EP1997103B1 - Method of coding a source audio signal, corresponding coding device, decoding method and device, signal, computer program products - Google Patents

Abstract

A method is provided for coding a source audio signal. The method includes the following steps: coding a quantization profile of coefficients representative of at least one transform of the source audio signal, according to at least to distinct coding techniques, delivering at least two sets of data representative of a quantization profile; selecting one of the sets of data representative of a quantization profile, as a function of a predetermined selection criterion; transmitting and/or storing the set of data representative of a selected quantization profile and an indicator representative of the corresponding coding technique.

Description

1. Field of the invention

The field of the invention is that of encoding and decoding digital audio signals, such as music or digitized speech signals.

More particularly, the invention relates to the quantization of the spectral coefficients of audio signals, by implementing a perceptual coding.

The invention applies in particular, but not exclusively, to systems implementing a hierarchical encoding of digital audio data, of the type of the "scalable" (or "scalable") coding / decoding system proposed as part of the MPEG standard. Audio (ISO / IEC 14496-3).

More generally, the invention finds applications in the field of sound and music quantization in an efficient manner, for storage, compression and transmission through transmission channels, for example wireless or hard-wired.

2. Solution of the prior art 2.1 Perceptual coding with transmission of a masking curve 2.1.1 audio compression and quantification

Audio compression is often based on certain hearing abilities of the human ear. The coding and quantization of an audio signal often takes this characteristic into account. We speak of perceptual coding, or coding according to a psycho-acoustic model of the human ear.

In particular, the ear is unable to separate two components of a signal transmitted at close frequencies, as well as in a reduced time interval. This property is called auditory masking. In addition, the ear has a hearing threshold, in a quiet environment, below which no sound will be perceived. The level of this threshold varies according to the frequency of the sound wave.

In the context of compression and / or transmission of digital audio signals, it is sought to determine a number of quantization use to quantize spectral coefficients composing the signal, without introducing excessive quantization noise, and thus adversely affect the quality of the coded signal. The objective is generally to reduce the number of quantization bits, so as to obtain an effective compression of the signal, it is therefore a question of finding a compromise between the sound quality and the level of compression of the signal.

In standard state-of-the-art techniques, the quantization principles then use a human ear-induced masking threshold and the masking property to determine the amount of maximum quantization noise that is acceptable to inject into the signal without it being perceived by the ear when the audio signal is restored, that is to say without introducing too much distortion.

2.1.2 Perceptive audio coding by transform

For an exhaustive description of transform audio coding, it is possible to refer in particular to the document Jayant, Johnson and Safranek, "Signal Compression Based on the Method of Human Perception", Proc. Of IEEE, Vol. 81, No. 10, PP. 1385-1422, October 1993 .

This technique exploits the model of frequency masking of the ear illustrated by the figure 1 , which presents an example of frequency representation of an audio signal and the threshold of masking of the ear. The abscissa axis 10 represents the frequencies f, in Hz, the ordinate axis 11 that of the sound intensity I, in dB. The ear breaks down the spectrum of a signal x ( t ) into critical bands 120, 121, 122, 123 in the frequency domain according to the Barks scale. The critical band 120 of index n of the signal x (t) and energy E _n then generates a mask 13 inside the band of index n and in the neighboring critical bands 122 and 123. The masking threshold 13 is proportional to the energy E _n of the component 120 "masking" and decreasing for the critical bands of indices lower and greater than n .

Components 122 and 123 are hidden in the example of figure 1 . In addition, component 121 is also hidden because it is below Absolute hearing threshold 14. An overall masking curve is thus obtained, by combining the absolute hearing threshold 14 and the masking thresholds associated with each of the components of the audio signal x (t) analyzed in critical bands. This masking curve represents the spectral density of maximum quantization noise that can be superimposed on the signal, during its coding, without it being perceptible by the human ear. A quantization step profile, also called, by abuse of language, injected noise profile, is then shaped during the quantization of the spectral coefficients resulting from the frequency transform of the source audio signal.

Figure 2 is a flowchart illustrating the principle of a classical perceptual coder. A time source audio signal x (t) is transformed in the frequency domain by a time-frequency transform block. A spectrum of the source signal, composed of spectral coefficients X _n is then obtained, which is analyzed by a psychoacoustic model 21, whose role is to determine the overall masking curve C of the signal, as a function of the absolute hearing threshold. as well as masking thresholds of each spectral component of the signal. The masking curve obtained makes it possible to know the quantity of quantization noise that can be injected and thus to determine the number of bits to be used for quantizing the spectral coefficients, or samples. This step of determining the number of bits is performed by a binary allocation block 22, which delivers a quantization step profile Δ _n for each coefficient X _n . The bit allocation seeks to reach the target bit rate by adjusting the quantization steps under the shaping constraint given by the masking curve C. The quantization pitches Δ _n are coded, in the form of scale factors F in particular, by this block 22 of binary allocation, to be transmitted as additional information in the bit stream T.

A quantization block 23 receives the spectral coefficients X _{n as} well as the quantization steps Δ _n determined, and then delivers quantized coefficients X _n .

Finally, a block 24 for coding and forming a bitstream centralizes the quantized spectral coefficients X _n and the scale factors F, to code them and thus to form a bit stream containing the useful data relating to the coded source audio signal as well as the data representative of scale factors.

2.2 Hierarchical construction of masking curves

The disadvantages of the prior art are presented below in the context of a hierarchical coding of digital audio data. However, the invention applies to all types of digital audio signal coders, implementing a quantification based on the psycho-acoustic model of the ear. These are not necessarily hierarchical.

Hierarchical coding consists in cascading several stages of coders, the first stage generating the coded version at the lowest rate at which the subsequent stages bring successive improvements for gradually increasing flows. In the particular case of coding audio signals, the improvement stages are conventionally based on transform perceptual coding as described in the previous section.

However, a disadvantage of transform perceptual coding in such a hierarchical approach lies in the fact that the scale factors obtained must be transmitted from the first level, or base level. They then represent a significant part of the bit rate allocated to the low bit rate, compared to the useful data.

To overcome this drawback, and thus save the transmission of the injected quantification noise profile, that is to say the scale factors, a so-called "implicit" masking technique has been proposed by J.Li in the document "Embedded Audio Coding (EAC) With Implicit Auditory Masking", ACM Multimedia 2002 . Such a technique relies on the hierarchical structure of the coding-decoding system to recursively estimate the masking curve at each refinement level, using an approximation of this curve, which is refined from level to level.

The update of the masking curve is thus repeated at each hierarchical level, from the transform coefficients quantized at the previous level.

Since the estimation of the masking curve is based on the quantized values of the coefficients of the time-frequency transform, it can be performed identically at the level of the encoder and the decoder: this has the advantage of avoiding transmit the profile of the quantization step, or quantization noise, to the decoder.

2.3 Disadvantages of the prior art

Even if the implicit masking technique, based on hierarchical coding, makes it possible to avoid transmitting the masking curve, and thus to gain in terms of bit rate compared to the conventional perceptual encoding according to which the profile of the quantization step is transmitted, the inventors have found that it nevertheless has several disadvantages.

