EP2517485B1 - Method of generating left and right surround sound signals on the basis of a stereo sound signal - Google Patents

Description

The invention relates to a method for generating left and right "surround" sound signals from a stereo sound signal intended to be broadcast by a 5.1 type sound system. The invention aims in particular to provide a method of processing the surround sound providing a good sound reproduction while consuming few resources.

The invention finds a particularly advantageous application in the field of sound processing for home cinema, stereos, computers, mobile phones or any other device having a sound diffusion system with limited or no computation capabilities. the treatment of sound.

It is recalled that a 5.1 broadcasting system is an acoustic system comprising two front channels (left and right), two rear channels (left and right), a Low Frequency Effect channel, and a central channel. The sound broadcast by this kind of system is a so-called "surround" sound that provides a feeling of envelopment to the listener.

A system for producing such "surround sound" is for example described in the document JP 2007 336118

The invention is described below in an application for type 5.1 acoustic systems, however it could be implemented with any other acoustic system based on the creation of sound signals to give a feeling of sound wrap to the sound. 'auditor.

• une composante décorrélée avec celle de l'autre signal destinée à être diffusée sur les voies avant d'un système de type 5.1 ;
• une composante corrélée et en phase par rapport à celle de l'autre signal destinée à être diffusée sur la voie centrale d'un système de type 5.1 ; et
• une composante dite composante « surround » corrélée et hors phase par rapport à celle de l'autre signal destinée normalement à être diffusée sur les voies arrière d'un système de type 5.1.

More precisely, it is possible to consider that the left and right signals forming a stereo signal are each composed of 3 distinct components:

• a component decorrelated with that of the other signal intended to be broadcast on the front channels of a type 5.1 system;
• a component correlated and in phase with that of the other signal intended to be broadcast on the central channel of a type 5.1 system; and
• a so-called "surround" component correlated and out of phase with that of the other signal normally intended to be broadcast on the rear channels of a type 5.1 system.

The object of the invention is to create these surround components in order to reproduce a good quality surround sound effect while limiting the calculation time during the extraction of these components.

For this purpose, in the method according to the invention, a subtraction of the stereo signals from its left and right is carried out with respect to the other, so as to eliminate the correlated common component and in phase of the stereo signals. Then, the signal resulting from this subtraction step is decorrelated by means of a decorrelation module, so as to obtain the left and right "surround" components of the stereo signal ready to be broadcast, if necessary after parametric equalization. on the rear tracks of a 5.1-type system.

In addition, the compromise between the processing power of the treatment and the "surround" sound effect obtained is excellent with the method according to the invention.

• on soustrait un des signaux stéréo à l'autre signal à l'aide d'un module de soustraction pour obtenir un signal unique de soustraction dans lequel les composantes corrélées et en phase des signaux stéréo ont été supprimées,
• on génère, à partir du signal de soustraction, un signal de son droit et un signal de son gauche décorrélés l'un par rapport à l'autre correspondant respectivement aux signaux « surround » de son droit et gauche.

The invention therefore relates to a method of generating left and right signals called "surround" signals from a stereo sound signal composed of a signal of its left and a signal of its right, characterized in that it has the following steps:

• subtracting one of the stereo signals to the other signal using a subtraction module to obtain a single subtraction signal in which the correlated and in-phase components of the stereo signals have been suppressed,
• from the subtraction signal, a signal of its right and a signal of its left uncorrelated with respect to the other corresponding respectively to the signals "surround" of its right and left are generated.

According to one implementation, the "surround" signals are intended to be broadcast on the rear channels of a 5.1 type sound system.

• on applique le signal de soustraction en entrée d'un premier et d'un deuxième bloc élémentaire, le signal de sortie de ces blocs correspondant respectivement au signal de son droit et au signal de son gauche surround,
• le signal de sortie de chaque bloc est la combinaison du signal d'entrée du bloc pondéré par un premier gain, et de la combinaison du signal de sortie du bloc pondéré par un deuxième gain et des signaux d'entrée du bloc retardée par une ligne à retard.

According to one embodiment, for generating, from the subtraction signal, the signals of its left and right "surround" uncorrelated with respect to each other,

• the input subtraction signal of a first and a second elementary block is applied, the output signal of these blocks corresponding respectively to the signal of its right and to the signal of its left surround,
• the output signal of each block is the combination of the input signal of the weighted block by a first gain, and the combination of the output signal of the weighted block by a second gain and the input signals of the delayed block by a line. late.

