Abstract:
The invention relates to a method for the sound processing of a stereophonic signal inside a motor vehicle. In a first implementation (“driver” mode) the stereophonic sound source is centered in the middle of the dashboard for the ‘driver’ listen position. For this purpose, delays (t 1 -t 4 ) are introduced into the frequency bands of the channels transmitted by the speakers, such that the driver appears to be at the center of a circle on which the car speakers are positioned. In a second implementation (“all passengers” mode), the phases of the signals of the two front channels are equalized, such that the sound source appears to be centered on the driver and the front passenger of the vehicle.

Description:
RELATED APPLICATIONS 
     This application is a §371 application from PCT/FR2008/051164 filed Jun. 25, 2008, which claims priority from French Patent Application No. 07 56279 filed Jul. 5, 2007, each of which is herein incorporated by reference in its entirety. 
     TECHNICAL FIELD OF THE INVENTION 
     The invention relates to a method for the sound processing of a stereophonic signal delivered inside a motor vehicle and a motor vehicle implementing this method. In particular, the object of the invention is to increase the listening quality of an audio track inside a vehicle. This audio track may contain, for example, a telephone conversation and/or music. 
     The invention is particularly advantageous when applied to sound processing methods implemented with audio systems having two input channels and four, five, or six output channels. 
     BACKGROUND OF THE INVENTION 
     In cars, the stereo signal, composed of a left sound signal (1 st  channel) and a right sound signal (2 nd  channel) generated by a stereophonic source (such as a car radio), is delivered through 4 channels. 
     Two channels (the front left and right channels) are delivered by the front transducers of the vehicle, while two other channels (the rear left and right channels) are delivered by the rear transducers. A fifth channel can also be generated and delivered by a transducer located in the center of the dashboard. 
     In the application, a transducer means a system that transforms an electric sound signal into an acoustic sound signal. 
     In general, a transducer connected to a given channel includes two speakers, which respectively deliver the high frequency part and the low frequency part of the electric sound signal transported by the channel. 
     Thus, a first speaker called a “tweeter” delivers the high frequency part of the channel signal, while a second speaker called a “woofer” delivers the low frequency part of the channel signal. 
     In a known way, certain transducers may be positioned so that the sound seems to come from the bottom of the vehicle, which does not provide a very pleasant listening experience for the passengers. 
     The invention makes it possible to solve this problem by positioning the sound image in the plane of each passenger&#39;s ears, in front of each passenger and/or in the middle of the dashboard of the vehicle. 
     OBJECT AND SUMMARY OF THE INVENTION 
     Thus, the object of the invention is to minimize the phase opposition effects between the left and right signals received at the location of at least one passenger&#39;s head. 
     In a first embodiment of the invention, called the “driver” mode, the stereophonic sound source is centered in the middle of the dashboard for the “driver” listening position. Thus, delays are introduced in the frequency bands of each speaker so that all of the speakers seem to be at the same distance as the one furthest from the driver. 
     In a second embodiment, called the “all passengers” mode, the resulting phase of the front channel signals and the phase of the rear channel signals perceived by the listeners are equalized so that the sound source seems to be centered in front of each passenger. Moreover, in this mode, delays are introduced in the front channel signals so as to time-align the “tweeter/woofer” pairs. 
     Thus, the invention relates to a method for the sound processing of a stereophonic signal inside a motor vehicle, the stereophonic signal being composed of a left electric sound signal and a right electric sound signal, wherein
         the phase of these electric sound signals is equalized so as to minimize the phase opposition effects in frequency bands of these left and right signals received at approximately the location of one passenger&#39;s head, and   the phase-equalized left electric sound signal and the phase-equalized right electric sound signal are respectively delivered by means of a front left transducer positioned in the front left part of the vehicle and a front right transducer positioned in the front right part of the vehicle.       

     According to one embodiment, in order to minimize the phase oppositions, filters are applied to the left electric sound signal and/or to the right electric sound signal so that the phase difference curve between the left and right electric sound signals received at the location of the passenger&#39;s head bypasses the points at which the left and right electric sound signals are in phase opposition. 
