CA1198683A - Method and apparatus for reproducing sound having a realistic ambient field and acoustic image - Google Patents
Method and apparatus for reproducing sound having a realistic ambient field and acoustic imageInfo
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- CA1198683A CA1198683A CA000430384A CA430384A CA1198683A CA 1198683 A CA1198683 A CA 1198683A CA 000430384 A CA000430384 A CA 000430384A CA 430384 A CA430384 A CA 430384A CA 1198683 A CA1198683 A CA 1198683A
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Abstract
METHOD AND APPARATUS FOR REPRODUCING SOUND HAVING
A REALISTIC AMBIENT FIELD AND ACOUSTIC IMAGE
ABSTRACT OF THE DISCLOSURE
Apparatus for reproducing sound having a realistic ambient field and acoustic image is used in a stereophonic sound reproduction system having a left channel output and a right channel output.
A right main speaker and a left main speaker are disposed at right and left main speaker locations, respectively, which are equidistantly spaced from a listening location along a listening axis perpendicular to a line joining the left and right main speakers. A right sub-speaker and a left sub-speaker are respectively disposed at right and left sub-speaker locations equidistantly spaced from the listening location, and further from the listening location than the main speaker. The left and right channel outputs are respectively coupled to the left and right main speakers. A
left channel minus right channel difference signal is coupled to the left sub-speaker and a right channel minus left channel difference signal is coupled to the right sub-speaker. In one embodi-ment, the main and sub-speakers for each channel are respectively incorporated in a common enclosure to fix the spacing therebetween. A
technique for determining optimal spacing between the main and sub-speakers and between the various speakers and the listening location is set forth.
A REALISTIC AMBIENT FIELD AND ACOUSTIC IMAGE
ABSTRACT OF THE DISCLOSURE
Apparatus for reproducing sound having a realistic ambient field and acoustic image is used in a stereophonic sound reproduction system having a left channel output and a right channel output.
A right main speaker and a left main speaker are disposed at right and left main speaker locations, respectively, which are equidistantly spaced from a listening location along a listening axis perpendicular to a line joining the left and right main speakers. A right sub-speaker and a left sub-speaker are respectively disposed at right and left sub-speaker locations equidistantly spaced from the listening location, and further from the listening location than the main speaker. The left and right channel outputs are respectively coupled to the left and right main speakers. A
left channel minus right channel difference signal is coupled to the left sub-speaker and a right channel minus left channel difference signal is coupled to the right sub-speaker. In one embodi-ment, the main and sub-speakers for each channel are respectively incorporated in a common enclosure to fix the spacing therebetween. A
technique for determining optimal spacing between the main and sub-speakers and between the various speakers and the listening location is set forth.
Description
~9~
BACKGROUND OF THE INVENTION
This invention pertains to a method and appa-ratus for reproducing sound Erom stereophonic source signals in which the reproduced sound has a realistic ambient field and acoustic image.
The present invention can best be understood and appreciated by setting forth a generalized dis-cussion of the manner in which stereophonic signals originate, as well as a generalized discussion of the manner in which sound is conventionally reproduced from a stereophonic signal source.
When live music is, for eYample, performed the listener perceives both the sonic qualities of the instruments and the performers and also the sonic quali-ties of the acoustic environment in which the music isperformed. Normal stereophonic recording and repro-ducing techniques retain much of the former, but most of the latter is lost.
The human auditory system localizes position through two mechanisms. Direction is perceived due to an interaural time delay or phase shift. Distance is perceived due to the time delay between an in tial sound and a similar reflected sound. A third, poorly under-stood mechanism, causes the ear to perceive only the ~5 first of two similar sounds when separated by a very short delay. This is called the precedence effect.
Through these mechanisms the listener perceives the direct sound reflected from the walls of the hall. Due to the direction and distance information contained in the reflected signals the listener forms a subliminal impression of the size and shape of the hall in which the performance is taking place.
~9~6~513 SUMMARY :3F THE INVENTION
Accordingly, it is ~n object of this invention to provide an apparatus and method for realistic reproduction of recorded ambience information regardless of the recording microphone placement .
- It is a more specific object of the present invention to provide an apparatus and methvd which is practical and inexpensive for realistic reproduction oE recorded ambieslce information as well as other signals off the central axis, regardle~s of the recording microphone placement.
