WO2002069326A1 - Multichannel signal spectral reconstruction method and device - Google Patents

Multichannel signal spectral reconstruction method and device Download PDF

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Publication number
WO2002069326A1
WO2002069326A1 PCT/FR2002/000619 FR0200619W WO02069326A1 WO 2002069326 A1 WO2002069326 A1 WO 2002069326A1 FR 0200619 W FR0200619 W FR 0200619W WO 02069326 A1 WO02069326 A1 WO 02069326A1
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Prior art keywords
spectral
band
channel
signal
component
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PCT/FR2002/000619
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French (fr)
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Pierrick Philippe
Patrice Collen
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France Telecom
Telediffusion De France Sa
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Priority to EP02706874A priority Critical patent/EP1362344A1/en
Publication of WO2002069326A1 publication Critical patent/WO2002069326A1/en

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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
    • G10L19/04—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
    • G10L19/06—Determination or coding of the spectral characteristics, e.g. of the short-term prediction coefficients

Definitions

  • the present invention relates to a method and a device for spectral reconstruction of a multi-channel audio signal, in particular of a stereophonic signal.
  • the invention also relates to a decoding device comprising this reconstruction device, an associated coding device and a coding / decoding system.
  • bit rate we mean the amount of information transmitted per unit of time, generally expressed in kbit / s.
  • Known rate reduction coders are for example transform type coders, CELP type coders and even parametric type coders, such as a parametric MPEG4 type coder.
  • the audio signal In bit rate reduction audio coding, the audio signal often has to undergo bandwidth limitation when the bit rate becomes low. This limitation of bandwidth is necessary to avoid the introduction of audible quantization noise into the encoded signal. It is therefore desirable to regenerate as far as possible the high frequency content of the original signal.
  • This technique is based on an analysis in sub-bands and a complex harmonic duplication. It implements phase and amplitude adjustment methods which are costly in calculation. In addition, the spectral weighting factors only roughly model the spectral envelope.
  • the stereophonic content is generally greatly altered. Indeed, if the transmission rate is insufficient, there is a tendency to transmit only signals with low stereophonic content.
  • stereo M / S stereo mid-side
  • stereo intensity in which, above a certain frequency, a monophonic audio signal is transmitted, generally corresponding to. a weighting of the left and right channels, with gain factors which describe the original energy ratios between the two channels.
  • the selective application by frequency bands of different gains for each audio channel makes it possible to recreate an impression of stereophonic signal.
  • the stereo part at low frequency may be of poor quality. If the frequency limit beyond which the stereo intensity technique is applied is lowered, the stereophonic content is degraded because the use of gain factors allows only a rough reconstruction of the stereo content. The cost of transmitting the gain factors becomes significant if a more detailed reconstruction of the stereo content is desired.
  • the application of different gains by sub-bands tends to create discontinuities.
  • the problem underlying the invention is to provide a method and a device for reconstructing a stereo signal, and more generally of a multi-channel audio signal, allowing a reconstruction of the stereophonic content, in particular for the high frequencies, and requiring only a small amount of data to transmit.
  • spatial (Vj L ) characterized in that, for at least one component, it comprises:
  • the composite signal comprises at least one monophonic component (M) in a first spectral band (Bi), the spectral whitening step provides a whitened monophonic signal and the shaping step uses a filter envelope having the characteristic spectral envelope of the channel to be reconstructed in said first band.
  • the composite signal comprises several spatial components (Ni L ), each spatial component being associated with a channel and at least one spatial component having a spectrum limited to a second spectral band, the reconstruction of the channel associated with the limited spectrum space component comprising:
  • the second spectral band (B) is a low frequency band and the third spectral band (B 3 ) is adjacent to the second.
  • the components of the composite signal come from the decoding of a multi-channel source signal coded by a spectrum limiting coder.
  • the characteristic of the envelope filter is obtained from information giving the spectral envelope of the corresponding channel of the source signal in the first and third bands.
  • the invention is also defined by a device for reconstructing at least one channel of a multi-channel audio signal, in particular of a stereo signal, from a composite signal which may include monophonic components and spatial components , the device comprising means for implementing the steps of the method defined above.
  • the invention is also defined by a device for coding a source audio signal with several channels, at least a first spectral band of said signal being coded in monophonic, the device further providing spectral envelope information for at least one channel. in said first band.
  • the invention is also defined by a device for limiting the spectrum of a source audio signal with several channels, the spectrum of at least one channel being limited to a second spectral band by coding, the device further providing a spectral envelope information of said channel in a band distinct from said second band.
  • the invention is also defined by a spectrum limitation coding device for a source audio signal with several channels, at least one first spectral band of said signal being coded in monophonic, the spectrum of at least one channel being limited by the coding at a second spectral band, distinct from the first, the device further providing information on the spectral envelope of said channel in the first band and in a third band distinct from said first and second bands.
  • the spectral envelope information of the second channel is transmitted in the form of a difference with that of the first channel.
  • the invention is also defined by a signal coming from a spectrum limitation coding device as defined above, the signal comprising at least for a first spectral band a coded monophonic component as well as a spectral envelope information. coded relative to the spectral envelope of an audio channel in said first band.
  • the invention is also defined by a signal from a coding device with spectrum limitation as defined above, the signal comprising at least for a second spectral band a coded spatial component relating to an audio channel as well as a coded spectral envelope information relating to the spectral envelope of said audio channel in a band distinct from said second band.
  • the invention is also defined by a signal originating from a spectrum limitation coding device as defined above, the signal comprising at least, for a first spectral band, a monophonic component and, for a second distinct spectral band of the first, a coded spatial component relating to an audio channel, as well as information of spectral envelope of said channel in the first band and in a third spectral band distinct from said first and second bands.
  • the invention is also defined by a device for decoding a coded multi-channel audio signal, the device comprising a decoder adapted to supply from the coded signal a composite signal which may include monophonic components and spatial components, and a device reconstruction as defined above.
  • the invention is also defined by a decoding device comprising a first decoder adapted to supply, from said signal defined above, a composite signal capable of comprising monophonic components and spatial components as well as a second decoder adapted to supply, to from said signal, spectral envelope information.
  • the invention is further defined by a system for coding / decoding an audio signal with several channels, comprising a coding device and a decoding device as defined above.
  • the stereophonic content in particular the high frequency stereophonic content can be reconstituted during decoding without or with a minimum transmission of the information linked to the high frequency content of the original signals.
  • the spectral shape of high frequency stereophonic signals can be modeled through two filters, one filter for each channel.
  • Envelope information can be transmitted at low cost, because one can easily measure the differences between two envelopes and thereby take advantage of possible redundancies between the spectral shapes modeled. Only one channel can be transmitted, the other channel can be reconstructed by bleaching the transmitted signal and applying an envelope filter.
  • the envelope information relating to the non-transmitted channel has a very low transmission cost.
  • FIG. . 1 schematically represents a device for reconstructing a stereophonic audio signal according to a first embodiment of the invention
  • Figs. 2a to 2c illustrate the processing carried out by the reconstruction device of FIG. 1
  • Fig. 3 schematically represents a device for reconstructing a stereophonic audio signal according to a second embodiment of the invention
  • Figs. 4a to 4d illustrate the processing carried out by the reconstruction device of FIG. 3
  • Fig. 5 schematically represents a device for reconstructing a stereophonic audio signal according to a third embodiment of the invention
  • Figs. 6a to 6d illustrate the processing carried out by the reconstruction device of FIG. 5.
  • the spectral reconstruction device can be applied to the spectral reconstruction of a stereophonic audio signal resulting from the decoding of a signal coded by an encoder with spectral band limitation. It can be any type of flow reduction encoder.
  • the encoder can be of the transform type (MPEG1, MPEG2 or MPEG4-GA), of the CELP type (ITU G72X), or even of the parametric type (parametric MPEG4).
  • the invention can also be applied to signals which have not previously been the subject of coding, for example, signals which have simply undergone subsampling and an alteration of their stereophonic content.
  • Fig. 1 describes a first embodiment of the invention.
  • the signal is coded by an encoder 100 and, after transmission of the coded signal by any means, the coded signal is decoded by a decoder 110.
  • the reconstruction of the stereophonic content of an audio signal and more generally of the different channels of a multi-channel audio signal is carried out by the modules 150, 155, 150, 115, 175 and the summers 180 ,. For reasons of simplification, only one channel i has been shown
  • the signal decoded by the module 110 has a monophonic component (M) and a spatial component N, L , associated with the channel i to be reconstructed, as illustrated in FIG. 2a.
  • the monophonic component can be a component common to several channels, for example a sum of several channels or even the signal of a predominant channel among a set of channels.
  • the component N, L will be a low frequency signal with limited band (B 2 ) and the monophonic signal (M) will occupy a band (Bl) adjacent to the first.
  • the spectrum of the monophonic part (M) is whitened using a whitening filter 150. It is known that under certain stationary hypotheses, a signal can be modeled as the result of the filtering of an excitation signal by a spectral envelope filter. If a description of the spectral envelope of the signal is available, it is possible to whiten its spectrum by passing it through a whitening filter with transfer function (approximately) inverse to the envelope function. An approximation of the initial excitation signal is thus obtained, free of the influence of the spectral shape in the band considered.
