Abstract:
A method of generating left (Ls) and right (Rs) sound signals known as “surround” signals from a stereo sound signal composed of a left sound signal and of a right sound signal. One of the stereo signals is subtracted from the other signal with the aid of a subtraction module to obtain a single subtraction signal (LR) in which the correlated and in-phase components of the stereo signals (L, R) have been removed. A right sound signal (Rs) and a left sound signal (Ls) are generated from the subtraction signal (L−R), these signals being decorrelated with respect to one another and corresponding respectively to the right and left sound “surround” signals. These “surround” signals (Ls, Rs) intend for broadcast on the rear channels of an acoustic system.

Description:
TECHNICAL FILED OF THE INVENTION 
       [0001]    The invention relates to a method for generating left and right “surround” sound signals from a stereo signal, said left and right “surround” sound signals being intended to be broadcasted by means of an acoustic system of the type 5.1. The purpose of the invention is in particular to propose a method for processing a surround sound in order to get a good sound rendering while using few resources. 
         [0002]    The invention finds a particularly advantageous application in the field of sound processing for home-cinema, stereo equipment, computers, mobile telephones or any other device having a sound broadcasting system with capacities of calculation, limited or not, for the sound processing. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is pointed out that a 5.1 broadcasting system is an acoustic system including two (left and right) front channels, two (left and right) rear channels, a Low Frequency Effect channel, as well as a central channel. The sound broadcasted by this kind of system is a so-called “surround” sound which provides a listener with a sound envelopment feeling. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0004]    The invention is described hereafter in an application for acoustic systems of the type 5.1, it could however be implemented with any other acoustic system based on the generation of sound signals aiming at providing a listener with a sound envelopment feeling. 
         [0005]    More precisely, it is possible to consider that the left and right sound signals forming a stereo signal are each made up of 3 distinct components:
       a component decorrelated relative to that of the other signal, said component being intended to be broadcast by the front channels of a system of the type 5.1;   a component correlated and in-phase relative to that of the other signal, said component being intended to be broadcasted by the central channel of a system of the type 5.1; and   a component called “surround” component correlated and out-phase relative to that of the other signal, said component being normally intended to be broadcasted by the rear channels of a system of the type 5.1.       
 
         [0009]    The purpose of the invention is to generate these surround components in order to reproduce a sound surround effect with a good quality while limiting computing time during the extraction of these components. 
         [0010]    For this purpose, in the method according to the invention, a subtraction of the left and right stereo sound signals from one another is carried out, so as to suppress the correlated and in-phase common component from the stereo signals. Then, the signal resulting from this step of subtraction is decorrelated by means of a decorrelation module, so as to obtain the left and right “surround” components of the stereo signal which are ready to be broadcasted, if necessary after a parametric equalization, by the rear channels of a system of the type 5.1. 
         [0011]    Moreover, the compromise between the computing power of the processing and the “surround” sound effect obtained is excellent with the method according to the invention. 
         [0012]    The invention thus relates to a method for generating left and right sound signals called “surround” signals from a stereo sound signal made up of a left sound signal and a right sound signal, characterized in that it comprises the following steps:
       one of the stereo signals is subtracted from the other signal by means of a subtraction module to obtain a single subtraction signal in which the correlated and in-phase components of the stereo signals have been suppressed,   a right sound signal and a left sound signal decorrelated relative to one another which respectively correspond to the right and left “surround” sound signals are generated from the subtraction signal.       
 
         [0015]    According to an embodiment, the “surround” signals are intended to be broadcasted by the rear channels of an acoustic system of the type 5.1. 
         [0016]    According to an embodiment, for generating from the subtraction signal the right and left “surround” sound signals decorrelated relative to one another,
       the subtraction signal is applied to the input of a first and a second elementary block, the output signal of these blocks respectively corresponding to the right surround sound signal and to the left surround sound signal,   the output signal of each block is the combination of the input signal of the block weighted by a first gain, and of the combination of the output signal of the block weighted by a second gain and the input signals of the block delayed by a delay line.       
 
