FR2839602A1 - Car improved stereophonic sound reproduction having sound correction process allowing inversion electrical phase where opposition phase listening position detected between channels. - Google Patents

Abstract

The stereophonic sound correction process has signal processing allowing inversion of electrical phase between left and right channels. The inversion is used where opposition phase acoustics are detected between channels in the listening position.

Description

Stereophonic sound reproduction device in a

vehicle and vehicle so equipped.

  The present invention is intended to improve sound reproduction in stereophonic in the case where the listeners are not located at equal distance from the left and right speakers, and mainly in motor vehicles. Indeed, in a classic automobile, the respective locations of the seats and the speakers mean that the listener is never located at equal distance from the left speakers and

  right unless it is installed in the middle of the rear seat.

  Thus, stereophonic reproduction which requires that the listener be located at equal distance from the two right and left speakers is therefore theoretically impossible by car,

commonly used places.

  The theory is moreover largely confirmed by experience because, by car, the sound seems most often

  come from the speaker closest to the listener.

  More precisely, the sounds coming from the speaker closest to the listener on the one hand the components of the sound image situated on the side of the sound scene correspond to this speaker which is normal, but in in addition, those which should be placed in the middle of the sound stage, which does not

is not at all.

  Only the components of the sound scene located on the opposite side of the wind (at least on installations of a certain quality), correctly perceived from the upper area

farthest speaker.

  The problem is therefore to correctly reproduce the sounds located in the center of the sound scene whose character is

essentially monophonic.

  In automotive Hi-Fi, stereophonic reproduction is considered correct when the center of the sound image is located -2 between the axis of symmetry of the vehicle and the axis of the driver's chair. When the vehicle has a video screen in the middle of the dashboard, it is preferable that the center of the image

sound seems to come from the screen.

  When the vehicle has video screens incorporated in the front seats and intended for rear passengers, it is preferable that at the rear of the vehicle, the center of the image

  be located in front of the listeners.

  When the vehicle does not have a video screen, two theoretically defensible wind positions, either opposite

  the listener is in the axis of symmetry of the vehicle.

  In practice, an intermediate positioning is a

good compromise.

  In order to improve the quality of the sound image produced by conventional configurations, several solutions have been proposed which can be divided into two main categories: on the one hand, symmetrical solutions which put the driver and the passenger on a level foot equality and on the other hand the asymmetric solutions which privilege an auditor (in general

  the driver) compared to the other.

  The known symmetrical solutions are mainly due to the number of trots: - the installation of a central fly speaker reproducing the sum of the right and left channels - the bi-stereo configuration which was the subject of French patent application N 2 495 883 - the SeatBox configuration which was the subject of French patent application N 2 808 407 The drawback of the first two solutions is that they require at least the installation of an additional loudspeaker. The third solution which recommends the installation of the loudspeakers under the listeners does not solve the problem of

  existing vehicles, and is not always feasible.

-3-

  There are two known asymmetric solutions.

  The first which was the subject of a French patent application N 2,599. 581 consists in installing a loudspeaker in a central position and making it reproduce one of the channels

  stereophonic, the other channel being reproduced by the loudspeakers

outdoor speakers.

  This solution is asymmetrical because the stereophonic is necessarily reversed for one of the two listeners. Its main drawback is that it requires an additional speaker. The second which is most used consists in delaying the signal of one of the channels by a fraction of a second equal to the time taken by the sound to travel the difference in distance between

  the listener and the two right and left speakers.

  The consequence of this solution is to virtually distance the speaker corresponding to the delayed channel and thus

  place the listener at the center of this virtual configuration.

  This solution most often gives a good result for the listener close to the delayed channel because the center of the sound image is found for him close to the axis of symmetry of the vehicle but this result stands to the detriment of the other listener who volt the farthest speaker away

even more...

  The present invention proposes to obtain a correct sound image from speaker locations

  conventional. Wile has the advantage of not requiring any built-in

  additional speaker, and perform either a symmetrical correction or an asymmetric correction depending on the

search result.