  • the module 155 is a spectral envelope estimation module for the monophonic signal in the Bi band. It can for example model the envelopes by an LPC analysis, as described in the article by J. Makhoul, entitled “Linear Prediction: a tutorial review”, Proceedings of the IEEE, Vol. 63, ⁇ ° 4, pp 561-580.
  • the spatial component N, and the bleached monophonic component are represented in FIG. 2b.
  • the bleached monophonic component is subjected to a spectral envelope step in the envelope filter 170,.
  • This envelope filter has for characteristic the spectral envelope of the original channel i in the band Bi.
  • spectral estimation means 105 associated with the coder performs a spectral estimation of the different channels in the band Bi. and provide information describing the envelopes of the different channels in this band.
  • the envelopes are coded differently. In other words, the envelope of a first channel is coded and those of the other channels are coded by difference, so as to take advantage of the similarity of the envelopes to reduce the redundancy in the information to be transmitted.
  • the information relating to the various envelopes is decoded in the module 175.
  • the decoded information e (V,) are for example LPC coefficients. They are supplied to the envelope filter 170 ,.
  • the spectral envelope of the channel i is obtained as the extrapolated, in the band Bi, of the spectral envelope of the spatial component N, L , in the band B 2 .
  • This variant is symbolized in broken lines by the extrapolation module 115, receiving the component V, L and supplying the extrapolated envelope to the envelope filter 170,.
  • the spatial component N, L is then added by means of the summator 180, to the monophonic component shaped to provide a reconstructed channel N ,.
  • the spectrum of the reconstructed pathway is illustrated in Fig. 2c.
  • This monophonic component can correspond to a channel or to a sum of channels, as seen above.
  • the different channels are reconstructed thanks to the shaping of the monophonic signal whitened by their respective spectral envelopes.
  • Fig. 3 describes a second embodiment of the invention.
  • the modules bearing the same references as in FIG. 1 have a function identical to that already described. For reasons of simplification, only the reconstruction of a channel i has been shown.
  • the decoder 110 supplies spatial components with limited spectra. This will typically be the case if the encoder 100 is a spectrum limiting encoder.
  • a spatial component N of spectrum limited to the band B 2 , as shown in FIG. 4a.
  • the module 160 is a spectral transposition module. Its function is to copy the spectral content of at least part of the band B 2 , called the source band, into a second band B 3 , called the target band.
  • the transposition operation is for example a simple translation of spectrum in the target band or else the combination of a reversal and a translation.
  • band B is a low frequency band and the target band is adjacent to the latter.
  • the spectral transposition operation has been illustrated in FIG. 4b.
  • the signal obtained at the output of 160 is a signal with spectrum limited to band B. It is subjected to spectral whitening in the whitening filter 150,.
  • the characteristic of the whitening filter is the inverse of the spectral envelope of the spatial component transposed in the band B 3 .
  • Module 1 15 estimates the coefficients of the whitening filter and supplies them to the latter.
  • the coefficients of the filter 150 are obtained from the spectral envelope of the channel i in the source band. It should be noted that the order of the spectral bleaching (150,) and spectral transposition (160,) modules can be reversed. The order chosen depends in particular on the desired whitening precision. The result of spectral bleaching is illustrated in Fig. 4c.
  • the spatial component with transposed and whitened spectrum is subjected to a spectral shaping step in the envelope filter 170,.
  • the characteristic of this envelope filter is the spectral envelope of the original channel i in band B 3 .
  • the information relating to the various envelopes is decoded in module 175.
  • the decoded information e (N,) are for example LPC coefficients. They are supplied to the envelope filter 170 ,.
  • the spectral envelope of channel i is obtained as the extrapolated, in band B 3 , of the spectral envelope of the spatial component N,, in band B.
  • This variant is symbolized by the link in broken lines between the module 115, and the envelope filter 170,.
  • the spatial component N, L is then added by means of the summator 181, to the signal from the filter 170, to provide a reconstructed channel N ,.
  • the spectrum of the reconstructed pathway is illustrated in Fig. 4d.
  • Fig. 5 describes a third embodiment of the invention.
  • the modules bearing the same references as those in FIG. 1 or of FIG. 3 have a function identical to that already described. For reasons of simplification, only the reconstruction of a channel i has been shown.
  • the decoder 110 supplies a monophonic component M as well as spatial components with limited spectra N,.
  • the monophonic component can be common to several or to all channels. Illustrated in FIG. 6 has a spatial component with limited spectrum N, L as well as the monophonic component M.
  • the spatial component (for example relating to a channel of a stereo signal) of the signal occupies a low frequency band. In the higher frequencies (spectral band Bl) the signal is coded in monophonic.
  • the monophonic component M at the output of the decoder 110, the monophonic component M, on the one hand, and the spatial component V, on the other hand, undergo separate processing.
  • the monophonic component is whitened by means of the whitening filter 150 as in FIG. 1.
  • the spatial component for its part, is the subject of a transposition of all or part of its spectral content in a target band B 3 , typically contiguous to the monophonic band B 15 as shown in FIG. 6b.
  • the source band of the spectral content is included in the band B 2 .
  • the source band is included in the band B 2 + B ⁇ , in other words at least part of the spectral content of the monophonic band can also be transposed.
  • the signal at the output of 160 has a spectrum limited to the band B 3 . It is bleached in the whitening filter 150, the transfer function of which is determined from the spectral estimation module 115, or, alternatively, by spectral envelope information of the original channel i in the source band given by the module. 175 decoding function.
  • the transfer function of the filter 150 is given by the inverse of the spectral envelope in the source band.
  • Fig. 6c illustrates the result of the laundering operations in bands B] and B 2 .
  • the whitened monophonic and spatial components are combined in the summator 180, and the sum is filtered by the envelope filter 170 ,.
  • the transfer function of this filter is given by spectral envelope information of the original channel i, in the band B1. + B 3 , provided by the envelope decoding module 175. As indicated more top, the envelopes of the different channels can be coded at 105 as differences.
  • the transfer function of the filter casing 170 is extrapolated in the B band. 1 + B 3 , of the spectral envelope of the component N, L.
  • the modules 105 and 175 are not necessary.
  • the spatial component N is combined with the signal from 170, by means of the summator 181, to provide a reconstructed channel N, the spectrum of which is shown in FIG. 6d.
  • the envelope filters can be applied in the time domain or in the frequency domain.
  • the device according to the invention has been shown in the form of functional modules, it goes without saying that all or part of this device can be produced by means of a single processor or a plurality of dedicated or non-dedicated processors.

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Abstract

The invention relates to a method for reconstructing at least one channel of a multichannel audio signal, especially a stereo signal, from a composite signal which can comprise monophonic components and spatial components. The inventive method comprises, for at least one component, a spectral whitening stage of said component in order to obtain a white audio signal, and a white signal spectral forming step involving an envelope filter having as a characteristic at least one part of the spectral envelope of the channel to be reconstructed. The invention also relates to a device for reconstructing a multichannel signal for the implementation of the inventive mentioned method; a decoding device including the above, an associated coding device, a coding /decoding device including the above and a coded signal.

Description

PROCEDE ET DISPOSITIF DE RECONSTRUCTION SPECTRALE DE SIGNAUX A PLUSIEURS VOIESMETHOD AND DEVICE FOR SPECTRAL RECONSTRUCTION OF MULTI-CHANNEL SIGNALS
La présente invention concerne un procédé et un dispositif de reconstruction spectrale d'un signal audio à plusieurs voies, notamment d'un signal stéréophonique.The present invention relates to a method and a device for spectral reconstruction of a multi-channel audio signal, in particular of a stereophonic signal.
L'invention concerne également un dispositif de décodage comprenant ce dispositif de reconstruction, un dispositif de codage associé et un système de codage/décodage lesThe invention also relates to a decoding device comprising this reconstruction device, an associated coding device and a coding / decoding system.
5 incluant.5 including.
Dans l'état de la technique de la transmission de signaux audio, il est connu de coder le signal à l'émission et de le décoder à la réception. Ce codage peut être un codage à réduction de débit. Par débit, on entend la quantité d'information transmise par unité de temps, généralement exprimée en kbits/s. Dans ce qui suit, on désigneraIn the state of the art of transmitting audio signals, it is known to code the signal on transmission and to decode it on reception. This coding can be a bit rate reduction coding. By bit rate, we mean the amount of information transmitted per unit of time, generally expressed in kbit / s. In what follows, we will designate
10 l'importance de la transmission d'une information en terme de débit par l'expression « coût en transmission », exprimée en kbits/s. Des codeurs à réduction de débit connus sont par exemple les codeurs de type par transformée, les codeurs de type CELP et même des codeurs de type paramétrique, comme un codeur de type MPEG4 paramétrique.10 the importance of the transmission of information in terms of bit rate by the expression “cost in transmission”, expressed in kbit / s. Known rate reduction coders are for example transform type coders, CELP type coders and even parametric type coders, such as a parametric MPEG4 type coder.