     According to one embodiment, in order to minimize the phase opposition effects, all-pass filters are applied to the left or right signal, these all-pass filters each having a cutoff frequency substantially equal to a middle frequency of the frequency band for which the left and right electric sound signals received are in phase opposition. 
     According to one embodiment, in order to minimize the phase opposition effects, pairs of all-pass filters are applied, one of the filters in the pair being applied to the left electric sound signal and the other filter in the pair being applied to the right electric sound signal, the filters in a pair having cutoff frequencies that surround a middle frequency of the frequency band for which the left and right electric sound signals received are in phase opposition. 
     According to one embodiment, the all-pass filters are Infinite Impulse Response (IIR) type filters. 
     According to one embodiment, the filters are Finite Impulse Response (FIR) type filters, these filters each having a phase response, each having the curve of an inverted gate having a value of −180 degrees in a frequency band in which the signals received are in phase opposition. 
     According to one embodiment, the electric sound signals received are considered to be in phase opposition when the phase difference between these signals is equal to 180 degrees plus or minus 20 degrees modulo 360 degrees. 
     According to one embodiment, the phase opposition effects are minimized for a frequency band of between 20 hz and 2 kHz. 
     According to one embodiment, the frequency spectrum of the left and right electric sound signals is equalized so as to compensate for the acoustics in the front of the vehicle, by means of a spectrum correction module. 
     According to one embodiment, frequency bands of each electric sound signal are filtered, and delays are introduced in these frequency bands. The delays are chosen so as to time-align the speakers of the front left transducer and the speakers of the front right transducer delivering these frequency bands. 
     According to one embodiment, the low frequency part and the high frequency part of each electric sound signal are filtered, the delays being chosen so as to time-align the speakers respectively delivering the low and high frequency parts of the left electric sound signal, the delays being chosen so as to time-align the speakers respectively delivering the low and high frequency parts of the right electric sound signal. 
     According to one embodiment, the left and right delays applied to the high frequency speakers are identical, and the left and right delays applied to the low frequency speakers are identical, due to the geometry of the vehicle. However, in a variant, they could be different. 
     According to one embodiment, the frequency bands of the speakers correspond to the frequency bands of the filtered signals they deliver. 
     According to one embodiment, the frequency bands of the left electric sound signal are combined into a reconstructed left electric sound signal, this reconstructed left electric sound signal being delivered by the front left transducer. While the frequency bands of the right electric sound signal are combined into a reconstructed right electric sound signal, this reconstructed right electric sound signal being delivered by the front right transducer. 
     According to one embodiment, the frequency bands of the electric sound signals are volume-adjusted by gain cells. 
     According to one embodiment, a central electric sound signal is generated from the in-phase spectral components of left and right electric sound signals originating from a stereophonic source, this central electric sound signal being delivered, after the introduction of a delay and an adjustment of the level and volume, by a transducer positioned in the center of the dashboard. 
     According to one embodiment, the left electric sound signal and the right electric sound signal are obtained by subtracting the spectral components of the central electric sound signal from those of the original left electric sound signal and from those of the original right electric sound signal, respectively. 
     According to one embodiment, rear left and right electric sound signals are generated from substantially out-of-phase components of the left and right electric sound signals, these signals being delivered, after the introduction of a delay and an adjustment of the level and volume, by a rear left transducer and a rear right transducer, respectively. 
     The invention also relates to a method for the sound processing of a stereophonic signal inside a motor vehicle, the stereophonic signal being composed of a left electric sound signal and a right electric sound signal, wherein frequency bands of each electric sound signal are filtered, and delays are introduced in these frequency bands. 
     The delays are chosen so that the transducers delivering these frequency bands are virtually disposed on a circle, this circle having as its center the place where the driver is located and having a radius equal to the distance that separates the driver from the transducer furthest from the driver. 
