EP2489039A1

EP2489039A1 - Optimized low-throughput parametric coding/decoding

Info

Publication number: EP2489039A1
Application number: EP10785120A
Authority: EP
Inventors: Thi Minh Nguyet Hoang; Stéphane RAGOT; Balazs Kovesi
Original assignee: France Telecom SA
Current assignee: Orange SA
Priority date: 2009-10-15
Filing date: 2010-10-15
Publication date: 2012-08-22
Anticipated expiration: 2030-10-15
Also published as: US9167367B2; JP2013508743A; CN102656628B; BR112012008793B1; US20120207311A1; KR101646650B1; JP5752134B2; CN102656628A; EP2489039B1; WO2011045548A1; KR20120095920A; BR112012008793A2

Abstract

The present invention pertains to a method of parametric coding of a multichannel digital audio signal comprising a step of coding a signal arising from a channel reduction matrixing of the multichannel signal. The method of coding furthermore comprises the following steps: obtaining, per frame of predetermined length, spatial information parameters for the multichannel signal; dividing the spatial information parameters into a plurality of blocks of parameters; selecting a block of parameters as a function of the index of the current frame; coding the block of parameters selected for the current frame. The invention pertains also to a method of decoding the multichannel signal by decoding the blocks of parameters received per frame. It pertains to a coder and decoder implementing the respective methods of coding and decoding.

Description

Optimized low bit rate parametric coding / decoding

The present invention relates to the field of coding / decoding of digital signals.

The coding and decoding according to the invention is particularly suitable for the transmission and / or storage of digital signals such as audio-frequency signals (speech, music or other).

More particularly, the present invention relates to the parametric encoding / decoding of multichannel audio signals.

This type of coding / decoding is based on the extraction of spatial information parameters so that at decoding, these spatial characteristics can be reconstructed for the listener.

This type of parametric encoding applies in particular for a stereo signal. Such a coding / decoding technique is for example described in the document by Breebaart, J. and van de Par, S and Kohlrausch, A. and Schuijers, titled "Parametric Coding of Stereo Audio" in EURASIP Journal on Applied Signal Processing 2005: 9, 1305-1322. This example is repeated with reference to FIGS. 1 and 2 respectively describing an encoder and a parametric stereo decoder.

Thus, FIG. 1 describes an encoder receiving two audio channels, a left channel (denoted L for Left in English) and a right channel (denoted R for Right in English).

The channels L (n) and R (n) are processed by the blocks 101, 102 and 103, 104 respectively which perform a short-term Fourier analysis. The transformed signals L [jJ and R [j] are thus obtained.

The block 105 performs a channel reduction matrix or "Downmix" in English to obtain from the left and right signals, a sum signal, a mono signal in this case, in the frequency domain.

An extraction of spatial information parameters is also performed in block 105. The ICLD (InterChannel Level Difference) type parameters, also called interchannel intensity differences, characterize the energy ratios per frequency subband between the left and right channels.

They are defined in dB by the following formula:

where L [j] and R [j] correspond to the spectral (complex) coefficients of the L and R channels, the values B [k] and B [k + 1], for each frequency band k, define the sub-division. spectrum band and the symbol * indicates the complex conjugate.

A parameter of ICPD type (for "InterChannel Phase Difference" in English) also called phase difference by frequency subband, is defined according to the following relation:

icPD [.] = (Σ;; ^l _i - ^l L [y]. / _? ^* []) ₍₂₎ where indicates the argument (phase) of the complex operand.

An interchannel time lag called ICTD (for "interchannel time difference") can also be defined equivalent to ICPD.

An interchannel coherence parameter ICC (for "InterChannel Cohérence" in English) represents inter-channel correlation.

These ICLD, ICPD and ICC parameters are extracted from the stereo signals, by block 105.

The mono signal is passed in the time domain (blocks 106 to 108) after short-term Fourier synthesis (inverse FFT, windowing and OverLap-Add or OLA) and a mono coding (block 109) is realized. . In parallel, the stereo parameters are quantized and coded in block 110.

In general, the spectrum of the signals (L [y], /? []) Is divided according to a nonlinear frequency scale of the ERB (Rectangular Bandwidth Equivalent) or Bark type, with a number of subbands typically ranging from 20 to 34. This scale defines the values of B (k) and B (k + 1) for each subband k. The settings (ICLD, ICPD, ICC) are encoded by scalar quantization possibly followed by entropy coding or differential coding. For example, in the article cited above, the ICLD is encoded by a non-uniform quantizer (ranging from -50 to +50 dB) with differential coding; the non-uniform quantization step exploits the fact that the higher the value of the ICLD, the lower the auditory sensitivity to variations of this parameter.