15 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 régénérer dans la mesure du possible le contenu haute fréquence du signal original.15 In bit rate reduction audio coding, the audio signal often has to undergo bandwidth limitation when the bit rate becomes low. This limitation of bandwidth is necessary to avoid the introduction of audible quantization noise into the encoded signal. It is therefore desirable to regenerate as far as possible the high frequency content of the original signal.
Il est connu de l'état de la technique, et notamment du document WO-A- 9857436, le fait de régénérer le contenu spectral haute fréquence du signal original en procédant à une transposition harmonique du spectre basse fréquence du signal décodé vers les hautes fréquences. Cette transposition est effectuée en recopiant la valeur spectrale d'un fondamental à fk à toutes les fréquences de la série harmonique n*fk. La forme du spectre haute fréquence ainsi obtenue est ajustée en appliquant des facteurs de pondération spectrale.It is known from the state of the art, and in particular from document WO-A-9857436, the fact of regenerating the high frequency spectral content of the original signal by carrying out a harmonic transposition of the low frequency spectrum from the decoded signal to the high frequencies . This transposition is carried out by copying the spectral value of a fundamental at fk at all the frequencies of the harmonic series n * fk. The shape of the high frequency spectrum thus obtained is adjusted by applying spectral weighting factors.
Cette technique est fondée sur une analyse en sous-bandes et une duplication harmonique complexe. Elle met en œuvre des méthodes d'ajustement de phase et d'amplitude coûteuses en calcul. En outre, les facteurs de pondération spectrale ne modélisent que grossièrement l'enveloppe spectrale. Lorsque des signaux audio stéréophoniques sont transmis avec des procédés de codage audio à réduction de débit de l'état de la technique, le contenu stéréophonique est généralement fortement altéré. En effet, si le débit de transmission est insuffisant, on tend à ne transmettre que des signaux à faible contenu stéréophonique.This technique is based on an analysis in sub-bands and a complex harmonic duplication. It implements phase and amplitude adjustment methods which are costly in calculation. In addition, the spectral weighting factors only roughly model the spectral envelope. When stereophonic audio signals are transmitted with prior art bitrate reduction audio coding methods, the stereophonic content is generally greatly altered. Indeed, if the transmission rate is insufficient, there is a tendency to transmit only signals with low stereophonic content.
Il est connu du domaine du traitement de signaux stéréophoniques une technique appelée M/S stéréo (mid-side stéréo) selon laquelle sont transmis des signaux correspondant à la moitié respectivement de la somme et de la différence des voies gauche et droite. Au décodage, les voies gauche et droite sont reconstituées à partir de ces signaux. Lorsque les voies gauche et droite sont fortement corrélées, le terme de différence sera faible et pourra donc être transmis à faible coût. Cependant, à très faible débit, le terme de différence est quantifié à zéro. Au décodage, on obtient alors un signal monophonique.It is known in the field of stereophonic signal processing a technique called stereo M / S (stereo mid-side) according to which signals corresponding to half the sum and the difference of the left and right channels are transmitted respectively. On decoding, the left and right channels are reconstructed from these signals. When the left and right channels are strongly correlated, the difference term will be small and can therefore be transmitted at low cost. However, at very low bit rate, the difference term is quantified to zero. On decoding, a monophonic signal is then obtained.
Une autre technique connue dans le domaine du traitement de signaux stéréophoniques est la stéréo d'intensité dans laquelle, au dessus d'une certaine fréquence, on transmet un signal audio monophonique, correspondant généralement à. une pondération des voies gauche et droite, avec des facteurs de gain qui décrivent les rapports d'énergie originaux entre les deux voies. Au décodage, l'application sélective par bandes de fréquences de gains différents pour chaque voie audio permet de recréer une impression de signal stéréophonique. Cependant, là encore, à bas débit, la partie stéréo en basse fréquence risque d'être de mauvaise qualité. Si la limite fréquentielle au delà de laquelle on applique la technique de stéréo d'intensité est abaissée, le contenu stéréophonique est dégradé car l'utilisation de facteurs de gains ne permet qu'une reconstitution grossière du contenu stéréo. Le coût en transmission des facteurs de gain devient important si l'on souhaite une reconstitution plus fine du contenu stéréo. En outre, l'application de gains différents par sous-bandes tend à créer des discontinuités.Another technique known in the field of stereophonic signal processing is stereo intensity in which, above a certain frequency, a monophonic audio signal is transmitted, generally corresponding to. a weighting of the left and right channels, with gain factors which describe the original energy ratios between the two channels. When decoding, the selective application by frequency bands of different gains for each audio channel makes it possible to recreate an impression of stereophonic signal. However, here again, at low bit rate, the stereo part at low frequency may be of poor quality. If the frequency limit beyond which the stereo intensity technique is applied is lowered, the stereophonic content is degraded because the use of gain factors allows only a rough reconstruction of the stereo content. The cost of transmitting the gain factors becomes significant if a more detailed reconstruction of the stereo content is desired. In addition, the application of different gains by sub-bands tends to create discontinuities.
Le problème à la base de l'invention est de fournir un procédé et un dispositif de reconstruction d'un signal stéréo, et de manière plus générale d'un signal audio à plusieurs voies, permettant une reconstitution du contenu stéréophonique, en particulier pour les hautes fréquences, et ne nécessitant qu'une faible quantité de données à transmettre.The problem underlying the invention is to provide a method and a device for reconstructing a stereo signal, and more generally of a multi-channel audio signal, allowing a reconstruction of the stereophonic content, in particular for the high frequencies, and requiring only a small amount of data to transmit.
Ce problème est résolu par un procédé de reconstruction d'au moins une voie d'un signal audio à plusieurs voies, en particulier d'un signal stéréo, à partir d'un signal composite pouvant comporter des composantes monophoniques (M) et des composantes spatiales (VjL) caractérisé en ce que, pour au moins une composante, il comprend :This problem is solved by a method of reconstructing at least one channel of a multi-channel audio signal, in particular of a stereo signal, from a composite signal which may include monophonic components (M) and components. spatial (Vj L ) characterized in that, for at least one component, it comprises:
- une étape de blanchiment spectral de ladite composante pour obtenir un signal blanchi ;a step of spectral bleaching of said component to obtain a bleached signal;
- une étape de mise en forme du spectre du signal blanchi au moyen d'un filtre d'enveloppe ayant pour caractéristique au moins une partie de l'enveloppe spectralea step of shaping the spectrum of the whitened signal by means of an envelope filter having as characteristic at least part of the spectral envelope
(e(Nj)) de la voie à reconstruire.(e (Nj)) of the route to be reconstructed.
Selon un premier mode de réalisation, le signal composite comprend au moins une composante monophonique (M) dans une première bande spectrale (Bi), l'étape de blanchiment spectral fournit un signal monophonique blanchi et l'étape de mise en forme utilise un filtre d'enveloppe ayant pour caractéristique l'enveloppe spectrale de la voie à reconstruire dans ladite première bande.According to a first embodiment, the composite signal comprises at least one monophonic component (M) in a first spectral band (Bi), the spectral whitening step provides a whitened monophonic signal and the shaping step uses a filter envelope having the characteristic spectral envelope of the channel to be reconstructed in said first band.
Selon un second mode de réalisation, le signal composite comprend plusieurs composantes spatiales (NiL), chaque composante spatiale étant associée à une voie et au moins une composante spatiale ayant un spectre limité à une seconde bande spectrale, la reconstruction de la voie associée à la composante spatiale à spectre limité comprenant:According to a second embodiment, the composite signal comprises several spatial components (Ni L ), each spatial component being associated with a channel and at least one spatial component having a spectrum limited to a second spectral band, the reconstruction of the channel associated with the limited spectrum space component comprising:
- une étape de transposition de tout ou partie du contenu spectral de ladite composante spatiale dans une troisième bande spectrale distincte de ladite seconde bande spectrale pour fournir une composante à spectre transposé ; - une étape de blanchiment spectral avant ou après l'étape de transposition de sorte que la composante à spectre transposé est blanchie ;a step of transposing all or part of the spectral content of said spatial component into a third spectral band distinct from said second spectral band to provide a component with transposed spectrum; - a spectral bleaching step before or after the transposition step so that the component with transposed spectrum is bleached;
- une étape de mise en forme du spectre de la composante à spectre transposé et blanchi au moyen d'un filtre d'enveloppe ayant pour caractéristique l'enveloppe spectrale de la voie à reconstruire dans ladite troisième bande. Avantageusement, la seconde bande spectrale (B ) est une bande basse fréquence et la troisième bande spectrale (B3) est adjacente à seconde.a step of shaping the spectrum of the component with transposed and whitened spectrum by means of an envelope filter having as characteristic the spectral envelope of the channel to be reconstructed in said third band. Advantageously, the second spectral band (B) is a low frequency band and the third spectral band (B 3 ) is adjacent to the second.