     According to one embodiment, the low frequency part and the high frequency part of each electric sound signal are filtered, the transducers each comprising a low frequency speaker and a high frequency speaker, the delays being chosen so as to time-align the speakers respectively delivering the low and high frequency parts of the left electric sound signal. 
     The delays are chosen so as to time-align the speakers respectively delivering the low and high frequency parts of the right electric sound signal. The left and right delays applied to the high frequency speakers are identical, and the left and right delays applied to the low frequency speakers are identical. 
     The invention also relates to a motor vehicle comprising a sound source generating a stereo signal inside a car, this stereo signal being composed of a left electric sound signal and a right electric sound signal, these left and right electric sound signals being processed by the method according to the invention so as to be respectively delivered by a front left transducer comprising only one speaker and a front right transducer comprising only one speaker. 
     According to one embodiment, the front left and right speakers are wide-band speakers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood by reading the following description and examining the accompanying figures These figures are given merely as examples and do not in any way limit the invention. They show: 
         FIG. 1 : a schematic functional representation of an audio system implementing the “driver” mode according to the invention; 
         FIG. 2 : a schematic functional representation of an audio system implementing the “all passengers” mode according to the invention; 
         FIG. 3 : a schematic functional representation of an audio system according to the invention with 2 input channels and 6 output channels; 
         FIGS. 4-5 : schematic representations of the virtual location of the center of the sound image when the method according to the invention is implemented in “driver” mode and when the method according to the invention is implemented in “all passengers” mode, respectively. 
         FIG. 6 : a graphical representation of the phase difference between the front left and right signals received at the location of one passenger&#39;s head, before and after phase correction; 
         FIG. 7 : a graphical representation of a phase response of an “all pass” filter used to minimize the phase opposition between the acoustic signals received at the location of one passenger&#39;s head; 
         FIG. 8 : graphical representations of the phase responses of two “all pass” filters and their combination, as well as the phase response of a Finite Impulse Response filter. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The same elements retain the same references from one figure to another. 
       FIG. 1  shows a schematic functional representation of an audio system implementing the “driver” mode, which makes it possible to position the center of the sound image for a listening position in the driver&#39;s seat of the vehicle. 
     The audio system according to the invention has two input channels  2  and  3  and four output channels  20 ,  25 ,  34 ″ and  35 ″, respectively delivered by the transducers  21 ,  26 ,  39 ,  41 . 
     More precisely, a sound source  1 , such as a CD player, generates a stereo signal composed of a left electric sound signal  2  and a right electric sound signal  3  (2 input channels). 
     These signals  2  and  3  are applied as input to a module  4 . 1  for correcting the sound level spectrum. This module  4 . 1  equalizes the spectrum of the signals  2  and  3 . 
     For this purpose, the module  4 . 1  comprises a filter for smoothing the perceived spectral response of the electric sound signals  2  and  3  so that all of the frequencies emitted at a given power tend to be perceived by the driver at the same level of amplitude. 
     In one embodiment, in order to calculate the coefficients of the filter of the module  4 . 1 , for example “peak/notch” type filters, a known signal is delivered via the front left and right transducers  21 ,  26  and the signal is recorded at the location of the driver&#39;s head by means of a microphone. From this is deduced a transfer function called the “vehicle transfer function,” and using the inverse transfer function of the “vehicle transfer function,” the coefficients of the filter are parameterized in so that the defects in the spectrum of the recorded signal are compensated in such a way as to reconstruct the spectrum of the initial signal. 
     This module  4 . 1  thus creates a spectral shape that compensates for the acoustics of the vehicle so that the sound signals delivered in the front of the vehicle by the transducers  21 ,  26  and perceived by the driver (after the passage of the sound signals into the vehicle) have a spectrum as close as possible to that of the original sound signal. 
     An equalized left electric sound signal  5  and an equalized right electric sound signal  6  are obtained as output from the module  4 . 1 . These signals  5  and  6  are applied as input to a block  7  for spatially correcting the signals  5  and  6 . 
