment.
Fig. 3 illustrates an apparatus for generating a decorrelated signal according to an
embodiment. The apparatus comprises a transient separator 310, a transient decorrelator
320, a conventional decorrelator 330 and a combination unit 340. The transient handling
approach of this embodiment aims to generate decorrelated signals from applause-like
audio signals, e.g., for the application in the upmix-process of spatial audio decoders.
In Fig. 3, an input signal is fed into a transient separator 310. The input signal may have
been transformed to a frequency domain, e.g., by. applying a hybrid QMF filter bank. The
transient separator 310 may decide for each considered signal component of the input
signal whether it comprises a transient. Furthermore, the transient separator 310 may be
arranged to feed the considered signal portion either into the transient decorrelator 320, if
the considered signal portion comprises a transient (signal component si), or it may feed
the considered signal portion into the conventional decorrelator 330, if the considered
signal portion does not comprise a transient (signal component s2). The transient separator
310 may also be arranged to split the considered signal portion depending on the existence
of a transient in the considered signal portion and provide them partially to the transient
decorrelator 320 and partially to the conventional decorrelator 330.
In an embodiment, the transient decorrelator 320 decorrelates signal component si
according to a transient decorrelation method which is particularly suitable to decorrelate
transient signal components. For example, the decorrelation of the transient signal
components may be carried out by applying phase information, e.g., by applying phase
terms. A decorrelation method where phase terms are applied on transient signal
components is explained below with respect to the embodiment of Fig. 5. Such a
decorrelation method may also be employed as a transient decorrelation method of the
transient decorrelator 320 of the embodiment of Fig. 3.
Signal component s2, which comprises non-transient signal portions, is fed into the
conventional decorrelator 330. The conventional deccorrelator 330 may then decorrelate
signal component s2 according to a conventional decorrelation method, for example, by
applying lattice allpass structures, e.g., a lattice IIR (infinite impulse response) filter.
After being decorrelated by the conventional decorrelator 330, the decorrelated signal
component from the conventional decorrelator 330 is fed into the combining unit 340. The
decorrelated transient signal component from the transient decorrelator 320 is also fed into
the combining unit 340. The combining unit 340 then combines both decorrelated signal
components, e.g. by adding both signal components, to obtain a decorrelated combination
signal.
In general, a method decorrelating a signal comprising transients according to an
embodiment may be conducted as follows:
In a separation step, the input signal is separated into two components: one component si
comprises the transients of the input signal, another component s2 comprises the remaining
(non-transient) part of the input signal. The non-transient component s2 of the signal may
be processed like in systems without applying the decorrelation method of the transient
decorrelator of this embodiment. I.e.: the transient-free signal s2 may be fed to one or
several conventional decorrelating signal processing structures like lattice IIR allpass
structures.
Moreover, the signal component comprising the transients (the transient stream si) is fed
to a "transient decorrelator" structure that decorrelates the transient stream while
maintaining the special signal properties better than the conventional decorrelating
structures. The decorrelation of the transient stream is carried out by applying phase
information at a high temporal resolution. Preferably, the phase information comprises
phase terms. Furthermore, it is preferred that the phase information may be provided by an
encoder.
Furthermore, the output signals of both the conventional decorrelator and the transient
decorrelator are combined to form the decorrelated signal which might be utilized in the
upmix-process of spatial audio coders. The elements (hll5 h12 , h2 1, h22) of the mixingmatrix
(Mmix) of the spatial audio decoder may remain unchanged.
Fig. 4 illustrates an apparatus for decoding an apparatus input signal according to an
embodiment, wherein the apparatus input signal is fed into the transient separator 410. The
apparatus comprises the transient separator 410, a transient decorrelator 420, a
conventional decorrelator 430, combining unit 440 and a mixer 450. The transient
separator 410, the transient decorrelator 420, the conventional decorrelator 430 and the
combining unit 440 of this embodiment may be similar to the transient separator 310, the
transient decorrelator 320, the conventional decorrelator 330 and the combining unit 340 of
the embodiment of Fig. 3, respectively. A decorrelated combination signal generated by
the combining unit 440 is fed into a mixer 450 as a first mixer input signal. Furthermore,
the apparatus input signal that has been fed into the transient separator 410 is also fed into
the mixer 450 as a second mixer input signal. Alternatively, the apparatus input signal is
not directly fed into the mixer 450, but a signal derived from the apparatus input signal is
fed into the mixer 450. A signal may be derived from the apparatus input signal, for
example, by applying a conventional signal processing method to the apparatus input
signal, e.g. applying a filter. The mixer 450 of the embodiment of Fig. 4 is adapted to
generate output signals based on the input signals and a mixing rule. Such a mixing rule
may be, for example, to multiply the input signals and a mixing matrix, for example by
applying the formula
The mixer 450 may generate the output channels L, R on the basis of correlation/coherence
parameter data, e.g., Inter-Channel Correlation/Coherence (ICC), and/or level difference
parameter data, e.g., Inter Channel Level Difference (ILD). For example, the coefficients
of a mixing matrix may depend on the correlation/coherence parameter data and/or the
level difference parameter data. In the embodiment of Fig. 4, the mixer 450 generates the
two output channels L and R. However, in alternative embodiments, the mixer may
generate a plurality of output signals, for example 3, 4, 5, or 9 output signals, which may
be surround sound signals.
