EP1599868B1 - Method and device for the spectral reconstruction of an audio signal - Google Patents

Method and device for the spectral reconstruction of an audio signal Download PDF

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Publication number
EP1599868B1
EP1599868B1 EP04716626A EP04716626A EP1599868B1 EP 1599868 B1 EP1599868 B1 EP 1599868B1 EP 04716626 A EP04716626 A EP 04716626A EP 04716626 A EP04716626 A EP 04716626A EP 1599868 B1 EP1599868 B1 EP 1599868B1
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Prior art keywords
frequency
coder
coded
decoder
core
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German (de)
French (fr)
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EP1599868A1 (en
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Pierrick Philippe
Jean-Bernard Rault
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Orange SA
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France Telecom SA
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    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; SPEECH OR AUDIO CODING OR DECODING
    • G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
    • G10L19/02—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis using spectral analysis, e.g. transform vocoders or subband vocoders
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; SPEECH OR AUDIO CODING OR DECODING
    • G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
    • G10L19/005—Correction of errors induced by the transmission channel, if related to the coding algorithm
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; SPEECH OR AUDIO CODING OR DECODING
    • G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
    • G10L19/02—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis using spectral analysis, e.g. transform vocoders or subband vocoders
    • G10L19/0204—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis using spectral analysis, e.g. transform vocoders or subband vocoders using subband decomposition
    • G10L19/0208—Subband vocoders
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; SPEECH OR AUDIO CODING OR DECODING
    • G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
    • G10L19/04—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis using predictive techniques
    • G10L19/16—Vocoder architecture
    • G10L19/18—Vocoders using multiple modes
    • G10L19/24—Variable rate codecs, e.g. for generating different qualities using a scalable representation such as hierarchical encoding or layered encoding