• pour le premier bloc élémentaire, on a : $${\mathrm{s}}_{\mathrm{1}}\left(\mathrm{n}\right)={\mathrm{e}}_{\mathrm{1}}\left(\mathrm{n}\right)\mathrm{.}{\mathrm{<figure-callout\; id="1"\; label="g"\; filenames="imgf0001.png"\; state="\{\{state\}\}">g</figure-callout>}}_{\mathrm{1}}+{\mathrm{<figure-callout\; id="1"\; label="s"\; filenames="imgf0001.png"\; state="\{\{state\}\}">s</figure-callout>}}_{\mathrm{1}}\left(\mathrm{n}-\mathrm{D}\mathrm{1}\right)\mathrm{.}{\mathrm{<figure-callout\; id="2"\; label="g"\; filenames="imgf0001.png"\; state="\{\{state\}\}">g</figure-callout>}}_{\mathrm{2}}+{\mathrm{<figure-callout\; id="1"\; label="e"\; filenames="imgf0001.png"\; state="\{\{state\}\}">e</figure-callout>}}_{\mathrm{1}}\left(\mathrm{n}-\mathrm{D}\mathrm{1}\right),$$
  
  • e₁ étant le signal d'entrée du premier bloc correspondant au signal de soustraction,
  • s₁ étant le signal de sortie du premier bloc correspondant à un des signaux de son surround (droit ou gauche),
  • g₁, g₂ étant respectivement les valeurs du premier gain et du deuxième gain du premier bloc,
  • D₁ étant la valeur du nombre d'échantillons de retard introduits par la ligne à retard, et
• pour le deuxième bloc élémentaire, on a : $${\mathrm{s}}_2\left(\mathrm{n}\right)={\mathrm{e}}_2\left(\mathrm{n}\right)\mathrm{.}{\mathrm{g}}_3+{\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="imgf0001.png"\; state="\{\{state\}\}">s</figure-callout>}}_2\left(\mathrm{n}-\mathrm{D}2\right)\mathrm{.}{\mathrm{g}}_4+{\mathrm{<figure-callout\; id="2"\; label="e"\; filenames="imgf0001.png"\; state="\{\{state\}\}">e</figure-callout>}}_2\left(\mathrm{n}-\mathrm{D}\mathrm{2}\right),$$
  
  • e₂ étant le signal d'entrée du deuxième bloc correspondant au signal de soustraction,
  • s₂ étant le signal de sortie du deuxième bloc correspondant à l'autre signal de son surround (droit si s₁ correspond au gauche ou gauche si s₁ correspond au droit),
  • g₃, g₄ étant respectivement les valeurs du premier gain et du deuxième gain du deuxième bloc,
  • D2 étant la valeur du nombre d'échantillons de retard introduits par la ligne à retard.

According to one implementation:

• for the first elementary block, we have: $${\mathrm{<figure-callout\; id="1"\; label="s"\; filenames="imgf0001.png"\; state="\{\{state\}\}">s</figure-callout>}}_{\mathrm{1}}\left(\mathrm{not}\right)={\mathrm{<figure-callout\; id="1"\; label="e"\; filenames="imgf0001.png"\; state="\{\{state\}\}">e</figure-callout>}}_{\mathrm{1}}\left(\mathrm{not}\right)\mathrm{.}{\mathrm{boy\; Wut}}_{\mathrm{1}}+{\mathrm{<figure-callout\; id="1"\; label="s"\; filenames="imgf0001.png"\; state="\{\{state\}\}">s</figure-callout>}}_{\mathrm{1}}\left(\mathrm{not}-\mathrm{D}\mathrm{1}\right)\mathrm{.}{\mathrm{boy\; Wut}}_{\mathrm{2}}+{\mathrm{<figure-callout\; id="1"\; label="e"\; filenames="imgf0001.png"\; state="\{\{state\}\}">e</figure-callout>}}_{\mathrm{1}}\left(\mathrm{not}-\mathrm{D}\mathrm{1}\right),$$
  