At the decoder 200, the mono signal is decoded (block 201), a de-correlator is used (block 202) to produce two versions M (n) and M '(n) of the decoded mono signal. These two signals passed in the frequency domain (blocks 203 to 206) and the decoded stereo parameters (block 207) are used by the stereo synthesis (block 208) to reconstruct the left and right channels in the frequency domain. These channels are finally reconstructed in the time domain (blocks 209 to 214).

In stereo signal coding techniques, a stereo intensity coding technique (Intensity Stereo Coding) consists of coding the sum (M) channel as well as the ICLD energy ratios as defined above.

Stereo intensity coding exploits the fact that the perception of high frequency components is mainly related to the temporal (energy) envelopes of the signal.

For mono signals, there are also quantization techniques with or without memory such as "Coded Pulse Modulation" (MIC) coding or its adaptive version called "Adaptive Differential Coded Pulse Modulation" (ADPCM).

Of particular interest here is ITU-T Recommendation G.722, which uses ADAPM for Adaptive Differential Pulse Code Modulation (ADPCM).

The input signal of a G.722-type encoder is in an expanded band with a minimum bandwidth of [50-7000 Hz] with a sampling frequency of 16 kHz. This signal is decomposed into two sub-bands [0-4000 Hz] and [4000-8000 Hz] obtained by decomposition of the signal by so-called quadrature mirror filters. Quadrature Mirror Filters (QMF) in English, then each of the subbands is separately encoded by an ADPCM encoder.

The low band is coded by a 6, 5 and 4 bit nested code ADPCM coding while the high band is coded by a 2 bit ADPCM coder per sample. The total bit rate is 64, 56 or 48 bit / s depending on the number of bits used for decoding the low band.

Recommendation G.722 was first used in the Integrated Services Digital Network (ISDN) and then in the enhanced HD voice telephony (HD) or HD voice enhanced telephony applications in English.

A quantized signal frame according to the G.722 standard consists of 6, 5 or 4 bit low band (0-4000 Hz) and 2 high band (4000-8000 Hz) coded quantization indices. Since the transmission frequency of the scalar indices is 8 kHz in each subband, the bit rate is 64, 56 or 48 kbit / s. In the G.722 standard, the 8 bits are distributed as follows: 2 bits for the high band, 6 bits for the low band. The last or last two bits of the low band can be "stolen" or replaced by data.

ITU-T has recently launched a standardization activity called G.722- SWB (as part of the Q.10 / 16 question described for example in document ITU-T: Annex Q10.J Terms of Reference ( ITU-T G.722 and ITU-T G.711WB, January 2009, WD04_G722G71 1SWBToRr3.doc) Extending the G.722 Recommendation in two ways:

- An extension of the acoustic band of 50-7000 Hz (enlarged band) at 50-14000 Hz (super-wide band). In English the super-enlarged band is called

Superwideband (SWB).

- An extension of mono to stereo. This stereo extension can extend wide-band mono coding or super-wide band mono coding.

In the context of G.722-SWB, G.722 coding operates with short 5 ms frames. We are particularly interested here in the stereo extension of the G.722 broadband coding

Two G.722 stereo extension modes are to be tested in the G.722-SWB standardization:

- A G.722 stereo extension at 56 kbit / s with an additional bit rate of 8 kbit / s, or 64 kbit / s in total

- A 64 kbit / s G.722 extension with an additional bit rate of 16 kbit / s, or 80 kbit / s in total

The spatial information represented by the ICLD or other parameters requires a bit rate (additional stereo extension) all the more important as the coding frames are short.

By way of example, in the context of G.722-SWB normalization, assuming that a stereo extension of G.722 (wide band) is performed by the intensity coding technique, we obtain the next stereo expansion rate.

For a G.722 (mono) sum signal with a frame of 5 ms and a division of the broadband spectrum (0-8000 Hz) in 20 sub-bands, 20 ICLD parameters to be transmitted every 5 ms are obtained. It can be assumed that these ICLD parameters are coded with a bit rate (average) of the order of 4 bits per subband. The stereo extension rate of G.722 thus becomes 20 x 4 bits / 5 ms = 16 kbit / s. Thus the stereo extension of G.722 by ICLD with 20 sub-bands leads to an additional bit rate of the order of 16 kbit / s. However, according to the state of the art, the coding of the ICLD alone is not generally sufficient to achieve good stereo quality.

This example thus illustrates the difficulty of performing a stereo extension of an encoder such as G.722 with short frames (of 5 ms).

Direct encoding of the ICLD (without other parameters) gives an additional bit rate (stereo extension) around 16 kbit / s which is already the maximum possible bit rate for the G.722 extension.

There is therefore a need to represent a stereo signal or more generally multichannel, effectively, at a rate as low as possible, with acceptable quality, when coding frames are short. The present invention improves the situation.