Definitions

  • the present invention relates to a method and a device for encoding and decoding an audio signal employing spectrum reconstruction techniques.
  • the invention lies in the improvement of the decoding of an audio signal encoded by a spectral band limiting coder, called a core coder.
  • This coding may be a rate reduction coding.
  • Known rate reduction encoders are, for example, transform type encoders such as MPEG1, MPEG2 or MPEG4-GA encoders, CELP type encoders and even parametric type encoders, such as a parametric MPEG4 type encoder.
  • the audio signal In audio reduction rate coding, the audio signal must often be bandwidth-restricted when the bit rate becomes low. This bandwidth limitation is necessary to avoid the introduction of audible quantization noise into the coded signal. It is therefore desirable to complete as far as possible the complete spectral content of the original signal.
  • Band widening is known from the state of the art, such as the spectral broadening method known as the High Frequency Regeneration (HFR) method.
  • the decoded, low-bandwidth low-frequency signal is subjected to a non-linear device to obtain a signal enriched in harmonics.
  • This signal after whitening and shaping based on information describing the spectral envelope of the full-band signal before coding, allows the generation of a high frequency signal corresponding to the high frequency content of the signal before coding.
  • These systems perform a time adaptation of the cutoff frequency between the low frequency band coded by an encoder, called the core coder, and the high frequency band coded by an HFR system, called a band extension coder.
  • the core encoder and the band extension encoder distribute the bandwidth according to the appropriate cutoff frequency.
  • This type of system is particularly interesting for coding audio signals.
  • Certain communication networks such as the Internet, wireless communication networks and others do not guarantee a perfect routing of data between the sender and the recipient. Some data may never reach the recipient or arrive too late. Arriving too late, the recipient considers them lost.
  • the invention attempts to overcome the disadvantages of the prior art by proposing a coding method according to claim 1 and the corresponding device according to claim 9.
  • the decoded signal will be of better quality, no spectral component of the signal is missing, the frequency spectrum decoded with the extension decoder being modified according to the cutoff frequency of the signal decoded by the core encoder.
  • the portion of the frequency spectrum of the decoded audio signal with a heart decoder is the lower part of the frequency spectrum of the audio signal.
  • the information representative of at least one cut-off frequency of the signal decoded by the heart decoder is obtained by making an estimate of the high cut-off frequency of the signal decoded by the heart decoder.
  • the information representative of at least one cut-off frequency of the signal decoded by the heart decoder is obtained from information included in the data stream comprising the coded digital signal.
  • the heart decoder is a hierarchical decoder and information is obtained representative of the bandwidth of the signal decoded by the heart decoder for each layer of the decoded signal.
  • the decoder can adapt the processing to each coding layer, the decoder has this information at each layer and can thus modify the frequency spectrum decoded with the extension decoder according to this information.
  • the invention according to claims 17 and 18 also relates to the computer program stored on an information carrier, said program comprising instructions for implementing the previously described processing method, when it is loaded and executed by a user. computer system.
  • the Fig. 1a represents a frequency spectrum of an audio signal to be encoded.
  • the low frequencies of the spectrum ( Fig. 1b ) are encoded by a core encoder, while the high frequencies are encoded by an extension encoder. This part of the high frequencies is represented in Fig. 1 C .
  • the Fig. first represents the frequency spectrum of a decoded audio signal with a heart decoder, the encoded audio signal having been transmitted over a network and some data has been lost.
  • the information encoded by the extension encoder is much more limited in number.
  • the frequency spectrum of an audio signal transmitted over a network and decoded with an extension decoder is considered correct. This is represented at Fig. 1f .
  • the Fig. 2a represents the frequency spectrum of the total audio signal to be encoded by a hierarchical core encoder and an extension encoder.
  • a hierarchical core encoder will successively encode different subparts of the frequency spectrum of the audio signal to be encoded.
  • a first part of the spectrum for example the part containing the lowest frequency components, such as the spectrum represented in Fig. 2b , will be coded.
  • the first layer This is called the first layer. Then another part containing additional frequency components will be coded. This is the second layer, she is represented at the Fig. 2c .
  • the information representative of the lowest frequencies is generally transmitted in the first layers.
  • the other layers are, for example, then transmitted in an order according to the frequencies of the spectrum that they represent.
  • some of the layers of the transmitted layers have a higher priority character than others.
  • the layers with the lowest frequencies are considered as priority
  • the layers with the highest frequencies are considered as the lowest priorities.
  • the layers with the lowest frequencies are associated with efficient error correction codes, ensuring good decoding, so no transmission losses.
  • the Fig. 2d represents the part of the spectrum allocated to the band extension encoder, it is identical to that described in Fig. first .
  • Figs. 2f and 2g represent the frequency spectra of a decoded audio signal with a hierarchical heart decoder comprising two hierarchy layers, the coded audio signal having been transmitted over a network and some layers of which have been lost.
  • the part of the spectrum allocated to the band extension encoder is identical to that described in Fig. 1 C . She is represented at Fig 2h .
  • the Fig. 3a represents the frequency spectrum of the total audio signal to be encoded by a core encoder and an extension encoder according to the invention.
  • the core encoder encodes the low frequency components of the frequency spectrum of the audio signal, this is shown in FIG. Fig. 3b .
  • the extension coder not only encodes the high frequency components of the frequency spectrum of the audio signal to be coded but also a portion of the low frequency components that the core code coder . These components are represented at Fig. 3c .
  • the Fig. 3d represents the frequency spectrum of an audio signal decoded with a heart decoder, the coded audio signal having been transmitted over a network and some layers 31 have been lost.
  • An estimate of the bandwidth of the audio signal decoded by the heart decoder is performed, if it is different from that expected, the heart decoder informs the extension decoder of the missing bandwidth.
  • the extension decoder adapts the decoding so that the decoding also applies to the missing bandwidth.
  • Fig 3rd is represented the frequency spectrum equivalent to the coded information received by the extension decoder. This spectrum is composed of components 32,33 and 34.
  • the information corresponding to the component 34 is sufficient for the decoding.
  • the Fig. 4a represents a block diagram describing the coding device according to the invention.
  • the coding device consists of a digital analog converter 400 which converts the analog signal to be encoded into a digital signal.
  • a digital analog converter 400 which converts the analog signal to be encoded into a digital signal.
  • the analog-to-digital converter is not necessary.
  • the digital signal is delivered to the heart encoder which encodes this signal.
  • the core encoder is for example a rate reduction encoder as conforming to one of the MPEG1, MPEG2 or MPEG4-GA standards, or a CELP type encoder, a hierarchical coder, or even a parametric MPEG4 encoder.
  • the output of the core encoder represents the data of the signal covering the frequency spectrum such as that represented in FIG. Fig. 3b .
  • the band extension encoder is for example an HFR (High Frequency Regeneration) type encoder, for example SBR (Spectral Band Replication) as described in FIG. document "Audio Engineering Society, convention paper 5553", presented at the 112th AES Convention by Mr Martin Dietz .
  • HFR High Frequency Regeneration
  • SBR Spectrum Band Replication
  • the output of the band extension encoder represents the signal envelope data covering the frequency spectrum such as that shown in FIG. Fig. 3c .
  • a cutoff frequency adjustment module 402 is connected to the tape extension encoder 403 and the core encoder 401.
  • This module 402 defines the frequency spectrum that the extension encoder takes into account for coding.
  • This module 402 determines this spectrum as a function of the high cut-off frequency of the core encoder 401, and of a variable frequency band which enables the decoder according to the invention to be able to overcome the possible transmission losses.
  • variable frequency band is adjusted so as to guarantee the correct Recomposing the signal for layers that do not have a robust error correction code.
  • the frequency spectrum of the core encoder 401 can be adjusted from the frequency spectrum of the extension encoder 403.
  • the module 402 defines the frequency spectrum that the core encoder 401 takes into account for the coding.
  • This module 402 defines this spectrum as a function of the low cut-off frequency of the extension encoder 403 and of a variable frequency band which enables the decoder according to the invention to be able to overcome the possible transmission losses.
  • the coding device also comprises a multiplexer 404 which multiplexes the audio signals coded by the core coder 401 and by the extension coder 403.
  • the module 402 transfers to the multiplexer 404 the information representative of the passband of the core encoder 401 or its cutoff frequencies, or even the low cutoff frequency of the extension encoder 403 so that these included in the transmitted data.
  • the inclusion is performed in the case of a hierarchical coder for each coding layer.
  • the multiplexed data is then transferred to a network transmission module which, for example in the case of a radio transmission, applies error correcting codes to the multiplexed data and transmits them to the network 405.
  • the Fig. 4b represents a block diagram describing the main elements of a hierarchical heart encoder.
  • This hierarchical coder can replace the encoder 401 previously described with reference to the Fig. 4a .
  • a hierarchical heart encoder usually subdivides the frequency spectrum to be coded into different layers.
  • a layer represents a frequency band of the spectrum to be coded.
  • the number of layers is variable and allows a progressive transmission of the coded signal.
  • the encoder consists of a first encoder 410 which encodes the lowest part of the frequency spectrum of the original signal.
  • the encoded information is transferred to a multiplexer 416 which transfers this data to the multiplexer 404.
  • module 402 transfers to the multiplexer 404 the information representative of the bandwidth of the core encoder 410 so that they are included in the data stream associated with this layer.
  • the coded information is also transferred to a decoder 411. This decoder decodes this information and then transmits it to a subtraction circuit 413 which will subtract the decoded signal from the original signal.
  • the original signal has previously been delayed by a delay equal to the coding time of the encoder 410 and the decoding time of the decoder 411.
  • the signal obtained at the output of the subtracter circuit is then the original signal in which the previously coded low frequency components have been removed to the coding residue.
  • This signal is again coded by an encoder 415 which may be of the same type as the encoder 410.
  • the frequency components of the signal greater than those coded by the encoder 410 are coded.
  • the encoded information is transferred to a multiplexer 416 which transfers this data to the multiplexer 404.
  • the module 402 transfers to the multiplexer 404 the information representative of the bandwidth of the core encoder 415 so that they are included in the data stream associated with this layer. It can also transfer the total number of coding layers, the high or low cutoff frequency of the core encoder 415.
  • the elements 410, 411, 413 and 414 will have to be duplicated for each additional layer.
  • each encoder may be variable: It should also be noted that the invention is applicable for mono, stereo or multi-channel audio signals.
  • the bandwidth information transmitted by the coder can be transmitted in a conjugated manner or in a preferential mode, the bandwidth of each of the channels can be deduced from the other channels by a differential coding.
  • the Fig. 5 represents a block diagram describing the decoding device according to the invention.
  • the decoding device consists of a demultiplexer 510 which separates the signals received via the network 405 into data intended for the heart decoder 511 and data intended for the extension decoder 512. It also extracts signals received, the information representative of the bandwidth of the core encoder 401 of the coding device, the encoders 410 and 415 if the signal has been encoded with a hierarchical coder, or even the low cutoff frequency of the encoder extension 403 of the coding device if these were included in the transmitted data.
  • the heart decoder 511 decodes the data to provide a decoded signal such as the signal shown in FIG. Fig. 3d .
  • the core decoder 511 is, for example, a decoder such as one of the MPEG1, MPEG2 or MPEG4-GA standards, or a CELP type decoder, a hierarchical decoder, or even a parametric MPEG4 decoder.
  • the heart decoder 511 comprises a module 511b for obtaining information representative of at least one cutoff frequency which estimates, according to a first embodiment, the frequency spectrum of the signal received by the latter.
  • the module 511b does this for example by performing a time-frequency transformation on the decoded signal and determining the frequency from which the signal energy becomes negligible. Preferably, this can be done with the assistance of a perception model.
  • the decoder 511 more precisely its 511b module then transfers information representative of the cutoff frequency or the bandwidth to the extension decoder 512.
  • the extension decoder 512 selects, from the representative information transmitted by the decoder 511, among the coded data received from the multiplexer 510, the data corresponding to a representation of the spectral envelope greater than the determined frequency. by the encoder 511.
  • the core decoder 511 more precisely the module 511b for obtaining information representative of at least one cut-off frequency obtains from the demultiplexer 510, according to a second embodiment, the information representative of the bandwidth of the core encoder 401 or encoders 410 and 415 of the coding device, or even the number of layers of the coded signal, or even the low cutoff frequency of the encoder 403 extension encoder if they were included in the data transmitted.
  • the module 511 b checks, in the case where it is a hierarchical decoder, if each layer is well received and in the negative transfers information representative of the bandwidth of one or more lost layers to the extension decoder 512.
  • the extension decoder 512 selects, from the representative information transmitted by the module 511b, among the coded data received from the multiplexer 510, the data corresponding to the envelope of the signal corresponding to a representation of the spectral envelope of the frequencies. higher than the lowest frequency corresponding to the lost frequency bands.
  • the extension decoder corrects the losses due to the network either on losses affecting the last layers received or losses affecting an intermediate layer.
  • the band extension decoder 512 is for example a HFR (High Frequency Regeneration) type decoder, for example a SBR (Spectral Band Replication) type decoder as described in the document "Audio Engineering Society, convention paper 5553", presented at the 112th AES Convention by Mr Martin Dietz .
  • HFR High Frequency Regeneration
  • SBR Spectrum Band Replication
  • the extension decoder 512 decodes all the information received. A selection among the decoded data is performed so as to keep only those corresponding to a representation of the spectral envelope greater than the frequency determined by the encoder 511.
  • the envelope decoded by the extension decoder 512 or selected is transferred to a gain control module 515.
  • the signal decoded by the heart decoder 511 is sent to a transposition module 513 which generates a signal in the high frequencies of the spectrum from the low frequency decoded signal.
  • This signal is fed into the gain control module 515 to allow adjustment of the high frequency signal envelope.
  • the adjusted envelope signal is then added to the decoded signal by the heart decoder 511 with an adder 516.
  • the adder 516 may in a preferred mode favor certain frequency components by multiplying for example certain components by coefficients.
  • the signal decoded by the heart decoder 511 has been previously delayed by a delay equal to the difference in processing time between the summed signals. This delay is performed by the delay circuit 514.
  • the frequency spectrum of the signal obtained is thus similar to that of the Fig. 3f .
  • the summation signal can then be converted to analog form using a digital to analog converter 517.
  • the Fig. 6 represents the algorithm performed according to the invention at the coder.
  • the invention as described with reference to FIGS. preceding is also feasible in software form in which a processor executes the executable code associated with the steps E1 to E7 of the algorithm of the Fig. 6 .
  • the processor When the coding device is powered up, and more particularly in the case of using a computer as coding device, the processor reads from the read-only memory of the computer or an information medium such as a compact disc CD-ROM, the instructions of the program corresponding to steps E1 to E7 of the Fig. 6 and loads RAM RAM to run them.
  • step E1 on receiving audio data to be coded, the processor determines the bandwidth of the core encoder or at least one cutoff frequency.
  • the bandwidth of the core coder may or may not be variable in time depending for example on the load of the core coder.
  • the processor encodes the data according to an algorithm called core coding according to one of the MPEG1, MPEG2 or MPEG4-GA standards, or of the CELP type, of the hierarchical type, or even of the parametric MPEG4 type.
  • Step E2 consists in verifying whether, and in the case of a hierarchical coding, all the layers have been coded or not.
  • the processor repeats the step E1 for each layer of the coded audio signal.
  • step E3 the processor determines a frequency margin.
  • This margin can be predetermined and stored in a register or be in the form of a variable.
  • This variable depends, for example, on the type of error correction that will be applied to the coded data during transmission on the network.
  • the processor determines in step E4, from the margin and the high cutoff frequency of the core encoder, the low cutoff frequency of the extension encoder.
  • the processor transfers this information to the extension coding routine at step E5.
  • step E6 the processor stores this information.
  • the processor executes the extension coding by coding the data whose spectrum is greater than the information transferred to the step E5.
  • the band extension coding is for example a High Frequency Regeneration (HFR) type coding, for example SBR (Spectral Band Replication) as described in the document. "Audio Engineering Society, convention paper 5553", presented at the 112th AES Convention by Mr Martin Dietz .
  • HFR High Frequency Regeneration
  • step E7 which consists of multiplexing the audio signals encoded in step E1 and the audio signals encoded in step E7 to form a stream of coded data transmitted over a network.
  • the processor inserts in the coded and transmitted data stream, the information stored in step E6 or inserts one or more of the following information: heart coder bandwidth, coder bandwidth extension, low and high frequency of each coding layer, number of coding layers if a hierarchical coder is used.
  • Insertion is performed in the case of a hierarchical coder for each coding layer.
  • the processor returns to step E1 waiting for new audio data to be coded.
  • the Fig. 7 represents the algorithm performed according to the invention at the level of the decoder.
  • the processor When the reception device is powered up, and more particularly in the case of using a computer as a receiving device, the processor reads from the read-only memory of the computer or of an information medium such as a compact disc CD-ROM, the instructions of the program corresponding to steps E10 to E15 of the Fig. 7 and loads RAM RAM to run them.
  • the processor on receiving audio data to be decoded, separates the signals received via the network 405 into data intended for the heart decoder and data intended for the extension decoder. It also extracts signals received, information representative of the bandwidth or at least one cut-off frequency of the core encoder encoding the audio signal, or encoders encoding the audio signal if the signal has been coded with an encoder hierarchy, or even the low cut-off frequency of the extension encoder encoding the audio signal if they have been included in the transmitted data.
  • step E7 The processor then proceeds to decode this data.
  • the processor proceeds to decode the data according to a decoding algorithm called core decoding as conforming to one of the MPEG1, MPEG2 or MPEG4-GA standards, or of the CELP type, a hierarchical decoding, or even a parametric MPEG4 type decoding. .
  • core decoding as conforming to one of the MPEG1, MPEG2 or MPEG4-GA standards, or of the CELP type, a hierarchical decoding, or even a parametric MPEG4 type decoding.
  • step E12 is a step of obtaining information representative of at least one cutoff frequency which estimates, according to a first embodiment, the frequency spectrum of the signal received by this one. This is done for example by performing a time-frequency transformation on the decoded signal in step E11 and determining the frequency from which the signal energy becomes negligible. Preferably, this can be done with the assistance of a perception model.
  • the processor obtains the information extracted in step E1 and verifies, in the case where the latter is a hierarchical decoder, whether each layer is well received and, if not, transfers information representative of the bandwidth. from one or more layers lost to the extension decoder.
  • step E13 consists of an adaptation of the low cutoff frequency of the expansion decoder so that it compensates the losses due to the network.
  • the adaptation is performed from the information representative of the cutoff frequency or the bandwidth obtained in step E12 or if the decoding of step E11 is a hierarchical decoding of the information representative of the bandwidth or a cutoff frequency of one or more lost layers.
  • step E14 the processor goes to step E14 and decodes according to a so-called extension decoding algorithm the data corresponding to the frequencies higher than this previously determined low cutoff frequency.
  • the processor selects, from the adapted frequency, among the data separated in step E1 and intended for the extension decoding, the data corresponding to the envelope of the signal corresponding to a representation of the spectral envelope of the frequencies greater than the lowest frequency corresponding to the lost frequency bands.
  • the extension decoding corrects the losses due to the network either on losses affecting the last layers received or losses affecting an intermediate layer.
  • the extension decoding is a band extension decoding algorithm, for example a High Frequency Regeneration (HFR) type decoding, for example a Spectral Band Replication (SBR) type decoding as described in the document.
  • HFR High Frequency Regeneration
  • SBR Spectral Band Replication
  • the processor returns to step E10 while waiting for new audio data to be decoded.

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Abstract

The method involves coding a part of a frequency spectrum of an audio signal with a core coder. A complementary part of the frequency spectrum of the audio signal is coded with an extension coder. A part of the spectrum coded with the core encoder is also encoded with the extension encoder. A cut-off frequency of the core coder is determined and is transferred with a coded digital signal on a network. Independent claims are also included for the following: (a) a method of spectral reconstruction of a coded audio signal (b) a device for coding an audio signal (c) a device for spectral reconstruction of a coded audio signal (d) a data signal representing a coded audio signal (e) a computer program stored on a computer medium comprising instruction enabling to implement coding method (f) a computer program stored on a computer medium comprising instruction enabling to implement a spectral reconstruction method.

Description

La présente invention concerne un procédé et un dispositif de codage et de décodage d'un signal audio employant des techniques de reconstruction de spectre.The present invention relates to a method and a device for encoding and decoding an audio signal employing spectrum reconstruction techniques.