Indeed, the masking model implemented simultaneously with the encoder and the decoder is necessarily fixed, and therefore can not be adapted precisely to the nature of the signal. For example, a single masking factor is used, regardless of the tonal character or not of the components of the spectrum to be encoded.

In addition, masking curves are calculated under a hypothesis of stationarity of the signal, and poorly applied to transient portions and sound attacks.

Moreover, since the masking curves are obtained at each level from the coefficients or coefficient residues quantized at the previous levels, the masking curve of the first levels is incomplete, since certain portions of the spectrum are not yet coded. This incomplete curve does not necessarily represent an optimal form of the profile of the quantization step for the hierarchical level considered.

3. Presentation of the invention

The invention relates to a method for coding a source audio signal according to claim 1.

The invention thus relies on a new and inventive approach to coding the coefficients of a source audio signal, making it possible to reduce the bit rate allocated to the transmission of quantization steps while keeping a quantization noise profile injected as close as possible to that given by a masking curve calculated from the complete knowledge of the signal.

The invention proposes a selection between different possible modes of calculating the quantization step profile. It can thus make a selection between several quantization step profile templates, or injected noise profiles. This choice is indicated by an indicator, for example a signal contained in the bitstream formed by the encoder and transmitted to the system for reproducing the audio signal, the decoder.

The selection criterion can take into account, in particular, the efficiency of each quantization step profile and the bit rate required to code the corresponding data set.

Thus, a compromise is made between the bit rate necessary to transport the data representative of the signal, and the distortion affecting the signal.

The quantization is optimized, while minimizing the bit rate necessary for the transmission of data representative of the profile of the quantization step, and not providing direct information on the audio signal itself.

In other words, at the encoder, the choice of a quantization mode is made by comparing a reference masking curve, estimated from the audio signal to be encoded, with the noise profiles associated with each of the modes. of quantification.

It results from the technique of the invention a better compression efficiency compared to prior techniques, and therefore increased perceived quality.

For at least a first of the coding techniques, the data set may correspond to a parametric representation of the quantization step profile.

In other words, among the proposed techniques for quantizing the coefficients of a transformed audio signal, it is possible to represent the quantization step profile parametrically.

In a particular embodiment, the parametric representation is formed of at least one line segment, characterized by a slope and a value at the origin.

A second of the coding techniques can deliver a constant quantization step profile.

This encoding mode therefore proposes to code the quantization step profile based on a signal-to-noise ratio (SNR), and not on a signal masking curve.

According to a third advantageous coding technique, the quantization step profile corresponds to an absolute hearing threshold.

In other words, the set of data representative of the quantization step profile may be empty and no data relating to the quantization step profile is transmitted from the encoder to the decoder. The threshold of absolute hearing is known to the decoder.

According to a fourth coding technique, the set of data representative of the quantization step profile can comprise all the quantization steps implemented.

This fourth coding technique corresponds to the case where the quantization step profile is determined as a function of the signal masking curve, known only to the encoder, and fully transmitted to the decoder. The requested bit rate is important, but the signal quality is optimal.

According to a particular embodiment, the coding implements a hierarchical processing delivering at least two hierarchical coding levels, comprising a basic level and at least one level of refinement comprising refinement information with respect to the basic level or at least one level of refinement. previous refinement level.

In this case, a fifth coding technique provides that the set of data representative of the quantization step profile is obtained, at a given level of refinement, taking into account data constructed at the previous hierarchical level.

The invention thus effectively applies to hierarchical coding, and proposes to code the quantization step profile according to a technique of refining it at each hierarchical level.

The selection step can be implemented at each level of hierarchical coding.

In the case where the coding method delivers frames of coefficients, the selection step can be implemented for each of the frames.

The signaling can thus be carried out not only for each processing frame, but, in the particular application of a hierarchical coding of the data, for each level of refinement.

In other cases, the coding can be implemented on groups of frames, of predefined or variable sizes. It can also be expected that the current profile remains unchanged until a new indicator has been transmitted.

The invention also relates to a coding device for a source audio signal, comprising means for implementing such a method.

The invention also relates to a computer program product for implementing the coding method as described above.

The invention also relates to a coded signal representative of a source audio signal according to claim 10.

Such a signal may in particular comprise data relating to at least two hierarchical levels obtained by a hierarchical processing, comprising a basic level and at least one level of refinement comprising refinement information relative to the basic level or to a level of refinement. previous, and includes an indicator representative of a coding technique for each level.

When the signal according to the invention is organized in frames of successive coefficients, it may comprise an indicator representative of the coding technique used for each of the frames.

The invention furthermore relates to a method for decoding such a signal, according to claim 14.

Such a decoding method also comprises a step of constructing a reconstructed audio signal, representative of the source audio signal, taking into account the reconstructed quantization step profile.

For at least a first of the coding techniques, the data set can correspond to a parametric representation of the quantization step profile, and the reconstruction step delivers a reconstructed quantization step profile in the form of at least one right segment.

For at least one second of the coding techniques, the data set may be empty and the reconstruction step delivers a constant quantization step profile.

For at least a third of the coding techniques, the data set may be empty, and the reconstruction step outputs a quantization step profile corresponding to an absolute hearing threshold.

For at least a fourth of the coding techniques, the data set may comprise all the quantization steps implemented during the coding method described above, and the construction step delivers a quantization step profile under the form of a set of quantization steps implemented during the coding process.

In a particular embodiment, the decoding method may implement hierarchical processing delivering two hierarchical decoding levels, comprising a base level and at least one refinement level comprising refinement information with respect to the base level or a previous level of refinement.

For at least one fifth of the coding techniques, the reconstruction step delivers a quantization step profile obtained, at a level of given refinement, taking into account data built at the previous hierarchical level.

The invention also relates to a device for decoding a coded signal representative of a source audio signal, comprising means for implementing the decoding method described above.

The invention also relates to a computer program product for implementing the decoding method as described above.

4. List of figures

• la figure 1 illustre le seuil de masquage fréquentiel ;
• la figure 2 est un organigramme simplifié du codage perceptif par transformée selon l'état de la technique ;
• la figure 3 illustre un exemple de signal selon l'invention ;
• la figure 4 est un organigramme simplifié du procédé de codage selon l'invention ;
• la figure 5 est un organigramme simplifié du procédé de décodage selon l'invention ;
• les figures 6A et 6B illustrent schématiquement un dispositif de codage et un dispositif de décodage mettant en oeuvre l'invention.

Other features and advantages of the invention will appear more clearly on reading the following description of a particular embodiment, given as a simple illustrative and nonlimiting example, and the appended drawings, among which:

• the figure 1 illustrates the frequency masking threshold;
• the figure 2 is a simplified flowchart of perceptual coding by transform according to the state of the art;
• the figure 3 illustrates an example signal according to the invention;
• the figure 4 is a simplified flowchart of the coding method according to the invention;
• the figure 5 is a simplified flowchart of the decoding method according to the invention;
• the Figures 6A and 6B schematically illustrate a coding device and a decoding device embodying the invention.