Fig. 5 depicts a system overview of the transient handling approach in a l-to-2 (OTT)
upmix system of an embodiment, e.g., a l-to-2 box of an MPS (MPEG Surround) spatial
audio decoder. The parallel signal path for the separated transients according to an
embodiment is comprised in the U-shaped transient handling box. An apparatus input
signal DMX is fed into a transient separator 510. The apparatus input signal may be
represented in a frequency domain. For example, a time domain input signal may have
been transformed into a frequency domain by applying a QMF filter bank as used in
MPEG Surround. The transient separator 510 may then feed the components of the
apparatus input signal DMX into a transient decorrelator 520 and/or into a lattice IIR
decorrelator 530. The components of the apparatus input signal are then decorrelated by
the transient decorrelator 520 and/or the lattice IIR decorrelator 530. Afterwards, the
decorrelated signal components Dl and D2 are combined by a combining unit 540, e.g., by
adding both signal components, to obtain a decorrelated combination signal D. The
decorrelated combination signal is fed into a mixer 552 as a first mixer input signal D.
Furthermore, the apparatus input signal DMX (or alternatively: a signal derived from the
apparatus input signal DMX) is also fed into the mixer 552 as a second mixer input signal.
The mixer 552 then generates a first and a second "dry" signal, depending on the apparatus
input signal DMX. The mixer 552 also generates a first and second "wet" signal depending
on the decorrelated combination signal D. The signals, generated by the mixer 552 may
also be generated based on transmitted parameters, e.g., correlation/coherence parameter
data, e.g., Inter-Channel Correlation/Coherence (ICC), and/or level difference parameter
data, e.g., Inter Channel Level Difference (ILD). In an embodiment, the signals generated
by the mixer 552 may be provided to a shaping unit 554 which shapes the provided signals
based on provided temporal shaping data. In other embodiments, no signal shaping takes
place. The generated signals are then provided to a first 556 or second 558 adding unit
which combine the provided signals to generate a first output signal L and a second output
signal R, respectively.
The processing principles shown in Fig. 5 may be applied in mono-to-stereo upmix
systems (e.g., stereo audio coders) as well as in multi-channel setups (e.g., MPEG
Surround). In embodiments, the proposed transient handling scheme may be applied as an
upgrade to existing upmix systems without large conceptual changes of the upmix system,
since only a parallel decorrelator signal path is introduced without altering the upmix
process itself.
Signal separation into the transient and non-transient component is controlled by
parameters that might be generated in an encoder and/or the spatial audio decoder. The
transient decorrelator 520 utilizes phase information, e.g., phase terms that might be
obtained in an encoder or in the spatial audio decoder. Possible variants for obtaining
transient handling parameters (i.e.: transient separation parameters like transient positions
or separation strength and transient decorrelation parameters like phase information) are
described below.
The input signal may be represented in a frequency domain. For example, a signal may
have been transformed to a frequency domain by employing an analysis filter bank. A
QMF filter bank may be applied to obtain a plurality of subband signals from a time
domain signal.
For best perceptual quality, the transient signal processing may be preferably restricted to
signal frequencies in a limited frequency range. One example would be to limit the
processing range to frequency band indices k > 8 of a hybrid QMF filter bank as used in
MPS, similar to the frequency band limitation of guided envelope shaping (GES) in MPS.
In the following, embodiments of a transient separator 520 are explained in more detail.
The transient separator 510 splits the input signal DMX into transient and non-transient
components si and s2, respectively. The transient separator 510 may employ transient
separation information for splitting the input signal DMX, for example a transient
separation parameter b[h] . The splitting of the input signal DMX may be done in a way
such that the sum of the component, sl+s2 5 equals the input signal DMX:
s [n] =DMX [r -b [h]
s2[n]=DMX[n] -(l -fi[n])
where n is the time index of downsampled subband signals and valid values for the time
variant transient separation parameter b[h] are in the range [0, 1]. b[h] may be a frequency
independent parameter. A transient separator 510 which is adapted to separate an apparatus
input signal based on a frequency independent separation parameter may feed all subband
signal portions with time index n either to the transient decorrelator 520 or into the second
decorrelator depending on the value of b[h] .
Alternatively, b[h] may be a frequency dependent parameter. A transient separator 510
which is adapted to separate an apparatus input signal based on a frequency dependent
transient separation information may process subband signal portions with the same time
index differently, if their corresponding transient separation information differ.
Furthermore, the frequency dependency may, e.g., be used to limit the frequency range of
the transient processing as mentioned in the section above.
In an embodiment, the transient separation information may be a parameter which either
indicates that a considered signal portion of an input signal DMX comprises a transient or
which indicates that the considered signal portion does not comprise a transient. The
transient separator 510 feeds the considered signal portion into the transient decorrelator
520, if the transient separation information indicates that the considered signal portion
comprises a transient. Alternatively, the transient separator 10 feeds the considered signal
portion into the second decorrelator, e.g. the lattice IIR decorrelator 530, if the transient
separation information indicates that the considered signal portion comprises a transient.
For example, a transient separation parameter b [h] may be employed as transient
separation information which may be a binary parameter, n is the time index of a
considered signal portion of the input signal DMX. b [h] may be either 1 (indicating that the
considered signal portion shall be fed into the transient decorrelator) or 0 (indicating that
the considered signal portion shall be fed into the second decorrelator). Restricting b [h] to
b {0, 1} results in hard transient/non-transient decisions, i.e.: components that are
treated as transients are fully separated from the input ( b = 1).