     More precisely, these signals  5  and  6  are respectively applied as input to a high pass type filter  9  and a low pass type filter  10 . A left high frequency electric sound signal  5   a  and a right high frequency electric sound signal  6   a  are obtained as output from the filter  9 . A left low frequency electric sound signal  5   b  and a right low frequency electric sound signal  6   b  are obtained as output from the filter  10 . 
     The cutoff frequencies of the filters  9  and  10  correspond to the cutoff frequencies of the speakers used to deliver the filtered signals. In one embodiment, these cutoff frequencies are substantially identical. In other words, the frequency bands of the filtered signals correspond to the frequency bands of the speakers delivering these filtered signals. 
     In this case, two speakers  22 . 1 ,  22 . 2  and  27 . 1 ,  27 . 2  are connected to each channel in order to respectively deliver the high frequency bands and the low frequency bands. In a variant, for a vehicle comprising 3 speakers per channel, respectively delivering a high, middle and low frequency sound signal, the left and right electric sound signals are each respectively filtered by 3 filters, each of which corresponds to one of the frequency bands of these 3 speakers (high, middle or low). 
     The signals  5   a ,  5   b  and  6   a ,  6   b  are then each applied as input to a delay cell  13 . 1 - 13 . 4 . The delays t 1 -t 4  introduced are set as a function of the positioning of the speakers in the car, particularly as a function of the distance between them and the driver. 
     More precisely, delays t 1 -t 4  are introduced in the signals  5   a ,  5   b  and  6   a ,  6   b  so that all of the front speakers seem to be located at the same distance RHPmax as the transducer  41  furthest from the head of the driver  62  (see  FIG. 4 ). 
     Thus, the frequency band intended to be delivered by the furthest speaker is not delayed, while the frequency bands delivered by the speakers closer to the driver&#39;s head are delayed by a delay such that the sound delivered by these closer speakers seems to be perceived at the level of the driver&#39;s head at the same time as the signal from the furthest speaker furthest is perceived. In other words, the frequency bands are delayed in such a way that the sounds delivered by all of the speakers are perceived at the same time at the location of the driver&#39;s head. 
     The driver  62  is thus located in the center of a circle C of radius RHPmax on which the images S 1 -S 4  from the speakers  22 . 1 ,  22 . 2 ,  27 . 1 ,  27 . 2  are located, as illustrated in  FIG. 4 . 
     In practice, the distance that separates each speaker from the driver is first measured and as a function of this measurement, a delay is introduced in the frequency bands delivered by the speakers other than the one that is furthest away, so that all of the speakers seem to be located at the distance RHPmax of the furthest speaker. 
     In the driver mode, placing all of the transducers the same distance away from the driver (at least one of the passengers) completely cancels out the phase opposition effects, which are not very pleasant to the ear. 
     The delayed signals  5   a ′,  6   a ′,  6   b ′ and  6   b ′ observable as output from the cells  13 . 1 - 13 . 4  are applied as input to gain cells  15 . 1 - 15 . 4 . These cells  15 . 1 - 15 . 4  adjust the volume of the high and low frequency sound signals. To do this, the delayed signals are multiplied by coefficients K 1 -K 4 , for example between 0 and 1. 
     The processed left high-frequency electric sound signal  5   a ″ observable as output from the cell  15 . 1  and the processed left low frequency electric sound signal  5   b ″ observable as output from the cell  15 . 3  are applied as input to an adder  17 . 1 . 
     A reconstructed left electric sound signal  20  is then observable as output from this adder  17 . 1 . This signal  20  corresponds to the front left channel (first output channel) delivered by a transducer  21  comprising two speakers  22 . 1  and  22 . 2  positioned in the front left part of the vehicle. 
     The first speaker  22 . 1  (the “tweeter”) delivers the high frequency part of the signal  20 , while the second speaker  22 . 2  (“the woofer”) delivers the low frequency part of the signal  20 . 