Selon un troisième mode de réalisation, les composantes du signal composite sont issues du décodage d'un signal source à plusieurs voies codé par un codeur à limitation de spectre. Pour la reconstruction de la voie à reconstruire, la caractéristique du filtre d'enveloppe est obtenue à partir d'une information donnant l'enveloppe spectrale de la voie correspondante du signal source dans les première et troisième bandes.According to a third embodiment, the components of the composite signal come from the decoding of a multi-channel source signal coded by a spectrum limiting coder. For the reconstruction of the channel to be reconstructed, the characteristic of the envelope filter is obtained from information giving the spectral envelope of the corresponding channel of the source signal in the first and third bands.
L'invention est également définie par un dispositif de reconstruction d'au moins une voie d'un signal audio à plusieurs voies, en particulier d'un signal stéréo, à partir d'un signal composite pouvant comporter des composantes monophoniques et des composantes spatiales, le dispositif comprenant des moyens pour mettre en oeuvre les étapes du procédé défini ci-dessus.The invention is also defined by a device for reconstructing at least one channel of a multi-channel audio signal, in particular of a stereo signal, from a composite signal which may include monophonic components and spatial components , the device comprising means for implementing the steps of the method defined above.
L'invention est également définie par un dispositif de codage d'un signal audio source à plusieurs voies, au moins une première bande spectrale dudit signal étant codée en monophonique, le dispositif fournissant en outre une information d'enveloppe spectrale pour au moins une voie dans ladite première bande.The invention is also defined by a device for coding a source audio signal with several channels, at least a first spectral band of said signal being coded in monophonic, the device further providing spectral envelope information for at least one channel. in said first band.
L'invention est également définie par un dispositif de codage à limitation de spectre d'un signal audio source à plusieurs voies, le spectre d'au moins une voie étant limité à une seconde bande spectrale par le codage, le dispositif fournissant en outre une information d'enveloppe spectrale de ladite voie dans une bande distincte de ladite seconde bande.The invention is also defined by a device for limiting the spectrum of a source audio signal with several channels, the spectrum of at least one channel being limited to a second spectral band by coding, the device further providing a spectral envelope information of said channel in a band distinct from said second band.
L'invention est également définie par un dispositif de codage à limitation de spectre d'un signal audio source à plusieurs voies, au moins une première bande spectrale dudit signal étant codée en monophonique, le spectre d'au moins une voie étant limité par le codage à une seconde bande spectrale, distincte de la première, le dispositif fournissant en outre une information d'enveloppe spectrale de ladite voie dans la première bande et dans une troisième bande distincte desdites première et seconde bandes. Avantageusement, pour au moins une première voie et une seconde voie, l'information d'enveloppe spectrale de la seconde voie est transmise sous forme de différence avec celle de la première voie.The invention is also defined by a spectrum limitation coding device for a source audio signal with several channels, at least one first spectral band of said signal being coded in monophonic, the spectrum of at least one channel being limited by the coding at a second spectral band, distinct from the first, the device further providing information on the spectral envelope of said channel in the first band and in a third band distinct from said first and second bands. Advantageously, for at least a first channel and a second channel, the spectral envelope information of the second channel is transmitted in the form of a difference with that of the first channel.
L'invention est également définie par un signal issu d'un dispositif de codage à limitation de spectre tel que défini ci-dessus, le signal comportant au moins pour une première bande spectrale une composante monophonique codée ainsi qu'une information d'enveloppe spectrale codée relative à l'enveloppe spectrale d'une voie audio dans ladite première bande.The invention is also defined by a signal coming from a spectrum limitation coding device as defined above, the signal comprising at least for a first spectral band a coded monophonic component as well as a spectral envelope information. coded relative to the spectral envelope of an audio channel in said first band.
L'invention est également définie par un signal issu d'un dispositif de codage à limitation de spectre tel que défini ci-dessus, le signal comportant au moins pour une seconde bande spectrale une composante spatiale codée relative à une voie audio ainsi qu'une information d'enveloppe spectrale codée relative à l'enveloppe spectrale de ladite voie audio dans une bande distincte de ladite seconde bande.The invention is also defined by a signal from a coding device with spectrum limitation as defined above, the signal comprising at least for a second spectral band a coded spatial component relating to an audio channel as well as a coded spectral envelope information relating to the spectral envelope of said audio channel in a band distinct from said second band.
L'invention est également définie par un signal issu d'un dispositif de codage à limitation de spectre tel que défini ci-dessus, le signal comportant au moins, pour une première bande spectrale, une composante monophonique et, pour une seconde bande spectrale distincte de la première, une composante spatiale codée relative à une voie audio, ainsi qu'une information d'enveloppe spectrale de ladite voie dans la première bande et dans une troisième bande spectrale distincte desdites première et seconde bandes.The invention is also defined by a signal originating from a spectrum limitation coding device as defined above, the signal comprising at least, for a first spectral band, a monophonic component and, for a second distinct spectral band of the first, a coded spatial component relating to an audio channel, as well as information of spectral envelope of said channel in the first band and in a third spectral band distinct from said first and second bands.
L'invention est également définie par un dispositif de décodage d'un signal audio à plusieurs voies codé, le dispositif comprenant un décodeur adapté à fournir à partir du signal codé un signal composite pouvant comporter des composantes monophoniques et des composantes spatiales, et un dispositif de reconstruction tel que défini plus haut.The invention is also defined by a device for decoding a coded multi-channel audio signal, the device comprising a decoder adapted to supply from the coded signal a composite signal which may include monophonic components and spatial components, and a device reconstruction as defined above.
L'invention est également définie par un dispositif de décodage comprenant un premier décodeur adapté à fournir, à partir dudit signal défini plus haut, un signaL composite pouvant comporter des composantes monophoniques et des composantes spatiales ainsi qu'un second décodeur adapté à fournir, à partir dudit signal, une information d'enveloppe spectrale.The invention is also defined by a decoding device comprising a first decoder adapted to supply, from said signal defined above, a composite signal capable of comprising monophonic components and spatial components as well as a second decoder adapted to supply, to from said signal, spectral envelope information.
Enfin, l'invention est encore définie par un système de codage/décodage d'un signal audio à plusieurs voies, comprenant un dispositif de codage et un dispositif de décodage comme définis plus haut. De manière générale, grâce au procédé et au dispositif de reconstruction selon l'invention, le contenu stéréophonique, notamment le contenu stéréophonique haute fréquence peut être reconstitué lors du décodage sans ou avec une transmission minimale des informations liées au contenu haute fréquence des signaux originaux. La forme spectrale de signaux stéréophoniques haute fréquence peut être modélisée par le biais de deux filtres, un filtre pour chaque voie. Des informations d'enveloppe peuvent être transmises à faible coût, car l'on peut facilement mesurer les différences entre deux enveloppes et tirer ainsi parti des redondances éventuelles entre les formes spectrales modélisées. Une seule voie peut être transmise, l'autre voie pouvant être reconstruite par blanchiment du signal transmis et application d'un filtre d'enveloppe. Les informations d'enveloppe relatives à la voie non transmise ont un coût de transmission très faible.Finally, the invention is further defined by a system for coding / decoding an audio signal with several channels, comprising a coding device and a decoding device as defined above. In general, thanks to the reconstruction method and device according to the invention, the stereophonic content, in particular the high frequency stereophonic content can be reconstituted during decoding without or with a minimum transmission of the information linked to the high frequency content of the original signals. The spectral shape of high frequency stereophonic signals can be modeled through two filters, one filter for each channel. Envelope information can be transmitted at low cost, because one can easily measure the differences between two envelopes and thereby take advantage of possible redundancies between the spectral shapes modeled. Only one channel can be transmitted, the other channel can be reconstructed by bleaching the transmitted signal and applying an envelope filter. The envelope information relating to the non-transmitted channel has a very low transmission cost.
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: la Fig. 1 représente schématiquement un dispositif de reconstruction d'un signal audio stéréophonique selon un premier mode de réalisation de l'invention ; les Figs. 2a à 2c illustrent le traitement effectué par le dispositif de reconstruction de la Fig. 1 ; la Fig. 3 représente schématiquement un dispositif de reconstruction d'un signal audio stéréophonique selon un second mode de réalisation de l'invention ; les Figs. 4a à 4d illustrent le traitement effectué par le dispositif de reconstruction de la Fig. 3 ; la Fig. 5 représente schématiquement un dispositif de reconstruction d'un signal audio stéréophonique selon un troisième mode de réalisation de l'invention ; les Figs. 6a à 6d illustrent le traitement effectué par le dispositif de reconstruction de la Fig. 5.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 made in relation to the accompanying drawings, among which: FIG. . 1 schematically represents a device for reconstructing a stereophonic audio signal according to a first embodiment of the invention; Figs. 2a to 2c illustrate the processing carried out by the reconstruction device of FIG. 1; Fig. 3 schematically represents a device for reconstructing a stereophonic audio signal according to a second embodiment of the invention; Figs. 4a to 4d illustrate the processing carried out by the reconstruction device of FIG. 3; Fig. 5 schematically represents a device for reconstructing a stereophonic audio signal according to a third embodiment of the invention; Figs. 6a to 6d illustrate the processing carried out by the reconstruction device of FIG. 5.