  • e ₁ being the input signal of the first block corresponding to the subtraction signal,
  • where s ₁ is the output signal of the first block corresponding to one of the surround sound signals (right or left),
  • g ₁ , g ₂ being respectively the values of the first gain and the second gain of the first block,
  • D ₁ being the value of the number of delay samples introduced by the delay line, and
• for the second elementary block, we have: $${\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="imgf0001.png"\; state="\{\{state\}\}">s</figure-callout>}}_2\left(\mathrm{not}\right)={\mathrm{<figure-callout\; id="2"\; label="e"\; filenames="imgf0001.png"\; state="\{\{state\}\}">e</figure-callout>}}_2\left(\mathrm{not}\right)\mathrm{.}{\mathrm{boy\; Wut}}_3+{\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="imgf0001.png"\; state="\{\{state\}\}">s</figure-callout>}}_2\left(\mathrm{not}-\mathrm{D}2\right)\mathrm{.}{\mathrm{boy\; Wut}}_4+{\mathrm{<figure-callout\; id="2"\; label="e"\; filenames="imgf0001.png"\; state="\{\{state\}\}">e</figure-callout>}}_2\left(\mathrm{not}-\mathrm{D}\mathrm{2}\right),$$
  
  • e ₂ being the input signal of the second block corresponding to the subtraction signal,
  • s ₂ being the output signal of the second block corresponding to the other surround sound signal (right if s ₁ corresponds to the left or left if s ₁ corresponds to the right),
  • g ₃ , g ₄ being respectively the values of the first gain and the second gain of the second block,
  • D2 being the value of the number of delay samples introduced by the delay line.

According to one implementation, the gain values inside a block are opposite to each other, the value of the first gain being opposite to the value of the second gain.

According to one implementation, the gain values of the first block are opposite to the gain values of the second block, the value of the first gain of the first block being opposite to the value of the first gain of the first block. second block; while the value of the second gain of the first block is opposite to the value of the second gain of the second block.

According to one implementation, the gain values of the first and second elementary blocks have the same absolute value.

According to one implementation, the first gain of the first block and the second gain of the second block are equal to g; while the second gain of the first block and the first gain of the second block are worth -g.

According to one implementation, the delay introduced by the line of the first block and the delay by the line of the second block are equal.

According to one embodiment, the subtraction signal is firstly filtered using a high frequency filter and only the filtered high frequency part is applied to the input of the elementary blocks.

• on filtre la partie basse fréquence des signaux gauche et droit du signal stéréo,
• on retarde les parties basses fréquences ainsi filtrées d'un retard à l'aide de troisièmes lignes à retard, et
• on somme la partie basses fréquences ainsi retardée avec les signaux de sortie des blocs élémentaires pour obtenir le signal de son droit surround et le signal de son gauche surround.

According to one implementation,

• the low frequency part of the left and right signals of the stereo signal is filtered,
• the low frequency parts thus filtered are delayed by a delay with the aid of third delay lines, and
• the low frequency part thus delayed is summed with the output signals of the elementary blocks to obtain the signal of its surround right and the signal of its left surround.

According to one embodiment, the phase and the gain of the output signals of each elementary block are modified as a function of the frequency by means of parametric filtering cells to modify the sound perception of the left and right surround sound signals.

• on génère les signaux de son « surround » à l'aide du procédé selon l'une des revendications précédentes et on les applique sur les voies arrière du système acoustique.

The invention further relates to a method of generating surround signals to be broadcast by an acoustic system consisting of a front / rear speaker from a stereo sound signal composed of a left signal and a signal. of his right, characterized in that it presents the following step:

• the "surround" sound signals are generated by the method according to one of the preceding claims and are applied to the rear channels of the acoustic system.

• Figure 1 : une représentation schématique d'un dispositif selon l'invention permettant l'extraction des composantes « surrounds » d'un signal de son stéréo dans une version basique ;
• Figure 2 : une représentation graphique d'un dispositif selon l'invention permettant l'extraction des composantes « surrounds » d'un signal de son stéréo dans une version perfectionnée ;
• Figure 3 : une représentation schématique d'un dispositif selon l'invention permettant de générer, à partir d'un signal de son stéréo, des signaux de son diffusables par un système acoustique de type 5.1.