In accordance w;th one embodiment of the invention~ in a stereophon;c sound reproduction system having a left channel output and a right channel output, a right main speaker and a left main speaker are provided respectively at right and left main speaker locations which are
BACKGROUND OF THE INVENTION
This invention pertains to a method and appa-ratus for reproducing sound Erom stereophonic source signals in which the reproduced sound has a realistic ambient field and acoustic image.
The present invention can best be understood and appreciated by setting forth a generalized dis-cussion of the manner in which stereophonic signals originate, as well as a generalized discussion of the manner in which sound is conventionally reproduced from a stereophonic signal source.
When live music is, for eYample, performed the listener perceives both the sonic qualities of the instruments and the performers and also the sonic quali-ties of the acoustic environment in which the music isperformed. Normal stereophonic recording and repro-ducing techniques retain much of the former, but most of the latter is lost.
The human auditory system localizes position through two mechanisms. Direction is perceived due to an interaural time delay or phase shift. Distance is perceived due to the time delay between an in tial sound and a similar reflected sound. A third, poorly under-stood mechanism, causes the ear to perceive only the ~5 first of two similar sounds when separated by a very short delay. This is called the precedence effect.
Through these mechanisms the listener perceives the direct sound reflected from the walls of the hall. Due to the direction and distance information contained in the reflected signals the listener forms a subliminal impression of the size and shape of the hall in which the performance is taking place.
~9~6~513 SUMMARY :3F THE INVENTION
Accordingly, it is ~n object of this invention to provide an apparatus and method for realistic reproduction of recorded ambience information regardless of the recording microphone placement .
- It is a more specific object of the present invention to provide an apparatus and methvd which is practical and inexpensive for realistic reproduction oE recorded ambieslce information as well as other signals off the central axis, regardle~s of the recording microphone placement.
In accordance w;th one embodiment of the invention~ in a stereophon;c sound reproduction system having a left channel output and a right channel output, a right main speaker and a left main speaker are provided respectively at right and left main speaker locations which are
2~ equi~istantly spaced from a listening location.
The listening location is defined as a spatial position for accommodating a listener's head . facing the main speakers and having a right ear location and a left ear location along an ear axis, with the right and left ear locations separated along the ear axis by a maximum interaural so~nd distance of ~tmaX. and the 1istening lccation being defined as the point on the ear axi~ equidistant to the right and left ears. A right sub-speaker and a left sub~speaker are provided at right and left sub-speaker l~ca-tions which are equidistantly spaced from the listening location~ The right and left channel outputs are coupled respectively to the right and ~98~3 left main speakers~ A left channel minus right channel signal is developed and coupled to the left sub-speaker and a r:ight channel minus left channel signal is develop~d and coupled to the right sub-speaker. By care~ul selection of the distance between the main speakers and sub-speakers, sound reproduced by the system as perceived by a listener whose head i5 located generally at the listening location has a realis-tic acoust;c field and enhanced acoustic image.
Other objects and specific features of the method and apparatus of the present invention will become apparent from the detailed description of the invention in connection with the accompany-ing draw.ings.
BRXEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a diagram of the typical environment in which stereophonic recordings are made.
Fiqure 2 is a diagram illustating conven-tional stereophonic sound reproduction, ~nd showing interaural crosstalk paths.
Figure 3 is a diagram showing the apparent source as perceived by a listener for a sound source equidistant from the recording micr.ophones when the sound is reproduced over a pair of speakers.
Figure 4 i5 a ~iagram illustrating the location of apparent sources to a listener when a stereophonic recoeding is reproduced, taking into account reflection of sound from the walls of the ha~l in which the recording was made.
. ~
Figure 5 is a diagram illustrating a situation where path lengths to two recording microphones for reflected sounds is such that the difference in arrival times of the reflected sound of the two microphones is comparable to a possible value of interaural time delay.
Figure 6 is a diagram showing how each possi-ble value of interaural time delay corresponds to an angle of incidence for perceived sounds within a 190 arc.
Figure 7 is a diagram illustrating an off-axis source whose signal arrives at the right microphone ~t later than at the left microphone, where ~t is equal to the maximum possible interaural time delay.