5. Description of an embodiment of the invention 5.1 Structure of the encoder

An embodiment of the invention is described below in the particular application of a hierarchical coding. It is recalled that, in this scheme, the coding hierarchical cascading perceptive quantization steps out of a time-frequency transform (for example an MDCT for "Modified Discrete Cosine Transform" in English, or modified discrete cosine transform) of the source audio signal to be encoded.

An encoder according to this embodiment of the invention is described in connection with the figure 4 . A source audio signal x ( t ) is intended to be transformed in the frequency domain directly or indirectly. Indeed, optionally, the signal x ( t ) may first be coded in a coding step 40. Such a step is implemented by a "core" coder. In this case, this first coding step corresponds to a first hierarchical level of coding, that is to say the basic level. Such a "core" coder can implement a coding step 401, and a local decoding step 402. It then delivers a first bitstream 46 representative of the data of the coded audio signal at the lowest level of refinement. Different coding techniques can be envisaged to obtain the low bit rate, such as parametric coding such as the sinusoidal coding described in the document. B. den Brinker, E. Schuijers and W. Oomen, "Parametric coding for high-quality audio", in Proc. 112nd AES Convention, Munich, Germany, 2002 or coding by analysis-synthesis type CELP (for Code-Excited Linear Prediction in English) decit in the document M. Schroeder and B. Atal, "Code-excited linear prediction (CELP): High quality speech at very low bit rates", in Proc. IEEE Int. Conf. Acoust., Speech, Signal Processing, Tampa, pp. 937-940, 1985 .

A subtraction 403 is performed between the decoded samples by the local decoder 402 and the actual values of x ( t ), so as to obtain a time domain residue signal r ( t ).

dans le domaine fréquentiel, représentatifs des résidus délivrés par le codeur « coeur » 40, pour chaque bande critique d'indice k et pour le premier niveau hiérarchique.It is then this residual signal at the output of the low-rate encoder 40 (or "core" encoder) that is transformed from the time space to the frequency space in step 41. Spectral coefficients are obtained ${\mathrm{<figure-callout\; id="1"\; label="R\; k"\; filenames="imgf0002.png"\; state="\{\{state\}\}">R\; </figure-callout>}}_k^{},$

in the frequency domain, representative of the residues delivered by the "core" coder 40, for each critical band of index k and for the first hierarchical level.

associée à une mise en oeuvre 422 d'un modèle psycho-acoustique, chargé de déterminer une première courbe de masquage pour le premier niveau de raffinement. On obtient alors en sortie de l'étape 421 de codage des coefficients de résidus quantifiés ${\hat{R}}_k^{\left(1\right)},$

lesquels sont soustraits (423) aux coefficients $R_k^{\left(1\right)}$

d'origine issus de l'étape 40 de codage « coeur». De nouveaux coefficients $R_k^{\left(2\right)}$

sont obtenus, qui sont eux-mêmes quantifiés et codés à l'étape 431 de codage du niveau 43 suivant. Là encore, un modèle 432 psycho-acoustique est mis en oeuvre et met à jour le seuil de masquage en fonction des coefficients ${\hat{R}}_k^{\left(1\right)}$

de résidus quantifiés précédemment.The next coding level stage 42 contains a residue coding step 421 ${\mathrm{<figure-callout\; id="1"\; label="R\; k"\; filenames="imgf0002.png"\; state="\{\{state\}\}">R\; </figure-callout>}}_k^{},$

associated with an implementation 422 of a psychoacoustic model, responsible for determining a first masking curve for the first level of refinement. At the output of the coding step 421, quantized residual coefficients are thus obtained. ${\hat{R}}_{\mathrm{<figure-callout\; id="1"\; label="k"\; filenames="imgf0002.png"\; state="\{\{state\}\}">k</figure-callout>}}^{\left(1\right)},$

which are subtracted (423) from the coefficients $R_k^{}$${}_{\left(1\right)}^{}$

origin from step 40 coding "heart". New coefficients $R_k^{}$${}_{\left(2\right)}^{}$

are obtained, which are themselves quantized and coded at the next level 431 encoding step 431. Here again, a psychoacoustic model 432 is implemented and updates the masking threshold as a function of the coefficients. ${\hat{R}}_{\mathrm{<figure-callout\; id="1"\; label="k"\; filenames="imgf0002.png"\; state="\{\{state\}\}">k</figure-callout>}}^{\left(1\right)}$

previously quantified residues.

In summary, the basic coding step 40 ("core" coder) allows the transmission and decoding, in a terminal, of a low bit rate version of the audio signal. The successive stages 42, 43 of quantization of the residues in the transformed domain constitute improvement layers, making it possible to construct a hierarchical bit stream from the low rate level to a desired maximum rate.

According to the invention, as illustrated by figure 4 an indicator ψ ⁽¹⁾ , ψ ⁽²⁾ is associated with each psychoacoustic model 422, 432 of each coding level, for each of the quantization stages. The value of this indicator is specific to each stage and controls the calculation mode of the quantization step profile. It is placed in the header 441 and 451 quantized spectral coefficient frames 442, 452 in the bitstreams 44, 45 associated and formed at each coding level 42, 43 improved.

An example structure of a signal obtained according to this coding technique is illustrated by the figure 3 . The signal is organized into blocks or frames of data 31 each comprising a header 32 and a data field 33. A block corresponds for example to the data (contained in the field 33) of a hierarchical level for a time interval predetermined. The header 32 may include several signaling information, decoding assistance ... It comprises at least, according to the invention, the information Ψ.

5.2 Structure of the decoder

In relation to the figure 5 , the decoding method implemented according to the invention is described, in the case of a hierarchical decoding of the signal of the figure 3 .

Similar to the coding method presented in connection with the figure 4 , the decoding comprises several levels 50, 51, 52 of decoding refinement.

A first decoding step 501 receives a bit stream 53 containing the data 530 representative of the indicator ψ ⁽¹⁾ of the first level, determined during the first coding step and transmitted to the decoder. The bitstream further contains the data 531 representative of the spectral coefficients of the audio signal.