     Likewise, the processed right high frequency electric sound signal  6   a ″ observable as output from the cell  15 . 2 , and the processed right low frequency electric sound signal  6   b ″ observable as output from the cell  15 . 4  are applied as input to an adder  17 . 2 . 
     A reconstructed left electric sound signal  25  is then observable as output from this adder  17 . 2 . This signal  25  corresponds to the front right channel (second output channel) delivered by a transducer  26  comprising two speakers  27 . 1  and  27 . 2  positioned in the front right part of the vehicle. 
     The first speaker  27 . 1  (the “tweeter”) delivers the high frequency part of the signal  25 , while the second speaker  27 . 2  (“the woofer”) delivers the low frequency part of the signal  25 . 
     The high frequency and low frequency parts of the signals  20  and  25  delivered by the speakers  22 . 1 ,  22 . 2  and  27 . 1 ,  27 . 2  correspond, as seen above, to the frequency bands filtered by the high frequency and low frequency filters  9  and  10 . 
     In a variant, the high frequency electric sound signals  5   a ″ and  6   a ″ are respectively delivered by a transducer  29  and  30  comprising only one speaker  31 ,  32  having a high frequency band. While the transducers  21  and  26  directly deliver the signals  5   b ″ and  6   b ″. Thus, there is one speaker per channel, not two speakers per channel. In this case, the adders  17 . 1  and  17 . 2  are eliminated. 
     Furthermore, the signals  2  and  3  are applied as input to a second module  4 . 2  for correcting the level spectrum. Like the module  4 . 1  for the front channels  20 ,  25  of the vehicle, this module  4 . 2  compensates for the acoustics of the vehicle for the rear channels  34 ″,  35 ″ of the vehicle. Equalized left and right electric sound signals  34 ,  35  are observable as output from the module  4 . 2 . 
     These signals  34  and  35  are applied as input to a second block  7   b  is for the spatial correction of the signals  34  and  35 . 
     More precisely, these signals  34  and  35  (the third and fourth output channels) are respectively applied as input to the delay cells  13 . 5  and  13 . 6 . These cells  13 . 5 ,  13 . 6  each introduce a delay t 5  and t 6  in the signals  34  and  35  so that all of the transducers seem to be virtually at the distance RHPmax of the speaker furthest from the driver, as illustrated by  FIG. 4 . 
     The signals  34 ′ and  35 ′ observable as output from the delay cells are applied as input to a gain cell  15 . 5 ,  15 . 6 , which adjusts the volume of the signals  34 ′,  35 ′ by multiplying them by a gain K 5 , K 6 . 
     The processed electric sound signals  34 ″ and  35 ″ observable as output from the cells  15 . 5  and  15 . 6  are respectively applied as input to a rear transducer  39  and  41  in order to be delivered. 
     The transducers  39  and  41  each comprise a speaker  40 . 1  and  42 . 1  for delivering the signals  34 ″,  35 ″, respectively. 
     In a variant, the rear transducers  39 ,  41  comprise several speakers. 
     In a variant, the system has only two front channels transporting the signals  20 ,  25 , and no rear channel transporting the signals  34 ″,  35 ″. 
     In a variant, the spectrum correction modules  4 . 1  and  4 . 2  are not used, the signals  2  and  3  in that case being directly applied as input to the block  7  and the cells  13 . 5 ,  13 . 6 . 
     In the “all passengers” embodiment of  FIG. 2 , before being applied as input to the modules  4 . 1  and  4 . 2 , the signals  2  and  3  are applied as input to a phase equalization module  45 . Phase-equalized left and right electric sound signals  2   bis  and  3   bis  are obtained as output from the module  45 . These signals  2   bis  and  3   bis  are then processed by the blocks  4 . 1  and  7  prior to being delivered by the front transducers  21  and  26  and processed by the blocks  4 . 2  and  7   bis  prior to being delivered by the rear transducers  39  and  41 . 