Le dispositif de reconstruction spectrale selon l'invention peut s'appliquer à la reconstruction spectrale d'un signal audio stéréophonique issu du décodage d'un signal codé par un codeur à limitation de bande spectrale. Il peut s'agir de tout type de codeur à réduction de débit. Le codeur peut être de type par transformée (MPEG1, MPEG2 ou MPEG4-GA), de type CELP (ITU G72X), ou même de type paramétrique (MPEG4 paramétrique). L'invention peut également s'appliquer à des signaux qui n'ont pas préalablement fait l'objet d'un codage, par exemple, des signaux ayant simplement subi un sous-échantillonnage et une altération de leur contenu stéréophonique.The spectral reconstruction device according to the invention can be applied to the spectral reconstruction of a stereophonic audio signal resulting from the decoding of a signal coded by an encoder with spectral band limitation. It can be any type of flow reduction encoder. The encoder can be of the transform type (MPEG1, MPEG2 or MPEG4-GA), of the CELP type (ITU G72X), or even of the parametric type (parametric MPEG4). The invention can also be applied to signals which have not previously been the subject of coding, for example, signals which have simply undergone subsampling and an alteration of their stereophonic content.
La Fig. 1 décrit un premier mode de réalisation de l'invention. Le signal est codé par un codeur 100 et, après transmission du signal codé par un moyen quelconque, le signal codé est décodé par un décodeur 110.Fig. 1 describes a first embodiment of the invention. The signal is coded by an encoder 100 and, after transmission of the coded signal by any means, the coded signal is decoded by a decoder 110.
Après décodage dans le décodeur 110, une reconstruction du contenu stéréophonique d'un signal audio et plus généralement des différentes voies d'un signal audio multi-voie est effectuée par les modules 150, 155, 150,, 115,, 175 et les sommateurs 180,. Pour des raisons de simplification, seule une voie i a été représentéeAfter decoding in the decoder 110, the reconstruction of the stereophonic content of an audio signal and more generally of the different channels of a multi-channel audio signal is carried out by the modules 150, 155, 150, 115, 175 and the summers 180 ,. For reasons of simplification, only one channel i has been shown
(par exemple la voie droite d'un signal stéréo).(for example the right channel of a stereo signal).
Le signal décodé par le module 1 10 comporte une composante monophonique (M) et une composante spatiale N,L, associée à la voie i à reconstruire, comme illustré en Fig. 2a. La composante monophonique peut être une composante commune à plusieurs voies, par exemple une somme de plusieurs voies ou encore le signal d'une voie prépondérante parmi une ensemble de voies. Typiquement, la composante N,L sera un signal basse fréquence à bande limitée (B2) et le signal monophonique (M) occupera une bande (Bl) adjacente à la première.The signal decoded by the module 110 has a monophonic component (M) and a spatial component N, L , associated with the channel i to be reconstructed, as illustrated in FIG. 2a. The monophonic component can be a component common to several channels, for example a sum of several channels or even the signal of a predominant channel among a set of channels. Typically, the component N, L will be a low frequency signal with limited band (B 2 ) and the monophonic signal (M) will occupy a band (Bl) adjacent to the first.
Le spectre de la partie monophonique (M) est blanchi à l'aide d'un filtre blanchisseur 150. On sait que sous certaines hypothèses de stationnante, un signal peut être modélisé comme le résultat du filtrage d'un signal d'excitation par un filtre d'enveloppe spectrale. Si l'on dispose d'une description de l'enveloppe spectrale du signal, il est possible de blanchir son spectre en le faisant passer dans un filtre blanchisseur de fonction de transfert (approximativement) inverse à la fonction d'enveloppe. On obtient ainsi une approximation du signal d'excitation initial, débarrassée de l'influence de la forme spectrale dans la bande considérée.The spectrum of the monophonic part (M) is whitened using a whitening filter 150. It is known that under certain stationary hypotheses, a signal can be modeled as the result of the filtering of an excitation signal by a spectral envelope filter. If a description of the spectral envelope of the signal is available, it is possible to whiten its spectrum by passing it through a whitening filter with transfer function (approximately) inverse to the envelope function. An approximation of the initial excitation signal is thus obtained, free of the influence of the spectral shape in the band considered.
Le module 155 est un module d'estimation d'enveloppe spectrale pour le signal monophonique dans la bande Bi. Il peut par exemple modéliser les enveloppes par une analyse LPC, telle que décrite dans l'article de J. Makhoul, intitulé « Linear Prédiction : a tutorial review », Proceedings of the IEEE, Vol. 63, Ν°4, pp 561-580.The module 155 is a spectral envelope estimation module for the monophonic signal in the Bi band. It can for example model the envelopes by an LPC analysis, as described in the article by J. Makhoul, entitled “Linear Prediction: a tutorial review”, Proceedings of the IEEE, Vol. 63, Ν ° 4, pp 561-580.
La composante spatiale N, et la composante monophonique blanchie sont représentés en Fig. 2b. La composante monophonique blanchie est soumise à une étape d'enveloppe spectrale dans le filtre d'enveloppe 170,. Ce filtre d'enveloppe a pour caractéristique l'enveloppe spectrale de la voie i originale dans la bande Bi. Selon une première variante de réalisation, des moyens d'estimation spectrale 105 associés au codeur effectue une estimation spectrale des différentes voies dans la bande Bi. et fournissent des informations décrivant les enveloppes des différentes voies dans cette bande. Avantageusement, les enveloppes sont codées de manière différentielle. Autrement dit, l'enveloppe d'une première voie est codée et celles des autres voies sont codées par différence, de manière à tirer profit de la similarité des enveloppes pour réduire la redondance dans l'information à transmettre. Du côté du décodeur, les informations relatives aux différentes enveloppes sont décodées dans le module 175. Les informations décodées e(V,) sont par exemple des coefficients LPC. Elles sont fournies au filtre d'enveloppe 170,.The spatial component N, and the bleached monophonic component are represented in FIG. 2b. The bleached monophonic component is subjected to a spectral envelope step in the envelope filter 170,. This envelope filter has for characteristic the spectral envelope of the original channel i in the band Bi. According to a first alternative embodiment, spectral estimation means 105 associated with the coder performs a spectral estimation of the different channels in the band Bi. and provide information describing the envelopes of the different channels in this band. Advantageously, the envelopes are coded differently. In other words, the envelope of a first channel is coded and those of the other channels are coded by difference, so as to take advantage of the similarity of the envelopes to reduce the redundancy in the information to be transmitted. On the side of the decoder, the information relating to the various envelopes is decoded in the module 175. The decoded information e (V,) are for example LPC coefficients. They are supplied to the envelope filter 170 ,.
Selon une seconde variante de réalisation, l'enveloppe spectrale de la voie i est obtenue comme l'extrapolée, dans la bande Bi, de l'enveloppe spectrale de la composante spatiale N,L , dans la bande B2. Cette variante est symbolisée en traits discontinus par le module d'extrapolation 1 15, recevant la composante V,L et fournissant l'enveloppe extrapolée au filtre d'enveloppe 170,.According to a second alternative embodiment, the spectral envelope of the channel i is obtained as the extrapolated, in the band Bi, of the spectral envelope of the spatial component N, L , in the band B 2 . This variant is symbolized in broken lines by the extrapolation module 115, receiving the component V, L and supplying the extrapolated envelope to the envelope filter 170,.
La composante spatiale N,L est ensuite ajoutée au moyen du sommateur 180, à la composante monophonique mise en forme pour fournir une voie reconstruite N,. Le spectre de la voie reconstruite est illustré en Fig. 2c.The spatial component N, L is then added by means of the summator 180, to the monophonic component shaped to provide a reconstructed channel N ,. The spectrum of the reconstructed pathway is illustrated in Fig. 2c.
Un cas particulier important est celui où la largeur de bande B2 est nulle, c'est-à- dire celui où seule une composante monophonique est transmise. Cette composante monophonique peut correspondre à une voie ou à une somme de voies, comme vu plus haut. Les différentes voies sont reconstruites grâce à la mise en forme du signal monophonique blanchi par leurs enveloppes spectrales respectives. Ainsi, pour un signal stéréo, on peut reconstruire la voie gauche à partir de la voie droite, ou bien les voies droite et gauche à partir de la voie somme. On recrée ainsi un effet stéréo à partir d'un signal transmis en mono.An important particular case is that where the bandwidth B 2 is zero, that is to say that where only a monophonic component is transmitted. This monophonic component can correspond to a channel or to a sum of channels, as seen above. The different channels are reconstructed thanks to the shaping of the monophonic signal whitened by their respective spectral envelopes. Thus, for a stereo signal, we can reconstruct the left channel from the right channel, or the right and left channels from the sum channel. We thus recreate a stereo effect from a signal transmitted in mono.