In another embodiment, the transient separator 510 is adapted to partially feed a considered
signal portion of the apparatus input signal into the transient decorrelator 520 and to
partially feed the considered signal portion into the second decorrelator 530. The amount
of the considered signal portion that is fed into the transient separator 520 and the amount
of the considered signal portion that is fed into the second decorrelator 530 depends on
transient separation information. In an embodiment, b [h] has to be in the range [0, 1]. In a
further embodiment, b [h] may be restricted to b [h] [0, b c] , where b <1, results in a
partial separation of the transients, leading to a less pronounced effect of the transient
handling scheme. Therefore, changing h c allows to fade between the output of the
conventional upmix processing without transient handling and the upmix processing
including the transient handling.
In the following, a transient decorrelator 520 according to an embodiment is explained in
more detail.
A transient decorrelator 520 according to an embodiment creates an output signal that is
sufficiently decorrelated to the input. It does not alter the temporal structure of single
claps/transients (no temporal smearing, no delay). Instead, it leads to a spatial distribution
of the transient signal components (after the upmix process), which is similar to the spatial
distribution in the original (non-coded) signal. The transient decorrelator 520 may allow
for bit rate vs. quality trade-offs (e.g., fully random spatial transient distribution at low
bitrate ® close to the original (near-transparent) at high bit rate). Furthermore, this is
achieved with low computational complexity.
As has been explained above, on the encoder side, a "reverse" mixing matrix may be
employed to create a downmix signal and a residual signal, e.g., from the two channels of a
stereo signal. While the downmix signal may be transmitted to the decoder, the residual
signal may be discarded. According to an embodiment, the phase difference between the
residual signal and the downmix signal may be determined, e.g., by an encoder, and may
be employed by a decoder when decorrelating a signal. By this, it may then be possible to
reconstruct an "artificial" residual signal, by applying the original phase of the residual on
the downmix.
A corresponding decorrelation method of the transient decorrelator 520 according to an
embodiment will be explained in the following:
According to a transient decorrelation method, a phase term may be employed.
Decorrelation is achieved by simply multiplying the transient stream by phase terms at
high temporal resolution, e.g., at subband signal time resolution in transform domain
systems like MPS:
In this equation, n is the time index of downsampled subband signals. Df ideally reflects
the phase difference between downmix and residual. Therefore, the transient residuals are
replaced by a copy of the transients from the downmix, modified such that they exhibit the
original phase.
Applying the phase information inherently results in a panning of the transients to the
original position in the upmix process. As an illustrative example consider the case ICC=0,
ILD=0: The transient part of the output signals then reads:
L[n] = c ]+Dl[n = c s[n] (l + ej "])
For Df=0 this results in L=2c*s, R=0, whereas Df=p leads to L=0, R=2c*s. Other values
of Df, ICC, and ILD lead to different level and phase relations between the rendered
transients.
The Df[h] values may be applied as frequency independent broadband parameters or as
frequency dependent parameters. In case of applause-like signals without tonal
components, broadband Df[h] values may be advantageous due to lower data rate demands
and consistent handling of broadband transients (consistency over frequency).
The transient handling structure of Fig. 5 is arranged such that only the conventional
decorrelator 530 is bypassed regarding the transient signal components while the mixing
matrix remains unaltered. Thus, the spatial parameters (ICC, ILD) are inherently also taken
into account for the transient signals, e.g.: the ICC automatically controls the width of the
rendered transient distribution.
Considering the aspect of how to obtain phase information, in an embodiment, phase
information may be received from an encoder.
Fig. 6 illustrates an embodiment of an apparatus for generating a decorrelated signal. The
apparatus comprises a transient separator 610, a transient decorrelator 620, a conventional
decorrelator 630, a combining unit 640 and a receiving unit 650. The transient separator
610, the conventional decorrelator 630 and the combining unit 640 are similar to the
transient separator 310, the conventional decorrelator 330 and the combining unit 340 of
the embodiment shown in Fig. 3. However, Fig. 6 furthermore illustrates a receiving unit
650 which is adapted to receive phase information. The phase information may have been
transmitted by an encoder (not shown). For example, an encoder may have computed the
phase difference between residual and downmix signals (relative phase of the residual
signal with respect to a downmix). The phase difference may have been calculated for
certain frequency bands or broadband (e.g., in a time domain). The encoder may
appropriately code the phase values by uniform or non-uniform quantization and
potentially lossless coding. Afterwards, the encoder may transmit the coded phase values
to the spatial audio decoding system. Obtaining the phase information from an encoder is
advantageous as the original phase information is then available in a decoder (except for
the quantization error).
The receiving unit 650 feeds the phase information into the transient decorrelator 620
which uses the phase information when it decorrelates a signal component. For example,
the phase information may be a phase term and the transient decorrelator 620 may multiply
a received transient signal component by the phase term.
In case of transmitting phase information Df[h] from the encoder to the decoder, the
required data rate can be reduced as follows:
The phase information Df[h] may be applied only to the transient signal components in the
decoder. Therefore, the phase information only needs to be available in the decoder as long
as there are transient components in the signal to be decorrelated. The transmission of the
phase information can thus be limited by the encoder such that only the necessary
information is transmitted to the decoder. This can be done by applying a transient
detection in the encoder as described below. Phase information Df[h] is only transmitted
for points in time n, for which transients have been detected in the encoder.
Considering the aspect of transient separation, in an embodiment, transient separation may
be encoder driven.
According to an embodiment, the transient separation information (also referred to as
"transient information") may be obtained from an encoder. The encoder may apply
transient detection methods as described in Andreas Walther, Christian Uhle, Sascha Disch
"Using Transient Suppression in Blind Multi-channel Up-mix Algorithms," in Proc. 122nd
AES Convention, Vienna, Austria, May 2007 either to the encoder input signals or to the
downmix signals. The transient information is then transmitted to the decoder and
preferably obtained e.g., at the time resolution of downsampled subband signals.