Figure 8 illustrates the apparent source that would appear to a listener for the situation shown in Figure 7 when the recording were reproduced on a pair of speakers.
Figure 9 is a diagram showing use of main speakers and sub-speakers in accordance with the invention.
Figure 10 is a diagram illustrating an appa-rent source location as produced by the arrangement of Figure 9.
Figure 11 illustrates an embodiment of the invention in which the sub-speakers and main speakers are commonly mounted in respective enclosures.
Referring to Figure 1, there is illustrated a source S, spaced from a listener P in an enviroment which includes a plurality of walls, Wl, W2, W3. In such an enviroment the listener will of course perceive sounds from the source S along a direct path DPl. Also, the listener will perceive sounds reflected from the walls of the environment, illustrated in Figure 1 by the path RP2 to the listener P. In stereophonic recording, micro-phones S as shown in Figure 1. If the source S is equi-distant from the microphones, then both microphones will pick up sounds from the source S along direct paths DP2 and DP3. In addition, the hall ambience information wi'l be recorded by the left and right microphones ML and MR
in addition to the direct sound from the source. This is illustrated by the reflected paths RP3 and RP4 from the point Pl on wall Wl.
Turning now to Figure 2, there is i]lustrated what happens when the sounds recorded by the microphones are in Figure 1 are reproduced by loudspeakers LS and Rs positioned in the same position relative to the listener P as the recording microphones. In Figure 2 the listener P is shown as having a left ear Le and a right ear Re.
If the sound recorded as in Figure 1 was initially equidistant from the two microphones, the sound will reach each microphone at the same time.
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Accordingly~ in reproducing the sound, a listener equidistant from the twQ speakers LS and RS will hear the reproduced direct sound from the left speaker in the left ear (pa~h A) at the same time as the same s~und from the right speaker is heard in the righ~ ear (path B~. The precedence effec~
will tend to reduce perception of interaural crosstalk paths a and b., The listener P, hearing the same sound in both ears at once will localiæe the sound as being directly in front of and between the speakers, as shown in Figure 3.
Referring again for a moment to Figure 1, consider a sound reflec~ed from the point Pl on the wall ~1 of the hall. The reflec~ed sound from the secondary source reaches the left microphone ML first via the path RP3. This sound is delayed relative to the direct sound along path DP2, partially preserving the distance information about the reflection from Pl. The sound from Pl at some time thereafter reaches the right micro-phone MR along path RP4 after a further delay and further reduction in loudness. In this case, the delay corresponds approximately to the distance MD
between the microphones. Turning now to Figure 4, there is illustrated what the listener P will hear with respect tv both the direct and reflec~ed ~ound illustrated in Figure 1. When reproduced by the loudspeakers LS and RS the listener will first hear the direct sound from the source at the same tîme in both ears, corresponding to the apparent source shown in Figure 4. The listener will then hear the delayed sound corresponding to the reflection from Pl being recorded by the left microphone and reproduced by the let speaker first ~n the left ear Le and then in the right ear d Re. The initial delay caused by the longer path taken by the reflection in reachiny the left microphone ML gives the listener an impression of the dis~ance between the orisinal source, Pl, and himself. However~ the interaural delay ~t~
(correspondin~ to the time it takes sound to travel between a listener's ears) gi~es the impression that the reflected sound has come rom a point behind and in the same direction as the left speaker; illustrated as the first apparent point Pl in Figure 4. For reference, the location of ~he ac~ual point Pl is also in Figure 4~ After a further d~lay, the lis~ener will hear the reflected sound reproduced by the right speaker 15 RS. Since the additional delay (corresponding to the distance MD in Figure 1) is much greater than any possible interaural delay (except for the case of a very small microphone spacing) this sound will create a second apparent point Pl behind and in the same direction as the right speaker, as illustrated in Figure 4. However, it has been observed in experiments that the listener mainly perceives the direction information of tne first apparent point source Pl, largely ignoring the second. Thus the listener perceives the sound as coming primarily from the direction of the left speaker or slightly inside the left speaker if the loudness of the second apparent point source Pl is significant compared to the first, This analysis describes the effect on an~y other sound sources recorded b~ the two microphones such that the difference in arrival times at the two microphones is greater than the maximum possible interaural t ime delay .