La Fig. 3 décrit un second mode de réalisation de l'invention. Les modules portant les mêmes références que sur la Fig. 1 ont une fonction identique à celle déjà décrite. Pour des raisons de simplification, seule la reconstruction d'une voie i a été représentée.Fig. 3 describes a second embodiment of the invention. The modules bearing the same references as in FIG. 1 have a function identical to that already described. For reasons of simplification, only the reconstruction of a channel i has been shown.
Dans ce mode de réalisation, le décodeur 1 10 fournit des composantes spatiales à spectres limités. Ce sera typiquement le cas si le codeur 100 est un codeur à limitation de spectre. Nous considérerons une composante spatiale N, de spectre limité à la bande B2 , comme représenté en Fig. 4a. Le module 160, est un module de transposition spectrale. Sa fonction est de recopier le contenu spectral d'une partie au moins de la bande B2, dite bande source, dans un seconde bande B3, dite bande cible. L'opération de transposition est par exemple une simple translation de spectre dans la bande cible ou bien la combinaison d'un retournement et d'une translation. Typiquement, la bande B est une bande basse fréquence et la bande cible est adjacente à cette dernière. L'opération de transposition spectrale a été illustrée en Fig. 4b.In this embodiment, the decoder 110 supplies spatial components with limited spectra. This will typically be the case if the encoder 100 is a spectrum limiting encoder. We will consider a spatial component N, of spectrum limited to the band B 2 , as shown in FIG. 4a. The module 160 is a spectral transposition module. Its function is to copy the spectral content of at least part of the band B 2 , called the source band, into a second band B 3 , called the target band. The transposition operation is for example a simple translation of spectrum in the target band or else the combination of a reversal and a translation. Typically, band B is a low frequency band and the target band is adjacent to the latter. The spectral transposition operation has been illustrated in FIG. 4b.
Le signal obtenu en sortie de 160, est un signal à spectre limité à la bande B . Il est soumis à un blanchiment spectral dans le filtre blanchisseur 150,. Selon une première variante, la caractéristique du filtre blanchisseur est l'inverse de l'enveloppe spectrale de la composante spatiale transposée dans la bande B3. Le module 1 15, estime les coefficients du filtre blanchisseur et les fournit à ce dernier. Alternativement, les coefficients du filtre 150, sont obtenus à partir de l'enveloppe spectrale de la voie i dans la bande source. Il faut noter que l'ordre des modules de blanchiment spectral (150,) et de transposition spectrale (160,) peut être interverti. L'ordre choisi dépend notamment de la précision de blanchiment souhaitée. Le résultat du blanchiment spectral est illustré en Fig. 4c.The signal obtained at the output of 160 is a signal with spectrum limited to band B. It is subjected to spectral whitening in the whitening filter 150,. According to a first variant, the characteristic of the whitening filter is the inverse of the spectral envelope of the spatial component transposed in the band B 3 . Module 1 15 estimates the coefficients of the whitening filter and supplies them to the latter. Alternatively, the coefficients of the filter 150 are obtained from the spectral envelope of the channel i in the source band. It should be noted that the order of the spectral bleaching (150,) and spectral transposition (160,) modules can be reversed. The order chosen depends in particular on the desired whitening precision. The result of spectral bleaching is illustrated in Fig. 4c.
La composante spatiale à spectre transposé et blanchi est soumise à une étape de mise en forme spectrale dans le filtre d'enveloppe 170,. Ce filtre d'enveloppe a pour caractéristique l'enveloppe spectrale de la voie i originale dans la bande B3.The spatial component with transposed and whitened spectrum is subjected to a spectral shaping step in the envelope filter 170,. The characteristic of this envelope filter is the spectral envelope of the original channel i in band B 3 .
Les informations relatives aux différentes enveloppes sont décodées dans le module 175. Les informations décodées e(N,) sont par exemple des coefficients LPC. Elles sont fournies au filtre d'enveloppe 170,.The information relating to the various envelopes is decoded in module 175. The decoded information e (N,) are for example LPC coefficients. They are supplied to the envelope filter 170 ,.
Selon une seconde variante de réalisation, l'enveloppe spectrale de la voie i est obtenue comme l'extrapolée, dans la bande B3, de l'enveloppe spectrale de la composante spatiale N, , dans la bande B . Cette variante est symbolisée par la liaison en traits discontinus entre le module 115, et le filtre d'enveloppe 170,.According to a second alternative embodiment, the spectral envelope of channel i is obtained as the extrapolated, in band B 3 , of the spectral envelope of the spatial component N,, in band B. This variant is symbolized by the link in broken lines between the module 115, and the envelope filter 170,.
La composante spatiale N,L est ensuite ajoutée au moyen du sommateur 181, au signal issu du filtre 170, pour fournir une voie reconstruite N,. Le spectre de la voie reconstruite est illustré en Fig. 4d.The spatial component N, L is then added by means of the summator 181, to the signal from the filter 170, to provide a reconstructed channel N ,. The spectrum of the reconstructed pathway is illustrated in Fig. 4d.
La Fig. 5 décrit un troisième mode de réalisation de l'invention. Les modules portant les mêmes références que celles de la Fig. 1 ou de la Fig. 3 ont une fonction identique à celle déjà décrite. Pour des raisons de simplification, seule la reconstruction d'une voie i a été représentée. Dans ce mode de réalisation, le décodeur 1 10 fournit une composante monophonique M ainsi que des composantes spatiales à spectres limités N, . La composante monophonique peut être commune à plusieurs ou à toutes les voies. On a illustré en Fig. 6a une composante spatiale à spectre limité N,L ainsi que la composante monophonique M. La composante spatiale (par exemple relative à une voie d'un signal stéréo) du signal occupe une bande basse fréquence. Dans les plus hautes fréquences (bande spectrale Bl) le signal est codé en monophonique.Fig. 5 describes a third embodiment of the invention. The modules bearing the same references as those in FIG. 1 or of FIG. 3 have a function identical to that already described. For reasons of simplification, only the reconstruction of a channel i has been shown. In this embodiment, the decoder 110 supplies a monophonic component M as well as spatial components with limited spectra N,. The monophonic component can be common to several or to all channels. Illustrated in FIG. 6 has a spatial component with limited spectrum N, L as well as the monophonic component M. The spatial component (for example relating to a channel of a stereo signal) of the signal occupies a low frequency band. In the higher frequencies (spectral band Bl) the signal is coded in monophonic.
En sortie du décodeur 1 10, la composante monophonique M, d'une part, et la composante spatiale V, , d'autre part, subissent des traitements distincts. La composante monophonique est blanchie au moyen du filtre blanchisseur 150 comme dans la Fig. 1. La composante spatiale, quant à elle, fait l'objet d'une transposition de tout ou partie de son contenu spectral dans une bande cible B3, typiquement contiguë à la bande monophonique Bl5 comme représenté en Fig. 6b. Selon une première variante, la bande source du contenu spectral est incluse dans la bande B2. Selon une seconde variante, non représentée, la bande source est incluse dans la bande B2+Bι , autrement dit au moins une partie du contenu spectral de la bande monophonique peut être également transposée. Le signal en sortie de 160, possède un spectre limité à la bande B3. Il est blanchi dans le filtre blanchisseur 150, dont la fonction de transfert est déterminée à partir du module d'estimation spectrale 115, ou, alternativement, par une information d'enveloppe spectrale de la voie i originale dans la bande source donnée par le module de décodage 175. La fonction de transfert du filtre 150, est donnée par l'inverse de l'enveloppe spectrale dans la bande source. Là aussi l'ordre du module de transposition 160, et du module de blanchiment 150, peut être inversé. La Fig. 6c illustre le résultat des opérations de blanchiment dans les bandes B] et B2. Les composantes monophonique et spatiale blanchies sont combinées dans le sommateur 180, et la somme est filtrée par le filtre d'enveloppe 170,. Selon une première variante, la fonction de transfert de ce filtre est donnée par une information d'enveloppe spectrale de la voie i originale, dans la bande B1.+B3, fournie par le module de décodage d'enveloppe 175. Comme indiqué plus haut, les enveloppes des différentes voies peuvent être codées en 105 sous forme de différences.At the output of the decoder 110, the monophonic component M, on the one hand, and the spatial component V, on the other hand, undergo separate processing. The monophonic component is whitened by means of the whitening filter 150 as in FIG. 1. The spatial component, for its part, is the subject of a transposition of all or part of its spectral content in a target band B 3 , typically contiguous to the monophonic band B 15 as shown in FIG. 6b. According to a first variant, the source band of the spectral content is included in the band B 2 . According to a second variant, not shown, the source band is included in the band B 2 + Bι, in other words at least part of the spectral content of the monophonic band can also be transposed. The signal at the output of 160 has a spectrum limited to the band B 3 . It is bleached in the whitening filter 150, the transfer function of which is determined from the spectral estimation module 115, or, alternatively, by spectral envelope information of the original channel i in the source band given by the module. 175 decoding function. The transfer function of the filter 150 is given by the inverse of the spectral envelope in the source band. Here too, the order of the transposition module 160, and of the bleaching module 150, can be reversed. Fig. 6c illustrates the result of the laundering operations in bands B] and B 2 . The whitened monophonic and spatial components are combined in the summator 180, and the sum is filtered by the envelope filter 170 ,. According to a first variant, the transfer function of this filter is given by spectral envelope information of the original channel i, in the band B1. + B 3 , provided by the envelope decoding module 175. As indicated more top, the envelopes of the different channels can be coded at 105 as differences.