The transient information may preferably comprise a simple binary (transient/nontransient)
decision for each signal sample in time. This information may preferably also be
represented by the transient positions in time and the transient durations.
The transient information may be losslessly coded (e.g., run-length coding, entropy
coding) to reduce the data rate that is necessary to transmit the transient information from
the encoder to the decoder.
The transient information may be transmitted as broadband information or as frequency
dependent information at a certain frequency resolution. Transmitting the transient
information as broadband parameters reduces the transient information data rate and
potentially improves the audio quality due to consistent handling of broadband transients.
Instead of the binary (transient/non-transient) decision, also the strength of the transients
may be transmitted, e.g., quantized in two or four steps. The transient strength may then
control the separation of the transients in the spatial audio decoder as follows: Strong
transients are fully separated from the IIR lattice decorrelator input, whereas weaker
transients are only partially separated.
The transient information may only be transmitted, if the encoder detects applause-like
signals, e.g., using applause detection systems as described in Christian Uhle, "Applause
Sound Detection with Low Latency", in Audio Engineering Society Convention 127, New
York, 2009.
The detection result for the similarity of the input signal to applause-like signals may also
be transmitted at a lower time resolution (e.g., at the spatial parameters update rate in
MPS) to the decoder to control the strength of the transient separation. The applause
detection result may be transmitted as a binary parameter (i.e., as a hard decision) or as a
non-binary parameter (i.e., as a soft decision). This parameter controls the separationstrength
in the spatial audio decoder. Therefore, it allows to (hardly or gradually) switch
on/off the transient handling in the decoder. This allows avoiding artifacts that might
occur, e.g., when applying a broadband transient handling scheme to signals that contain
tonal components.
Fig. 7 illustrates an apparatus for decoding a signal according to an embodiment. The
apparatus comprises a transient separator 710, a transient decorrelator 720, a lattice IIR
decorrelator 730, a combining unit 740, a mixer 752, an optional shaping unit 754, a first
adding unit 756 and a second adding unit 758, which correspond to the transient separator
510, the transient decorrelator 520, the lattice IIR decorrelator 530, the combining unit
540, the mixer 552 the optional shaping unit 554, the first adding unit 556 and the second
adding unit 558 of the embodiment of Fig. 5, respectively. In the embodiment of Fig. 7, an
encoder obtains phase information and transient position information and transmits the
information to an apparatus for decoding. No residual signals are transmitted. Fig. 7
illustrates a l-to-2 upmix configuration like an OTT box in MPS. It may be applied in a
stereo codec for upmixing from a mono downmix to a stereo output according to an
embodiment. In the embodiment of Fig. 7, three transient handling parameters are
transmitted as frequency independent parameters from the encoder to the decoder, as can
be seen in Fig. 7:
A first transient handling parameter to be transmitted is the binary transient/non-transient
decision of a transient detector running in the encoder. It is used to control the transient
separation in the decoder. In a simple scheme, the binary transient/non-transient decision
may be transmitted as a binary flag per subband time sample without further coding.
A further transient handling parameter to be transmitted is the phase value (or the phase
values) Df [h] that is needed for the transient decorrelator. Df is only transmitted for times
n, for which transients have been detected in the encoder. Df values are transmitted as
indices of a quantizer with a resolution of, e.g. 3 bit per sample.
Another transient handling parameter to be transmitted is the separation strength (i.e., the
effect strength of the transient handling scheme). This information is transmitted at the
same temporal resolution as the spatial parameters ILD, ICC.
The necessary bit rate BR for transmitting transient separation decisions and broadband
phase information from the encoder to the decoder can be estimated for MPS-like systems
as:
BR = BRtransient separation flags +BR {f s /64) + -Q-f /64 = {l + -Q)-f s /64 ,
where s is the transient density (fraction of time slots (=subband time samples) that are
marked as transients), Q is the number of bits per transmitted phase value, and fs is the
sampling rate. Note that (fs/64) is the sampling rate of the downsampled subband signals.
E {s} < 0.25 has been measured for a set of several representative applause items, where
E{.} denotes the mean over the item duration. A reasonable compromise between
exactness of the phase values and parameter bit rate is Q=3. To reduce the parameter data
rate, the ICCs and ILDs may be transmitted as broadband cues. The transmission of the
ICCs and ILDs as broadband cues is especially applicable for non-tonal signals like
applause.
Additionally, the parameters for signaling the separation strength are transmitted at the
update rate of the ICCs/ILDs. For long spatial frames in MPS (32 times 64 samples) and 4-
step quantized separation strengths, this results in an additional bit rate of
BR-transientseparationstrength ·3 ) · 2 .
The separation strength parameter may be derived in an encoder from the results of signal
analysis algorithms that assess the similarity to applause-like signals, the tonality, or other
signal characteristics that indicate potential benefits or problems when applying the
transient decorrelation of the embodiment.
The transmitted parameters for transient handling may be subject to lossless coding to
reduce redundancy, resulting in a lower parameter bit rate (e.g., run-length coding of
transient separation information, entropy coding).
Returning to the aspect of obtaining phase information, in an embodiment, phase
information may be obtained in a decoder.
In such an embodiment, the apparatus for decoding does not obtain phase information from
an encoder, but may determine the phase information itself. Therefore, it is not necessary
to transmit phase information what results in a reduced overall transmission rate.