Referring to Figure 5, for some reflected sounds the path lengths to the two microphones ML
and MR will be such tha~ the di~ferences in arrival times of the reflected sound at the two 5 microphones will be comparable to a possible val~e of interaural time delay., Thus~ the refelected sound from point P2 to the left microphone ML
~long path d' would be approximately equal to the path leng~h c ' to ~he r ight microphone MR plus the 10 interaural time delay at. Thlls, assume that d' equals ~ t. When this occurs, the arrival of the reproduced sou nd f rom the two speakers at the correspondin~ ears at slightly different tirnes will have the same effect as an interaural time delay giving the listener a definite impression of the direction and distance of the reflected sound3 Referring to Figure 6, as there illustrated each possible value of interaural time delay corres-ponds to an angle of incidence for the perceived sound within a 180 arc. As the difference in arrival times at the mirophones approaches the maximum possible value of the intera~ral delay, the apparent direction of the sound wo~l3 swiny rapidly to the right or left. In practice this is limited by the listening angle of the loudspeakers.
~hen the time difference of the sounds arriving at the respective ears approaches the interaural delay corresponding to the listening angle of the speakers~ the interaural crosstalk signal of the opposite speaker ~rad~ally takes precedence effectively limiting the apparent sound so~rces to within th~ listenin~ angle of the speaker~
It sh~uld be apparent at this point that ~11 sound sources, ambient or otherwise, who~e signals arrive at the respective microphones with ~;
a time difference greater than the interaural time delay corresponding to the listening angle of the reprc>ducing speakers will appear to 'che listener as apparent sources behind and in the same general direction as one of the speakers as shown in Figure 4. The delayed signal appearing in the other channel; being lower in loudnes~" will have only slight effect ln drawing the apparent source inside the spea~ers. This has ben confirmed by 1~ experiments which show that, in fact, the apparen~c sound source remains s~bstantially within the listening angle defined by the speakers.
The existence of interaural crosstalk has long been known and discussed at some length in the l;terature. Additionally, there are several recent patents which have disclosed meth~ds and techniyues for eliminating interaural crosstalk, without however making a complete analysis of the consequences of so doing.
~0 One such prior art patent is United States Patent ~o. 4,0~,675 to ~obayashi et al.
This patent discloses a means for cancellin~
interaural crosstalk using inverted and ~elayed versions of the left and right stereo signals fed to a second pair of speakers arranged to produce the correct geometry. As explained in Uni~ed States Patent No~ 4,218,505 to Carver, the Ko~ayashi et al device is only partially effective.
Carver discloses in United States Patent 4 ,218 ,505
The listening location is defined as a spatial position for accommodating a listener's head . facing the main speakers and having a right ear location and a left ear location along an ear axis, with the right and left ear locations separated along the ear axis by a maximum interaural so~nd distance of ~tmaX. and the 1istening lccation being defined as the point on the ear axi~ equidistant to the right and left ears. A right sub-speaker and a left sub~speaker are provided at right and left sub-speaker l~ca-tions which are equidistantly spaced from the listening location~ The right and left channel outputs are coupled respectively to the right and ~98~3 left main speakers~ A left channel minus right channel signal is developed and coupled to the left sub-speaker and a r:ight channel minus left channel signal is develop~d and coupled to the right sub-speaker. By care~ul selection of the distance between the main speakers and sub-speakers, sound reproduced by the system as perceived by a listener whose head i5 located generally at the listening location has a realis-tic acoust;c field and enhanced acoustic image.
Other objects and specific features of the method and apparatus of the present invention will become apparent from the detailed description of the invention in connection with the accompany-ing draw.ings.
BRXEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a diagram of the typical environment in which stereophonic recordings are made.
Fiqure 2 is a diagram illustating conven-tional stereophonic sound reproduction, ~nd showing interaural crosstalk paths.
Figure 3 is a diagram showing the apparent source as perceived by a listener for a sound source equidistant from the recording micr.ophones when the sound is reproduced over a pair of speakers.
Figure 4 i5 a ~iagram illustrating the location of apparent sources to a listener when a stereophonic recoeding is reproduced, taking into account reflection of sound from the walls of the ha~l in which the recording was made.