According to the quantized coefficients, or the quantized coefficient residues, and the value of ψ ⁽¹⁾ received, a psychoacoustic model is implemented in a first step 502, to determine a first estimate of the masking curve, and thus a quantization step profile that is used to process the residuals of the spectral coefficients available to the decoder at this stage of the decoding process.

pour chaque bande critique d'indice k permettent une mise à jour du modèle psycho-acoustique au niveau 51 suivant, dans une étape 512, qui affine alors la courbe de masquage et donc le profil des pas de quantification. Ce raffinement prend donc en compte la valeur de l'indicateur ψ⁽²⁾ pour le niveau 2, contenu en en-tête 540 du train binaire 54 transmis par le codeur correspondant, les résidus quantifiés au niveau précédent ainsi que des données 541 quantifiées relatives aux résidus de niveau 2, comprises dans le train binaire 54.Spectral coefficient residues obtained ${\hat{R}}_{\mathrm{<figure-callout\; id="1"\; label="k"\; filenames="imgf0002.png"\; state="\{\{state\}\}">k</figure-callout>}}^{\left(1\right)}$

for each critical band of index k allow an update of the psycho-acoustic model at the next level 51, in a step 512, which then refines the masking curve and thus the profile of the quantization steps. This refinement therefore takes into account the value of the indicator ψ ⁽²⁾ for the level 2, contained in the header 540 of the bit stream 54 transmitted by the corresponding coder, the residues quantized at the previous level as well as the quantized data 541 relating to level 2 residues included in bit stream 54.

quantifiés sont obtenus en sortie du second niveau 51 de décodage. Ils sont additionnés (56) aux résidus ${\hat{R}}_k^{\left(1\right)}$

du niveau précédent, mais aussi injectés au niveau 52 suivant, qui, de façon similaire, affinera la précision sur les coefficients spectraux ainsi que sur le profil des pas de quantification, à partir d'une étape 521 de décodage, et de la mise en oeuvre d'un modèle psycho-acoustique dans une étape 522. Ce niveau reçoit de plus un train binaire 55· envoyé par le codeur contenant la valeur de l'indicateur 55 ψ⁽³⁾ et le spectre quantifié 551.Residues ${\hat{R}}_{\mathrm{<figure-callout\; id="2"\; label="k"\; filenames="imgf0002.png"\; state="\{\{state\}\}">k</figure-callout>}}^{\left(2\right)}$

quantized are obtained at the output of the second decoding level 51. They are added (56) to the residues ${\hat{R}}_{\mathrm{<figure-callout\; id="1"\; label="k"\; filenames="imgf0002.png"\; state="\{\{state\}\}">k</figure-callout>}}^{\left(1\right)}$

of the previous level, but also injected at the next level 52, which similarly will fine-tune the accuracy of the spectral coefficients as well as the profile of the quantization steps, from a decoding step 521, and the implementation of a psychoacoustic model in a step 522. This level further receives a bit stream 55 · sent by the encoder containing the value of the indicator 55 ψ ⁽³⁾ and the quantified spectrum 551.

quantifiés obtenus sont additionnés aux résidus ${\hat{R}}_k^{\left(2\right)},$

et ainsi de suite.Residues ${\hat{R}}_{\mathrm{<figure-callout\; id="3"\; label="k"\; filenames="imgf0002.png"\; state="\{\{state\}\}">k</figure-callout>}}^{\left(3\right)}$

quantified data are added to the residuals ${\hat{R}}_{\mathrm{<figure-callout\; id="2"\; label="k"\; filenames="imgf0002.png"\; state="\{\{state\}\}">k</figure-callout>}}^{\left(2\right)},$

And so on.

In summary, the psychoacoustic model is updated as the coefficients are decoded by the successive refinement levels. Reading the indicator Ψ transmitted by the encoder then makes it possible to reconstruct the noise profile (or quantization) by each quantization stage.

The steps of updating the psychoacoustic model and quantifying the spectral coefficients common to the coding method and to the decoding method according to a particular embodiment are described below in detail. The step of determining the value of the indicator ψ, carried out at the coding, is then detailed, followed by the step of reconstructing the quantization steps at the decoder.

5.3 Update of the psychoacoustic model

It is recalled that a psycho-acoustic model takes into account the sub-bands in which the ear breaks down an audio signal and thus determines masking thresholds by using psychoacoustic information. These thresholds are used to determine the quantization step of the spectral coefficients.

In the context of the present invention, the step (implementation in steps 422, 432 of the coding method and in the steps 502, 512, 522 of the decoding method) of updating the masking curve by the psycho-acoustic model remains unchanged regardless of the value of the indicator ψ on the choice of the profile of the quantization step.

On the other hand, it is the way in which this masking curve updated by the psychoacoustic model is used which is conditioned by the value of the indication Ψ to define the profile of the quantization step implemented for quantify the spectral coefficients (or residual coefficients determined at a previous refinement level).

d'un signal audio x(t), où k représente l'indice fréquentiel de la transformée temps-fréquence. Ce spectre est initialisé au premier niveau de raffinement de quantification, par les données disponibles en sortie de l'étape de codage mise en oeuvre par le codeur coeur. Aux niveaux de quantification suivants, le spectre ${\hat{X}}_k^{\left(l\right)}$

est actualisé à partir des coefficients résiduels ${\hat{R}}_k^{\left(l-1\right)}$

quantifiés en sortie du niveau de raffinement précédent selon la formule suivante : ${\hat{X}}_k^{\left(l\right)}={\hat{X}}_k^{\left(l-1\right)}+{\hat{R}}_k^{\left(l-1\right)},$

avec k = 0,...,N - 1, où N est la taille de la transformée dans le domaine fréquentiel.The psychoacoustic model uses at each quantization level (in the particular application of a hierarchical coding-decoding system) index 1 , the estimated spectrum ${\hat{X}}_k^{\left(l\right)}$

an audio signal x ( t ), where k represents the frequency index of the time-frequency transform. This spectrum is initialized at the first level of quantization refinement, by the data available at the output of the coding step implemented by the core coder. At the following quantification levels, the spectrum ${\hat{X}}_k^{\left(l\right)}$

is updated from the residual coefficients ${\hat{R}}_k^{\left(l-1\right)}$

quantized at the output of the preceding refinement level according to the following formula: ${\hat{X}}_k^{\left(l\right)}={\hat{X}}_k^{\left(l-1\right)}+{\hat{R}}_k^{\left(l-1\right)},$

with k = 0, ..., N - 1, where N is the size of the transform in the frequency domain.

avec le motif de masquage obtenu par le modèle psycho-acoustique, on peut construire un seuil de masquage associé au signal x(t).By convolution of the spectrum ${\hat{X}}_k^{\left(l\right)}$

with the masking pattern obtained by the psycho-acoustic model, we can build a masking threshold associated with the signal x ( t ).

estimée à l'étage de quantification d'indice l s'obtient alors comme le maximum entre le seuil de masquage associé au signal x(t) et la courbe d'audition absolue.The masking curve ${\hat{M}}_k^{\left(l\right)}$

estimated at the index quantization stage l is then obtained as the maximum between the masking threshold associated with the signal x ( t ) and the absolute auditory curve.

Moreover, the coding and decoding methods each comprise an initialization step Init of the psycho-acoustic model during its first implementation (step 422 of the coding method and step 502 of the decoding method), from the data transmitted by the heart coder.

Several scenarios are possible according to the type of "core" encoder implemented, some examples of which are described in the appendix.

5.4 Quantification of the spectral coefficients

Before describing precisely a technique for determining the best value of the indicator Ψ which determines the choice of the quantization step profile, we first detail the way in which the number of bits to be allocated is calculated in order to quantify each spectral coefficient of the quantization step. audio signal, that is to say once the profile of the quantization step is known.