  In addition, it acts by a signal processing which can be carried out by a DSP already assigned to other tasks, it is to

  say that its industrial cost can be very low, or even zero.

  It is generally accepted that in a vehicle, if the sound most often comes only from the speaker closest to the listener -4, it is that the loudest speaker

  close is necessarily the one we hear best.

  In reality, things are not so simple. Indeed, when delaying the signal of a channel according to a method that was mentioned above, the center of the sound scene comes, for the listener closest to the delayed channel, to be placed in a position close to the axis of symmetry of the vehicle, and without

  for that have to modify the balance adjustment.

  It is therefore not, contrary to what is commonly accepted, the level of the signal reaching the listener that makes the speaker closest to him omnipresent. And, careful listening made it possible to assimilate the phenomenon to a listening

reverse phase stereophonic.

  Indeed, when the phase of one of the channels is reversed in a domestic stereophonic installation, the sound image is destroyed and the sound is focused on the loudspeakers. And, when one deviates laterally from the normal listening position, the sound tends, on modulations of a monophonic nature,

  to focus on the nearest speaker.

  Hence the assumption that in a vehicle, there were significant phenomena of acoustic phase conflict at the listening locations producing an effect similar to that of

electrical phase reversal.

  Indeed, these conflicts exist and they can be highlighted by relatively simple means. For this, it is enough to make two readings of response curves. The first with the two speakers in phase and the second with the two speakers out of phase. There is a legitimate suspicion of phase conflict when the level of the out-of-phase curve exceeds that

of the phase curve.

  Figure 1 reproduces the behavior of the configuration of the original speakers located at the rear of a Renault Clio vehicle, the microphone being placed at the head of one

rear passengers.

  The curve in solid line corresponds to the reading of the high-

  speakers in phase and the dotted curve corresponds to the out-of-phase reading. There is a phase conflict out of two

octaves between 250 and 1000 Hz.

  These curves clearly show the existence of complex acoustic phenomena which generate phase conflicts much more important than theoretical phase conflicts.

  generated by the only difference in distance between the

speakers and listeners.

  The idea of the invention is that by causing electrical phase inversions between the channels in the frequency bands where the acoustic phase conflicts occur, one must restore the coherence of the phases between the channels and

  by the same the structure of the sound image.

  The invention firstly provides a stereophonic diffusion method in a vehicle using restitution means

  sound modulation of a basic signal.

  According to a general characteristic of the invention, local inversions of phases between the channels are carried out by appropriate means, in the zones of the sound spectrum or wind detected acoustic phase conflicts between the channels.

  right and left at listening places.

  The subject of the invention is also a vehicle fitted with an audio frequency stereo device, comprising a basic signal source, and means for restoring the modulation.

  sound correspond to the basic signal.

  According to a general characteristic of the invention, the stereophonic audiofrequence device comprises at least one means making it possible to reverse the phase on one of the channels on a portion of the sound spectrum correspond to an acoustic phase conflict between the right and left channels at listening places. FIG. 2 illustrates a stereophonic diffusion method in accordance with the invention comprising a modulation source 1, a signal processing device 2 comprising a means -6 of local phase reversal, a power amplifier 3 and

  means forming loudspeakers 4 and 5.

  Figure 3 illustrates a non-limiting example of

  realization of a local phase inversion.

  This example uses the property of the second order complementary Sallen-Key pass-low and low-pass filters to be in phase opposition to the crossover frequency, so that to reconstruct the entire audio spectrum in one all-pass filter, the phase of the pass part should be reversed

  tape, which is exactly what we want to do.

  Suppose we want to reverse the phase of a channel between a low frequency Fb and a high frequency Fh. The frequencies Fb and Fh show the frequencies of start and end of a

phase conflict zone.

  In this example, illustrated by FIG. 3, the filter 1 is a haul-pass filter tuned to the frequency Fb. Filter 2

  is a low-pass filter tuned to the frequency Fh.

  At the output of filter 2, we therefore have a bandpass filter between the frequencies Fb and Fh whose phase we reverse by injecting it at the negative input of an amplifier

  operational 5 mounts as summator-subtractor.