In an embodiment, phase information is obtained in an MPS based decoder from "Guided
Envelope Shaping (GES)" data. This is only applicable if GES data is transmitted, i.e., if
the GES feature is activated in an encoder. The GES feature is available e.g., in MPS
systems. The ratio of GES envelope values between the output channels reflects panning
positions for the transients at high time resolution. The GES envelope ratio (GESR) can be
mapped to the phase information needed for the transient handling. In GES, the mapping
may be performed according to a mapping rule obtained empirically from building
statistics of the phase-relative-to-GESR-distribution for a representative set of appropriate
test signals. Determining the mapping rule is a step for designing the transient handling
system, not a run time process when applying the transient handling system. Therefore, it
is advantageous that there is no need to spend additional transmission costs for the phase
data if GES data is needed for the application of the GES feature anyway. Bitstream
backward compatibility is achieved with MPS bitstreams/decoders. However, phase
information extracted from GES data is not as exact (e.g.: the sign of the estimated phase is
unknown) as the phase information that might be obtained in the encoder.
In a further embodiment, phase information may also be obtained in a decoder, but from
transmitted non-fullband residuals. This is applicable, e.g., if band limited residual signals
are transmitted (typically covering a frequency range up to a certain transition frequency)
in an MPS coding scheme. In such an embodiment, the phase relation between the
downmix and transmitted residual signal in the residual band(s) is calculated, i.e., for
frequencies for which residual signals are transmitted. Furthermore, the phase information
from the residual band(s) to the non-residual band(s) is extrapolated (and/or possibly
interpolated). One possibility is to map the phase relation obtained in the residual band(s)
to a global frequency independent phase relation value that is then used for the transient
decorrelator. This results in the benefit that no additional transmission costs arise for the
phase data, if non-full band residuals are transmitted anyway. However, it has to be
considered, that the correctness of the phase estimate depends on the width of the
frequency band(s) where residual signals are transmitted. The correctness of the phase
estimates also depends on the consistency of the phase relation between the downmix and
the residual signal along the frequency axis. For clearly transient signals, high consistency
is usually encountered.
In a further embodiment, phase information is obtained in a decoder employing additional
correction information transmitted from the encoder. Such an embodiment is similar to the
two previous embodiments (phase from GES, phase from residuals), but additionally, it is
necessary to generate correction data in the encoder which is transmitted to the decoder.
The correction data allows for reducing the phase estimation error that may occur in the
two variants described before (phase from GES, phase from residuals). Furthermore, the
correction data may be derived from estimating the decoder-side phase estimation error in
the encoder. The correction data may be this (potentially coded) estimated estimation error.
Furthermore, with respect to the phase-estimation-from-GES-data approach, the correction
data may simply be the correct sign of the encoder-generated phase values. This allows
generating phase terms with the correct sign in the decoder. The benefit of such an
approach is that due to the correction data, the exactness of the phase information
recoverable in the decoder is much closer to that of the encoder generated phase
information. However, the entropy of the correction information is lower than the entropy
of the correct phase information itself. Thus, the parameter bit rate is lowered when
compared to directly transmitting the phase information obtained in the encoder.
In another embodiment, phase information/terms are obtained from a (pseudo-) random
process in a decoder. The benefit of such an approach is that there is no need to transmit
any phase information with high temporal resolution. This results in a reduced data rate. In
an embodiment, a simple method is to generate phase values with a uniform random
distribution in the range [- 180°, 180°].
In a further embodiment, the statistical properties of the phase distribution in the encoder
are measured. These properties are coded and then transmitted (at low time resolution) to
the decoder. Random phase values are generated in the decoder which are subject to the
transmitted statistical properties. These properties might be the mean, variants, or other
statistical measures of the statistical phase distribution.
When more than one decorrelator instance is running in parallel (e.g., for a multichannel
upmix), care has to be taken to ensure mutually decorrelated decorrelator outputs. In an
embodiment, wherein multiple vectors of (pseudo-) random phase values (instead of a
single vector) are generated for all but the first decorrelator instance, a set of vectors is
selected that results in the least correlation of the phase value across all decorrelator
instances.
In case of transmitting phase correction information from the encoder to the decoder, the
required data rate can be reduced as follows:
The phase correction information only needs to be available in the decoder as long as there
are transient components in the signal to be decorrelated. The transmission of the phase
correction information can thus be limited by the encoder such that only the necessary
information is transmitted to the decoder. This can be done by applying a transient
detection in the encoder as has been described above. Phase correction information is only
transmitted for points in time n, for which transients have been detected in the encoder.
Returning to the aspect of transient separation, in an embodiment, transient separation may
be decoder driven.
In such an embodiment, transient separation information may also be obtained in the
decoder, e.g., by applying a transient detection method as described in Andreas Walther,
Christian Uhle, Sascha Disch "Using Transient Suppression in Blind Multi-channel Up¬
mix Algorithms," in Proc. 122nd AES Convention, Vienna, Austria, May 2007 to the
downmix signal that is available in the spatial audio decoder before upmixing to a stereo or
multichannel output signal. In this case, no transient information has to be transmitted,
which saves transmission data rate.
However, performing the transient detection in decoding might cause issues when, e.g.,
standardizing the transient handling scheme: for example, it might be hard to find a
transient detection algorithm which results in exactly the same transient detection results
when being implemented on different architectures/platforms involving different numerical
precisions, rounding schemes, etc. Such a predictable decoder behavior is often mandatory
for standardization. Furthermore, the standardized transient detection algorithm might fail
for some input signals, causing intolerable distortions in the output signals. It might then be
difficult to correct the failing algorithm after standardization without building a decoder
that is not conforming to the standard. This issue might be less severe if at least a
parameter controlling the transient separation strength is transmitted at low time resolution
(e.g., at the spatial parameter update rate of MPS) from the encoder to the decoder.