5.4.1 Binary allocation

des coefficients résiduels $R_k^{\left(l\right)}$

en entrée de l'étage de quantification d'indice l s'obtiennent à partir du profil de pas de quantification, noté ${\mathrm{\Delta }}_n^{\left(l\right)}$

selon les équations suivantes : $${\mathit{rq}}_k^{\left(l\right)}=Q\left(g_l\frac{R_k^{\left(l\right)}}{{\mathrm{\Delta }}_n^{\left(l\right)}}\right)\mathrm{pour}\mathit{kOffset}\left(n\right)\le k\le \mathit{kOffset}\left(n+1\right)$$

et $${\hat{R}}_k^{\left(l\right)}=\frac{{\mathrm{\Delta }}_n^{\left(l\right)}}{g_l}{Q}^{-1}\left({\mathit{rq}}_k^{\left(l\right)}\right)\mathrm{pour}\mathit{kOffset}\left(n\right)\le k\le \mathit{kOffset}\left(n+1\right)$$

où ${\mathit{rq}}_k^{\left(l\right)}$

sont des coefficients à valeurs entières et kOffset(n) désigne l'indice fréquentiel initial de la bande critique d'indice n.We are here in the general case of a quantization law Q, which may for example correspond to the rounding to the nearest integer. Quantified values ${\hat{R}}_k^{\left(l\right)}$

residual coefficients $R_k^{\left(l\right)}$

at the input of the quantization stage of index l are obtained from the quantization step profile, noted ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}$

according to the following equations: $${\mathit{rq}}_k^{\left(l\right)}=Q\left({\mathrm{boy\; Wut}}_l\frac{R_k^{\left(l\right)}}{{\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}}\right)\mathrm{for}\mathit{kOffset}\left(\mathrm{not}\right)\le k\le \mathit{kOffset}\left(\mathrm{not}+1\right)$$

and $${\hat{R}}_k^{\left(l\right)}=\frac{{\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}}{{\mathrm{boy\; Wut}}_l}{Q}^{-1}\left({\mathit{rq}}_k^{\left(l\right)}\right)\mathrm{for}\mathit{kOffset}\left(\mathrm{not}\right)\le k\le \mathit{kOffset}\left(\mathrm{not}+1\right)$$

or ${\mathit{rq}}_k^{\left(l\right)}$

are integer coefficients and kOffset ( n ) denotes the initial frequency index of the critical band of index n .

The coefficient g _l corresponds to a constant gain making it possible to adjust the level of the quantization noise injected parallel to the profile given by ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}\mathrm{.}$

According to a first approach, this gain g _l is determined by an allocation loop in order to reach a target bit rate assigned to each quantization level of index l . It is then transmitted to the decoder in the bitstream at the output of the quantization stage.

According to a second approach, the gain g _l is a function of the single index refinement level l and this function is known to the decoder.

5.4.2 Quantization step profiles

de pas de quantification à partir d'un choix entre plusieurs techniques de codage, ou modes de calculs de ce profil. La sélection est indiquée par la valeur de l'indicateur ψ, transmis dans le train binaire. Selon la valeur de cet indicateur, le profil du pas de quantification est soit totalement transmis, soit partiellement, soit pas du tout. Dans ce dernier cas, le profil du pas de quantification est estimé au niveau du décodeur.The coding and decoding methods according to the invention then propose to determine a profile ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}$

no quantization from a choice between several coding techniques, or modes of calculation of this profile. The selection is indicated by the value of the indicator ψ transmitted in the bit stream. Depending on the value of this indicator, the quantization step profile is either fully transmitted, partially, or not at all. In the latter case, the profile of the quantization step is estimated at the decoder.

de pas de quantification utilisé par l'étage de quantification d'indice l est calculé à partir de la courbe de masquage disponible à cet étage et de l'indicateur ψ^(l) en entrée.The profile ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}$

The quantization step used by the index quantization stage 1 is calculated from the masking curve available at this stage and from the input indicator ψ ^{( 1 )} .

In a particular embodiment, the indicator ψ ^{( 1 )} is coded on 3 bits, to indicate five coding techniques different from the profile of the quantization step.

On dit qu'on quantifie au sens du rapport signal sur bruit (SNR).For a value of the indicator ψ ^{( l )} = 0, the masking curve estimated by the psychoacoustic model is not used and the profile of the quantization steps is uniform according to the formula ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}=\mathit{coast}\mathrm{.}$

We say that we quantify in the sense of the signal-to-noise ratio (SNR).

où Q_k désigne le seuil d'audition absolu.For a value of the indicator ψ ^{( l )} = 1, the quantization step profile is defined only from the absolute hearing threshold according to the equation ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}={\displaystyle \underset{k=\mathit{kOffset}\left(\mathrm{not}\right)}{\overset{\mathit{kOffset}\left(\mathrm{not}+1\right)-1}{\Sigma }}}{Q}_k,$

where Q _k denotes the absolute hearing threshold.

In this case, the encoder transmits to the decoder no information relating to the quantization step.

estimée par le modèle psycho-acoustique à l'étage d'indice l qui est utilisée pour définir le profil des pas de quantification selon l'équation ${\mathrm{\Delta }}_n^{\left(l\right)}={\displaystyle \sum _{k=\mathit{kOffset}\left(n\right)}^{\mathit{kOffset}\left(n+1\right)-1}}{\hat{M}}_k^{\left(l\right)}\mathrm{.}$

On note que ce mode n'est possible que dans l'application particulière où une construction hiérarchique de la courbe de masquage est mise en oeuvre dans le système de codage-décodage du signal audio.For a value of the indicator ψ ^{( l )} = 2, it is the masking curve ${\hat{M}}_k^{\left(l\right)}$

estimated by the psycho-acoustic model at the index stage l which is used to define the profile of the quantization steps according to the equation ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}={\displaystyle \underset{k=\mathit{kOffset}\left(\mathrm{not}\right)}{\overset{\mathit{kOffset}\left(\mathrm{not}+1\right)-1}{\Sigma }}}{\hat{M}}_k^{\left(l\right)}\mathrm{.}$

Note that this mode is only possible in the particular application where a hierarchical construction of the masking curve is implemented in the encoding-decoding system of the audio signal.

For a value of the indicator ψ ^{( l )} = 3, the profile of the quantization steps is then defined from a prototype of a parameterizable curve known to the decoder. According to a particular application, but not exclusively, this protoype is an affine straight line, in dB for each critical band of index n , of slope α. We denote D _n (α), with: log ₂ ( D _n (α)) = α n + K, where K is a constant.