. ~
Figure 5 is a diagram illustrating a situation where path lengths to two recording microphones for reflected sounds is such that the difference in arrival times of the reflected sound of the two microphones is comparable to a possible value of interaural time delay.
Figure 6 is a diagram showing how each possi-ble value of interaural time delay corresponds to an angle of incidence for perceived sounds within a 190 arc.
Figure 7 is a diagram illustrating an off-axis source whose signal arrives at the right microphone ~t later than at the left microphone, where ~t is equal to the maximum possible interaural time delay.
Figure 8 illustrates the apparent source that would appear to a listener for the situation shown in Figure 7 when the recording were reproduced on a pair of speakers.
Figure 9 is a diagram showing use of main speakers and sub-speakers in accordance with the invention.
Figure 10 is a diagram illustrating an appa-rent source location as produced by the arrangement of Figure 9.
Figure 11 illustrates an embodiment of the invention in which the sub-speakers and main speakers are commonly mounted in respective enclosures.
Referring to Figure 1, there is illustrated a source S, spaced from a listener P in an enviroment which includes a plurality of walls, Wl, W2, W3. In such an enviroment the listener will of course perceive sounds from the source S along a direct path DPl. Also, the listener will perceive sounds reflected from the walls of the environment, illustrated in Figure 1 by the path RP2 to the listener P. In stereophonic recording, micro-phones S as shown in Figure 1. If the source S is equi-distant from the microphones, then both microphones will pick up sounds from the source S along direct paths DP2 and DP3. In addition, the hall ambience information wi'l be recorded by the left and right microphones ML and MR
in addition to the direct sound from the source. This is illustrated by the reflected paths RP3 and RP4 from the point Pl on wall Wl.
Turning now to Figure 2, there is i]lustrated what happens when the sounds recorded by the microphones are in Figure 1 are reproduced by loudspeakers LS and Rs positioned in the same position relative to the listener P as the recording microphones. In Figure 2 the listener P is shown as having a left ear Le and a right ear Re.
If the sound recorded as in Figure 1 was initially equidistant from the two microphones, the sound will reach each microphone at the same time.
~' 4 ~ ~ r~
Accordingly~ in reproducing the sound, a listener equidistant from the twQ speakers LS and RS will hear the reproduced direct sound from the left speaker in the left ear (pa~h A) at the same time as the same s~und from the right speaker is heard in the righ~ ear (path B~. The precedence effec~
will tend to reduce perception of interaural crosstalk paths a and b., The listener P, hearing the same sound in both ears at once will localiæe the sound as being directly in front of and between the speakers, as shown in Figure 3.
Referring again for a moment to Figure 1, consider a sound reflec~ed from the point Pl on the wall ~1 of the hall. The reflec~ed sound from the secondary source reaches the left microphone ML first via the path RP3. This sound is delayed relative to the direct sound along path DP2, partially preserving the distance information about the reflection from Pl. The sound from Pl at some time thereafter reaches the right micro-phone MR along path RP4 after a further delay and further reduction in loudness. In this case, the delay corresponds approximately to the distance MD
between the microphones. Turning now to Figure 4, there is illustrated what the listener P will hear with respect tv both the direct and reflec~ed ~ound illustrated in Figure 1. When reproduced by the loudspeakers LS and RS the listener will first hear the direct sound from the source at the same tîme in both ears, corresponding to the apparent source shown in Figure 4. The listener will then hear the delayed sound corresponding to the reflection from Pl being recorded by the left microphone and reproduced by the let speaker first ~n the left ear Le and then in the right ear d Re. The initial delay caused by the longer path taken by the reflection in reachiny the left microphone ML gives the listener an impression of the dis~ance between the orisinal source, Pl, and himself. However~ the interaural delay ~t~
(correspondin~ to the time it takes sound to travel between a listener's ears) gi~es the impression that the reflected sound has come rom a point behind and in the same direction as the left speaker; illustrated as the first apparent point Pl in Figure 4. For reference, the location of ~he ac~ual point Pl is also in Figure 4~ After a further d~lay, the lis~ener will hear the reflected sound reproduced by the right speaker 15 RS. Since the additional delay (corresponding to the distance MD in Figure 1) is much greater than any possible interaural delay (except for the case of a very small microphone spacing) this sound will create a second apparent point Pl behind and in the same direction as the right speaker, as illustrated in Figure 4. However, it has been observed in experiments that the listener mainly perceives the direction information of tne first apparent point source Pl, largely ignoring the second. Thus the listener perceives the sound as coming primarily from the direction of the left speaker or slightly inside the left speaker if the loudness of the second apparent point source Pl is significant compared to the first, This analysis describes the effect on an~y other sound sources recorded b~ the two microphones such that the difference in arrival times at the two microphones is greater than the maximum possible interaural t ime delay .