         [0019]    According to an embodiment:
       for the first elementary block:       
 
         [0000]        s   1 ( n ) =e   1 ( n ) .g   1   +S   1 ( n−D 1) .g   2   +e   1 ( n−D 1), 
         [0021]    e 1  being the input signal of the first block corresponding to the subtraction signal, 
         [0022]    s 1  being the output signal of the first block corresponding to one of the surround sound signals (right or left), 
         [0023]    g 1 , g 2  being respectively the values of the first gain and the second gain of the first block, 
         [0024]    n being the n th  harmonic sample, 
         [0025]    D 1  being the value of the number of delay samples introduced by the delay line, and
       for the second elementary block:       
 
         [0000]        s   2 ( n ) e   2 ( n ) .g   3   +s   2 ( n−D   2 ) .g   4   +e   2 ( n−D 2), 
         [0027]    e 2  being the input signal of the second block corresponding to the subtraction signal, 
         [0028]    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 one or left if s 1  corresponds to the right one), 
         [0029]    g 3 , g 4  being respectively the values of the first gain and the second gain of the second block, 
         [0030]    n being the n th  harmonic sample 
         [0031]    D2 being the value of the number of delay samples introduced by the delay line. 
         [0032]    According to an embodiment, the gain values inside a block are opposite one another, the value of the first gain being opposite the value of the second gain. 
         [0033]    According to an embodiment, the gain values of the first block are opposite the gain values of the second block, the value of the first gain of the first block being opposite the value of the first gain of the second block; while the value of the second gain of the first block is opposite the value of the second gain of the second block. 
         [0034]    According to an embodiment, the gain values of the first and second elementary block have the same absolute value. 
         [0035]    According to an embodiment, 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 equal to −g. 
         [0036]    According to an embodiment, the delay introduced by the delay line of the first block and the delay introduced by the delay line of the second block are equal to each other. 
         [0037]    According to an embodiment, the subtraction signal is first filtered by means of a high-pass filter and only the high frequency part filtered is applied to the inputs of the elementary blocks. 
         [0038]    According to an embodiment,
       the low frequency part of the left and right signals of the stereo signal is filtered,   the low frequency parts thus filtered is delayed with a delay by means of third delay lines, and   the low frequency part thus delayed is added to the output signals of the elementary blocks to obtain the right surround sound signal and the left surround sound signal.       
 
         [0042]    According to an embodiment, the phase and the gain of the output signals of each elementary block is modified according to the frequency by means of parametric filtering cells for modifying the sound perception of the left and right surround sound signals. 
         [0043]    The invention moreover relates to a method for generating surround signals to be broadcasted by an acoustic system composed of a front/rear loudspeaker from a stereo sound signal made up of a left sound signal and a right sound signal, wherein said method includes the following step:
       the “surround” sound signals are generated by means of said method for generating left and right sound signals called “surround” signals from a stereo sound signal made up of a left sound signal and a right sound signal and said “surround” sound signals are applied to the rear channels of the acoustic system.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]    The invention will be better understood when reading the following description and examining the annexed figures. These figures are given only as an illustration but by no means as a restriction of the invention. They show: 
           [0046]      FIG. 1 : a schematic representation of a device according to the invention allowing the extraction of the “surrounds” components of a stereo sound signal in a basic version; 
           [0047]      FIG. 2 : a graphical representation of a device according to the invention allowing the extraction of the “surrounds” components of a stereo sound signal in an improved version; and 
           [0048]      FIG. 3 : a schematic representation of a device according to the invention allowing to generate from a stereo sound signal sound signals that can be broadcasted by an acoustic system of the type 5.1. 
       