The value of the slope α is chosen by correlation with the reference masking curve, calculated at the encoder from a spectral analysis of the signal to be coded. Its quantified value α is then transmitted to the decoder and used to define the profile of the quantization steps according to the formula: ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}=D_{\mathrm{not}}\left(\hat{\alpha }\right)\mathrm{.}$

déterminé à l'étape de codage est entièrement transmis au décodeur. Les pas de quantification sont par exemple définis à partir de la courbe de masquage de référence M_k calculée au codeur à partir du signal audio source à coder. On a alors : ${\mathrm{\Delta }}_n^{\left(l\right)}={\displaystyle \sum _{k=\mathit{kOffset}\left(n\right)}^{\mathit{kOffset}\left(n+1\right)-1}}{M}_k\mathrm{.}$

Finally, for a value of the indicator ψ ^{( l )} = 4, the profile of the quantization steps ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}$

determined at the coding step is fully transmitted to the decoder. The quantization steps are for example defined from the reference masking curve M _k calculated at the encoder from the source audio signal to be encoded. We then have: ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}={\displaystyle \underset{k=\mathit{kOffset}\left(\mathrm{not}\right)}{\overset{\mathit{kOffset}\left(\mathrm{not}+1\right)-1}{\Sigma }}}{M}_k\mathrm{.}$

5.5 Determination of the value of the indicator ψ

The invention proposes a particular technique for judiciously choosing the value of the indicator ψ, and therefore the quantization step profile to be applied for coding and decoding an audio signal. This choice is made at the coding step, for each quantization level (in the case of a hierarchical coding) of index l .

Le choix d'une valeur de l'indicateur ψ consiste à trouver le meilleur compromis entre l'optimalité du profil de pas de quantification, vis-à-vis de la distorsion perçue, et la minimisation du débit alloué à la transmission du profil des pas de quantification.Indeed, it is known that at a given quantization stage, the optimal quantization pitch profile with respect to the perceived distortion between the signal to be encoded and the reconstructed signal is obtained from the calculation of the masking curve. reference, based on the psycho-acoustic model and given by the formula: ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}={\displaystyle \underset{k=\mathit{kOffset}\left(\mathrm{not}\right)}{\overset{\mathit{kOffset}\left(\mathrm{not}+1\right)-1}{\Sigma }}}{M}_k^{\left(l\right)}\mathrm{.}$

The choice of a value of the indicator ψ consists of finding the best compromise between the optimality of the quantization step profile, with respect to the perceived distortion, and the minimization of the bit rate allocated to the transmission of the profile of the no quantification.

A cost function is introduced, to obtain such a compromise: $$\mathrm{VS}\left(\psi \right)=d\left({\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}\left(\psi \right),{\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}\left(\psi =4\right)\right)+\theta \left(\psi \right)\mathrm{with}\psi =0,1,2,3,4.$$

This function makes it possible to take into account the efficiency of each of the coding techniques of the quantization step profile.

est une mesure de distance entre le profil de pas de quantification associé à chacune des valeurs de l'indicateur ψ(ψ = 0,1,2,3,4) considérées et le profil optimal (associé à la valeur de l'indicateur Ψ = 4, correspondant à la transmission de la courbe de masquage de référence). Cette distance peut se mesurer comme le surcoût, en bits, associé à l'utilisation d'un profil de masquage « sous optimal ». Cette fonction coût est calculée selon la formule : $$d\left({\mathrm{\Delta }}_n^{\left(l\right)}\left(\psi \right),{\mathrm{\Delta }}_n^{\left(l\right)}\left(\psi =4\right)\right)={\displaystyle \sum _n}\left|{\log }_2\left({\mathrm{\Delta }}_n^{\left(l\right)}\left(\psi \right)\right)-{\log }_2\left({\mathrm{\Delta }}_n^{\left(l\right)}\left(\psi =4\right)\right)-{\log }_2\left(\frac{G_1}{G_2}\right)\right|$$

avec : $G_1={\displaystyle \sum _n}{\mathrm{\Delta }}_n^{\left(l\right)}\left(\psi \right)$

et $G_2={\displaystyle \sum _n}{\mathrm{\Delta }}_n^{\left(l\right)}\left(\psi =4\right)\mathrm{.}$

The first term $d\left({\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}\left(\psi \right),{\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}\left(\psi =4\right)\right)$

is a measure of the distance between the quantization step profile associated with each of the values of the indicator ψ (ψ = 0,1,2,3,4) considered and the optimal profile (associated with the value of the indicator Ψ = 4, corresponding to the transmission of the reference masking curve). This distance can be measured as the additional cost, in bits, associated with the use of a "suboptimal" masking profile. This cost function is calculated according to the formula: $$d\left({\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}\left(\psi \right),{\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}\left(\psi =4\right)\right)={\displaystyle \underset{\mathrm{not}}{\Sigma }}\left|{\log }_2\left({\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}\left(\psi \right)\right)-{\mathrm{<figure-callout\; id="2"\; label="log"\; filenames="imgf0002.png"\; state="\{\{state\}\}">log</figure-callout>}}_2\left({\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}\left(\psi =4\right)\right)-{\mathrm{<figure-callout\; id="2"\; label="log"\; filenames="imgf0002.png"\; state="\{\{state\}\}">log</figure-callout>}}_2\left(\frac{{\mathrm{<figure-callout\; id="1"\; label="BOY\; WUT"\; filenames="imgf0002.png"\; state="\{\{state\}\}">BOY\; WUT</figure-callout>}}_1}{{\mathrm{<figure-callout\; id="2"\; label="BOY\; WUT"\; filenames="imgf0002.png"\; state="\{\{state\}\}">BOY\; WUT</figure-callout>}}_2}\right)\right|$$

with: ${\mathrm{<figure-callout\; id="1"\; label="BOY\; WUT"\; filenames="imgf0002.png"\; state="\{\{state\}\}">BOY\; WUT</figure-callout>}}_1={\displaystyle \underset{\mathrm{not}}{\Sigma }}{\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}\left(\psi \right)$

and ${\mathrm{<figure-callout\; id="2"\; label="BOY\; WUT"\; filenames="imgf0002.png"\; state="\{\{state\}\}">BOY\; WUT</figure-callout>}}_2={\displaystyle \underset{\mathrm{not}}{\Sigma }}{\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}\left(\psi =4\right)\mathrm{.}$

The ratio of the gains G ₁ and G ₂ makes it possible to standardize the profiles of the quantization step with respect to each other.

des pas de quantification. En d'autres termes, il représente le nombre de bits additionnels (hormis ceux codant l'indicateur Ψ) devant être transmis au décodeur pour permettre la reconstruction des pas de quantification. Soit :

• θ(ψ) est nul pour ψ = 0,1,2 (correspondant respectivement aux techniques de codage de quantification constante, de seuil d'audition absolu et de courbe de masquage ré estimée lors de l'étape de décodage) ;
• θ(ψ) représente le nombre de bits codant α̂ lorsque ψ = 3 (correspondant à la technique de codage paramétrique du profil de pas de quantification) ;
• θ(ψ) est le nombre de bits codant les pas de quantification ${\mathrm{\Delta }}_n^{\left(l\right)}$
  
  définis à partir de la courbe de référence, lorsque ψ = 4 (correspondant à la transmission complète des pas de quantification du codeur vers le décodeur).

The second term θ (ψ) represents the additional cost in bits associated with the transmission of the profile ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}\left(\psi \right)$

quantization steps. In other words, it represents the number of additional bits (except those encoding the indicator Ψ) to be transmitted to the decoder to allow the reconstruction of quantization steps. Is :

• θ (ψ) is zero for ψ = 0,1,2 (corresponding respectively to the constant quantization, absolute hearing threshold and masking curve re estimation techniques during the decoding step);
• θ (ψ) represents the number of bits encoding α when ψ = 3 (corresponding to the parametric coding technique of the quantization step profile);
• θ (ψ) is the number of bits coding the quantization steps ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}$
  
  defined from the reference curve, when ψ = 4 (corresponding to the complete transmission of quantization steps from the encoder to the decoder).