  Filter 3 is the low-pass filter complementary to filter 1 and filter 4 is the high-pass filter complementary to filter 2. The inlet of filter 4 is taken at the output of filter 1 to be in phase with the input. of filter 2. The outputs of filters 2 and 4 are injected at the positive input of

amplifier 5.

  The resistances of the 5-wind summator-subtractor calculated to obtain the same gain in absolute value for the filters

  pass-pass, low-pass and band-pass.

  At the output of the subtractor summator 5 we do have an all-pass filter whose phase is reversed between the

frequencies Fb and Fh.

  FIG. 4 is the reproduction of an oscillogram representing a burst of sinusoidal signal at a frequency situated between Fb and Fh. This figure has two curves, one in solid line is the input signal, the other in dotted line is the output signal of the inverter all-pass filter

in Figure 3.

  The comparison of these two curves clearly shows that the phase inversion is done by phase shift of 180. It is a phase inversion comparable to

  a delay compared to the other channel.

  When this all-pass filter is introduced into one channel, leaving the other unchanged, the two listening positions show a disappearance of the impression of out of phase in this zone of the sound spectrum, an improvement in timbre, and, sound image is corrected asymmetrically by favoring the

  listening position on the side of the all-pass filter.

  The result is comparable to that obtained by delaying the signal of a channel over the entire band of the spectrum.

  audio according to a method mentioned above.

  To obtain a symmetrical correction, we will carry out on the other channel in the frequency zone considered,

  the same slip of 180 but without phase inversion.

  An exemplary embodiment of such a device is illustrated in FIG. 5. It uses the property of Sallen-Key complementary filters of the third order to be in phase

at the connection frequency.

  The device of FIG. 5 is identical to that of FIG. 3 with the exception of the filters which wind of order 3 instead of being of order 2 and of the operational amplifier 5 of output mounted as a simple summator, because there is no signal to reverse. At the output of summator 5 we do have a pass filter

  all without phase inversion between the frequencies Fb and Fh.

  -8 Figure 6 is the oscillogram which represents the

  behavior of the reconstructed signal at the output of the pass filter

  all done with Sallen-Key filters of order 3. The solid curve represents a burst of sinusoidal signal at a frequency located between Fb and Fh at the input of the filter and the

  dotted curve represents the signal at the output of the filter.

  There is indeed a delay without phase change.

  FIG. 7 is the oscillogram which represents the signals of the left and right channels processed respectively by the

  all-pass filters in Figures 3 and 5.

  The two signals wind clearly in phase opposition but without time offset, which ensures the system symmetry

sought.

  Listening to the system fitted with two all-pass filters, we notice at the two listening positions a disappearance of the impression of out of phase and an improvement in timbres, and, the center of the sound image tends to become move closer to the axis

  of symmetry of the vehicle for the two listening positions.

  However, in certain unfavorable configurations, when the loudspeakers are very badly positioned, the action of the symmetrical device may be disappointing. In this case, we may prefer to use asymmetric correction, especially if

  the driver is alone in his car.

  The transition from one to the other of these options may be

  do this by simply retracting the non-inverting delay filter.

Claims (4)

  1- Device for broadcasting stereophonic sound messages in a vehicle of the type comprising at least one modulation source, signal processing means, a power amplifier and two loudspeaker means, characterized in that the processing means of the wind agency signal to allow electrical phase inversion between the right and left channels in frequency or wind zones detected acoustic phase oppositions between these channels at listening places.   2- Device for broadcasting stereophonic sound messages 1 5 in a vehicle according to claim 1 and characterized in that the phase inversion is carried out without sliding of relative phase between the channels.   3- Device for broadcasting stereophonic sound messages in a vehicle according to claim 1 and characterized in that the phase inversion is carried out with sliding of relative phase between the channels.   4- Vehicle fitted with a stereophonic audio-frequency device comprising a basic signal source, and means for restoring a sound modulation correspond to the basic signal and characterized by the fact that the restoring means comprise at least one phase inverter in a frequency band or wind present acoustic phase oppositions   between the right and left channels at listening locations