    
    
       [0049]    Identical elements have the same reference throughout the figures. 
       DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0050]      FIG. 1  shows a device  10  according to the invention making it possible to generate a left sound signal L s  and a right sound signal R s  called “surround” signals from a stereo sound signal formed of a left sound signal L and of a right sound signal R. 
         [0051]    One considers that the left sound signal L and the right sound signal R are respectively formed of a sum of 3 components:
       L=L 0 +C+S for the left signal L, and   R=R 0 +C+(−S) for the right signal R.   The components L 0  and S 0  are the components decorrelated relative to one another,   The component C is common to both signals L and R since it corresponds to the correlated and in-phase components of the signals L and R,   The component S corresponds to the correlated and out-phase component of the signals L and R.       
 
         [0057]    The purpose of the invention is to isolate the component S and to generate two decorrelated components in order to broadcast them in channels distinct from those in which the stereo signals L and R are broadcasted in order to provide the listener with an envelopment effect (cf.  FIG. 3 ). 
         [0058]    For this purpose, the left L and right R signals are applied to the input of a subtracter  11  in order to suppress the left component C from the L and right R signals and to keep only the component L 0 , R 0  and the component S of the stereo signals L and R. Here, the right sound signal R is subtracted from the left sound signal L (L−R), but it would be possible to carry out the opposite operation (R−L). 
         [0059]    The subtraction signal L−R obtained at the output of the subtracter  11  is then applied to the input of a decorrelation module  12  of the signal which makes it possible to generate from the subtraction signal L−R two signals decorrelated relative to one another: the left “surround” sound signal L s  and the right “surround” sound signal R s . 
         [0060]    For this purpose, the decorrelation module  12  is made of two elementary blocks  13 . 1 - 13 . 2  to the inputs of which the subtraction signal L−R is applied, the output s 1 , S 2  of these blocks  13 . 1 ,  13 . 2  respectively corresponding to the right surround sound signal Rs and to the left surround sound signal Ls. The output signal s 1  (resp. s 2 ) of each block  13 . 1  (resp.  13 . 2 ) depends on the input signal e 1  (resp. e 2 ) of the block weighted by a first gain g 1  (resp. g 3 ), and of the combination of the input signals e 1  (resp. e 2 ) and output signals s 1  (resp. s 2 ) of the block weighted by a second gain g 2  (resp. g 4 ), delayed by a delay line  14 . 1  (resp.  14 . 2 ). 
         [0061]    According to an embodiment, for each elementary block  13 . 1 ,  13 . 2 , the input signal e 1 , e 2  is applied to the input of a first adder  16 . 1 ,  16 . 2  and is applied to an input of a second adder  17 . 1 ,  17 . 2  after being 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 adder  16 . 1 ,  16 . 2  after being multiplied by the second gain g 2 , g 4 , the output signal of the first adder  16 . 1 ,  16 . 2  being applied to the input of the 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 thus to the right R s  or left L s  surround sound signal. 
         [0062]    Thus for the first elementary block  13 . 1 : 
         [0000]        s   1 ( n ) =e   1 ( n ) .g   1   +s   1 ( N−D 1) .g   2   +e   1 ( n−D 1) 
         [0063]    e 1  being the input signal of the first block  13 . 1  corresponding to the subtraction signal (L−R), 
         [0064]    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 ), 
         [0065]    g 1 , g 2  being respectively the values of the first gain and the second gain of the first block  13 . 1 , 
         [0066]    n being the n th  harmonic sample, 
         [0067]    D1 being the value of the number of delay samples introduced by the delay line  14 . 1 . 
         [0068]    For the second elementary block  13 . 2 : 
         [0000]        s   2 ( n ) e   2 ( n ) .g   3   +s   2 ( n−D 2) .g   4   +e   2 ( n−D 2) 
         [0069]    e 2  being the input signal of the second block  13 . 2  corresponding to the subtraction signal (L−R), 
         [0070]    s 2  being the output signal of the second block  13 . 2  corresponding to the other surround sound signal (right Rs if s 1  corresponds to the left one; or left Ls if s 1  corresponds to the right one), 
         [0071]    g 3 , g 4  being respectively the values of the first gain and the second gain of the second block  13 . 2 , 