For this purpose, it proposes in one embodiment, a parametric encoding method of a multichannel digital audio signal comprising a coding step (G.722 Cod) of a signal resulting from a channel reduction matrix for the multichannel signal. . The method is such that it further comprises the following steps:

obtaining (Obt.), per frame of predetermined length, spatial information parameters of the multichannel signal;

division (Div) of the spatial information parameters into a plurality of parameter blocks;

- selection (St.) of a parameter block according to the index of the current frame;

- coding (Q) of the parameter block selected for the current frame.

Thus, the spatial information parameters are divided into several blocks, coded over several frames. The coding rate is therefore spread over several frames, the coding of this information is therefore at a lower rate.

The various particular embodiments mentioned below may be added independently or in combination with each other, to the steps of the method defined above.

In one embodiment, the spatial information parameters are obtained by the following steps:

- Frequency transformation (Fen., FFT) of the multichannel signal to obtain the spectra of the multichannel signal, per frame;

-decoupage (D), per frame, of the spectra of the multichannel signal, in a plurality of frequency subbands,

- calculation of spatial information parameters by frequency subbands.

The division of spatial information parameters is performed according to the frequency sub-bands obtained by cutting.

This block distribution is performed according to the defined frequency subbands, so as to optimize the use of these parameters and minimize the impact on the multichannel signal quality. Advantageously, said spatial information parameters are defined as the energy ratio between the channels of the multichannel signal.

These parameters make it possible to better define the directions of the sound sources and thus to define, for example for a stereo signal, the characteristics of the left and right signals reconstructed during decoding.

In a particular embodiment, the coding of a block of spatial information parameters is performed by non-uniform scalar quantization.

This quantization is adapted to use a minimum of additional bit rate to a multichannel extension of the coding.

In a first embodiment, the parameter division step makes it possible to obtain two blocks, a first block corresponding to the parameters of the first frequency sub-bands and a second block corresponding to the parameters of the last frequency sub-bands obtained by cutting.

In another particular embodiment, the step of dividing the parameters makes it possible to obtain two blocks interleaving the parameters of the different frequency sub-bands.

This distribution of the parameters is therefore performed simply and efficiently. The distribution of the parameters on two contiguous blocks brings the advantage of being able to make a conventional differential coding.

Advantageously, the coding of the first block and the second block is performed according to whether the frame to be coded is of even index or odd index.

Thus, the refreshing of the parameters is carried out according to a short rhythm, which makes it possible not to bring about perceptual degradation during the decoding.

In another embodiment, the method further includes a main component analysis step for obtaining the spatial information parameters including a rotation angle parameter and an energy ratio between a main component and a signal of a component. atmosphere.

This particular mode of obtaining spatial information parameters also makes it possible to take into account the correlations existing between different channels of the multichannel signal. The invention also applies to a parametric decoding method of a multichannel digital audio signal comprising a step of decoding (G.722 Dec) a signal resulting from a channel reduction matrix for the multichannel signal. The method is such that it further comprises the following steps:

decoding spatial information parameters received for a current frame of predetermined length of decoded signal;

storage of the decoded parameters for the current frame;

obtaining decoded and stored parameters of at least one preceding frame and associating these parameters with those decoded for the current frame;

- Rebuilding the multichannel signal from the decoded signal and the combination of parameters obtained for the current frame.

Thus, at decoding, the spatial information parameters are received on several successive frames and are decoded successively without requiring too much extra bitrate.

Obtaining these spatial parameters makes it possible to obtain the reconstruction of good quality of the multichannel signal.

In the same way as for the coding method, the decoded and stored parameters of a preceding frame correspond to the parameters of the first frequency sub-bands of the decoding frequency band and the decoded parameters of the current frame correspond to the parameters of the last sub-bands of frequencies obtained by cutting or vice versa.

The invention also relates to an encoder implementing the coding method comprising a coding module (304) of a signal resulting from a channel reduction matrix for the multichannel signal. The encoder is such that it further comprises:

a module for obtaining, for a frame of predetermined length, spatial information parameters of the multichannel signal;

a module for dividing the spatial information parameters into a plurality of parameter blocks;

a module for selecting a parameter block according to the index of the current frame; an encoding module of the parameter block selected for the current frame.

The invention also relates to a decoder implementing the decoding method and comprising a module for decoding a signal from a channel reduction matrix for the multichannel signal. The decoder further comprises:

a spatial information parameter decoding module received for a current frame of predetermined length of decoded signal;

storage space for storing decoded parameters for the current frame;

a module for obtaining the decoded and stored parameters of at least one preceding frame and for associating these parameters with those decoded for the current frame;

a module for reconstructing the multichannel signal from the decoded signal and the combination of parameters obtained for the current frame.