In a further embodiment, transient separation is also decoder driven and non-fullband
residuals are transmitted. In this embodiment, the decoder driven transient separation may
be refined by employing obtained phase estimates from transmitted non-fullband residuals
(see above). Note that this refinement can be applied in the decoder without transmitting
additional data from the encoder to the decoder.
In this embodiment, the phase terms that are applied in a transient decorrelator are obtained
by extrapolating the correct phase values from the residual bands to frequencies where no
residuals are available. One method is to calculate a (potentially e.g. signal power
weighted) mean phase value from the phase values that can be calculated for those
frequencies where residual signals are available. The mean phase value may then be
applied as a frequency independent parameter in the transient decorrelator.
As long as the correct phase relation between the downmix and the residual is frequency
independent, the mean phase value represents a good estimate of the correct phase value.
However; in the case of a phase relation that is not consistent along the frequency axis, the
mean phase value may be a less correct estimate, potentially leading to incorrect phase
values and audible artifacts.
The consistency of the phase relation between the downmix and the transmitted residual
along the frequency axis can therefore be used as a reliability measure of the extrapolated
phase estimate that is applied in the transient decorrelator. To lower the risk of audible
artifacts, the consistency measure obtained in the decoder may be used to control the
transient separation strength in the decoder, e.g. as follows:
Transients, for which the corresponding phase information (i.e. the phase information for
the same time index n) is consistent along frequency, are fully separated from the
conventional decorrelator input and are fully fed into the transient decorrelator. Since large
phase estimation errors are unlikely, the full potential of the transient handling is used.
Transients, for which the corresponding phase information is less consistent along
frequency, are only partially separated, leading to a less prominent effect of the transient
handling scheme.
Transients, for which the corresponding phase information is very inconsistent along
frequency, are not separated, leading to the standard behavior of a conventional upmix
system without the proposed transient handling. Thus, no artifacts due to large phase
estimation errors can occur.
The consistency measures for the phase information may be deducted, e.g. from the
(potentially signal power weighted) variance of standard deviation of the phase
information along frequency.
Since only few frequencies may be available for which the residual signals are transmitted,
the consistency measure may have to be estimated from only few samples along frequency,
leading to a consistency measure that only seldom reaches extreme values ("perfectly
consistent" or "perfectly inconsistent"). Thus, the consistency measure may be linearly or
non-linearly distorted before being used to control the transient separation strength. In an
embodiment, a threshold characteristic is implemented as illustrated in Fig. 8, right
example.
Fig. 8 depicts different exemplary mappings from phase consistency measures to transient
separation strengths, illustrating the impact of the variants for obtaining transient handling
parameters on the robustness to transient misclassification. The variants for obtaining the
transient separation information and the phase information listed above differ in parameter
data rate and therefore represent different operating points in term of overall bit rate of a
codec implementing the proposed transient handling technique. Apart from this, the choice
of the source for obtaining the phase information also affects aspects such as the robustness
to false transient classifications: handling a non-transient signal as a transient causes much
less audible distortions if the correct phase information is applied in the transient handling.
Thus, a signal classification error causes less severe artifacts in the scenario of transmitted
phase values when compared to the scenario of random phase generation in the decoder.
Fig. 9 is a One-To-Two system overview with transient handling according to a further
embodiment, wherein narrow band residual signals are transmitted. The phase data Df is
estimated from the phase relation between the downmix (DMX) and the residual signal in
the frequency band(s) of the residual signal. Optionally, phase correction data is
transmitted to lower the phase estimation error.
Fig. 9 illustrates a transient separator 910, a transient decorrelator 920, a lattice IIR
decorrelator 930, a combining unit 940, a mixer 952 an optional shaping unit 954, a first
adding unit 956 and a second adding unit 958, which correspond to the transient separator
510, the transient decorrelator 520, the lattice IIR decorrelator 530, the combining unit
540, the mixer 552 the optional shaping unit 554, the first adding unit 556 and the second
adding unit 558 of the embodiment of Fig. 5, respectively. The embodiment of Fig. 8
furthermore comprises a phase estimation unit 960. The phase estimation unit 960 receives
an input signal DMX, a residual signal "residual" and optionally, phase correction data.
Based on the received information the phase information unit calculates phase data Df.
Optionally, the phase estimation unit also determines phase consistency information and
passes the phase consistency information to the transient separator 910. For example, the
phase consistency information may be used by the transient separator to control the
transient separation strength.
The embodiment of Fig. 9 applies the finding that if residuals are transmitted within the
coding scheme in a non-full band fashion, the signal power weighted mean phase
difference between the residual and the downmix re siduai_bands) may be applied as
broadband phase information to the separated transients (Df = A plow residuaijjands)- this
case, no additional phase information has to be transmitted, lowering the bit rate demand
for the transient handling. In the embodiment of Fig. 9, the phase estimate from the
residual bands may considerably deviate from the more precise broadband phase estimate
that is available in the encoder. An option is therefore to transmit phase correction data
(e.g., A tion A - A res iduatbands) so that the correct Df are available in the decoder.