The invention will be better understood on reading the description which follows and the examination of the figures which accompany it. These figures are given for illustrative but not limiting of the invention. They show :

• Figure 1 : a schematic representation of a device according to the invention for extracting the "surround" components of a stereo sound signal in a basic version;
• Figure 2 : a graphic representation of a device according to the invention for extracting the "surround" components of a stereo sound signal in an improved version;
• Figure 3 : a schematic representation of a device according to the invention for generating, from a stereo sound signal, sound signals diffusable by a 5.1 type acoustic system.

Identical elements retain the same reference from one figure to another.

The Figure 1 shows a device 10 according to the invention for generating a left sound signal L _S and a signal of its right R _S said "surround" signals from a stereo sound signal formed of a left sound signal L and a signal from his right R.

• L = L₀+C+S pour le signal de son gauche L, et
• R = R₀+C+(-S) pour le signal de son droit R.
• Les composantes L₀ et S₀ sont les composantes décorrélées entre elles,
• La composante C est commune aux deux signaux L et R puisqu'elle correspond aux composantes corrélées et en phase des signaux L et R
• La composante S correspond à la composante corrélée et hors phase des signaux L et R.

It is considered that the signal of its left L and the signal of its right R are formed respectively of a sum of 3 components:

• L = L ₀ + C + S for the left signal L, and
• R = R ₀ + C + (- S) for the signal of his right R.
• The components L ₀ and S ₀ are the components decorrelated between them,
• The component C is common to the two signals L and R since it corresponds to the correlated components and in phase of the signals L and R
• The S component corresponds to the correlated and out-of-phase component of the L and R signals.

The object of the invention is to isolate the component S and to create two decorrelated components in order to be able to broadcast them on channels distinct from those where the stereo signals L and R are broadcast in order to provide a enveloping effect to the listener ( cf. Figure 3 ).

For this purpose, the signals of its left L and right R are applied at the input of a subtractor 11 in order to suppress the component C of the left and right signals R and retain only the components L ₀ , R ₀ and the component S stereo signals L and R. Here, we subtract the signal from its right R to the left-hand signal L (LR), but it would be possible to perform the reverse operation (RL).

The subtraction signal LR obtained at the output of the subtracter 11 is then applied at the input of a signal decorrelation module 12 which makes it possible to create, from the subtraction signal LR, two signals decorrelated with respect to each other. : the signal of its left "surround" L _S and the signal of its right "surround" R _S.

For this purpose, the decorrelation module 12 is formed of two input 13.1-13.2 blocks from which the subtraction signal LR is applied, the output s1, s2 of these blocks 13.1, 13.2 respectively corresponding to the signal of its surround right R _S and to the signal of his left surround L _S. The output signal s ₁ (resp., S ₂ ) of each block 13.1 (respectively 13.2) is a function of the input signal e ₁ (respectively e ₂ ) of the weighted block by a first gain g ₁ (respectively g ₃ ), and of the combination of the input signals e1 (respectively e2) and of the output s ₁ (respectively s ₂ ) of the weighted block by a second gain g ₂ (respectively g ₄ ), delayed by a delay line 14.1 (respectively 14.2).

According to one embodiment, for each elementary block 13.1, 13.2, the input signal e ₁ , e ₂ is applied at the input of a first adder 16.1, 16.2 and applied to an input of a second adder 17.1, 17.2 after having been multiplied by the first gain g ₁ , g ₃ . The output signal s ₁ , s ₂ of the block is applied to another input of the first summer 16.1, 16.2 after having been multiplied by the second gain g ₂ , g ₄ , the output signal of the first summer 16.1, 16.2 being applied in input of delay line 14.1, 14.2. The output signal of the delay line 14.1, 14.2 is applied to another input of the second adder 17.1, 17.2, the output signal of this second adder 17.1, 17.2 corresponding to the output signal s ₁ , s ₂ of the block and therefore to the right signal R _S or left L _S surround.

• e₁ étant le signal d'entrée du premier bloc 13.1 correspondant au signal de soustraction (L-R),
• s₁ étant le signal de sortie du premier bloc 13.1 correspondant à un des signaux de son surround (droit R_S ou gauche L_S),
• g₁, g₂ étant respectivement les valeurs du premier gain et du deuxième gain du premier bloc 13.1,
• D1 étant la valeur du nombre d'échantillons de retard introduits par la ligne à retard 14.1.