Plus particulièrement, l'invention se situe dans l'amélioration du décodage d'un signal audio codé par un codeur à limitation de bande spectrale, dit codeur coeur.More particularly, the invention lies in the improvement of the decoding of an audio signal encoded by a spectral band limiting coder, called a core coder.

Dans l'état de la technique de la transmission de signaux audio, il est bien connu de procéder, avant la transmission, à une opération de codage d'un signal original. Le signal reçu subit quant à lui une opération inverse de décodage. Ce codage peut être un codage à réduction de débit. Des codeurs à réduction de débit connus sont par exemple les codeurs de type par transformée tels que les codeurs MPEG1, MPEG2 ou MPEG4-GA, les codeurs de type CELP et même des codeurs de type paramétrique, comme un codeur de type MPEG4 paramétrique.In the state of the art of the transmission of audio signals, it is well known to proceed, before transmission, to a coding operation of an original signal. The received signal undergoes an inverse operation of decoding. This coding may be a rate reduction coding. Known rate reduction encoders are, for example, transform type encoders such as MPEG1, MPEG2 or MPEG4-GA encoders, CELP type encoders and even parametric type encoders, such as a parametric MPEG4 type encoder.

En codage audio à réduction de débit, le signal audio doit souvent subir une limitation de bande passante lorsque le débit binaire devient faible. Cette limitation de bande passante est nécessaire pour éviter l'introduction de bruit de quantification audible dans le signal codé. Il est alors souhaitable de compléter dans la mesure du possible le contenu spectral complet du signal original.In audio reduction rate coding, the audio signal must often be bandwidth-restricted when the bit rate becomes low. This bandwidth limitation is necessary to avoid the introduction of audible quantization noise into the coded signal. It is therefore desirable to complete as far as possible the complete spectral content of the original signal.

Il a notamment été présenté dans le document " Scalable audio coder based on quantizer units of MDCT coefficients" (JIN A ET AL, IEEE International Conference on acoustics, Speech, and Signal Processing, Proceedings, 15 mars 1999 pages 897-900 ) présente un codeur hiérarchique à réduction de débit dans lequel des coefficients MDCT sont quantifiés hiérarchiquement par des modules d'encodage. Chacun de ces modules définit un niveau de hiérarchie, dont la fréquence de coupure et la bande peut être modifiée.It has been presented in the document " Scalable audio coding based on quantizer units of MDCT coefficients "(JIN A AND AL, IEEE International Conference on Acoustics, Speech, and Signal Processing, Proceedings, March 15, 1999 pages 897-900 ) presents a rate reduction hierarchical coder in which MDCT coefficients are hierarchically quantized by encoding modules. Each of these modules defines a hierarchy level, whose cutoff frequency and band can be changed.

Il a également été présenté dans le document EP A-1037 196 un unique codeur qui sélectionne au moins une paire de composantes spectrales présentant une similitude maximale au sein d'un ensemble de composantes spectrales et inclut dans son flux binaire de sortie une information d'identification de chaque paire de composante sélectionnée, permettant au décodeur d'obtenir les indices supprimés pour l'autre composante de la paire.It was also presented in the document EP A-1037196 a single encoder which selects at least one pair of spectral components having maximum similarity within a set of spectral components and includes in its output bit stream identification information of each selected component pair, allowing the decoder to get the suppressed clues for the other component of the pair.

L'élargissement de bande est connu de l'état de la technique, comme par exemple, la méthode d'élargissement spectral connue sous le nom de méthode HFR (High-Frequency Regeneration). Le signal basse fréquence décodé, à bande limitée, est soumis à un dispositif non linéaire pour obtenir un signal enrichi en harmoniques. Ce signal, après blanchiment et mise en forme fondée sur une information décrivant l'enveloppe spectrale du signal pleine bande avant codage, permet la génération d'un signal haute fréquence correspondant au contenu haute fréquence du signal avant codage.Band widening is known from the state of the art, such as the spectral broadening method known as the High Frequency Regeneration (HFR) method. The decoded, low-bandwidth low-frequency signal is subjected to a non-linear device to obtain a signal enriched in harmonics. This signal, after whitening and shaping based on information describing the spectral envelope of the full-band signal before coding, allows the generation of a high frequency signal corresponding to the high frequency content of the signal before coding.

Sont aussi connus les systèmes de codage audio numérique qui utilisent des techniques de reconstruction de spectre hautes fréquences au niveau du codeur ainsi qu'au niveau du décodeur.Also known are digital audio coding systems that use high frequency spectrum reconstruction techniques at the encoder as well as at the decoder.

Ces systèmes effectuent une adaptation dans le temps de la fréquence de coupure entre la bande de fréquence basse codée par un codeur, dit codeur coeur, et la bande de fréquence haute codée par un système HFR, dit codeur d'extension de bande.These systems perform a time adaptation of the cutoff frequency between the low frequency band coded by an encoder, called the core coder, and the high frequency band coded by an HFR system, called a band extension coder.

Dans ce cas, le codeur coeur et le codeur d'extension de bande se répartissent la bande passante en fonction de la fréquence de coupure adaptée.In this case, the core encoder and the band extension encoder distribute the bandwidth according to the appropriate cutoff frequency.

Ce type de système est particulièrement intéressant pour le codage des signaux audio.This type of system is particularly interesting for coding audio signals.

Certains réseaux de communication tels que le réseau Internet, les réseaux de communication sans fils et autres ne garantissent pas un parfait acheminement des données entre l'émetteur et le destinataire. Certaines données peuvent ainsi ne jamais arriver au destinataire ou arriver trop tard à celui-ci. En arrivant trop tard, le destinataire les considère comme perdues.Certain communication networks such as the Internet, wireless communication networks and others do not guarantee a perfect routing of data between the sender and the recipient. Some data may never reach the recipient or arrive too late. Arriving too late, the recipient considers them lost.

Dans ces réseaux, la bande passante disponible pour acheminer les données varie aussi en permanence et de façon importante.In these networks, the bandwidth available to route the data also varies continuously and significantly.

Dans d'autres réseaux, tels que les réseaux hertziens, certaines des données parmi les données transmises ont un caractère plus prioritaire que d'autres. A celles-ci sont associés des codes correcteurs d'erreurs performants, assurant un bon décodage, donc pas de pertes de transmission. D'autres par contre, sont moins importantes et des codes correcteurs d'erreurs moins performants, voire pas, leur sont associés. Ces dernières sont soumises aux aléas du réseau et le décodage risque de ne pas être réalisable.In other networks, such as radio networks, some of the data in the transmitted data is more of a priority than others. To these are associated efficient error-correcting codes, ensuring good decoding, so no transmission losses. Others, on the other hand, are less important and less efficient error correction codes, if any, are associated with them. These are subject to the vagaries of the network and decoding may not be feasible.

Dans certains systèmes de codage tels que ceux utilisés dans la norme MPEG4, il se peut, suite à des erreurs de transmission, que l'on ne puisse plus décoder le signal d'une certaine bande de fréquence du spectre du signal codé, ces composantes fréquentielles sont alors perdues.In some coding systems such as those used in the MPEG4 standard, it is possible, due to transmission errors, that the signal of a certain frequency band of the spectrum of the coded signal can no longer be decoded. Frequencies are lost.

Ainsi, même si le codage du signal audio a été effectué de la meilleure façon, le décodage de signaux transmis sur de tels réseaux comporte un certain nombre de défauts liés à ces réseaux.Thus, even if the coding of the audio signal has been done in the best way, the decoding of signals transmitted on such networks has a number of defects related to these networks.

L'invention tente de résoudre les inconvénients de l'art antérieur en proposant un procédé de codage selon la revendication 1 et le dispositif correspondant selon la revendication 9.The invention attempts to overcome the disadvantages of the prior art by proposing a coding method according to claim 1 and the corresponding device according to claim 9.

Corrélativement l'invention propose un procédé et un dispositif de reconstruction spectrale selon les revendications 6 et 14, dans lequel une partie du spectre de fréquence du signal audio est décodée avec un codeur à limitation de bande spectrale dit codeur coeur et dans lequel la partie complémentaire du spectre de fréquence du signal audio est décodée avec un codeur d'extension, caractérisé en ce que le dispositif comporte :

  • des moyens d'obtention d'informations représentatives d'au moins une fréquence de coupure du signal décodé par le décodeur coeur,
  • des moyens de sélection, parmi les données à décoder ou décodées avec le décodeur d'extension, de données pertinentes pour le décodage en fonction des informations obtenues.
Correlatively, the invention proposes a method and a spectral reconstruction device according to claims 6 and 14, wherein a portion of the frequency spectrum of the audio signal is decoded with a core-coded spectral band limiting encoder and wherein the complementary portion of the frequency spectrum of the audio signal is decoded with an extension encoder, characterized in that the device comprises:
  • means for obtaining information representative of at least one cut-off frequency of the signal decoded by the heart decoder,
  • means for selecting, from among the data to be decoded or decoded with the extension decoder, data relevant for the decoding according to the information obtained.

Ainsi, le signal décodé va être de meilleure qualité, aucune composante spectrale du signal ne manque, le spectre en fréquence décodé avec le décodeur d'extension étant modifié conformément à la fréquence de coupure du signal décodé par le codeur coeur.Thus, the decoded signal will be of better quality, no spectral component of the signal is missing, the frequency spectrum decoded with the extension decoder being modified according to the cutoff frequency of the signal decoded by the core encoder.

Plus particulièrement, la partie du spectre de fréquence du signal audio décodée avec un décodeur coeur est la partie basse du spectre en fréquence du signal audio.More particularly, the portion of the frequency spectrum of the decoded audio signal with a heart decoder is the lower part of the frequency spectrum of the audio signal.

Avantageusement, les informations représentatives d'au moins une fréquence de coupure du signal décodé par le décodeur coeur sont obtenues en effectuant une estimation de la fréquence de coupure haute du signal décodé par le décodeur coeur.Advantageously, the information representative of at least one cut-off frequency of the signal decoded by the heart decoder is obtained by making an estimate of the high cut-off frequency of the signal decoded by the heart decoder.

Ainsi, il n'est pas nécessaire d'inclure d'informations supplémentaires dans le signal codé et transmis, moins d'informations transitent sur le réseau.Thus, it is not necessary to include additional information in the coded and transmitted signal, less information flows over the network.

Avantageusement, les informations représentatives d'au moins une fréquence de coupure du signal décodé par le décodeur coeur sont obtenues à partir d'informations incluses dans le flot de données comprenant le signal numérique codé.Advantageously, the information representative of at least one cut-off frequency of the signal decoded by the heart decoder is obtained from information included in the data stream comprising the coded digital signal.

Ainsi, la rapidité de traitement au niveau du décodeur est augmentée, tout en simplifiant celui-ci.Thus, the speed of processing at the decoder is increased, while simplifying it.

Plus particulièrement, le décodeur coeur est un décodeur hiérarchique et on obtient des informations représentatives de la bande passante du signal décodé par le décodeur coeur pour chaque couche du signal décodé.More particularly, the heart decoder is a hierarchical decoder and information is obtained representative of the bandwidth of the signal decoded by the heart decoder for each layer of the decoded signal.

Ainsi, le décodeur peut adapter le traitement à chaque couche de codage, le décodeur dispose de cette information à chaque couche et peut ainsi modifier le spectre en fréquence décodé avec le décodeur d'extension en fonction de ces informations.Thus, the decoder can adapt the processing to each coding layer, the decoder has this information at each layer and can thus modify the frequency spectrum decoded with the extension decoder according to this information.