However, since Apo rection may show a lower entropy than Df, the necessary parameter
data rate may be lower than the rate that would be needed for transmitting Df. (This
concept is similar to the general use of prediction in coding: instead of coding data directly,
a predication error with lower entropy is coded. In the embodiment of Fig. 9, the prediction
step is the extrapolation of the phase from the residual frequency bands to non-residual
bands). The consistency of the phase difference in the residual frequency bands
(A residuai_ an s along the frequency axis may be used to control the transient separation
strength.
In embodiments, a decoder may receive phase information from an encoder, or the decoder
may itself determine the phase information. Furthermore, the decoder may receive
transient separation information from an encoder, or the decoder may itself determine the
transient separation information.
In embodiments, an aspect of the transient handling is the application of the "semantic
decorrelation" concept decribed in WO/20 10/0 17967 together with the "transient
decorrelator", which is based on multiplying the input with phase terms. The perceptual
quality of rendered applause-like signals is improved since both processing steps avoid
altering the temporal structure of transient signals. Furthermore, the spatial distribution of
transients as well as phase relations between the transients is reconstructed in the output
channels. Furthermore, embodiments are also computationally efficient and can readily be
integrated into PS- or MPS- like upmix systems. In embodiments, the transient handling
does not affect the mixing matrix process, so that all spatial rendering properties that are
defined by the mixing matrix are also applied to the transient signal.
In embodiments, a novel decorrelation scheme is applied which is particularly suited for
the application in upmix systems, which is particularly suited to the application of spatial
audio coding schemes like PS or MPS and which improves the perceptual quality of the
output signals in the case of applause-like signals, i.e. signals that contain dense mixtures
of spatially distributed transients and/or may be seen as a particularly enhanced
implementation of the generic "semantic decorrelation" framework. Furthermore, in
embodiments a novel decorrelation scheme is comprised that reconstructs the
spatial/temporal distribution of the transients similar to the distribution in the original
signal, preserves the temporal structure of the transient signals, allows for varying the bit
rate versus quality trade-off and/or is ideally suited for a combination with MPS features
like non-full-band residuals or GES. The combinations are complementary, i.e.:
information of standard MPS features is reused for the transient handling.
Fig. 10 illustrates an apparatus for encoding an audio signal having a plurality of channels.
Two input channels L, R are fed into a downmixer 1010 and into a residual signal
calculator 1020. In other embodiments, a plurality of channels is fed into the downmixer
1010 and the residual signal calculator 1020, e.g., 3, 5 or 9 surround channels. The
downmixer 1010 then downmixes the two channels L, R, to obtain a downmix signal. For
example, the downmixer 1010 may employ a mixing matrix and conduct a matrix
multiplication of the mixing matrix and the two input channels L, R, to obtain the
downmix signal. The downmix signal may be transmitted to a decoder.
Furthermore, the residual signal generator 1020 is adapted to calculate a further signal
which is referred to as residual signal. Residual signals are signals which can be used to
regenerate the original signals by additionally employing the downmix signal and an
upmix matrix. When, for example, N signals are downmixed to 1 signal, the downmix is
typically 1 of the N components which result from the mapping of the N input signals. The
remaining components resulting from the mapping (e.g., N-l components) are the residual
signals and allow reconstructing the original N signals by an inverse mapping. The
mapping may, for example, be a rotation. The mapping shall be conducted such that the
downmix signal is maximized and the residual signals are minimized, e.g., similar as a
principal axis transformation. E.g., the energy of the downmix signal shall be maximized
and the energies of the residual signals shall be minimized. When downmixing 2 signals to
1 signal, the downmix is normally one of the two components which result from the
mapping of the 2 input signals. The remaining component resulting from the mapping is
the residual signal and allows reconstructing the original 2 signals by an inverse mapping.
In some cases, the residual signal may represent an error associated with representing the
two signals by their downmix and associated parameters. For example, the residual signal
may be an error signal which represents the error between original channels L, R and
channels L', R', resulting from upmixing the downmix signal that was generated based on
the original channels L and R.
In other words, a residual signal can be considered as a signal in the time domain or a
frequency domain or a subband domain, which together with the downmix signal alone or
with the downmix signal and parametric information allows a correct or nearly correct
reconstruction of an original channel. Nearly correct has to be understood that the
reconstruction with the residual signal having an energy greater than zero is closer to the
original channel compared to a reconstruction using the downmix without the residual
signal or using the downmix and the parametric information without the residual signal.
Furthermore, the encoder comprises a phase information calculator 1030. The downmix
signal and the residual signal are fed into the phase information calculator 1030. The phase
information calculator then calculates information on a phase difference between the
downmix and the residual signal to obtain phase information. For example, the phase
information calculator may apply functions that calculate a cross-correlation of the
downmix and the residual signal.
Moreover, the encoder comprises an output generator 1040. The phase information
generated by the phase information calculator 1030 is fed into the output generator 1040.
The output generator 1040 then outputs the phase information.
In an embodiment the apparatus further comprises a phase information quantizer for
quantizing the phase information. The phase information generated by the phase
information calculator may be fed into the phase information quantizer. The phase
information quantizer then quantizes the phase information. For example, the phase
information may be mapped to 8 different values, e.g., to one of the values 0, 1, 2, 3, 4, 5, 6
or 7. The values may represent the phase differences 0, /4, p/2, 3p/4, p, 5p/4, 3p/2 and
7p/4, respectively. The quantized phase information may then be fed into the output
generator 1040.
In a further embodiment, the apparatus moreover comprises a lossless encoder. The phase
information from the phase information calculator 1040 or the quantized phase information
from the phase information quanztizer may be fed into the lossless encoder. The lossless
encoder is adapted to encode phase information by applying lossless encoding. Any kind of
lossless coding scheme may be employed. For example, the encoder may employ
arithmetic coding. The lossless encoder then feeds the losslessly encoded phase
information into the output generator 1040.