Thus for the first elementary block 13.1, we have: $${\mathrm{<figure-callout\; id="1"\; label="s"\; filenames="imgf0001.png"\; state="\{\{state\}\}">s</figure-callout>}}_{\mathrm{1}}\left(\mathrm{not}\right)={\mathrm{<figure-callout\; id="1"\; label="e"\; filenames="imgf0001.png"\; state="\{\{state\}\}">e</figure-callout>}}_{\mathrm{1}}\left(\mathrm{not}\right)\mathrm{.}{\mathrm{boy\; <figure-callout\; id="1"\; label="Wut"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Wut</figure-callout>}}_{\mathrm{1}}+{\mathrm{<figure-callout\; id="1"\; label="s"\; filenames="imgf0001.png"\; state="\{\{state\}\}">s</figure-callout>}}_{\mathrm{1}}\left(\mathrm{not}-\mathrm{D}\mathrm{1}\right)\mathrm{.}{\mathrm{boy\; <figure-callout\; id="2"\; label="Wut"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Wut</figure-callout>}}_{\mathrm{2}}+{\mathrm{<figure-callout\; id="1"\; label="e"\; filenames="imgf0001.png"\; state="\{\{state\}\}">e</figure-callout>}}_{\mathrm{1}}\left(\mathrm{not}-\mathrm{D}\mathrm{1}\right)$$

• e ₁ being the input signal of the first block 13.1 corresponding to the subtraction signal (LR),
• s ₁ being the output signal of the first block 13.1 corresponding to one of the surround sound signals (right R _S or left L _S ),
• g ₁ , g ₂ being respectively the values of the first gain and the second gain of the first block 13.1,
• D1 being the value of the number of delay samples introduced by the delay line 14.1.

• e₂ étant le signal d'entrée du deuxième bloc 13.2 correspondant au signal de soustraction (L-R),
• s₂ étant le signal de sortie du deuxième bloc 13.2 correspondant à l'autre signal de son surround (droit R_S si s₁ correspond au gauche ; ou gauche L_S si s₁ correspond au droit),
• g₃, g₄ étant respectivement les valeurs du premier gain et du deuxième gain du deuxième bloc 13.2,
• D2 étant la valeur du nombre d'échantillons de retard introduits par la ligne à retard 14.2.

For the second elementary block 13.2, we have: $${\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="imgf0001.png"\; state="\{\{state\}\}">s</figure-callout>}}_2\left(\mathrm{not}\right)={\mathrm{<figure-callout\; id="2"\; label="e"\; filenames="imgf0001.png"\; state="\{\{state\}\}">e</figure-callout>}}_2\left(\mathrm{not}\right)\mathrm{.}{\mathrm{boy\; Wut}}_3+{\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="imgf0001.png"\; state="\{\{state\}\}">s</figure-callout>}}_2\left(\mathrm{not}-\mathrm{D}2\right)\mathrm{.}{\mathrm{boy\; Wut}}_4+{\mathrm{<figure-callout\; id="2"\; label="e"\; filenames="imgf0001.png"\; state="\{\{state\}\}">e</figure-callout>}}_2\left(\mathrm{not}-\mathrm{D}\mathrm{2}\right)$$

• e ₂ being the input signal of the second block 13.2 corresponding to the subtraction signal (LR),
• s ₂ being the output signal of the second block 13.2 corresponding to the other surround sound signal (right R _S if s ₁ corresponds to the left, or left L _S if s ₁ corresponds to the right),
• g ₃ , g ₄ being respectively the values of the first gain and the second gain of the second block 13.2,
• D2 being the value of the number of delay samples introduced by the delay line 14.2.

Preferably, within the same block 13.1 (respectively 13.2), the first gain g ₁ (respectively g ₃ ) and the second gain g ₂ (respectively g ₄ ) have opposite values, one for each report to the other. Each block 13.1, 13.2 then behaves like an all-pass type filter which does not modify the gain of the input signal e ₁ , e ₂ but only its phase.

In addition, the gains g ₁ , g ₂ of the first block 13.1 and the gains g ₃ , g ₄ of the second block 13.2 preferably have opposite values from each other. Thus, the value of the first gain g ₁ of the first block 13.1 is opposite to the value of the first gain g ₃ of the second block 13.2; while the value of the second gain g ₂ of the first block 13.1 is opposite to the value of the second gain g ₄ of the second block 13.2.