     For this purpose, the module  45  comprises a filter that corrects the phase defects perceived by the passengers. In one embodiment, in order to calculate the coefficients of the filter of the module  45 , a known signal whose phase response is zero is delivered by means of front left and right transducers  21 ,  26  positioned non-symmetrically relative to a passenger, for example the driver. In fact, the distance from one of the transducers  21 ,  26  to the passenger&#39;s head is different than the distance from the other transducer  21 ,  26  to the passengers head. 
     The signal emitted from the left channel via the transducer  21  is recorded by means of a microphone at the location of one passenger&#39;s head, and from this is deduced the phase response φL of the received left channel signal indicating the variation in the phase of the received left signal as a function of the frequency. 
     Likewise, the signal emitted from the right channel via the transducer  26  is recorded by means of the microphone at the location of one passenger&#39;s head, and from this is deduced the phase response φR of the received right channel signal indicating the variation in the phase of the received right signal as a function of the frequency. 
     The phase responses φL and φR are for example calculated from the Fourier transform of the signal received. 
     The phase difference φL−φR between the left and right signals received by the microphone is then deduced by performing a subtraction between the two phase responses obtained φL−φR. The curve C 1  representing this phase difference as a function of the frequency has a linear shape, as shown in  FIG. 6 . 
     The frequency bands A-C that are out-of-phase with this phase difference, i.e. the frequency bands for which the phase difference between the left and right signals received is equal to 180 degrees plus or minus 20 degrees and modulo 360 degrees, are then determined. 
     The coefficients of the filters  45 . 1  and  45 . 2  of the block  45  respectively applied to the left electric sound signal  2  and to the right electric sound signal  3 , which are for example “all pass” type filters, are then parameterized so as to minimize the phase opposition effects in these frequency bands. These all-pass filters are for example IIR (Infinite Impulse Response) type filters. 
     Minimizing the phase oppositions between signal received from the left channel and the signal received from the right channel gives all of the passengers in the vehicle the impression that the transducers  21 ,  26  are positioned symmetrically relative to each of them, which increases the quality of their listening experience. 
     The phase response of the all-pass filter G 1  shown in  FIG. 6  goes from 0 to minus 360 degrees, passing through an inflection point (which corresponds to the cutoff frequency) for which the phase equals minus 180 degrees. 
     Applying to one of the electric signals  2 ,  3  all-pass filters whose cutoff frequency fc is equal to the middle frequency f 1 , f 2  of the out-of-phase band in question introduces 180-degree phase delays at the points where the signals received are in phase opposition. This eliminates the frequency bands in which the left and right signals received are in phase opposition. 
     The curve C 2  thus represents the phase difference when an all-pass filter of cutoff frequency f 1  has been applied to one of the left or right electric sound signals, while the curve C 3  represents the phase difference when all-pass filters, respectively of cutoff frequency f 1  and f 2 , have been applied to one of the electric signals. It is noted that the curves C 1 -C 3  are spaced apart from each other by a 360-degree angle. 
     In a variant, a combination of two all-pass filters G 2 , G 3 , respectively applied to the phase of the left electric sound signal  2  and the right electric sound signal  3 , is used. The cutoff frequencies fc 1 , fc 2  surround the middle frequency f 1 , f 2  of the out-of-phase frequency band, as shown in  FIG. 8   a.    
     The combination of these filters G 2  and G 3  makes it possible to obtain a filter G 4 , shown in  FIG. 8   b , having a phase response that falls progressively from zero to a minimum of minus 180 degrees and then rises back to zero (the shape of an inverted Gauss curve), thus following the value of the phase difference D between the curves G 2  and G 3  of  FIG. 8   a.    
     Applying these pairs of filters thus allows the phase difference curve G 4  (represented by a dotted line) to locally deviate from the frequency values f 1 , f 2  for which the signals received are in phase opposition, then return to the curve C 1 . In other words, using these pairs of all-pass filters makes it possible to locally suppress the frequency bands A-C in phase opposition. 
     In practice, the out-of-phase frequency bands are corrected in the [20 Hz, 2000 Hz] range. 