With respect to the decoder and encoder and the methods of the described embodiments
the following is mentioned:
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.
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 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 or a non-transitory storage medium.
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.
Claims
1. An apparatus for decoding a signal comprising:
a transient separator (310; 410; 510; 610; 710; 910) for separating an apparatus
input signal into a first signal component and into a second signal component such
that the first signal component comprises transient signal portions of the input
signal and such that the second signal component comprises non-transient signal
portions of the input signal;
a transient decorrelator (320; 420; 520; 620; 720; 920) for decorrelating the first
signal component according to a first decorrelation method to obtain a first
decorrelated signal component;
a second decorrelator (330; 430; 530; 630; 730; 930) for decorrelating the second
signal component according to a second decorrelation method to obtain a second
decorrelated signal component, wherein the second decorrelation method is
different from the first decorrelation method;
a combining unit (340; 440; 540; 640; 740; 940) for combining the first
decorrelated signal component and the second decorrelated signal component to
obtain a decorrelated combination signal; and
a mixer (450; 552; 752; 952), being adapted to receive mixer input signals and
being adapted to generate output signals based on the mixer input signals and a
mixing rule;
wherein the combining unit (340; 440; 540; 640; 740; 940) and the mixer (450;
552; 752; 952) are arranged so that the decorrelated signal is fed into the mixer
(450; 552; 752; 952) as a first mixer input signal and that the apparatus input signal
or a signal derived from the apparatus input signal is fed into the mixer (450; 552;
752; 952) as a second mixer input signal.
2. An apparatus according to claim 1,
wherein the mixer (450; 552; 752; 952) is furthermore adapted to receive
correlation/coherence parameter data indicating a correlation or coherence between
two signals and wherein the mixer (450; 552; 752; 952) is furthermore adapted to
generate the output signals based on the correlation/coherence parameter data.
3. An apparatus according to claim 1 or 2,
wherein the mixer (450; 552; 752; 952) is furthermore adapted to receive level
difference parameter data indicating an energy difference between two signals and
wherein the mixer (450; 552; 752; 952) is furthermore adapted to generate the
output signals based on the level difference parameter data.
4. An apparatus according to one of the preceding claims,
wherein the mixer (450; 552; 752; 952) is adapted to employ a mixing rule which
comprises the rule to multiply the first and second mixer input signal by a mixing
matrix.
5. An apparatus according to one of the preceding claims,
wherein the combining unit (340; 440; 540; 640; 740; 940) is adapted to combine
the first decorrelated signal component and the second decorrelated signal
component by adding the first decorrelated signal component and the second
decorrelated signal component.
6. An apparatus according to one of the preceding claims,
wherein the transient separator (310; 410; 510; 610; 710; 910) is adapted to either
feed a considered signal portion of the apparatus input signal into the transient
decorrelator (320; 420; 520; 620; 720; 920) or to feed the considered signal portion
into the second decorrelator (330; 430; 530; 630; 730; 930) depending on transient
separation information which either indicates that the considered signal portion
comprises a transient or which indicates that the considered signal portion does not
comprise a transient.
7. An apparatus according to one of claims 1 to 5,
wherein the transient separator (310; 410; 510; 610; 710; 910) is adapted to partially
feed a considered signal portion of the apparatus input signal into the transient
decorrelator (320; 420; 520; 620; 720; 920) and to partially feed the considered
signal portion into the second decorrelator (330; 430; 530; 630; 730; 930), and
wherein the amount of the considered signal portion that is fed into the transient
separator and the amount of the considered signal portion that is fed into the second
decorrelator depend on transient separation information.
8. An apparatus according to one of the preceding claims,
wherein the transient separator (310; 410; 510; 610; 710; 910) is adapted to
separate an apparatus input signal which is represented in a frequency domain.
An apparatus according to one of the preceding claims,
wherein the transient separator (310; 410; 510; 610; 710; 910) is adapted to
separate the apparatus input signal into a first signal component and into a second
signal component based on a frequency independent transient separation
information.
An apparatus according to one of the preceding claims,
wherein the transient separator (310; 410; 510; 610; 710; 910) is adapted to
separate the apparatus input signal into a first signal component and into a second
signal component based on a frequency dependent transient separation information.
An apparatus according to one of the preceding claims,
wherein the apparatus furthermore comprises a receiving unit (650) which is
adapted to receive the phase information from an encoder; and wherein the transient
decorrelator (320; 420; 520; 620; 720; 920) is adapted to apply the phase
information from the encoder to the first signal component.
An apparatus according to one of the preceding claims,
wherein the second decorrelator (330; 430; 530; 630; 730; 930) is a lattice IIR
decorrelator.
A method for decoding a signal comprising:
separating an apparatus input signal into a first signal component and into a second
signal component such that the first signal component comprises transient signal
portions of the apparatus input signal and such that the second signal component
comprises non-transient signal portions of the apparatus input signal;
decorrelating the first signal component according to a first decorrelation method to
obtain a first decorrelated signal component;
decorrelating the second signal component according to a second decorrelation
method to obtain a second decorrelated signal component, wherein the second
decorrelation method is different from the first decorrelation method;
combining the first decorrelated signal component and the second decorrelated
signal component to obtain a decorrelated combination signal; and
generating output signals based on a mixing rule, the decorrelated signal and the
apparatus input signal.
A computer program implementing a method according to claim 13.