5.6 Reconstruction of quantization steps in the decoding process

The reconstruction of the quantization step profile at an index quantization stage 1 is performed according to the data transmitted by the decoder.

• si ψ^(l) = 4, le décodeur lit l'ensemble des pas de quantification ${\mathrm{\Delta }}_n^{\left(l\right)};$
  
• si ψ^(l) = 3, le paramètre α̂ est lu et le profil du pas de quantification est calculé au décodeur selon la formule précédemment introduite : ${\mathrm{\Delta }}_n^{\left(l\right)}=D_n\left(\hat{\alpha }\right);$
  
• si Ψ^(l) = 2, le décodeur calcule le profil du pas de quantification selon la formule précédemment introduite : ${\mathrm{\Delta }}_n^{\left(l\right)}={\displaystyle \sum _{k=\mathit{kOffset}\left(n\right)}^{\mathit{kOffset}\left(n+1\right)-1}}{\hat{M}}_k^{\left(l\right)}$
  
  à partir de la courbe de masquage ${\hat{M}}_k^{\left(l\right)}$
  
  reconstruite à l'étage d'indice l (construction récursive) ;
• si ψ^(l) = 1, le décodeur calcule le profil du pas de quantification selon la formule précédemment introduite : ${\mathrm{\Delta }}_n^{\left(l\right)}={\displaystyle \sum _{k=\mathit{kOffset}\left(n\right)}^{\mathit{kOffset}\left(n+1\right)-1}}{Q}_k$
  
  basée sur le seuil d'audition absolu ;
• si ψ^(l) = 0 , le décodeur calcule le profil du pas de quantification selon la formule précédemment introduite : ${\mathrm{\Delta }}_n^{\left(l\right)}=\mathit{cte}\mathrm{.}$
  

First, whatever the coding technique of the quantization step chosen, that is to say the value of the indicator Ψ ^{( 1 )} , the decoder decodes the value of this indicator present in the header of the bit stream received for each frame, then read the value of the gain of adjustment g _l . We then distinguish cases according to the value of the indicator:

• if ψ ^{( l )} = 4, the decoder reads all the quantization steps ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)};$
  
• if ψ ^{( l )} = 3, the parameter α is read and the profile of the quantization step is calculated at the decoder according to the formula previously introduced: ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}=D_{\mathrm{not}}\left(\hat{\alpha }\right);$
  
• if Ψ ^{( l )} = 2, the decoder calculates the profile of the quantization step according to the previously introduced formula: ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}={\displaystyle \underset{k=\mathit{kOffset}\left(\mathrm{not}\right)}{\overset{\mathit{kOffset}\left(\mathrm{not}+1\right)-1}{\Sigma }}}{\hat{M}}_k^{\left(l\right)}$
  
  from the masking curve ${\hat{M}}_k^{\left(l\right)}$
  
  rebuilt on the floor of index l (recursive construction);
• if ψ ^{( l )} = 1, the decoder calculates the profile of the quantization step according to the formula previously introduced: ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}={\displaystyle \underset{k=\mathit{kOffset}\left(\mathrm{not}\right)}{\overset{\mathit{kOffset}\left(\mathrm{not}+1\right)-1}{\Sigma }}}{Q}_k$
  
  based on the absolute hearing threshold;
• if ψ ^{( l )} = 0, the decoder calculates the profile of the quantization step according to the formula previously introduced: ${\mathrm{\Delta }}_{\mathrm{not}}^{\left(l\right)}=\mathit{coast}\mathrm{.}$
  

transmis dans le train binaire sont décodés (relatifs aux données utiles des coefficients spectraux ou de leurs résiduels), les valeurs quantifiées ${\hat{R}}_k^{\left(l\right)}$

des coefficients résiduels à l'étage d'indice l s'obtiennent selon les formules introduites au paragraphe 5.5.1 de la présente description, relatif à l'allocation binaire.Once the quantization steps are calculated at the decoding step, and the previously introduced coefficients ${\mathit{rq}}_k^{\left(l\right)}$

transmitted in the bitstream are decoded (relative to the useful data of the spectral coefficients or their residuals), the quantified values ${\hat{R}}_k^{\left(l\right)}$

residual coefficients at the stage of index l are obtained according to the formulas introduced in paragraph 5.5.1 of the present description, relating to the binary allocation.

5.7 Implementing devices

The method of the invention can be implemented a coding device, whose structure is presented in relation to the Figure 6A .

Such a device comprises a memory M 600, a processing unit 601, equipped for example with a microprocessor, and controlled by the computer program Pg 602. At initialization, the code instructions of the computer program 602 are for example loaded into a RAM memory before being executed by the processor of the processing unit 601. The processing unit 601 receives as input a source audio signal to be coded 603. The microprocessor μP of the processing unit 601 implements the coding method described above, according to the instructions of the program Pg 602. The processing unit 601 outputs a bitstream 604 including in particular quantized data representative of the coded source audio signal, data representative of a quantization step profile, and finally representative data of the ψ indicator.

The invention also relates to a device for decoding a coded signal representative of a source audio signal according to the invention, whose simplified overall structure is schematically illustrated by the Figure 6B . It comprises a memory M 610, a processing unit 611, equipped for example with a microprocessor, and controlled by the computer program Pg 612. At initialization, the code instructions of the computer program 612 are for example loaded into a RAM before being executed by the processor of the processing unit 611. The processing unit 611 receives as input a bitstream 613, comprising data representative of a coded source audio signal, representative data a quantization step profile and representative data of the Ψ indicator. The microprocessor μP of the processing unit 611 implements the decoding method according to the instructions of the program Pg 612, to deliver a reconstructed audio signal 612.

ANNEX

The psychoacoustic model can be initialized in several ways, depending on the type of "core" coder implemented in the base level coding step.

1 Initialization from the parameters transmitted by a sinusoidal encoder

des composantes sinusoïdales du signal.A sinusoidal encoder models the audio signal by a sum of sinusoids of varying frequencies and amplitudes over time. The quantized values of the frequencies and amplitudes are transmitted to the decoder. From these values, we can build the spectrum ${\hat{X}}_{\mathrm{<figure-callout\; id="0"\; label="k"\; filenames="imgf0001.png"\; state="\{\{state\}\}">k</figure-callout>}}^{\left(0\right)}$

sinusoidal components of the signal.