It also relates to a computer program comprising code instructions for implementing the steps of the encoding method as described and to a computer program comprising code instructions for implementing the steps of a decoding method. as described, when these are executed by a processor.

The invention finally relates to a storage means readable by a processor storing a computer program as described.

Other features and advantages of the invention will appear more clearly on reading the following description, given solely by way of nonlimiting example, and with reference to the appended drawings, in which:

FIG. 1 illustrates an encoder implementing a parametric coding known from the state of the art and previously described;

FIG. 2 illustrates a decoder implementing a parametric decoding known from the state of the art and previously described;

FIG. 3 illustrates an encoder according to one embodiment of the invention, implementing a coding method according to one embodiment of the invention; FIG. 4 illustrates a decoder according to one embodiment of the invention, implementing a decoding method according to one embodiment of the invention;

FIG. 5 illustrates the division of a digital audio signal into frames in an encoder implementing a coding method according to one embodiment of the invention;

FIG. 6 illustrates a coding method and an encoder according to another embodiment of the invention; and

FIGS. 7a and 7b respectively illustrate a device able to implement the coding method and the decoding method according to one embodiment of the invention.

With reference to FIG. 3, a first embodiment of a stereo signal coder embodying a coding method according to a first embodiment is now described.

This parametric stereo encoder operates in wideband with stereo signals sampled at 16 kHz with 5 ms frames. Each channel (L and R) is first pre-filtered by a high pass filter (HPF) eliminating the components below 50 Hz (blocks 301 and 302). Then a mono signal (M) is calculated by the block 303, an exemplary embodiment of which is given in the form:

M (n) = ^ (L '(n) + R' (n))

This signal is encoded (block 304) by a G.722 type encoder, as described, for example, in ITU-T Recommendation G.722, 7 kHz audio-coding within 64 kbit / s, Nov. 1988.

The delay introduced in the G.722 type coding is 22 samples at 16 kHz. The L and R channels are aligned in time (blocks 305 and 308) with a delay of T = 22 samples and analyzed in frequency per transform, for example by discrete Fourier transform with sinusoidal overlapping windowing. the example here is 50% (blocks 306, 307 and 309, 310). Each window thus covers 2 frames of 5 ms or 10 ms (160 samples).

The division of the signal into frames is defined with reference to FIG. 5. This figure illustrates the fact that the analysis window (solid line) of 10 ms covers the current frame of index t and the future frame of index t. +1 and the fact that a 50% overlap is used between the window of the current frame and the window (dotted line) of the previous frame.

Taking into account the future frame induces an additional algorithmic delay of 5 ms to the encoder.

For the frame t, the spectra obtained, L [t, j] and R [, ./] (j = 0 .. 9), at the output of the blocks 307 and 310 of FIG. 3, comprise 80 complex samples, with a resolution of 100 Hz per frequency band.

The block 31 1 for extracting spatial information parameters is now detailed.

The latter comprises, in the case of the processing in the frequency domain, a first module 313 for cutting the spectra L [t, j] and? [, ./ ^' ] in a predetermined number of frequency subbands, for example here in 20 subbands according to the scale defined below:

{B (k)} = o, .., 2o = [0, 1, 2, 3, 4, 5, 6, 7, 9, 11, 13, 16, 19, 23, 27, 31, 37, 44, 52, 61, 80]

This scale delimits (in number of Fourier coefficients) the frequency subbands of index k - 0 to 19. For example the first subband (k = 0) goes from the coefficient B (k) = 0 to B (k + l) -l = 0; it is therefore reduced to a single coefficient (100 Hz).

In the same way, the last sub-band (k = l9) goes from the coefficient B (k) = 6 l h B (k + 1) -

1 = 79, it has 19 coefficients (1900 Hz).

The module 314 comprises means for obtaining the spatial information parameters of the stereo signal.

For example, the parameters obtained are the interchannel intensity difference parameters, ICLD. For each frame of index t, the ICLD of the sub-band k = 0, ..., 19 is calculated according to the equation:

ICLD [/, Â:] = 10.1og dB (3)

where al [t, k] and a _R ² [t, k \ represent the energy of the left channel respectively

(L) and right channel (R).

In a particular embodiment, these energies are calculated as follows:

This formula amounts to combining the energy of two successive frames, which corresponds to a temporal support of 10 ms (15 ms if we count the effective temporal support of two successive windows).

The module 314 therefore produces a series of ICLD parameters defined previously.

These ICLD parameters are divided into the division module 315, into several blocks. In the embodiment illustrated here, the parameters are divided into two blocks according to the following two parts: {1CLD i, fcl] and {ICLD [f, A: 1]

The division of ICLD parameters into contiguous blocks makes it possible to perform a differential coding of the scalar quantization indices.