L'invention selon les revendications 17 et 18 concerne aussi le programme d'ordinateur stocké sur un support d'informations, ledit programme comportant des instructions permettant de mettre en oeuvre le procédé de traitement précédemment décrit, lorsqu'il est chargé et exécuté par un système informatique.The invention according to claims 17 and 18 also relates to the computer program stored on an information carrier, said program comprising instructions for implementing the previously described processing method, when it is loaded and executed by a user. computer system.

Les caractéristiques de l'invention mentionnées ci-dessus, ainsi que d'autres, apparaîtront plus clairement à la lecture de la description suivante d'un exemple de réalisation, ladite description étant faite en relation avec les dessins joints, parmi lesquels :

  • les Figs. 1a à 1d représentent les différents spectres en fréquence d'un signal audio codé avec un codeur coeur et un codeur d'extension ;
  • les Figs. 1e à 1g représentent les différents spectres en fréquence d'un signal audio transmis sur un réseau et décodé avec un décodeur coeur et un décodeur d'extension ;
  • les Figs. 2a à 2e représentent les différents spectres en fréquence d'un signal audio codé avec un codeur coeur hiérarchique et un codeur d'extension ;
  • les Figs. 2f à 2i représentent les différents spectres en fréquence d'un signal audio transmis sur un réseau et décodé avec un décodeur coeur hiérarchique et un décodeur d'extension ;
  • les Figs. 3a à 3c représentent les différents spectres en fréquence d'un signal audio codé avec un codeur coeur et un codeur d'extension selon invention ;
  • les Figs. 3d à 3f représentent les différents spectres en fréquence d'un signal audio transmis sur un réseau et décodé avec un décodeur coeur et un décodeur d'extension selon l'invention ;
  • la Fig. 4a représente un schéma bloc décrivant le dispositif de codage selon l'invention ;
  • la Fig. 4b représente un schéma bloc décrivant les principaux éléments d'un codeur hiérarchique coeur ;
  • la Fig. 5 représente un schéma bloc décrivant le dispositif de décodage selon l'invention,
  • la Fig. 6 représente selon l'invention, l'algorithme effectué au niveau du codeur ;
  • la Fig. 7 représente selon l'invention, l'algorithme effectué au niveau du décodeur.
The characteristics of the invention mentioned above, as well as others, will appear more clearly on reading the following description of an exemplary embodiment, said description being given in relation to the attached drawings, among which:
  • the Figs. 1a to 1d represent the different frequency spectra of an audio signal encoded with a core encoder and an extension encoder;
  • the Figs. 1st to 1g represent the different frequency spectra of an audio signal transmitted over a network and decoded with a heart decoder and an extension decoder;
  • the Figs. 2a to 2e represent the different frequency spectra of an audio signal coded with a hierarchical core coder and an extension coder;
  • the Figs. 2f to 2i represent the different frequency spectra of an audio signal transmitted over a network and decoded with a hierarchical heart decoder and an extension decoder;
  • the Figs. 3a to 3c represent the different frequency spectra of an audio signal encoded with a core encoder and an extension encoder according to the invention;
  • the Figs. 3d to 3f represent the different frequency spectra of an audio signal transmitted over a network and decoded with a heart decoder and an extension decoder according to the invention;
  • the Fig. 4a represents a block diagram describing the coding device according to the invention;
  • the Fig. 4b represents a block diagram describing the main elements of a hierarchical heart encoder;
  • the Fig. 5 represents a block diagram describing the decoding device according to the invention,
  • the Fig. 6 represents according to the invention, the algorithm performed at the coder;
  • the Fig. 7 represents according to the invention, the algorithm performed at the decoder.

La Fig. 1a représente un spectre en fréquence d'un signal audio devant être codé. Conformément aux codeurs utilisant des combinaisons de codeurs tels que l'association codeur coeur/codeur d'extension, les fréquences basses du spectre (Fig. 1b), sont codées par un codeur coeur, tandis que les fréquences hautes sont codées par un codeur d'extension. Cette partie des fréquences hautes est représentée en Fig. 1c.The Fig. 1a represents a frequency spectrum of an audio signal to be encoded. According to the encoders using combinations of encoders such as the core encoder / extension encoder association, the low frequencies of the spectrum ( Fig. 1b ) are encoded by a core encoder, while the high frequencies are encoded by an extension encoder. This part of the high frequencies is represented in Fig. 1 C .

Le cumul des fréquences hautes et basses donne alors un spectre total représenté en Fig. 1d qui est identique sinon similaire au spectre de la Fig. 1a.The combination of high and low frequencies then gives a total spectrum represented in Fig. 1d which is identical if not similar to the spectrum of the Fig. 1a .

Lorsqu'un tel signal audio codé est transmis sur un réseau, certaines données parmi l'ensemble des données transmises sont perdues.When such a coded audio signal is transmitted over a network, some of the data transmitted is lost.

Ce qui est par exemple le cas de certains systèmes de codage tels que ceux utilisés dans la norme MPEG4. Suite à des erreurs de transmission, il n'est plus possible de décoder le signal à partir d'une certaine fréquence du spectre du signal codé. Les informations représentatives des composantes du spectre en fréquence supérieur à cette fréquence sont alors considérées comme perdues.This is for example the case of some coding systems such as those used in the MPEG4 standard. As a result of transmission errors, it is no longer possible to decode the signal from a certain frequency of the spectrum of the coded signal. The information representative of the components of the frequency spectrum higher than this frequency are then considered lost.

La Fig. 1e représente le spectre en fréquence d'un signal audio décodé avec un décodeur coeur, le signal audio codé ayant été transmis sur un réseau et certaines données 10 ont été perdues.The Fig. first represents the frequency spectrum of a decoded audio signal with a heart decoder, the encoded audio signal having been transmitted over a network and some data has been lost.

Ce type de perte est particulièrement gênant pour les informations codées par le codeur coeur. L'absence des données 10 constitue un trou dans le spectre des fréquences décodées et ce trou crée des nuisances sonores importantes telles que des sifflements à de la restitution du signal sonore.This type of loss is particularly troublesome for the information coded by the heart coder. The absence of data 10 constitutes a hole in the spectrum of decoded frequencies and this hole creates significant noise, such as whistling sound restitution.

Les informations codées par le codeur d'extension sont beaucoup plus limitées quant à leur nombre.The information encoded by the extension encoder is much more limited in number.

Elles sont soit incluses avec les données codées par le codeur coeur, soit transmises de manière indépendante.They are either included with the data encoded by the core encoder, or are transmitted independently.

Dans notre exemple, le spectre en fréquence d'un signal audio transmis sur un réseau et décodé avec un décodeur d'extension est considéré comme correct. Ceci est représenté à la Fig. 1f.In our example, the frequency spectrum of an audio signal transmitted over a network and decoded with an extension decoder is considered correct. This is represented at Fig. 1f .

La reconstruction du signal audio respectivement par le décodeur coeur et le décodeur d'extension fait apparaître en Fig. 1g un spectre en fréquence comportant des composantes fréquentielles 10 ayant disparues.The reconstruction of the audio signal respectively by the heart decoder and the extension decoder Fig. 1g a frequency spectrum comprising frequency components 10 having disappeared.

Ces composantes fréquentielles disparues 10 entachent de façon importante la qualité de reproduction du signal audio.These missing frequency components greatly taint the quality of reproduction of the audio signal.

La Fig. 2a représente le spectre en fréquence du signal audio total devant être codé par un codeur coeur hiérarchique et un codeur d'extension.The Fig. 2a represents the frequency spectrum of the total audio signal to be encoded by a hierarchical core encoder and an extension encoder.

Un codeur coeur hiérarchique va coder successivement différentes sous-parties du spectre en fréquence du signal audio à coder.A hierarchical core encoder will successively encode different subparts of the frequency spectrum of the audio signal to be encoded.

Une première partie du spectre, par exemple la partie contenant les composantes fréquentielles les plus basses, telle que le spectre représenté en Fig. 2b, va être codée.A first part of the spectrum, for example the part containing the lowest frequency components, such as the spectrum represented in Fig. 2b , will be coded.

Celle-ci est appelée la première couche. Ensuite, une autre partie contenant des composantes fréquentielles additionnelles va être codée. C'est la seconde couche, elle est représentée à la Fig. 2c.This is called the first layer. Then another part containing additional frequency components will be coded. This is the second layer, she is represented at the Fig. 2c .

Ainsi, dans de tels systèmes de transmission de données audio, les informations représentatives des fréquences les plus basses sont généralement transmises dans les premières couches. Les autres couches sont, par exemple, transmises alors dans un ordre fonction des fréquences du spectre qu'elles représentent.Thus, in such audio data transmission systems, the information representative of the lowest frequencies is generally transmitted in the first layers. The other layers are, for example, then transmitted in an order according to the frequencies of the spectrum that they represent.

Dans les réseaux de diffusion de données de type hertzien, certaines des couches parmi les couches transmises ont un caractère plus prioritaire que d'autres. En général, les couches comportant les fréquences les plus basses sont considérées comme prioritaires, les couches comportant les fréquences les plus hautes sont considérées comme les moins prioritaires.In wireless data broadcasting networks, some of the layers of the transmitted layers have a higher priority character than others. In general, the layers with the lowest frequencies are considered as priority, the layers with the highest frequencies are considered as the lowest priorities.

Aux couches comportant les fréquences les plus basses, sont associés des codes correcteurs d'erreurs performants, assurant un bon décodage, donc pas de pertes de transmission.The layers with the lowest frequencies are associated with efficient error correction codes, ensuring good decoding, so no transmission losses.

Des codes correcteurs d'erreurs moins performants sont associés aux couches comportant les fréquences les plus hautes. Ces dernières sont soumises aux aléas du réseau et le décodage risque de ne pas être réalisable.Less efficient error correcting codes are associated with the layers having the highest frequencies. These are subject to the vagaries of the network and decoding may not be feasible.

La Fig. 2d représente la partie du spectre allouée au codeur d'extension de bande, elle est identique à celle décrite en Fig. 1e.The Fig. 2d represents the part of the spectrum allocated to the band extension encoder, it is identical to that described in Fig. first .

Le cumul des trois spectres des Figs. 2b, 2c et 2d donne alors un spectre total représenté en Fig. 2e qui est identique sinon similaire au spectre de la Fig. 2a.The sum of the three spectra of Figs. 2b, 2c and 2d then gives a total spectrum represented in Fig. 2nd which is identical if not similar to the spectrum of the Fig. 2a .

Les Figs. 2f et 2g représentent les spectres en fréquence d'un signal audio décodé avec un décodeur coeur hiérarchique comportant deux couches de hiérarchie, le signal audio codé ayant été transmis sur un réseau et dont certaines couches ont été perdues.The Figs. 2f and 2g represent the frequency spectra of a decoded audio signal with a hierarchical heart decoder comprising two hierarchy layers, the coded audio signal having been transmitted over a network and some layers of which have been lost.