Selon une seconde variante, la fonction de transfert du filtre d'enveloppe 170, est l'extrapolée, dans la bande B.1+B3, de l'enveloppe spectrale de la composante N,L. Dans ce cas, et si la fonction du filtre blanchisseur 150, est donnée par le module d'estimation 1 15,, on voit que les modules 105 et 175 ne sont pas nécessaires. La composante spatiale N, est combinée au signal issu de 170, au moyen du sommateur 181, pour fournir une voie reconstruite N, dont le spectre est représenté en Fig. 6d.In a second variant, the transfer function of the filter casing 170, is extrapolated in the B band. 1 + B 3 , of the spectral envelope of the component N, L. In this case, and if the function of the whitening filter 150, is given by the estimation module 115, it can be seen that the modules 105 and 175 are not necessary. The spatial component N, is combined with the signal from 170, by means of the summator 181, to provide a reconstructed channel N, the spectrum of which is shown in FIG. 6d.
On notera que, dans les trois modes de réalisation décrits ci-dessus, les filtres d'enveloppe peuvent être appliqués dans le domaine temporel ou dans le domaine fréquentiel.Note that, in the three embodiments described above, the envelope filters can be applied in the time domain or in the frequency domain.
Bien que le dispositif selon l'invention ait été représenté sous la forme de modules fonctionnels, il va de soi que tout ou partie de ce dispositif peut être réalisé au moyen d'un processeur unique ou une pluralité de processeurs dédiés ou non. Although the device according to the invention has been shown in the form of functional modules, it goes without saying that all or part of this device can be produced by means of a single processor or a plurality of dedicated or non-dedicated processors.

Claims

REVENDICATIONS
1) Procédé de reconstruction d'au moins une voie d'un signal audio à plusieurs voies, en particulier d'un signal stéréo, à partir d'un signal composite pouvant comporter des composantes monophoniques (M) et des composantes spatiales (N,L) caractérisé en ce que, pour au moins une composante, il comprend :1) Method for reconstructing at least one channel of a multi-channel audio signal, in particular a stereo signal, from a composite signal which may include monophonic components (M) and spatial components (N, L ) characterized in that, for at least one component, it comprises:
- une étape de blanchiment spectral (150, 150,) de ladite composante pour obtenir un signal blanchi ;- a spectral bleaching step (150, 150,) of said component to obtain a bleached signal;
- une étape de mise en forme (170, 170,) du spectre du signal blanchi au moyen d'un filtre d'enveloppe ayant pour caractéristique au moins une partie de l'enveloppe spectrale (e(N,)) de la voie à reconstruire.a step of shaping (170, 170,) of the spectrum of the whitened signal by means of an envelope filter having as characteristic at least part of the spectral envelope (e (N,)) of the channel to rebuild.
2) Procédé de reconstruction selon la revendication 1, caractérisé en ce que le signal composite comprend au moins une composante monophonique (M) dans une première bande spectrale (Bi.) , l'étape de blanchiment spectral (150) fournissant un signal monophonique blanchi et l'étape de mise en forme (170,) utilisant un filtre d'enveloppe ayant pour caractéristique l'enveloppe spectrale de la voie à reconstruire dans ladite première bande.2) A reconstruction method according to claim 1, characterized in that the composite signal comprises at least one monophonic component (M) in a first spectral band (Bi.), The spectral bleaching step (150) providing a bleached monophonic signal and the shaping step (170,) using an envelope filter having as characteristic the spectral envelope of the channel to be reconstructed in said first band.
3) Procédé de reconstruction selon la revendication 1, caractérisé en ce que le signal composite comprend plusieurs composantes spatiales (N,L), chaque composante spatiale étant associée à une voie et au moins une composante spatiale ayant un spectre limité à une seconde bande spectrale, la reconstruction de la voie associée à la composante spatiale à spectre limité comprenant: - une étape de transposition (160,) de tout ou partie du contenu spectral de ladite composante spatiale dans une troisième bande spectrale distincte de ladite seconde bande spectrale pour fournir une composante à spectre transposé ;3) A reconstruction method according to claim 1, characterized in that the composite signal comprises several spatial components (N, L ), each spatial component being associated with a channel and at least one spatial component having a spectrum limited to a second spectral band , the reconstruction of the channel associated with the limited spectrum space component comprising: - a step of transposition (160,) of all or part of the spectral content of said space component in a third spectral band distinct from said second spectral band to provide a transposed spectrum component;
- une étape de blanchiment spectral (150,) avant ou après l'étape de transposition de sorte que la composante à spectre transposé est blanchie ; - une étape de mise en forme du spectre (170,) de la composante à spectre transposé et blanchi au moyen d'un filtre d'enveloppe ayant pour caractéristique l'enveloppe spectrale de la voie à reconstruire dans ladite troisième bande. 4) Procédé de reconstruction selon la revendication 3, caractérisé en ce que la seconde bande spectrale (B2) est une bande basse fréquence et la troisième bande spectrale (B3) est adjacente à seconde.- a spectral bleaching step (150,) before or after the transposition step so that the component with transposed spectrum is bleached; - A step of shaping the spectrum (170,) of the component with transposed and whitened spectrum by means of an envelope filter having as characteristic the spectral envelope of the channel to be reconstructed in said third band. 4) A reconstruction method according to claim 3, characterized in that the second spectral band (B 2 ) is a low frequency band and the third spectral band (B 3 ) is adjacent to the second.
5) Procédé de reconstruction selon la revendication 1, caractérisé en ce que le signal composite comprend au moins une composante monophonique (M) à spectre limité à une première bande spectrale (Bi) et une pluralité de composantes spatiales (N,L) à spectres limités, chaque composante étant associée à une voie (i), la reconstruction d'une voie associée à une composante spatiale à spectre limité à une seconde bande spectrale comprenant:5) Reconstruction method according to claim 1, characterized in that the composite signal comprises at least one monophonic component (M) with spectrum limited to a first spectral band (Bi) and a plurality of spatial components (N, L ) with spectra limited, each component being associated with a channel (i), the reconstruction of a channel associated with a spatial component with a spectrum limited to a second spectral band comprising:
- une étape de transposition (160,) de tout ou partie du contenu spectral de ladite composante spatiale ou de la composante monophonique dans une troisième bande spectrale distincte desdites première et seconde bandes spectrales pour fournir une composante spatiale à spectre transposé; - une étape de blanchiment spectral de la composante monophonique blanchie- a transposition step (160,) of all or part of the spectral content of said spatial component or of the monophonic component in a third spectral band distinct from said first and second spectral bands to provide a spatial component with transposed spectrum; - a step of spectral bleaching of the bleached monophonic component
(150) et de ladite composante spatiale (150,) avant ou après l'étape de transposition, de sorte que la composante monophonique, d'une part, et la composante spatiale à spectre transposé, d'autre part, sont blanchies ;(150) and said spatial component (150,) before or after the transposition step, so that the monophonic component, on the one hand, and the spatial component with transposed spectrum, on the other hand, are whitened;
- une étape de mise en forme du spectre (170,) de la composante monophonique blanchie et du spectre de la composante spatiale à spectre transposé et blanchi par un filtre d'enveloppe ayant pour caractéristique l'enveloppe spectrale de la voie à reconstruire dans lesdites première et troisième bandes.- A step of shaping the spectrum (170,) of the bleached monophonic component and the spectrum of the spatial component with transposed and whitened spectrum by an envelope filter having as characteristic the spectral envelope of the channel to be reconstructed in said first and third bands.
6) Procédé de reconstruction selon la revendication 5, caractérisé en ce que la seconde bande spectrale (B ) est une bande basse fréquence, la première bande (Bi.) est adjacente à la seconde et la troisième bande (B3) est adjacente à la première.6) Reconstruction method according to claim 5, characterized in that the second spectral band (B) is a low frequency band, the first band (Bi.) Is adjacent to the second and the third band (B 3 ) is adjacent to the first one.
7) Procédé de reconstruction selon la revendication 6, caractérisé en ce que l'étape de blanchiment spectral est effectuée sur la composante monophonique et sur ladite composante spatiale à spectre limité au moyen d'un premier filtre blanchisseur ayant pour caractéristique l'inverse de l'enveloppe spectrale du signal monophonique dans la première bande et un second filtre blanchisseur ayant pour caractéristique l'inverse de l'enveloppe spectrale de ladite composante spatiale dans la seconde bande. 8) Procédé de reconstruction selon l'une des revendications 3 à 7, les composantes spatiales du signal composite étant issues du décodage d'un signal source à plusieurs voies codé par un codeur à limitation de spectre, caractérisé en ce que l'étape de blanchiment spectral de ladite composante spatiale est effectuée au moyen d'un filtre blanchisseur dont la caractéristique est obtenue au moyen d'une information donnant l'enveloppe spectrale de la voie correspondante du signal source.7) Reconstruction method according to claim 6, characterized in that the spectral bleaching step is carried out on the monophonic component and on said spatial component with limited spectrum by means of a first whitening filter having as characteristic the inverse of the spectral envelope of the monophonic signal in the first band and a second whitening filter having the characteristic of the inverse of the spectral envelope of said spatial component in the second band. 8) Reconstruction method according to one of claims 3 to 7, the spatial components of the composite signal being derived from the decoding of a source signal with several channels encoded by a spectrum limiting encoder, characterized in that the step of spectral whitening of said spatial component is carried out by means of a whitening filter whose characteristic is obtained by means of information giving the spectral envelope of the corresponding channel of the source signal.