The module 316 then makes a selection (St.) of a block to be encoded according to the index of the current frame to be coded.

In the example described here, for even-numbered frames t, the block {lCLD [i,]]) _{t = 0 g} is coded at 312 and transmitted, for frames r of odd index, the block

{lCLD [i, To:]} _{i = io} is coded in 312 and transmitted.

The coding of these blocks at 312 is carried out for example by non-uniform scalar quantization.

Thus, the coding of an ICLD block is achieved with: • 5 bits for the first ICLD parameter,

• 4 bits for the following 8 ICLD parameters,

• 3 bits for the last (tenth) ICLD parameter.

A more detailed example of realization is for example as below:

For the quantization table:

tab_ild_q5 [31] = {-50, -45, -40, -35, -30, -25, -22, -19, -16, -13, -10, -8, -6, -4, -2 , 0, 2, 4, 6, 8, 10, 13, 16, 19, 22, 25, 30, 35, 40, 45, 50} the 5-bit quantization of ICLD [t, k] consists in finding the quantization index i such that

i = arg minj = 0 ... 30 | ICLD [t, k] - tab_ild_q5 [j] | ^A 2 Similarly for the quantization table:

tab_ild_q4 [15] = {-16, -13, -10, -8, -6, -4, -2, 0, 2, 4, 6, 8, 10, 13, 16} ICLD 4-bit quantization [t, k] is to find the quantization index i such that

i = arg minj = 0 ... 15 | ICLD [t, k] - tab_ild_q4 | j] | ^A 2 Finally for the quantization table tab_ild_q3 [7] = {-16, -8, -4, 0, 4, 8, 16} the 3-bit quantization of ICLD [t, k] consists in finding the index of quantification such as

i = arg minj = 0 ... 15 | ICLD [t, k] - tab_ild_q3 [jJ | ^A 2

In total 5 + 8x4 +3 = 40 bits are therefore necessary for the coding of an ICLD block. The frame being 5 ms, we thus obtain 40 bits / 5 ms = 8 kbit / s as additional bit rate for the stereo coding extension.

This bit rate is therefore not too great and is sufficient to efficiently transmit the stereo parameters.

Two successive frames suffice in this embodiment to obtain the spatial information parameters of the multichannel signal, the length of two frames being most often the length of an analysis window for a 50% overlap frequency transformation. . Alternatively, a shorter recovery window could be used to reduce the delay introduced.

Thus, the coder described with reference to FIG. 3 implements a method of parametric encoding of a multichannel digital audio signal comprising a coding step (G.722 Cod) of a signal resulting from a channel reduction mastering of the channel. multichannel signal. The method further comprises the following steps:

- coding (Q) of the parameter block selected for the current frame.

In the embodiment described above, it was in the context of an expanded band encoder operating with a sampling frequency of 16 kHz and a particular subband cut.

In another possible embodiment, the encoder may operate at other frequencies (such as 32 kHz) and with different subband cutting.

One can also exploit the fact that the parameter ICLD [i, jt] for k = 0 can be neglected. Its calculation and therefore its coding can be avoided. In this case the coding of the ICLD parameters becomes:

for the frames of index t pair: coding of a block of 9 parameters {lCLD j /. λ:]} ^ by non-uniform scalar quantization with:

• 5 bits for the first parameter ICLD [Î, £] with fc = l

• 4 bits for the following 8 ICLD parameters

- for frames of odd t-index: coding of a block of 10 parameters {lCLD [f, À:]} _t as previously presented

• 5 bits for the first ICLD parameter, • 4 bits for the following 8 ICLD parameters,

• 3 bits for the last (tenth) ICLD parameter.

Thus, in this embodiment, 37 bits are used for frames of even t-index and 40 bits for frames of odd t-indexes.

Similarly, in an alternative embodiment, instead of dividing the parameters

ICLD in contiguous blocks, we can divide these parameters differently, for example in interleaving to obtain 2 parts: {lCLD [r, 2A:]} ^ and {lCLD [f, 2 £ + l]} _{t = Q.}

It should be noted that the coding method thus described is easily generalized in the case where the parameters are divided into more than 2 blocks. In an alternative embodiment, the ICLD parameters are divided into 4 blocks:

{lCLD [U]} _{t = lJ 19} .

The coding of the ICLD parameters is then distributed over 4 successive frames with storage of the parameters decoded in the previous frames during the decoding. The calculation of the ICLD must then be modified to include more than 2 frames in the calculation of the energies [t, k].

In this variant embodiment, the coding of the ICLD parameters can then use the following allocation:

• 5 bits for the first ICLD parameter

· 4 bits for the following 4 ICLD parameters

with a total of 21 bits per frame. The rate is therefore even lower than in the previous embodiment, the counterpart being that the ICLD parameters are updated in at least one block every 20 ms instead of every 10 ms. For some stereo parameters and depending on the type of signal, this variant may however introduce audible spatialization defects.