Lors de la transmission de la première couche, le spectre équivalent à cette couche n'a pas été entaché d'erreurs de transmission, comme cela est représenté à la Fig 2f.When transmitting the first layer, the spectrum equivalent to this layer was not tainted with transmission errors, as shown in FIG. Fig 2f .

Des données ont été perdues lors de la transmission de la seconde couche, le spectre équivalent à cette couche comporte des composantes fréquentielles, 25 à la Fig. 2g, absentes.Data was lost during the transmission of the second layer, the spectrum equivalent to this layer has frequency components, 25 at the Fig. 2g , absent.

La partie du spectre allouée au codeur d'extension de bande est identique à celle décrite en Fig. 1c. Elle est représentée à la Fig 2h.The part of the spectrum allocated to the band extension encoder is identical to that described in Fig. 1 C . She is represented at Fig 2h .

Ainsi, la reconstruction du signal audio respectivement par le codeur hiérarchique coeur et le décodeur d'extension fait apparaître en Fig. 2i un spectre en fréquence comportant des composantes fréquentielles 25 ayant disparues.Thus, the reconstruction of the audio signal respectively by the hierarchical core coder and the extension decoder makes it apparent that Fig. 2i a frequency spectrum having frequency components 25 that have disappeared.

La Fig. 3a représente le spectre en fréquence du signal audio total devant être codé par un codeur coeur et un codeur d'extension selon l'invention.The Fig. 3a represents the frequency spectrum of the total audio signal to be encoded by a core encoder and an extension encoder according to the invention.

Le codeur coeur code les composantes basses fréquences du spectre en fréquence du signal audio, ceci est représenté à la Fig. 3b.The core encoder encodes the low frequency components of the frequency spectrum of the audio signal, this is shown in FIG. Fig. 3b .

Contrairement à l'état de la technique, et selon l'invention, le codeur d'extension code non seulement les composantes hautes fréquences du spectre en fréquence du signal audio à coder mais aussi une partie 30 des composantes basses fréquences que le codeur coeur code. Ces composantes sont représentées à la Fig. 3c.In contrast to the state of the art, and according to the invention, the extension coder not only encodes the high frequency components of the frequency spectrum of the audio signal to be coded but also a portion of the low frequency components that the core code coder . These components are represented at Fig. 3c .

La Fig. 3d représente le spectre en fréquence d'un signal audio décodé avec un décodeur coeur, le signal audio codé ayant été transmis sur un réseau et dont certaines couches 31 ont été perdues.The Fig. 3d represents the frequency spectrum of an audio signal decoded with a heart decoder, the coded audio signal having been transmitted over a network and some layers 31 have been lost.

Une estimation de la bande passante du signal audio décodé par le décodeur coeur est effectuée, si celle-ci est différente de celle attendue, le décodeur coeur informe le décodeur d'extension de la bande passante manquante.An estimate of the bandwidth of the audio signal decoded by the heart decoder is performed, if it is different from that expected, the heart decoder informs the extension decoder of the missing bandwidth.

Le décodeur d'extension, avec cette information, adapte le décodage de manière à ce que le décodage s'applique aussi à la bande passante manquante.The extension decoder, with this information, adapts the decoding so that the decoding also applies to the missing bandwidth.

En Fig 3e, est représenté le spectre en fréquence équivalent aux informations codées reçues par le décodeur d'extension. Ce spectre est composé des composantes 32,33 et 34.In Fig 3rd , is represented the frequency spectrum equivalent to the coded information received by the extension decoder. This spectrum is composed of components 32,33 and 34.

Si aucune erreur de transmission liée à la variation de bande passante du réseau ou d'erreurs de transmission n'est arrivée, les informations correspondant à la composante 34 suffisent au décodage.If no transmission error related to the bandwidth variation of the network or transmission errors has arrived, the information corresponding to the component 34 is sufficient for the decoding.

Si la bande passante du réseau a varié ou des erreurs de transmission sont arrivées de façon telle que la composante 31 de la Fig. 3d est perdue, les informations correspondant aux composantes 33 et 34 sont nécessaires au décodage.If the bandwidth of the network has varied or transmission errors have arrived in such a way that component 31 of the Fig. 3d is lost, the information corresponding to components 33 and 34 are necessary for decoding.

Ainsi, la reconstruction du signal audio respectivement par le décodeur hiérarchique coeur et le décodeur d'extension fait apparaître en Fig. 3f un spectre en fréquence ne comportant plus de composantes fréquentielles manquantes. Ainsi, même lorsque le réseau comporte des variations importantes de bande passante, le signal audio décodé reste de qualité.Thus, the reconstruction of the audio signal respectively by the hierarchical heart decoder and the extension decoder makes it apparent that Fig. 3f a frequency spectrum no longer including missing frequency components. Thus, even when the network has significant variations in bandwidth, the decoded audio signal remains of quality.

La Fig. 4a représente un schéma bloc décrivant le dispositif de codage selon l'invention.The Fig. 4a represents a block diagram describing the coding device according to the invention.

Le dispositif de codage est constitué d'un convertisseur analogique numérique 400 qui convertit le signal analogique à coder en un signal numérique. Bien entendu, si les données sont déjà sous forme numérique, le convertisseur analogique numérique n'est pas nécessaire.The coding device consists of a digital analog converter 400 which converts the analog signal to be encoded into a digital signal. Of course, if the data is already in digital form, the analog-to-digital converter is not necessary.

Le signal numérique est délivré au codeur coeur qui encode ce signal. Le codeur coeur est par exemple un codeur à réduction de débit tel que conforme à l'un des standards MPEG1, MPEG2 ou MPEG4-GA, ou un codeur de type CELP, un codeur hiérarchique, voire même un codeur MPEG4 paramétrique.The digital signal is delivered to the heart encoder which encodes this signal. The core encoder is for example a rate reduction encoder as conforming to one of the MPEG1, MPEG2 or MPEG4-GA standards, or a CELP type encoder, a hierarchical coder, or even a parametric MPEG4 encoder.

La sortie du codeur coeur représente les données du signal couvrant le spectre en fréquence tel que celui représenté en Fig. 3b.The output of the core encoder represents the data of the signal covering the frequency spectrum such as that represented in FIG. Fig. 3b .

Ce même signal numérique est délivré au codeur d'extension de bande 403. Le codeur d'extension de bande est par exemple un codeur de type HFR (High Frequency Regeneration) , par exemple SBR (Spectral Band Replication) tel que décrit dans le document « Audio Engineering Society, convention paper 5553 », présenté à la 112ieme convention AES par Mr Martin Dietz .This same digital signal is delivered to the band extension encoder 403. The band extension encoder is for example an HFR (High Frequency Regeneration) type encoder, for example SBR (Spectral Band Replication) as described in FIG. document "Audio Engineering Society, convention paper 5553", presented at the 112th AES Convention by Mr Martin Dietz .

La sortie du codeur d'extension de bande représente les données de l'enveloppe du signal couvrant le spectre en fréquence tel que celui représenté en Fig. 3c.The output of the band extension encoder represents the signal envelope data covering the frequency spectrum such as that shown in FIG. Fig. 3c .

Un module d'ajustement de fréquence de coupure 402 est relié au codeur d'extension de bande 403 et au codeur coeur 401.A cutoff frequency adjustment module 402 is connected to the tape extension encoder 403 and the core encoder 401.

Ce module 402 définit le spectre en fréquence que le codeur d'extension prend en compte pour le codage.This module 402 defines the frequency spectrum that the extension encoder takes into account for coding.

Ce module 402 détermine ce spectre en fonction de la fréquence de coupure haute du codeur coeur 401, et d'une bande de fréquence variable qui permet au décodeur selon l'invention de pouvoir pallier les possibles pertes de transmission.This module 402 determines this spectrum as a function of the high cut-off frequency of the core encoder 401, and of a variable frequency band which enables the decoder according to the invention to be able to overcome the possible transmission losses.

Par exemple, dans le cas de l'utilisation d'un codeur hiérarchique et d'une transmission avec des codes correcteurs d'erreurs dont la robustesse est variable selon les couches transmises, la bande de fréquence variable est ajustée de manière à garantir la bonne recomposition du signal pour les couches ne disposant pas d'un code correcteur d'erreurs robuste.For example, in the case of the use of a hierarchical coder and a transmission with error correcting codes whose robustness is variable according to the transmitted layers, the variable frequency band is adjusted so as to guarantee the correct Recomposing the signal for layers that do not have a robust error correction code.

Il est à remarquer que dans une variante, le spectre en fréquence du codeur coeur 401 peut être ajusté à partir du spectre en fréquence du codeur d'extension 403.It should be noted that in one variant, the frequency spectrum of the core encoder 401 can be adjusted from the frequency spectrum of the extension encoder 403.

Dans ce cas, le module 402 définit le spectre en fréquence que le codeur coeur 401 prend en compte pour le codage. Ce module 402 définit ce spectre en fonction de la fréquence de coupure basse du codeur d'extension 403 et d'une bande de fréquence variable qui permet au décodeur selon l'invention de pouvoir pallier les possibles pertes de transmission.In this case, the module 402 defines the frequency spectrum that the core encoder 401 takes into account for the coding. This module 402 defines this spectrum as a function of the low cut-off frequency of the extension encoder 403 and of a variable frequency band which enables the decoder according to the invention to be able to overcome the possible transmission losses.

Le dispositif de codage comporte aussi un multiplexeur 404 qui multiplexe les signaux audio codés par le codeur coeur 401 et par le codeur d'extension 403.The coding device also comprises a multiplexer 404 which multiplexes the audio signals coded by the core coder 401 and by the extension coder 403.

Selon une variante de l'invention, le module 402 transfère au multiplexeur 404 les informations représentatives de la bande passante du codeur coeur 401 ou ses fréquences de coupures, voire même de la fréquence de coupure basse du codeur d'extension 403 pour que celles-ci soient inclues dans les données transmises.According to a variant of the invention, the module 402 transfers to the multiplexer 404 the information representative of the passband of the core encoder 401 or its cutoff frequencies, or even the low cutoff frequency of the extension encoder 403 so that these included in the transmitted data.

L'inclusion est effectuée dans le cas d'un codeur hiérarchique pour chaque couche de codage.The inclusion is performed in the case of a hierarchical coder for each coding layer.

Les données multiplexées sont alors transférées à un module de transmission réseau qui, par exemple dans le cas d'une transmission hertzienne, applique des codes correcteurs d'erreurs aux données multiplexées et transmet celles-ci sur le réseau 405.The multiplexed data is then transferred to a network transmission module which, for example in the case of a radio transmission, applies error correcting codes to the multiplexed data and transmits them to the network 405.

La Fig. 4b représente un schéma bloc décrivant les principaux éléments d'un codeur hiérarchique coeur.The Fig. 4b represents a block diagram describing the main elements of a hierarchical heart encoder.

Ce codeur hiérarchique peut remplacer le codeur 401 précédemment décrit en référence à la Fig. 4a.This hierarchical coder can replace the encoder 401 previously described with reference to the Fig. 4a .

Un codeur hiérarchique coeur subdivise habituellement le spectre de fréquence à coder en différentes couches. Une couche représente une bande de fréquence du spectre à coder. Le nombre de couches est variable et permet une transmission progressive du signal codé.A hierarchical heart encoder usually subdivides the frequency spectrum to be coded into different layers. A layer represents a frequency band of the spectrum to be coded. The number of layers is variable and allows a progressive transmission of the coded signal.