Referring to Figure 5, for some reflected sounds the path lengths to the two microphones ML
and MR will be such tha~ the di~ferences in arrival times of the reflected sound at the two 5 microphones will be comparable to a possible val~e of interaural time delay., Thus~ the refelected sound from point P2 to the left microphone ML
~long path d' would be approximately equal to the path leng~h c ' to ~he r ight microphone MR plus the 10 interaural time delay at. Thlls, assume that d' equals ~ t. When this occurs, the arrival of the reproduced sou nd f rom the two speakers at the correspondin~ ears at slightly different tirnes will have the same effect as an interaural time delay giving the listener a definite impression of the direction and distance of the reflected sound3 Referring to Figure 6, as there illustrated each possible value of interaural time delay corres-ponds to an angle of incidence for the perceived sound within a 180 arc. As the difference in arrival times at the mirophones approaches the maximum possible value of the intera~ral delay, the apparent direction of the sound wo~l3 swiny rapidly to the right or left. In practice this is limited by the listening angle of the loudspeakers.
~hen the time difference of the sounds arriving at the respective ears approaches the interaural delay corresponding to the listening angle of the speakers~ the interaural crosstalk signal of the opposite speaker ~rad~ally takes precedence effectively limiting the apparent sound so~rces to within th~ listenin~ angle of the speaker~
It sh~uld be apparent at this point that ~11 sound sources, ambient or otherwise, who~e signals arrive at the respective microphones with ~;
a time difference greater than the interaural time delay corresponding to the listening angle of the reprc>ducing speakers will appear to 'che listener as apparent sources behind and in the same general direction as one of the speakers as shown in Figure 4. The delayed signal appearing in the other channel; being lower in loudnes~" will have only slight effect ln drawing the apparent source inside the spea~ers. This has ben confirmed by 1~ experiments which show that, in fact, the apparen~c sound source remains s~bstantially within the listening angle defined by the speakers.
The existence of interaural crosstalk has long been known and discussed at some length in the l;terature. Additionally, there are several recent patents which have disclosed meth~ds and techniyues for eliminating interaural crosstalk, without however making a complete analysis of the consequences of so doing.
~0 One such prior art patent is United States Patent ~o. 4,0~,675 to ~obayashi et al.
This patent discloses a means for cancellin~
interaural crosstalk using inverted and ~elayed versions of the left and right stereo signals fed to a second pair of speakers arranged to produce the correct geometry. As explained in Uni~ed States Patent No~ 4,218,505 to Carver, the Ko~ayashi et al device is only partially effective.
Carver discloses in United States Patent 4 ,218 ,505
3~ an electronic device for cancelling interaural crosstalk. This device inverts one stereo signal, spllts it into several components, delays each component separately by a differ~nt amount and recombined these with a modified version of the 3 5 other ste re o s igna 1 ., Pe r f or mi ng th is operation on 1~
both s~ereo signals, Carver claims to effect a cancellation of interaural crosstalk and to create a ~dimensionalized effect."
Uni~ed States Pa~ent No. 4,199,Ç58 to Iwahara al~o discloses a technique for performin~
the interaural crosstalk cancellation. Iwahara uses a second pair of speakers tc) reproduce the cancellation signal, which is composed of a frequency and phase compensated version of the inverted main signal. This cancellation signal is fed to a speaker just outside the main speaker on the opposite side from which the cancellation signal was derived. The necessary delay is accomplished acoustically by the placement of the sub-spea~ers and detailed consideration is given to the phase and frequency compensation required to accomplish the cancellation. Additionally, a binaural signal input is specified. It will be seen later why a binaural input is essential to the correct function of an interaural crosstalk cancellation system.