However, the interest of transmitting the stereo or spatial parameters at a slower rate than that of the frames is always great. The imperfect auditory perception of interchanal energy variations is exploited. Finally, the encoding method thus described applies to the encoding of other parameters than the ICLD parameter. For example, the coherence parameter (ICC) can be calculated and transmitted selectively in a manner similar to the ICLD.

The two parameters can also be calculated and coded according to the coding method described above.

FIG. 4 illustrates a decoder in one embodiment of the invention as well as the decoding method that it implements.

The portion of bit stream scalable and received from the G.722 encoder is demultiplexed and decoded by a G.722 type decoder (block 401) in 56 or 64 kbit / s mode. The synthesized signal obtained corresponds to the mono signal M (n) in the absence of transmission errors.

A short-term discrete Fourier transform analysis with the same windowing as the encoder is performed on M (n) (blocks 402 and 403) to obtain the spectrum M [j].

The part of the bit stream associated with the stereo extension is also demultiplexed at block 404.

The operation of the synthesis block 405 is now detailed.

For even-numbered frames t, a first parameter block | lCLD ^q [f, *]] ₉ is decoded in the module 404 and these decoded parameters are stored in the module 412. For frames / odd index decodes in the module 404 a second block of parameters {ICLD ^q [f, £]} ^ ^ and stored in the module

412 these decoded parameters.

A more detailed example of realization is for example as below:

For the quantization table:

tab_ild_q5 [31] = {-50, -45, -40, -35, -30, -25, -22, -19, -16, -13, -10, -8, -6, -4, -2 , 0, 2, 4, 6, 8, 10, 13, 16, 19, 22, 25, 30, 35, 40, 45, 50) the decoding of a 5-bit index is to synthesize the parameter ICLD ⁴ [ t, k | as

ICLD ^q [t, kJ = tab_ild_q5 (i) Likewise for the quantization table:

tab_ild_q4 [15] = {- 16, -13, -10, -8, -6, -4, -2, 0, 2, 4, 6, 8, 10, 13, 16} the decoding of an index i to 4 bits is to synthesize the ICLD parameter ^q [t, k] as

ICLD ^q [t, kJ = tab_ild_q4 (i)

Finally for the quantization table tab_ild_q3 [7] = {- 16, -8, -4, 0, 4, 8, 16} the decoding of a 3-bit index consists in synthesizing the ICLD parameter ^q [t, k] as

ICLD ^q [t, k] = tab_ild_q3 (i)

In even-numbered frames, the missing part of the parameters is used in the module, the stored values jlCLD ^q [t -

the previous frame, ie: ICLD ^q [i, &] = ICLD ^q [1 - 1, £] for £ = 10 ... 19. Similarly, in odd-numbered frames, the missing portion | LCLD ^q [f-values stored in the previous frame are used.

The parameters for each of the frequency bands are thus obtained. The spectra of the left and right channels are reconstructed by the synthesis module 414 by applying the parameters jlCLD ^q [/ - l, *] J thus decoded by subband. This synthesis is carried out for example as follows:

rL [j] = _Cl [i, k] .M [jl

= B (k) ... B (k + l) - l (5)

R [j] = c ₂ [t, k] M [j]

with

or

c [tM = Q ^{, cw [, Mno} Note that the calculation of scale factors above is given as an example. Other means of expression of scale factors exist and can be implemented for the present invention.

The left and right channels L (n) and R (n) are reconstructed by inverse discrete Fourier transform (blocks 406 and 409) of the respective spectra L [j] and R [j] and addition-overlap (blocks 408 and 411) with sinusoidal windowing (blocks 407 and 410).

Thus, the decoder described with reference to FIG. 4, in the particular embodiment of decoding stereo signals, implements a method of parametric decoding of a multichannel digital audio signal comprising a decoding step (G.722 Dec). a signal from a channel reduction matrix for the multichannel signal. The method further comprises the following steps:

decoding (Q ^"1 ) spatial information parameters received for a current frame of predetermined length of decoded signal;

memorization (Mem) of the decoded parameters for the current frame;

obtaining (Comp.P) decoded and stored parameters of at least one preceding frame and associating these parameters with those decoded for the current frame;

- Reconstruction (Synth.) of the multichannel signal from the decoded signal and the combination of parameters obtained for the current frame.

In the case of a division in more than two blocks spatial information parameters, for example in 4 blocks as in an embodiment variant described above, all the decoded parameter blocks for 4 decoded frames are obtained.