2 Initialization from the parameters transmitted by a CELP encoder

où N est la taille de la transformée et P est le nombre de coefficients LPC transmis par le codeur CELP.From the linear prediction coding (LPC) coefficients α _m quantized and transmitted by a coder CELP (for "Code-excited linear prediction"), it is possible to deduce an envelope spectrum according to the following equation : ${\hat{X}}_{\mathrm{<figure-callout\; id="0"\; label="k"\; filenames="imgf0001.png"\; state="\{\{state\}\}">k</figure-callout>}}^{\left(0\right)}=\frac{1}{{\left|1-{\displaystyle \underset{m=1}{\overset{P}{\Sigma }}}{\mathrm{at}}_m\exp \left(-j\frac{2\mathit{\pi mk}}{\mathrm{NOT}}\right)\right|}^2}$

where N is the size of the transform and P is the number of LPC coefficients transmitted by the CELP coder.

3 Initialization from the decoded signal at the output of the core encoder

peut être simplement estimé à partir d'une analyse spectrale court-terme du signal décodé en sortie du codeur coeur.The initial spectrum ${\hat{X}}_{\mathrm{<figure-callout\; id="0"\; label="k"\; filenames="imgf0001.png"\; state="\{\{state\}\}">k</figure-callout>}}^{\left(0\right)}$

can be simply estimated from a short-term spectral analysis of the decoded signal at the output of the core coder.

peut être obtenu par addition du spectre d'enveloppe LPC défini selon l'équation précédente, et du spectre à court terme estimé à partir du résidu codé par un codeur CELP.A combination of these initialization methods is also possible. For example, the initial spectrum ${\hat{X}}_{\mathrm{<figure-callout\; id="0"\; label="k"\; filenames="imgf0001.png"\; state="\{\{state\}\}">k</figure-callout>}}^{\left(0\right)}$

can be obtained by adding the LPC envelope spectrum defined according to the preceding equation, and the estimated short-term spectrum from the coded residue by a CELP coder.

Claims (15)

1. Method for encoding a source audio signal, characterized in that it comprises the following steps:

   - encoding of a quantization step profile of coefficients representative of at least one transform of said source audio signal, according to at least two distinct encoding techniques, delivering at least two data sets representative of the quantization step profile;

   - selection of one of the data sets representative of the quantization step profile, as a function of a selection criterion making a compromise between a perceived distortion between said source audio signal to be encoded and signals reconstructed respectively on the basis of said data sets, and the bit rate necessary to encode said data sets, said selection criterion being obtained by comparison of a reference masking curve estimated on the basis of the audio signal to be encoded, with said data sets;

   - transmission and/or storage of said data set representative of the selected quantization step profile and of an indicator representative of the corresponding encoding technique.
2. Encoding method according to Claim 1, characterized in that said parametric representation is formed of at least one line segment, characterized by a slope and a value at the origin.
3. Encoding method according to either one of Claims 1 and 2, characterized in that a second of said encoding techniques delivers a constant quantization step profile.
4. Encoding method according to any one of Claims 1 to 3, characterized in that, according to a third encoding technique, said quantization step profile corresponds to an absolute hearing threshold.
5. Encoding method according to any one of Claims 1 to 4, characterized in that, according to a fourth encoding technique, said data set representative of the quantization step profile comprises all the quantization steps used.
6. Encoding method according to any one of Claims 1 to 5, characterized in that said encoding uses a hierarchical process delivering at least two hierarchical levels of encoding, comprising a base level and at least one refinement level comprising refinement information relative to said base level or to a previous refinement level.
7. Encoding method according to Claim 6, characterized in that, according to a fifth encoding technique, said data set representative of the quantization step profile is obtained, at a given refinement level, taking account of data constructed at the previous hierarchical level.
8. Device for encoding a source audio signal, characterized in that it comprises:

   - means for encoding a quantization step profile of coefficients representative of at least one transform of said source audio signal, delivering at least two data sets representative of the quantization step profile;

   - means for selecting one of said data sets representative of the quantization step profile, according to a selection criterion making a compromise between a perceived distortion between said source audio signal to be encoded and signals reconstructed respectively on the basis of said data sets and on the bit rate necessary to encode said data sets, said selection criterion being obtained by comparison of a reference masking curve, estimated on the basis of the audio signal to be encoded, with said data sets;

   - means for the transmission and/or storage of said data set representative of the selected quantization step profile and of an indicator representative of the corresponding encoding technique.
9. Computer program product that can be downloaded from a communication network and/or stored on a medium that can be read by computer and/or that can be executed by a microprocessor, characterized in that it comprises program code instructions for executing the encoding method according to at least one of Claims 1 to 7.
10. Encoded signal representative of a source audio signal, comprising data representative of a quantization step profile,
    characterized in that it comprises:

    - an indicator representative of a technique for encoding the quantization step profile used, chosen on the encoding from at least two available techniques, according to a selection criterion making a compromise between a perceived distortion between said source audio signal to be encoded and signals reconstructed respectively on the basis of the encoded quantization step profile according to said techniques and on the bit rate necessary to encode the quantization step profile according to said techniques, said selection criterion being obtained on the basis of a reference masking curve, estimated on the basis of the audio signal to be encoded;

    - a data set representative of said corresponding quantization step profile.
11. Signal according to Claim 10, characterized in that it comprises data relating to at least two hierarchical levels obtained by a hierarchical process, comprising a base level and at least one refinement level comprising refinement information relative to said base level or to a preceding refinement level,
    and in that it comprises an indicator representative of an encoding technique for each of said levels.
12. Signal according to either one of Claims 10 and 11, characterized in that it is organized into successive frames of coefficients, and in that it comprises an indicator representative of an encoding technique for each of said frames.
13. Method for decoding an encoded signal representative of a source audio signal, comprising data representative of quantization step profile, characterized in that it comprises the following steps:

    - extraction from said encoded signal:

    - of an indicator representative of a technique for encoding a quantization step profile used, chosen on encoding from at least two available techniques, according to a selection criterion making a compromise between a perceived distortion between said source audio signal to be encoded and signals reconstructed respectively on the basis of the encoded quantization step profile according to said techniques and on the bit rate necessary to encode the quantization step profile according to said techniques, said selection criterion being obtained on the basis of a reference masking curve estimated on the basis of the audio signal to be encoded;

    - of a data set representative of said corresponding quantization step profile;

    - reconstruction of said reconstructed quantization step profile, according to said data set and to the encoding technique designated by said indicator.
14. Device for decoding an encoded signal representative of a source audio signal, comprising data representative of a quantization step profile, characterized in that it comprises:

    - means for extracting from said encoded signal:

    - of an indicator representative of a technique for encoding a quantization step profile used, chosen on encoding from at least two available techniques, according to a selection criterion making a compromise between a perceived distortion between said source audio signal to be encoded and signals reconstructed respectively on the basis of the encoded quantization step profile according to said techniques and on the bit rate necessary to encode the quantization step profile according to said techniques, said selection criterion being obtained on the basis of a reference masking curve estimated on the basis of the audio signal to be encoded;

    - of a data set representative of said corresponding quantization step profile;

    - means for reconstructing said reconstructed quantization step profile according to said data set and the encoding technique designated by said indicator.
15. Computer program product that can be downloaded from a communication network and/or that can be stored on a medium that can be read by computer and/or that can be executed by a microprocessor, characterized in that it comprises program code instructions for executing the decoding method according to Claim 13.

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