Dans un souci de simplicité, seulement deux couches sont représentées ici. Le codeur est constitué d'un premier codeur 410 qui code la partie la plus basse du spectre en fréquence du signal original.For the sake of simplicity, only two layers are represented here. The encoder consists of a first encoder 410 which encodes the lowest part of the frequency spectrum of the original signal.

Les informations codées sont transférées à un multiplexeur 416 qui transfère ces données au multiplexeur 404.The encoded information is transferred to a multiplexer 416 which transfers this data to the multiplexer 404.

Il est à remarquer que le module 402 précédemment décrit transfère au multiplexeur 404 les informations représentatives de la bande passante du codeur coeur 410 pour que celles-ci soient inclues dans le flot de données associé à cette couche.It should be noted that the module 402 previously described transfers to the multiplexer 404 the information representative of the bandwidth of the core encoder 410 so that they are included in the data stream associated with this layer.

Ceci constitue alors la première couche du signal codé.This then constitutes the first layer of the coded signal.

Les informations codées sont aussi transférées à un décodeur 411. Ce décodeur décode ces informations pour ensuite les transmettre à un circuit de soustraction 413 qui va soustraire du signal original le signal décodé.The coded information is also transferred to a decoder 411. This decoder decodes this information and then transmits it to a subtraction circuit 413 which will subtract the decoded signal from the original signal.

Il est à remarquer que le signal original a été précédemment retardé d'un délai égal au temps de codage du codeur 410 et au temps de décodage du décodeur 411.It should be noted that the original signal has previously been delayed by a delay equal to the coding time of the encoder 410 and the decoding time of the decoder 411.

Le signal obtenu en sortie du circuit soustracteur est alors le signal original dans lequel les composantes basses fréquences précédemment codées ont été enlevées au résidu de codage près.The signal obtained at the output of the subtracter circuit is then the original signal in which the previously coded low frequency components have been removed to the coding residue.

Ce signal est de nouveau codé par un codeur 415 pouvant être de même type que le codeur 410. Ici, sont codées les composantes fréquentielles du signal supérieures à celles codées par le codeur 410.This signal is again coded by an encoder 415 which may be of the same type as the encoder 410. Here, the frequency components of the signal greater than those coded by the encoder 410 are coded.

Les informations codées sont transférées à un multiplexeur 416 qui transfère ces données au multiplexeur 404.The encoded information is transferred to a multiplexer 416 which transfers this data to the multiplexer 404.

Il est à remarquer que le module 402 précédemment décrit transfère au multiplexeur 404 les informations représentatives de la bande passante du codeur cceur 415 pour que celles-ci soient incluses dans le flot de données associé à cette couche. Il peut aussi transférer le nombre total de couches de codage, la fréquence de coupure haute ou basse du codeur coeur 415.It should be noted that the module 402 previously described transfers to the multiplexer 404 the information representative of the bandwidth of the core encoder 415 so that they are included in the data stream associated with this layer. It can also transfer the total number of coding layers, the high or low cutoff frequency of the core encoder 415.

Ceci constitue alors la seconde couche du signal codé.This then constitutes the second layer of the coded signal.

Il est à remarquer que si l'on désire augmenter le nombre de couches, les éléments 410,411, 413 et 414 devront être dupliqués pour chaque couche supplémentaire.It should be noted that if it is desired to increase the number of layers, the elements 410, 411, 413 and 414 will have to be duplicated for each additional layer.

Il est aussi à remarquer que le spectre de fréquence traité par chaque codeur peut être variable : Il est à remarquer aussi que l'invention est applicable pour les signaux audio de type monophonique, stéréophonique ou multivoie.It should also be noted that the frequency spectrum processed by each encoder may be variable: It should also be noted that the invention is applicable for mono, stereo or multi-channel audio signals.

Dans le cas de signaux multivoie, les informations de bande passante transmises par le codeur peuvent être transmises de manière conjuguée ou dans un mode préférentiel, la bande passante de chacune des voies peut être déduite des autres voies par un codage différentiel.In the case of multichannel signals, the bandwidth information transmitted by the coder can be transmitted in a conjugated manner or in a preferential mode, the bandwidth of each of the channels can be deduced from the other channels by a differential coding.

La Fig. 5 représente un schéma bloc décrivant le dispositif de décodage selon l'invention.The Fig. 5 represents a block diagram describing the decoding device according to the invention.

Le dispositif de décodage est constitué d'un démultiplexeur 510 qui sépare les signaux reçus par l'intermédiaire du réseau 405 en données destinées au décodeur coeur 511 et en données destinées au décodeur d'extension 512. Il extrait en outre des signaux reçus, les informations représentatives de la bande passante du codeur coeur 401 du dispositif de codage, des codeurs 410 et 415 si le signal a été codé avec un codeur hiérarchique, voire même de la fréquence de coupure basse du codeur d'extension 403 du dispositif de codage si celles-ci ont été incluses dans les données transmises.The decoding device consists of a demultiplexer 510 which separates the signals received via the network 405 into data intended for the heart decoder 511 and data intended for the extension decoder 512. It also extracts signals received, the information representative of the bandwidth of the core encoder 401 of the coding device, the encoders 410 and 415 if the signal has been encoded with a hierarchical coder, or even the low cutoff frequency of the encoder extension 403 of the coding device if these were included in the transmitted data.

Le décodeur coeur 511 décode les données pour fournir un signal décodé tel que le signal représenté à la Fig. 3d.The heart decoder 511 decodes the data to provide a decoded signal such as the signal shown in FIG. Fig. 3d .

Le décodeur coeur 511 est par exemple un décodeur tel que conforme à l'un des standards MPEG1, MPEG2 ou MPEG4-GA, ou un décodeur de type CELP, un décodeur hiérarchique, voire même un décodeur MPEG4 paramétrique.The core decoder 511 is, for example, a decoder such as one of the MPEG1, MPEG2 or MPEG4-GA standards, or a CELP type decoder, a hierarchical decoder, or even a parametric MPEG4 decoder.

Le décodeur coeur 511 comporte un module 511b d'obtention d'informations représentatives d'au moins une fréquence de coupure qui estime, selon un premier mode de réalisation, le spectre en fréquence du signal reçu par celui-ci. Le module 511b le réalise par exemple en effectuant une transformation temps fréquence sur le signal décodé et en déterminant la fréquence à partir de laquelle l'énergie du signal devient négligeable. Préférablement, ceci peut s'effectuer avec l'assistance d'un modèle de perception.The heart decoder 511 comprises a module 511b for obtaining information representative of at least one cutoff frequency which estimates, according to a first embodiment, the frequency spectrum of the signal received by the latter. The module 511b does this for example by performing a time-frequency transformation on the decoded signal and determining the frequency from which the signal energy becomes negligible. Preferably, this can be done with the assistance of a perception model.

Le décodeur 511, plus précisément son module 511b transfère ensuite une information représentative de la fréquence de coupure ou de la bande passante au décodeur d'extension 512.The decoder 511, more precisely its 511b module then transfers information representative of the cutoff frequency or the bandwidth to the extension decoder 512.

Le décodeur d'extension 512 sélectionne, à partir de l'information représentative transmise par le décodeur 511, parmi les données codées qu'il a reçues du multiplexeur 510, les données correspondant à une représentation de l'enveloppe spectrale supérieure à la fréquence déterminée par le codeur 511.The extension decoder 512 selects, from the representative information transmitted by the decoder 511, among the coded data received from the multiplexer 510, the data corresponding to a representation of the spectral envelope greater than the determined frequency. by the encoder 511.

De cette façon, les pertes liées à la transmission du signal codé sont compensées.In this way, the losses related to the transmission of the coded signal are compensated.

Le décodeur coeur 511, plus précisément le module 511b d'obtention d'informations représentatives d'au moins une fréquence de coupure obtient du démultiplexeur 510, selon un second mode de réalisation, les informations représentatives de la bande passante du codeur coeur 401 ou des codeurs 410 et 415 du dispositif de codage, voire le nombre de couches du signal codé, voire même de la fréquence de coupure basse du codeur d'extension 403 du dispositif de codage si celles-ci ont été incluses dans les données transmises.The core decoder 511, more precisely the module 511b for obtaining information representative of at least one cut-off frequency obtains from the demultiplexer 510, according to a second embodiment, the information representative of the bandwidth of the core encoder 401 or encoders 410 and 415 of the coding device, or even the number of layers of the coded signal, or even the low cutoff frequency of the encoder 403 extension encoder if they were included in the data transmitted.

A partir de ces données obtenues, le module 511 b vérifie, dans le cas où celui-ci est un décodeur hiérarchique, si chaque couche est bien reçue et dans la négative transfère une information représentative de la bande passante d'une ou des couches perdues au décodeur d'extension 512.From this data obtained, the module 511 b checks, in the case where it is a hierarchical decoder, if each layer is well received and in the negative transfers information representative of the bandwidth of one or more lost layers to the extension decoder 512.

Le décodeur d'extension 512 sélectionne, à partir de l'information représentative transmise par le module 511b, parmi les données codées reçues du multiplexeur 510, les données correspondant à l'enveloppe du signal correspondant à une représentation de l'enveloppe spectrale des fréquences supérieures à la fréquence la plus basse correspondant aux bandes de fréquences perdues.The extension decoder 512 selects, from the representative information transmitted by the module 511b, among the coded data received from the multiplexer 510, the data corresponding to the envelope of the signal corresponding to a representation of the spectral envelope of the frequencies. higher than the lowest frequency corresponding to the lost frequency bands.

Ainsi, le décodeur d'extension corrige les pertes dues au réseau que ce soit sur des pertes affectant les dernières couches reçues ou des pertes affectant une couche intermédiaire.Thus, the extension decoder corrects the losses due to the network either on losses affecting the last layers received or losses affecting an intermediate layer.

Le décodeur d'extension de bande 512 est par exemple un décodeur de type HFR (High Frequency Regeneration), par exemple un décodeur de type SBR (Spectral Band Replication) tel que décrit dans le document « Audio Engineering Society, convention paper 5553 », présenté à la 112ieme convention AES par Mr Martin Dietz .The band extension decoder 512 is for example a HFR (High Frequency Regeneration) type decoder, for example a SBR (Spectral Band Replication) type decoder as described in the document "Audio Engineering Society, convention paper 5553", presented at the 112th AES Convention by Mr Martin Dietz .

Il est à remarquer qu'en variante, le décodeur d'extension 512 décode la totalité des informations reçues. Une sélection parmi les données décodées est effectuée de manière à ne garder que celles correspondant à une représentation de l'enveloppe spectrale supérieure à la fréquence déterminée par le codeur 511.It should be noted that, in a variant, the extension decoder 512 decodes all the information received. A selection among the decoded data is performed so as to keep only those corresponding to a representation of the spectral envelope greater than the frequency determined by the encoder 511.

L'enveloppe décodée par le décodeur d'extension 512 ou sélectionnée est transférée à un module de contrôle de gain 515.The envelope decoded by the extension decoder 512 or selected is transferred to a gain control module 515.