9) Procédé de reconstruction selon l'une des revendications 3 à 7, caractérisé en ce que pour la reconstruction de ladite voie, la caractéristique du filtre d'enveloppe est extrapolée à partir de l'enveloppe spectrale de ladite composante spatiale dans la seconde bande.9) reconstruction method according to one of claims 3 to 7, characterized in that for the reconstruction of said channel, the characteristic of the envelope filter is extrapolated from the spectral envelope of said spatial component in the second band .
10) Procédé de reconstruction selon la revendication 3 ou 4, les composantes spatiales du signal composite étant issues du décodage d'un signal source à plusieurs voies codé par un codeur à limitation de spectre, caractérisé en ce que pour la reconstruction de ladite voie, la caractéristique du filtre d'enveloppe est obtenue à partir d'une information donnant l'enveloppe spectrale de la voie correspondante du signal source dans la troisième bande.10) Reconstruction method according to claim 3 or 4, the spatial components of the composite signal being obtained from the decoding of a source signal with several channels coded by a spectrum limiting coder, characterized in that for the reconstruction of said channel, the characteristic of the envelope filter is obtained from information giving the spectral envelope of the corresponding channel of the source signal in the third band.
1 1) Procédé de reconstruction selon l'une des revendications 5 à 9, les composantes du signal composite étant issues du décodage d'un signal source à plusieurs voies codé par un codeur à limitation de spectre, caractérisé en ce que pour la reconstruction de ladite voie, la caractéristique du filtre d'enveloppe est obtenue à partir d'une information donnant l'enveloppe spectrale de la voie correspondante du signal source dans les première et troisième bandes.1 1) Reconstruction method according to one of claims 5 to 9, the components of the composite signal being derived from the decoding of a source signal with several channels coded by a spectrum limiting encoder, characterized in that for the reconstruction of said channel, the characteristic of the envelope filter is obtained from information giving the spectral envelope of the corresponding channel of the source signal in the first and third bands.
12) Dispositif de reconstruction d'au moins une voie d'un signal audio à plusieurs voies, en particulier d'un signal stéréo, à partir d'un signal composite pouvant comporter des composantes monophoniques et des composantes spatiales, caractérisé en ce qu'il comprend des moyens pour mettre en oeuvre les étapes du procédé selon l'une des revendications précédentes. ιj) j-visposit uC cou gc u un signai audio source à plusieurs voies, au moins une première bande spectrale dudit signal étant codée en monophonique, caractérisé en ce qu'il fournit en outre une information d'enveloppe spectrale pour au moins une voie dans ladite première bande.12) Device for reconstructing at least one channel of a multi-channel audio signal, in particular a stereo signal, from a composite signal which may include monophonic components and spatial components, characterized in that it includes means for implementing the steps of the method according to one of the preceding claims. ιj) j-visposit uC cou gc u a source audio signal with several channels, at least a first spectral band of said signal being coded in monophonic, characterized in that it also provides spectral envelope information for at least one channel in said first band.
14) Dispositif de codage à limitation de spectre d'un signal audio source à plusieurs voies, le spectre d'au moins une voie étant limité à une seconde bande spectrale par le codage, caractérisé en ce qu'il fournit en outre une information d'enveloppe spectrale de ladite voie dans une bande distincte de ladite seconde bande.14) Device for coding spectrum limitation of a multi-channel audio source signal, the spectrum of at least one channel being limited to a second spectral band by coding, characterized in that it also provides information on spectral envelope of said channel in a band distinct from said second band.
15) Dispositif de codage à limitation de spectre d'un signal audio source à plusieurs voies, au moins une première bande spectrale dudit signal étant codée en monophonique, le spectre d'au moins une voie étant limité par le codage à une seconde bande spectrale, distincte de la première, caractérisé en ce qu'il fournit en outre une information d'enveloppe spectrale de ladite voie dans la première bande et dans une troisième bande distincte desdites première et seconde bandes.15) Device for coding spectrum limitation of a source audio signal with several channels, at least a first spectral band of said signal being coded in monophonic, the spectrum of at least one channel being limited by coding to a second spectral band , distinct from the first, characterized in that it also provides information on the spectral envelope of said channel in the first band and in a third band distinct from said first and second bands.
16) Dispositif de codage selon l'une des revendications 13 à 15, caractérisé en ce que, pour au moins une première voie et une seconde voie, l'information d'enveloppe spectrale de la seconde voie est transmise sous forme de différence avec celle de la première voie.16) Coding device according to one of claims 13 to 15, characterized in that, for at least a first channel and a second channel, the spectral envelope information of the second channel is transmitted in the form of a difference with that of the first way.
17) Signal issu d'un dispositif de codage à limitation de spectre selon la revendication 13 ou 16 en dépendance de 13, caractérisé en ce qu'il comporte au moins pour une première bande spectrale une composante monophonique codée ainsi qu'une information d'enveloppe spectrale codée relative à l'enveloppe spectrale d'une voie audio dans ladite première bande.17) Signal from a spectrum limitation coding device according to claim 13 or 16 depending on 13, characterized in that it comprises at least for a first spectral band a coded monophonic component as well as information of coded spectral envelope relative to the spectral envelope of an audio channel in said first band.
18) Signal issu d'un dispositif de codage à limitation de spectre selon la revendication 14 ou 16 en dépendance de 14, caractérisé en ce qu'il comporte au moins pour une seconde bande spectrale une composante spatiale codée relative à une voie audio ainsi qu'une information d'enveloppe spectrale codée relative à l'enveloppe spectrale de ladite voie audio dans une bande distincte de ladite seconde bande. 19) Signal issu d'un dispositif de codage à limitation de spectre selon la revendication 15 ou 16 en dépendance de 15, caractérisé en ce qu'il comporte au moins, pour une première bande spectrale, une composante monophonique et, pour une seconde bande spectrale distincte de la première, une composante spatiale codée relative à une voie audio, ainsi qu'une information d'enveloppe spectrale de ladite voie dans la première bande et dans une troisième bande spectrale distincte desdites première et seconde bandes.18) Signal from a spectrum limiting coding device according to claim 14 or 16 depending on 14, characterized in that it comprises at least for a second spectral band a coded spatial component relating to an audio channel as well as 'coded spectral envelope information relating to the spectral envelope of said audio channel in a band distinct from said second band. 19) Signal from a spectrum limitation coding device according to claim 15 or 16 depending on 15, characterized in that it comprises at least, for a first spectral band, a monophonic component and, for a second band spectral distinct from the first, a coded spatial component relating to an audio channel, as well as information on the spectral envelope of said channel in the first band and in a third spectral band distinct from said first and second bands.
20) Dispositif de décodage d'un signal audio à plusieurs voies codé, caractérisé en ce qu'il comprend un décodeur adapté à fournir à partir du signal codé un signal composite pouvant comporter des composantes monophoniques et des composantes spatiales et un dispositif de reconstruction selon la revendication 12.20) Device for decoding a coded multi-channel audio signal, characterized in that it comprises a decoder adapted to supply from the coded signal a composite signal which may include monophonic components and spatial components and a reconstruction device according to claim 12.
21) Dispositif de décodage d'un signal selon l'une des revendications 17 à 19, caractérisé en ce qu'il comprend un premier décodeur (1 10) adapté à fournir, à partir dudit signal, un signal composite pouvant comporter des composantes monophoniques et des composantes spatiales ainsi qu'un second décodeur (175) adapté à fournir, à partir dudit signal, une information d'enveloppe spectrale.21) Device for decoding a signal according to one of claims 17 to 19, characterized in that it comprises a first decoder (1 10) adapted to supply, from said signal, a composite signal which may include monophonic components and spatial components as well as a second decoder (175) adapted to provide, from said signal, spectral envelope information.
22) Système de codage/décodage d'un signal audio à plusieurs voies, caractérisé en ce qu'il comprend un dispositif de codage selon l'une des revendications 13 à 16 et un dispositif de décodage selon la revendication 20 ou 21. 22) System for coding / decoding an audio signal with several channels, characterized in that it comprises a coding device according to one of claims 13 to 16 and a decoding device according to claim 20 or 21.
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WO2022258036A1 (en) * 2021-06-11 2022-12-15 华为技术有限公司 Encoding method and apparatus, decoding method and apparatus, and device, storage medium and computer program

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