The bit rate of the stereo extension is therefore reduced and obtaining these parameters makes it possible to reconstruct a stereo signal of good quality.

It may also be noted that alternative techniques to the coding of the parameters (ICLD, ICPD, ICC) can be adopted to implement the coding method according to the invention. Thus, in an alternative embodiment, the module 314 of the parameter extraction block of FIG. 3 differs.

This module in this embodiment makes it possible to obtain other stereo parameters by applying a principal component analysis (PCA) such as that described in the article by Manuel Briand, David Virette and Nadine Martin entitled "Parametric coding of stereo audio based principal component analysis "published in the DAFX conference, 1991.

Thus, a principal component analysis is performed by subbands. The left and right channels thus analyzed are then rotated to obtain a main component and a qualified environment sub component. The stereo analysis produces, for each subband, a rotation angle parameter (Θ) and an energy ratio between the main component and the ambient signal {PCAR which stands for Principal Component to Ambience energy Ratio).

The stereo parameters then consist of the angle of rotation parameter and the energy ratio (Θ and PCAR).

FIG. 6 illustrates another embodiment of an encoder according to the invention.

Compared to the encoder of FIG. 3, it is here the block 303 for stamping or "downmix" that differs. In the example of Figure 3, the "downmix" operation has the advantage of being instantaneous and of minimal complexity.

However, this operation does not necessarily allow conservation of energy. An improvement of this "downmix" operation is possible in the time domain, for example with a calculation of the form M (n) =, L (n) + w ₂ R (n) and weights w ₁ and w ₂ adaptive , or else in frequency as represented here with reference to FIG.

The "downmix" operation here consists of the blocks 603a, 603b, 603c and 603d for the passage in the frequency domain.

The calculation of the mono signal is carried out in block 603e of "downmix" in which the signal is calculated in the frequency domain by the following formula:

where |. | represents the amplitude (complex module) and Z (.) the phase (complex argument).

Blocks 603f, 603g and 603h make it possible to bring the mono signal back into the time domain in order to be coded by block 304 as for the encoder illustrated in FIG.

An offset of T '= 80 + T samples is then obtained, an offset of 80 + 80 + 22 = 182 samples.

This offset makes it possible to synchronize the time frames of the left / right channels and those of the decoded mono signal.

The invention has been described here in the case of a G.722 encoder / decoder. it can obviously apply in the case of a modified G.722 encoder, for example including mechanisms of noise reduction ("noise feedback" in English) or including a scalable extension of G.722 with additional information. The invention can also be applied in the case of another mono encoder than the G.722 type such as for example a G.711.1 type encoder. In the latter case, the delay T must be adjusted to take into account the delay of the G.711.1 encoder.

Similarly, the time-frequency analysis of the embodiment described with reference to FIG. 3 could be replaced according to different variants:

- another windowing than sinusoidal windowing could be used,

- another covering than the 50% overlap between successive windows could be used

another frequency transform than the Fourier transform, for example the modified discrete cosine transform (MDCT) could be used.

The embodiments described previously dealt with the case of a multichannel signal of the stereo signal type, the embodiment of the invention also extends to the more general case of the coding of multichannel signals (with more than 2 audio channels) starting from a mono or even stereo downmix. In this case the coding of spatial information involves the coding and transmission of spatial information parameters. This is for example the case of 5.1 channel signals including a left channel (L), right (R), center (C), left rear (Ls for Left surround), right rear (Rs for Right surround), and subwoofer (LFE for Low Frequency Effects). The spatial information parameters of the multichannel signal then take into account the differences or the coherences between the different channels.

The encoders and decoders as described with reference to FIGS. 3, 4 and 6 may be integrated in a multimedia equipment of the living room decoder type, computer or communication equipment such as a mobile telephone or personal electronic organizer.

FIG. 7a represents an example of such multimedia equipment or coding device comprising an encoder according to the invention. This device comprises a PROC processor cooperating with a memory block BM having a storage and / or working memory MEM.

The memory block may advantageously comprise a computer program comprising code instructions for implementing the steps of the coding method within the meaning of the invention, when these instructions are executed by the processor PROC, and in particular the steps:

obtaining, for a frame of predetermined length, spatial information parameters of the multichannel signal;

dividing the spatial information parameters into a plurality of parameter blocks;

selecting a parameter block according to the index of the current frame;

coding of the parameter block selected for the current frame.

Typically, the description of FIG. 3 shows the steps of an algorithm of such a computer program. The computer program can also be stored on a memory medium readable by a reader of the device or downloadable in the memory space of the equipment. The device comprises an input module adapted to receive a multichannel signal S _m representing a sound scene, either by a communication network, or by reading a content stored on a storage medium. This multimedia equipment may also include means for capturing such a multichannel signal.