     In a variant, FIR or Finite Impulse Response type filters G 5  are used, making it possible to design the desired phase response, which phase response can have the curve of the combination of all-pass filters. Preferably, these filters each have a phase response having the curve of an inverted gate having a value of −180 degrees in a frequency band in which the left and right signals received are in phase opposition. 
     In practice, in order to develop such FIR filters, the frequency response desired in the frequency domain is first plotted, and an inverse Fourier transform is performed in order to obtain the impulse response of the filter in the time domain. 
     It is sufficient to perform the phase correction operation at the location of the head of one passenger, preferably the driver, in order for the effect associated with this correction to be perceived by all the passengers. 
     In essence, the vehicle is symmetrical between its left and right parts, so the perceived sound effect for the front passenger is the same as that perceived by the driver. Moreover, the vehicle is also symmetrical between its front and rear parts, so the sound effect associated with the phase correction of the left and right signals  2 ,  3  delivered in the rear is perceived equally by all of the rear passengers. 
     However, it would be possible to repeat the phase correction operation in the rear in order to adjust the settings of the method according to the invention. 
     Thus, the phase equalization is such that when the signals  20 ,  34 ″,  35 ″ and  25  are delivered, the passenger perceives the center of the sound image  67 ,  68 ,  69 ,  71  to be in front of him, as shown in  FIG. 5 . 
     In the “all passengers” embodiment, the delays t 1 -t 14  are introduced so as to time-align the “tweeter/woofer” pairs  22 . 1  and  22 . 2  as well as the pairs  27 . 1  and  27 . 2 . Time-alignment means introducing a delay in the signal from the closest speaker so that the sound wave emitted by the latter is perceived at the same time as the sound wave emitted by the speaker whose signal is not delayed. 
     The delays t 1  and t 2 , then t 3  and t 4 , are therefore identical in pairs, i.e., the left and right delays applied to the tweeters  22 . 1 ,  27 . 1  are identical (t 1 =t 2 ) and the left and right delays applied to the woofers  22 . 2 ,  27 . 2  are identical (t 3 =t 4 ). 
       FIG. 3  shows a variant wherein six input electric sound signals  51 - 55  are generated from two input electric sound signals  2  and  3 . These signals are generated by implementing the sound processing method described in the patent published as number WO 2006/125931. 
     More precisely, a central electric sound signal  55  that includes only the substantially in-phase spectral components of the left 2 and right 3 electric sound signals is generated. This signal  55  is first corrected by the spectrum correction module  4 . 3 . 
     Next, the signal obtained is delayed by the cell  13 . 7  by a delay t 7 , and volume-adjusted by the cell  15 . 7  in order to then be delivered by the transducer  61 . This transducer  61  includes one or two speakers  63 , depending on the vehicle model, and is preferably positioned in the center of the dashboard. 
     Furthermore, the front left electric sound signal  51  and the front right electric sound signal  52  are generated by subtracting the spectral components of the signal  55  from those of the left electric sound signal and from those of the right electric sound signal  3 , respectively. 
     The signals  51 ,  52 ,  53  and  54  are then processed in “driver” mode or in “all passengers” mode as described in  FIGS. 1 and 2 . 
     Another electric sound signal  56  can be created from the low frequency filtering of the left and right electric sound signals  2  and  3 . Like the others, this signal  56 , can be delayed by a delay cell  13 . 8  and volume-adjusted by a cell  15 . 8  prior to being delivered by a transducer  64  comprising a low frequency speaker  65 . 
     In a variant, a source such as a DVD player with 6 input signals (6 input channels) is already available. 
     In a variant, when there are 6 input channels available but only 2 or 4 output channels, the output channels correspond to a combination of the six available input channels. 
     It is noted that with the “all passengers” and “driver” modes, sound rendering using transducers  21 ,  26  and  39 ,  42  with one speaker is at least similar to sound rendering with no processing but with several speakers per transducer. 
     The use of the invention is particularly advantageous with entry level vehicles having only one speaker per transducer. In that case, the single speaker of the transducers  21  or  26  is preferably a wide band speaker.