Le signal décodé par le décodeur coeur 511 est envoyé à un module de transposition 513 qui génère un signal dans les hautes fréquences du spectre à partir du signal décodé basse fréquence.The signal decoded by the heart decoder 511 is sent to a transposition module 513 which generates a signal in the high frequencies of the spectrum from the low frequency decoded signal.

Ce signal est introduit dans le module de contrôle de gain 515 afin de permettre l'ajustement de l'enveloppe de signal haute fréquence.This signal is fed into the gain control module 515 to allow adjustment of the high frequency signal envelope.

Le signal d'enveloppe ajusté est ensuite additionné au signal décodé par le décodeur coeur 511 avec un additionneur 516.The adjusted envelope signal is then added to the decoded signal by the heart decoder 511 with an adder 516.

L'additionneur 516 peut dans un mode préféré favoriser certaines composantes fréquences en multipliant par exemple certaines composantes par des coefficients.The adder 516 may in a preferred mode favor certain frequency components by multiplying for example certain components by coefficients.

Il est à remarquer que le signal décodé par le décodeur coeur 511 a été préalablement retardé d'un délai égal à la différence de temps de traitement entre les signaux additionnés. Ce retard est effectué par le circuit de retard 514.It should be noted that the signal decoded by the heart decoder 511 has been previously delayed by a delay equal to the difference in processing time between the summed signals. This delay is performed by the delay circuit 514.

Le spectre en fréquence du signal obtenu est ainsi similaire à celui de la Fig. 3f.The frequency spectrum of the signal obtained is thus similar to that of the Fig. 3f .

Le signal de sommation peut être ensuite converti sous forme analogique à l'aide d'un convertisseur numérique analogique 517.The summation signal can then be converted to analog form using a digital to analog converter 517.

La Fig. 6 représente l'algorithme effectué selon l'invention au niveau du codeur.
L'invention telle que décrite en référence aux Figs. précédentes est aussi réalisable sous forme logicielle dans laquelle un processeur exécute le code exécutable associé aux étapes E1 à E7 de l'algorithme de la Fig. 6.The Fig. 6 represents the algorithm performed according to the invention at the coder.
The invention as described with reference to FIGS. preceding is also feasible in software form in which a processor executes the executable code associated with the steps E1 to E7 of the algorithm of the Fig. 6 .

A la mise sous tension du dispositif de codage, et plus particulièrement dans le cas d'une utilisation d'un ordinateur comme dispositif de codage, le processeur lit à partir de la mémoire morte de l'ordinateur ou d'un support d'informations tel qu'un disque compact CD-ROM, les instructions du programme correspondant aux étapes E1 à E7 de la Fig. 6 et les charge en mémoire vive RAM pour les exécuter.When the coding device is powered up, and more particularly in the case of using a computer as coding device, the processor reads from the read-only memory of the computer or an information medium such as a compact disc CD-ROM, the instructions of the program corresponding to steps E1 to E7 of the Fig. 6 and loads RAM RAM to run them.

A l'étape E1, à la réception de données audio à coder, le processeur détermine la bande passante du codeur coeur ou au moins une fréquence de coupure.In step E1, on receiving audio data to be coded, the processor determines the bandwidth of the core encoder or at least one cutoff frequency.

Il est à remarquer que la bande passante du codeur coeur peut ou pas être variable dans le temps en fonction par exemple de la charge du codeur coeur.It should be noted that the bandwidth of the core coder may or may not be variable in time depending for example on the load of the core coder.

A cette même étape, le processeur code les données selon un algorithme dit de codage coeur conforme à l'un des standards MPEG1, MPEG2 ou MPEG4-GA, ou de type CELP, de type hiérarchique, voire même de type MPEG4 paramétrique.At this same stage, the processor encodes the data according to an algorithm called core coding according to one of the MPEG1, MPEG2 or MPEG4-GA standards, or of the CELP type, of the hierarchical type, or even of the parametric MPEG4 type.

L'étape E2 consiste à vérifier si, et dans le cas d'un codage hiérarchique, toutes les couches ont été codées ou non.Step E2 consists in verifying whether, and in the case of a hierarchical coding, all the layers have been coded or not.

Dans la négative, et si le codage coeur est un codage hiérarchique, le processeur réitère l'étape E1 pour chaque couche du signal audio codé.In the negative, and if the core coding is a hierarchical coding, the processor repeats the step E1 for each layer of the coded audio signal.

Si la totalité des couches ont été codées, ou que le codage n'est pas un codage hiérarchique, l'algorithme passe à l'étape suivante E3.If all the layers have been coded, or the coding is not a hierarchical coding, the algorithm proceeds to the next step E3.

A l'étape E3, le processeur détermine une marge de fréquence. Cette marge peut être prédéterminée et mémorisée dans un registre ou être sous forme d'une variable.In step E3, the processor determines a frequency margin. This margin can be predetermined and stored in a register or be in the form of a variable.

Cette variable dépend par exemple du type de correction d'erreurs que l'on va appliquer aux données codées lors de leur transmission sur le réseau.This variable depends, for example, on the type of error correction that will be applied to the coded data during transmission on the network.

Cette marge déterminée, le processeur détermine à l'étape E4, à partir de la marge et de la fréquence de coupure haute du codeur coeur, la fréquence de coupure basse du codeur d'extension.This determined margin, the processor determines in step E4, from the margin and the high cutoff frequency of the core encoder, the low cutoff frequency of the extension encoder.

Cette opération réalisée, le processeur transfert cette information au sous-programme de codage d'extension à l'étape E5.When this is done, the processor transfers this information to the extension coding routine at step E5.

Enfin, selon un mode particulier de l'invention, à l'étape E6, le processeur mémorise cette information.Finally, according to a particular embodiment of the invention, in step E6, the processor stores this information.

Le processeur, à l'étape E7, exécute le codage d'extension en codant les données dont le spectre est supérieur à l'information transférée à l'étape E5. Le codage d'extension de bande est par exemple un codage de type HFR (High Frequency Regeneration), par exemple SBR (Spectral Band Replication) tel que décrit dans le document « Audio Engineering Society, convention paper 5553 », présenté à la 112ieme convention AES par Mr Martin Dietz .The processor, in step E7, executes the extension coding by coding the data whose spectrum is greater than the information transferred to the step E5. The band extension coding is for example a High Frequency Regeneration (HFR) type coding, for example SBR (Spectral Band Replication) as described in the document. "Audio Engineering Society, convention paper 5553", presented at the 112th AES Convention by Mr Martin Dietz .

Cette opération effectuée, le processeur passe à l'étape E7 qui consiste à multiplexer les signaux audio codés à l'étape E1 et les signaux audio codés à l'étape E7 pour former un flot de données codées et transmises sur un réseau.With this operation performed, the processor proceeds to step E7 which consists of multiplexing the audio signals encoded in step E1 and the audio signals encoded in step E7 to form a stream of coded data transmitted over a network.

Selon une variante de l'invention, le processeur insère dans le flot de données codées et transmises, l'information mémorisée à l'étape E6 ou insère une ou plusieurs des informations suivantes : bande passante du codeur coeur, bande passante du codeur d'extension, fréquence basse et haute de chaque couche de codage, nombre de couches de codage si un codeur hiérarchique est utilisé.According to a variant of the invention, the processor inserts in the coded and transmitted data stream, the information stored in step E6 or inserts one or more of the following information: heart coder bandwidth, coder bandwidth extension, low and high frequency of each coding layer, number of coding layers if a hierarchical coder is used.

L'insertion est effectuée dans le cas d'un codeur hiérarchique pour chaque couche de codage.Insertion is performed in the case of a hierarchical coder for each coding layer.

Ces opérations effectuées, le processeur retourne à l'étape E1 en attente de nouvelles données audio à coder.When these operations are performed, the processor returns to step E1 waiting for new audio data to be coded.

La Fig. 7 représente l'algorithme effectué selon l'invention au niveau du décodeur.The Fig. 7 represents the algorithm performed according to the invention at the level of the decoder.

L'invention telle que décrite en référence aux Figs. précédentes est aussi réalisable sous forme logicielle dans laquelle un processeur exécute le code associé aux étapes E10 à E15 de l'algorithme de la Fig. 7.The invention as described with reference to FIGS. preceding is also feasible in software form in which a processor executes the code associated with the steps E10 to E15 of the algorithm of the Fig. 7 .

A la mise sous tension du dispositif de réception, et plus particulièrement dans le cas d'une utilisation d'un ordinateur comme dispositif de réception, le processeur lit à partir de la mémoire morte de l'ordinateur ou d'un support d'informations tel qu'un disque compact CD-ROM, les instructions du programme correspondant aux étapes E10 à E15 de la Fig. 7 et les charge en mémoire vive RAM pour les exécuter.When the reception device is powered up, and more particularly in the case of using a computer as a receiving device, the processor reads from the read-only memory of the computer or of an information medium such as a compact disc CD-ROM, the instructions of the program corresponding to steps E10 to E15 of the Fig. 7 and loads RAM RAM to run them.

A l'étape E10, le processeur, à la réception de données audio à décoder, sépare les signaux reçus par l'intermédiaire du réseau 405 en données destinées au décodeur coeur et en données destinées au décodeur d'extension. Il extrait en outre des signaux reçus, les informations représentatives de la bande passante ou d'au moins une fréquence de coupure du codeur coeur ayant codé le signal audio, ou des codeurs ayant codé le signal audio si le signal a été codé avec un codeur hiérarchique, voire même la fréquence de coupure basse du codeur d'extension ayant codé le signal audio si celles- ci ont été incluses dans les données transmises.In step E10, the processor, on receiving audio data to be decoded, separates the signals received via the network 405 into data intended for the heart decoder and data intended for the extension decoder. It also extracts signals received, information representative of the bandwidth or at least one cut-off frequency of the core encoder encoding the audio signal, or encoders encoding the audio signal if the signal has been coded with an encoder hierarchy, or even the low cut-off frequency of the extension encoder encoding the audio signal if they have been included in the transmitted data.

Cette opération effectuée, le processeur passe à l'étape E7. Le processeur procède alors au décodage de ces données.Once this is done, the processor proceeds to step E7. The processor then proceeds to decode this data.

Le processeur procède au décodage des données selon un algorithme de décodage dit de décodage coeur tel que conforme à l'un des standards MPEG1, MPEG2 ou MPEG4-GA, ou de type CELP, un décodage hiérarchique, voire même un décodage de type MPEG4 paramétrique.The processor proceeds to decode the data according to a decoding algorithm called core decoding as conforming to one of the MPEG1, MPEG2 or MPEG4-GA standards, or of the CELP type, a hierarchical decoding, or even a parametric MPEG4 type decoding. .

Cette étape de décodage coeur effectuée, le processeur passe à l'étape E12 qui est une étape d'obtention d'informations représentatives d'au moins une fréquence de coupure qui estime, selon un premier mode de réalisation, le spectre en fréquence du signal reçu par celui-ci. Ceci est réalisé par exemple en effectuant une transformation temps fréquence sur le signal décodé à l'étape E11 et en déterminant la fréquence à partir de laquelle l'énergie du signal devient négligeable. Préférablement, ceci peut s'effectuer avec l'assistance d'un modèle de perception.This core decoding step performed, the processor goes to step E12 which is a step of obtaining information representative of at least one cutoff frequency which estimates, according to a first embodiment, the frequency spectrum of the signal received by this one. This is done for example by performing a time-frequency transformation on the decoded signal in step E11 and determining the frequency from which the signal energy becomes negligible. Preferably, this can be done with the assistance of a perception model.