We will also preferably choose gains for the first 13.1 and the second 13.2 block which have an identical absolute value g. Thus, preferably, the first gain g ₁ of the first block 13.1 and the second gain g ₄ of the second block 13.2 have a value g; while the second gain g ₂ of the first block 13.1 and the first g ₃ gain of the second block 13.2 has a value -g.

Preferably, the delays D1, D2 introduced by the delay line 14.1 of the first elementary block 13.1 and the delay line 14.2 of the second elementary block 13.2 are equal. However, it would be possible to choose delays D1, D2 having different durations.

In an exemplary embodiment, g = 0.4 and a delay of D1 and D2 of 176 samples with a sampling frequency of 44.1 kHz are chosen, such values making it possible to obtain a good sound reproduction.

In an improvement of the invention shown in the Figure 2 in addition, a stage 19 composed of two filters 20.1, 20.2 is used to isolate the low frequency part respectively of the L and R signals and of a filter 21 making it possible to isolate the high frequency part of the subtraction signal LR.

In this case, only the high frequency portion of the L-R signal is applied to the input of the decorrelation module 12. In one example, the cut-off frequencies of the low-frequency filters 20.1, 20.2 and the high-frequency filter 21 are of the order of 350 Hz.

The low frequency parts of the left and right signals are applied as third delay line inputs 23.1, 23.2 and the low frequency parts thus delayed are summed, if necessary after weighting by gains g ₇ , g ₈ respectively with the output signals s ₁ , s ₂ of the elementary blocks, so as to obtain signals of its right R _S and left L _S surround having improved sound reproduction. In one for example, the delay D3 applied by the third delay lines 23.1, 23.2 is 176 samples with a sampling frequency of 44.1 kHz

In addition, parametric equalizing cells 25.1, 25.2 are connected at the output of each elementary block 13.1, 13.2 before summation with the delayed low frequency part. These cells 25.1, 25.2 have the effect of modifying the perception of the output signals s ₁ , s ₂ of these blocks 13.1, 13.2, because even if the signals s ₁ , s ₂ have substantially identical levels, there are differences in their perception because of the decorrelation they exhibit relative to each other. As a result, it may be useful to modify these signals perceptively so that the overall auditory impression is the best possible.

For this purpose, the equalizing cells 25.1, 25.2 each comprise a filter 26.1, 26.2 whose type, the gain and the phase can be adjusted according to different frequency bands of the signals s ₁ , s ₂ and a gain g ₅ , g ₆ which acts on the entire spectrum of signals s ₁ , s ₂ . These parameters are adapted by sound engineers in particular according to the intended application.

The Figure 3 shows a use of the invention in the context of a 4-way (2-way front and 2-way rear) or 5-way sound system (with a central channel in addition) to obtain excellent sound reproduction while by limiting the computing power of sound processing.

More specifically, in the context of this use, the left stereo signals L and R right are applied to the input of the module 10 Figures 1 or 2 in order to extract the left surround signals L _S and right R _S which are broadcast on the rear channels 29.1, 19.2; while the initial left L and right S stereo signals are directly broadcast on the front channels 30.1, 30.2, where appropriate after parametric equalization performed using the modules 31.1, 31.2 (similar to the modules 25.1, 25.2 of the Figure 2 ).

Optionally, the common component C to the L and R signals is also extracted using a module 33 (an implementation example of such a module is given in the document FR-2886503 ) to be broadcast on the central lane 34.

Claims (10)

1. Method of generating left (L_s) and right (R_s) sound signals, called "surround" signals, from a stereo sound signal made up of a left sound signal (L) and a right sound signal (R), characterized in that it comprises the following steps:

   - one of the stereo signals (L, R) is subtracted from the other signal (L, R) using a subtraction module to obtain a single subtraction signal (L-R) in which the correlated and in-phase components of the stereo signals (L, R) have been eliminated,

   - the subtraction signal (L-R) is filtered using a high-frequency filter (21) and only the filtered high-frequency part is applied to the input of a first (13.1) and a second (13.2) individual block,

   - the output signal (s₁, s₂) of each block (13.1, 13.2) being the combination of the input signal (e₁, e₂) of the block weighted by a first gain (g₁, g₃), and of the combination of the output signal (s₁, s₂) of the block weighted by a second gain (g₂, g₄) and of the input signal (e₁, e₂) of the block delayed by a delay line (14.1, 14.2),