         [0072]    n being the n th  harmonic sample, 
         [0073]    D2 being the value of the number of delay samples introduced by the delay line  14 . 2 . 
         [0074]    Preferably, inside the same block  13 . 1  (resp.  13 . 2 ), the first gain g 1  (resp. g 3 ) and the second gain g 2  (resp. g 4 ) have values opposite one another. Each block  13 . 1 ,  13 . 2  behaves then as a filter of the all-pass type which does not modify the gain of the input signal e 1 , e 2  but only the phase thereof. 
         [0075]    Moreover, 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 values opposite one another. Thus, the value of the first gain g 1  of the first block  13 . 1  is opposite 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 the value of the second gain g 4  of the second block  13 . 2 . 
         [0076]    One 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 gain g 3  of the second block  13 . 2  have a value −g. 
         [0077]    Preferably, the delays D 1 , D 2  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 to each other. However, it would be possible to choose delays D 1 , D 2  with different durations. 
         [0078]    In an embodiment example, g=0.4 and a delay D 1  and D 2  of 176 samples at a sampling rate of 44.1 Khz are chosen, such values allowing to obtain a good sound rendering. 
         [0079]    In an improvement of the invention represented in  FIG. 2 , a stage  19  made up of two filters  20 . 1 ,  20 . 2  respectively allowing to isolate the low frequency part of the signals L and R and of a filter  21  allowing to isolate the high frequency part of the subtraction signal L−R is moreover used. 
         [0080]    In this case, only the high frequency part of the signal L−R is applied to the input of the decorrelation module  12 . In an example, the cut-off frequencies of the low-pass filters  20 . 1 ,  20 . 2  and of the high-pass filter  21  are about 350 Hz. 
         [0081]    The low frequency parts of the left and right signals are applied to the inputs of third delay lines  23 . 1 ,  23 . 2  and the low frequency parts thus delayed are added, if it is necessary after weighting with gains g 7 , g 8 , to the output signals s 1 , s 2  of the elementary blocks respectively, so as to obtain right R s  and left L s  surround sound signals with an improved sound rendering. In an example, the delay D 3  applied by the third delay lines  23 . 1 ,  23 . 2  is equal to 176 samples at a sampling rate of 44.1 KHz 
         [0082]    Moreover, parametric equalization cells  25 . 1 ,  25 . 2  are connected with the output of each elementary block  13 . 1 ,  13 . 2  before addition to the delayed low frequency part. These cells  25 . 1 ,  25 . 2  cause a modification of 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 the perception thereof because of the decorrelation relative to one another. Consequently, it can be useful to modify these signals from a perceptive point of view so that the general sound impression is as best as possible. 
         [0083]    For this purpose, each equalization cell  25 . 1 ,  25 . 2  comprises a filter  26 . 1 ,  26 . 2  whose type, gain and phase can be adjusted according to various frequency bands of the signals s 1 , s 2  and a gain g 5 , g 6  which acts on all the spectrum of the signals s 1 , s 2 . These parameters are adapted by sound engineers in particular according to the application considered. 
         [0084]      FIG. 3  shows a use of the invention within the framework of a sound broadcasting system with 4 channels (2 front channels and 2 rear channels) or with 5 channels (with an additional more central channel) making it possible to obtain an excellent sound rendering while limiting the computing power of the sound processing. 
         [0085]    More precisely, for this use, the left L and right R stereo signals are applied to the input of the module  10  in  FIG. 1  or  2  so as to extract the left L S  and right R S  “surround” signals which are broadcasted in the rear channels  29 . 1 ,  19 . 2 ; while the initial left L and right S stereo signals are directly broadcasted in the front channels  30 . 1 ,  30 . 2 , if necessary after a parametric equalization by means of the modules  31 . 1 ,  31 . 2  (similar to the modules  25 . 1 ,  25 . 2  in  FIG. 2 ). 
         [0086]    Optionally, the component C common to the signals L and R is also extracted by means of a module  33  (an example of implementation of such a module is given in document FR-2886503) in order to be broadcasted in the central channel  34 .