Selon un autre mode de réalisation, le processeur obtient les informations extraites à l'étape E1 et vérifie dans le cas où celui-ci est un décodeur hiérarchique, si chaque couche est bien reçue et dans la négative transfère une information représentative de la bande passante d'une ou des couches perdues au décodeur d'extension.According to another embodiment, the processor obtains the information extracted in step E1 and verifies, in the case where the latter is a hierarchical decoder, whether each layer is well received and, if not, transfers information representative of the bandwidth. from one or more layers lost to the extension decoder.

Cette opération effectuée, l'étape E13 consiste en une adaptation de la fréquence de coupure basse du décodeur d'extension de manière à ce que celui-ci compense les pertes dues au réseau. L'adaptation est effectuée à partir de l'information représentative de la fréquence de coupure ou de la bande passante obtenue à l'étape E12 ou si le décodage de l'étape E11 est un décodage hiérarchique de l'information représentative de la bande passante ou d'une fréquence de coupure d'une ou des couches perdues.This operation carried out, the step E13 consists of an adaptation of the low cutoff frequency of the expansion decoder so that it compensates the losses due to the network. The adaptation is performed from the information representative of the cutoff frequency or the bandwidth obtained in step E12 or if the decoding of step E11 is a hierarchical decoding of the information representative of the bandwidth or a cutoff frequency of one or more lost layers.

Cette opération effectuée, le processeur passe à l'étape E14 et décode selon un algorithme de décodage dit d'extension les données correspondant aux fréquences supérieures à cette fréquence de coupure basse précédemment déterminée.This operation carried out, the processor goes to step E14 and decodes according to a so-called extension decoding algorithm the data corresponding to the frequencies higher than this previously determined low cutoff frequency.

Le processeur sélectionne, à partir de la fréquence adaptée, parmi les données séparées à l'étape E1 et destinées au décodage d'extension, les données correspondant à l'enveloppe du signal correspondant à une représentation de l'enveloppe spectrale des fréquences supérieures à la fréquence la plus basse correspondant aux bandes de fréquences perdues.The processor selects, from the adapted frequency, among the data separated in step E1 and intended for the extension decoding, the data corresponding to the envelope of the signal corresponding to a representation of the spectral envelope of the frequencies greater than the lowest frequency corresponding to the lost frequency bands.

Ainsi, le décodage d'extension corrige les pertes dues au réseau que ce soit sur des pertes affectant les dernières couches reçues ou des pertes affectant une couche intermédiaire.Thus, the extension decoding corrects the losses due to the network either on losses affecting the last layers received or losses affecting an intermediate layer.

Le décodage d'extension est un algorithme de décodage d'extension de bande par exemple un décodage de type HFR (High Frequency Regeneration), par exemple un décodage de type SBR (Spectral Band Replication) tel que décrit dans le document « Audio Engineering Society, convention paper 5553 », présenté à la 112ieme convention AES par Mr Martin Dietz .The extension decoding is a band extension decoding algorithm, for example a High Frequency Regeneration (HFR) type decoding, for example a Spectral Band Replication (SBR) type decoding as described in the document. "Audio Engineering Society, convention paper 5553", presented at the 112th AES Convention by Mr Martin Dietz .

Enfin, les données décodées par le décodeur coeur et le décodeur d'extension sont additionnées pour former le signal audio décodé à l'étape E15.Finally, the data decoded by the heart decoder and the extension decoder are summed to form the audio signal decoded in step E15.

Ces opérations effectuées, le processeur retourne à l'étape E10 dans l'attente de nouvelles données audio à décoder.When these operations are performed, the processor returns to step E10 while waiting for new audio data to be decoded.

Claims (18)

  1. Method for coding an audio signal, in which a part of the frequency spectrum of the audio signal is coded with a spectral band limitation coder termed a core coder and in which the complementary part of the frequency spectrum of the audio signal is coded with a band extension coder, distinct from the core coder,
    characterized in that at least one spectrum part coded with the core coder is also coded with the band extension coder and in that the method comprises the steps of:
    - determining at least one cutoff frequency of the core coder with the aid of a frequency adjustment module taking account of the loading of the core coder,
    - determining parts of the spectrum that are coded with the core coder and the band extension coder respectively, on the basis of the cutoff frequency determined with the aid of the said adjustment module,
    the said parts overlapping in the vicinity of the said cutoff frequency, in such a way as to alleviate any loss of data during the transmission of the said spectrum part coded by the core coder,
    the said step of determining at least one cutoff frequency of the core coder by a frequency adjustment module comprising the steps of:
    - determining a frequency margin, it being possible for the said margin to be predetermined or stored in a register in the form of a variable,
    - on the basis of the said frequency margin, determining the high cutoff frequency of the core coder and the low cutoff frequency of the band extension coder delivering an item of information representative of the said low cutoff frequency of the band extension coder,
    - transferring the said item of information to the band extension coder,
    - storing the said item of information.
  2. Method according to Claim 1, characterized in that the method comprises a step of transferring the coded digital signal over a network and in that the or each frequency determined is transferred with the coded digital signal.
  3. Method according to Claim 1, characterized in that the core coder is a hierarchical coder and for each coding layer, at least one cutoff frequency of each coding layer is determined by the said frequency adjustment module.
  4. Method according to Claim 3, characterized in that the method comprises a step of transferring each coding layer of the coded digital signal over a network and in that the or each frequency determined by the said frequency adjustment module for the layer is transferred with the said layer.
  5. Method according to any one of Claims 1 to 4, characterized in that the part of the frequency spectrum of the audio signal that is coded with the core coder is the low part of the frequency spectrum of the audio signal.
  6. Method for the spectral reconstruction of an audio signal coded in the form of data, comprising:
    - data corresponding to low frequency components of the frequency spectrum of the audio signal, which are intended to be decoded with a spectral band limitation decoder termed a core decoder;
    - data, corresponding to a second part of the frequency spectrum of the audio signal, comprising high frequency components, which are intended to be decoded with a band extension decoder, distinct from the core decoder,
    characterized in that the said data intended for the said band extension decoder correspond not only to the said high frequency components, but also to a part of the said low frequency components of the frequency spectrum of the said audio signal which were coded by the core coder,
    and in that the method comprises the following steps:
    - estimating at least one high cutoff frequency of the signal decoded by the core decoder;
    - adapting a low cutoff frequency of the band extension decoder on the basis of the said at least one high cutoff frequency obtained; and
    - decoding by the band extension decoder of extension data corresponding to the frequencies above the said adapted low cutoff frequency.
  7. Method according to Claim 6, characterized in that the information representative of at least one cutoff frequency of the signal decoded by the core decoder is obtained on the basis of information included in the flow of data comprising the coded digital signal.
  8. Method according to Claim 6 or 7, characterized in that the core decoder is a hierarchical decoder and in that the method obtains information representative of the passband of the signal decoded by the core decoder for each layer of the decoded signal.
  9. Device for coding an audio signal, in which a part of the frequency spectrum of the audio signal is coded with a spectral band limitation coder termed a core coder and in which the complementary part of the frequency spectrum of the audio signal is coded with a band extension coder, distinct from the core coder, and in which at least one spectrum part coded with the core coder is also coded with the band extension coder and
    characterized in that it comprises:
    - a frequency adjustment module taking account of the loading of the core coder and determining at least one cutoff frequency of the core coder,
    - means for determining parts of the spectrum that are coded with the core coder and the band extension coder respectively, on the basis of the cutoff frequency determined with the aid of the said adjustment module,
    the said parts overlapping in the vicinity of the said cutoff frequency, in such a way as to alleviate any loss of data during the transmission of the said spectrum part coded by the core coder,
    - means for coding at least one spectrum part coded with the core coder with the band extension coder,
    the said coding device furthermore comprising:
    - means for determining a frequency margin, it being possible for the said margin to be predetermined or stored in a register in the form of a variable,
    - means for determining the high cutoff frequency of the core coder and the low cutoff frequency of the band extension coder on the basis of the said frequency margin, delivering an item of information representative of the said low cutoff frequency of the band extension coder,
    - means for transferring the said item of information to the said band extension coder,
    - means for storing the said item of information.
  10. Device according to Claim 9, characterized in that the device comprises means for transferring the coded digital signal over a network and in that the or each frequency determined is transferred with the coded digital signal.
  11. Device according to Claim 9, characterized in that the core coder is a hierarchical coder and for each coding layer, at least one cutoff frequency of each coding layer is determined by the said frequency adjustment module.
  12. Device according to Claim 11, characterized in that the device comprises means for transferring each layer of the coded digital signal over a network and in that the or each frequency determined by the said frequency adjustment module for the coding layer is transferred with the said coding layer.
  13. Device according to any one of Claims 9 to 12, characterized in that the part of the frequency spectrum of the audio signal that is coded with the core coder is the low part of the frequency spectrum of the audio signal.
  14. Device for the spectral reconstruction of an audio signal coded in the form of data, comprising:
    - a spectral band limitation decoder termed a core decoder able to decode data corresponding to high frequency components of the frequency spectrum of the audio signal,
    - a band extension decoder, distinct from the core decoder, able to decode data, corresponding to a second part of the frequency spectrum of the audio signal, comprising high frequency components,
    characterized in that the said data intended for the said band extension decoder correspond not only to the said high frequency components, but also to a part of the said low frequency components of the frequency spectrum of the said audio signal which were coded by the core coder,
    and in that the device comprises:
    - means for estimating at least one high cutoff frequency of the signal decoded by the core decoder;
    - means for adapting a low cutoff frequency of the band extension decoder on the basis of the said at least one high cutoff frequency obtained; and
    - means for decoding of the said band extension decoder, which are able to decode extension data corresponding to the frequencies above the adapted low cutoff frequency.
  15. Device according to Claim 14, characterized in that the information representative of at least one cutoff frequency of the signal decoded by the core decoder is obtained on the basis of information included in the flow of data comprising the coded digital signal.
  16. Device according to Claim 15, characterized in that the core decoder is a hierarchical decoder and in that the device will obtain information representative of at least one cutoff frequency of the signal decoded by the core decoder for each layer of the decoded signal.
  17. Computer program stored on an information medium, the said program comprising instructions making it possible to implement the coding method according to any one of Claims 1 to 5, when it is loaded and executed by a computer-based system.
  18. Computer program stored on an information medium, the said program comprising instructions making it possible to implement the method for reconstructing an audio signal according to any one of Claims 6 to 8, when it is loaded and executed by a computer-based system.
EP04716626A 2003-03-04 2004-03-03 Method and device for the spectral reconstruction of an audio signal Expired - Lifetime EP1599868B1 (en)

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FR0302730A FR2852172A1 (en) 2003-03-04 2003-03-04 Audio signal coding method, involves coding one part of audio signal frequency spectrum with core coder and another part with extension coder, where part of spectrum is coded with both core coder and extension coder
FR0302730 2003-03-04
PCT/FR2004/000488 WO2004081918A1 (en) 2003-03-04 2004-03-03 Method and device for the spectral reconstruction of an audio signal

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