   - the low-frequency part of the left (L) and right (R) signals of the stereo signal is filtered,

   - the duly filtered low-frequency parts are delayed by a delay (D3) using third delay lines (23.1, 23.2), and

   - the duly delayed low-frequency part is aggregated with the output signals (s₁, s₂) of the individual blocks (13.1, 13.2) to obtain the right surround sound signal (R_s) and the left surround sound signal (L_s).
2. Method according to Claim 1, characterized in that the "surround" signals (L_s, R_s) are intended to be broadcast over the rear channels of an acoustic system of 5.1 type.
3. Method according to Claim 1 or 2, characterized in that:

   - for the first individual block (13.1), the following applies: $$\mathrm{s}\mathrm{1}\left(\mathrm{n}\right)=\mathrm{e}\mathrm{1}\left(\mathrm{n}\right)\mathrm{.}\mathrm{g}\mathrm{1}+\mathrm{s}\mathrm{1}\left(\mathrm{n}-\mathrm{D}\mathrm{1}\right)\mathrm{.}\mathrm{g}\mathrm{2}+\mathrm{e}\mathrm{1}\left(\mathrm{n}-\mathrm{D}\mathrm{1}\right)$$

   

   e1 being the input signal of the first block corresponding to the subtraction signal (L-R),

   s1 being the output signal of the first block corresponding to one of the surround sound signals (right RS or left LS),

   g1, g2 being, respectively, the values of the first gain and of the second gain of the first block (13.1),

   D1 being the value of the number of delay samples introduced by the delay line (14.1), and

   - for the second individual block (13.2), the following applies: $$\mathrm{s}2\left(\mathrm{n}\right)=\mathrm{e}2\left(\mathrm{n}\right)\mathrm{.}\mathrm{g}3+\mathrm{s}2\left(\mathrm{n}-\mathrm{D}2\right)\mathrm{.}\mathrm{g}4+\mathrm{e}2\left(\mathrm{n}-\mathrm{D}2\right)$$

   

   e2 being the input signal of the second block corresponding to the subtraction signal (L-R),

   s2 being the output signal of the second block corresponding to the other surround sound signal (right if s1 corresponds to the left or left if s1 corresponds to the right),

   g3, g4 being, respectively, the values of the first gain and of the second gain of the second block (13.2),

   D2 being the value of the number of delay samples introduced by the delay line (14.2).
4. Method according to one of Claims 1 to 3, characterized in that the gain values within a block (13.1, 13.2) are opposite to one another, the value of the first gain (g₁, g₃) being opposite in relation to the value of the second gain (g₂, g₄).
5. Method according to one of Claims 1 to 4, characterized in that the gain values (g₁, g₂) of the first block (13.1) are opposite in relation to the gain values (g₃, g₄) of the second block (13.2), the value of the first gain (g₁) of the first block (13.1) being opposite to the value of the first gain (g₃) of the second block (13.2); whereas the value of the second gain (g₂) of the first block (13.1) is opposite to the value of the second gain (g₄) of the second block (13.2).
6. Method according to one of Claims 1 to 5, characterized in that the gain values of the first (g₁, g₂) and of the second (g₃, g₄) individual block have the same absolute value (g).
7. Method according to one of Claims 1 to 6, characterized in that the first gain (g₁) of the first block (13.1) and the second gain (g₄) of the second block have the value g; whereas the second gain (g₂) of the first block (13.1) and the first gain (g₃) of the second block have the value -g.
8. Method according to one of Claims 1 to 7, characterized in that the delay (D1) introduced by the line (14.1) of the first block (13.1) and the delay (D2) by the line (14.1) of the second block (14.2) are equal.
9. Method according to one of Claims 1 to 8, characterized in that the phase and the gain of the output signals (s₁, s₂) of each individual block (13.1, 13.2) are modified as a function of the frequency by means of parametric filtering cells (25.1, 25.2) to modify the audio perception of the left (L_s) and right (R_s) surround sound signals.
10. Method of generating surround signals intended to be broadcast by an acoustic system consisting of front/rear loudspeakers from a stereo sound signal made up of a left sound signal (L) and of a right sound signal (R), characterized in that it has the following step:

    - the "surround" sound signals are generated using the method according to one of the preceding claims and they are applied to the rear channels (29.1, 29.2) of the acoustic system.

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