The device comprises an output module capable of transmitting the coded spatial information parameters P _c and a sum signal Ss resulting from the coding of the multichannel signal.

In the same way, FIG. 7b illustrates an example of multimedia equipment or decoding device comprising a decoder according to the invention.

This device comprises a PROC processor cooperating with a memory block BM having a storage and / or working memory MEM.

The memory block may advantageously comprise a computer program comprising code instructions for implementing the steps of the decoding method in the sense of the invention, when these instructions are executed by the processor PROC, and in particular the steps of:

storage of the decoded parameters for the current frame;

Typically, the description of FIG. 4 repeats the steps of an algorithm of such a computer program. The computer program can also be stored on a memory medium readable by a reader of the device or downloadable in the memory space of the equipment.

The device comprises an input module able to receive the coded spatial information parameters P _c and a sum signal S _s originating, for example, from a communication network. These input signals can come from a reading on a storage medium.

The device comprises an output module capable of transmitting a multichannel signal decoded by the decoding method implemented by the equipment.

This multimedia equipment may also include speaker-type reproduction means or communication means capable of transmitting this multi-channel signal.

Obviously, such multimedia equipment may include both the encoder and the decoder according to the invention. The input signal then being the original multichannel signal and the output signal, the decoded multichannel signal.

Claims

1. A method of parametric coding of a multichannel digital audio signal comprising a coding step (G.722 Cod) of a signal resulting from a channel reduction matrix of the multichannel signal, characterized in that it also comprises the following steps:

- coding (Q) of the parameter block selected for the current frame.

Coding method according to claim 1, characterized in that the spatial information parameters are obtained by the following steps:

- calculation of spatial information parameters by frequency subbands.

3. Method according to claim 2, characterized in that the division of the spatial information parameters is performed as a function of the frequency sub-bands obtained by cutting.

4. Method according to claim 1, characterized in that said spatial information parameters are defined as the energy ratio between the channels of the multichannel signal.

5. Method according to claim 1, characterized in that the coding of a block of spatial information parameters is performed by non-uniform scalar quantization.

6. Method according to claim 3, characterized in that the step of dividing the parameters makes it possible to obtain two blocks, a first block corresponding to the parameters of the first frequency sub-bands and a second block corresponding to the parameters of the last sub-bands. frequency bands obtained by cutting.

7. Method according to claim 3, characterized in that the step of dividing the parameters makes it possible to obtain two blocks interleaving the parameters of the different frequency sub-bands.

8. Method according to one of claims 6 or 7, characterized in that the coding of the first block and the second block is performed according to whether the frame to be coded is even index or odd index.

9. The method of claim 1, characterized in that it further comprises a main component analysis step for obtaining the spatial information parameters comprising a rotation angle parameter and an energy ratio between a component main and a room signal.

10. A method of parametric decoding of a multichannel digital audio signal comprising a step of decoding (G.722 Dec) a signal resulting from a channel reduction matrix of the multichannel signal, characterized in that it further comprises the following steps:

decoding (Q ¹ ) received spatial information parameters for a current frame of predetermined length of decoded signal;

memorization (Mem) of the decoded parameters for the current frame; obtaining (Comp.P) decoded and stored parameters of at least one preceding frame and associating these parameters with those decoded for the current frame;

11. Method according to claim 10, characterized in that the decoded and stored parameters of a preceding frame correspond to the parameters of the first frequency sub-bands of the decoding frequency band and the decoded parameters of the current frame correspond to the parameters. last sub-bands of frequencies obtained by cutting or vice versa.

Computer program comprising code instructions for implementing the steps of an encoding method according to one of claims 1 to 9, when these are executed by a processor.

Computer program comprising code instructions for implementing the steps of a decoding method according to one of claims 10 to 11, when these are executed by a processor.

14. Parametric encoder of a multichannel digital audio signal comprising a coding module (304) for a signal resulting from a channel reduction matrix for the multichannel signal, characterized in that it further comprises:

- a obtaining module (314), per frame of predetermined length, spatial information parameters of the multichannel signal;

a division module (315) of the spatial information parameters into a plurality of parameter blocks;

a module for selecting (316) a parameter block as a function of the index of the current frame; an encoding module (312) of the parameter block selected for the current frame.

15. Parametric decoder of a multichannel digital audio signal comprising a module for decoding (401) a signal resulting from a channel reduction matrix for the multichannel signal, characterized in that it further comprises:

a decoding module (404) of spatial information parameters received for a current frame of predetermined length of decoded signal;

storage space (412) for storing decoded parameters for the current frame;

a module (413) for obtaining decoded and stored parameters of at least one preceding frame and for associating these parameters with those decoded for the current frame;

- A reconstruction module (414) of the multichannel signal from the decoded signal and the combination of parameters obtained for the current frame.