Assuming that a method or technique is successful in cancelling the interaural crosstalk, it should be examined what effect this would have 25 on the listener's perception of the reproduced sound. Referring to Figure 2, if the interaural crosstalk cancellation were successful, paths a and b to the opposite ears would be eliminated.
This would help the localization of sources equi-distant from the recording microphones (Figures land 3)~ As the sources moved off-center, however, ~he difference in arrival times at the two micr~phones increases corresponding ~o larger values of interaural time.delay and hence greater an~les of incidence as illustrated in Figu~e 6~
Since the cross~alk paths from the speakers have been cancelled outl7 the speakers give no direc-tiesnal information abc)ut themselves. The perceived direction of the apparen~ sound sourc:e 5 will depend only s~n the difference in arrival times of ~he signal at the $wo recording microphones and to a much 1 esser deg ree the relative loudness. Figure 7, for example, shows an off axis source whose signal arrives at the 10 right microphone ~t later than at the left microphone. In this example ~t is equal to the maximum possible interaural time delay. When reproduced, with crosstalk cancelled, the right channel signal will arrive at the right ear ~t later than ~he left signal at the left ear.
Fiqure 8 shows the apparent source displaced far to the left of the listener, which it would appear to the listener in such a circumstance.
It should be clear that for microphones spaced far apart only a small displacement of f the equidistant axis will be required to create an arrival time difference at the microphone equal to the maximum possible interaural time delay. This will result in a rather dramatic expansion of a small p~rtion of the renter of the stereo stage.
For sound sources further displaced and corespond-ing to time delays greater than the maximum possible interaural time delay, which will include most of the ambience information, the listener will have diffi~ulty localizing any apparent source~ In effect, the listener will be forced to perceive sounds as if he had ears placed at the recording microphone spacing and may perceive apparent sound sources within his own head ~hen the microphone spacing is large~ An acurate prediction of the effects of this situation is beyond the current state of the art of psycho-aco~stics and beyond the scope of this disc~ssion.
It is precisely because ~f this potential difficulty that the U.S~ Patent No. 4,199,658 to Iwahara specifies a binaural signal inputO That is to say, that the recording has been made with a microphone spacing equal to the ear spac;ng.
However, recordings made in this manner are extremely rare. It is also possible that the problem outlined above accounts for the unspeci-fied "dimensionalized effect" referred to by Carver in Un;ted States Patent No. 4,218,505. Use of any of the above-mentioned crosstalk cancella-tion systems with commonly available recordings might well result in the effect described by Carver:
"The overall effect of this is a rather startling creation of the impression that the sound is 'totally dimensianalized', in that the hearer someho~ appears to be ~within the so~nd' or in s~me manner surro~nded by the various so~rces of the sound. n (U.S .
Patent No. 4,218,585, column 9~ lines 35_39) . n Although this effect that Carver describes may be an interesting a~ral e~fect, it is not believed to give a realistic impression of the original performance, particularly in the reproduction of ambient information which constitutes the majority of far-off axis si~nals.
DE~CRIPTION OF THE PREFERRED EMBODIMENTS
~eferrlng now to Figure 9, there is shown a diagram of one embodiment of a sound reproduc-,~ tion system in accordance with the present :L3 invention. A left main speaker LMS and a right main speaker RMS are disposed at left and right main speaker locations along a speaker axis and the left and right main speakers are equidistantly spaced from a listening location. The listening location is defined as the point common to a listening axis perpendicular to thP speaker axis and equidistantly spaced from the main speakers, and to the ear axis at a polnt midway between the left ear Le and right ear Re of a person P.
A left sub-speaker LSS and a right sub-speaker RSS are also provided at left and right sub-speak2r locations which, in accordance with this one embodiment, are situated on the speaker axis. The left and right sub-speakers are also equi-distantly spaced with respect to the listening location.
As shown in Figure 9, the right and left main speakers are fed the right and ~eft channel stereo signals, respectively. The sub-speakers, positioned outside the left main speaker and outside the right main speaker are fed the difference siynals left channel minus rignt channel and right channel minus left channel, respectivelyO
Applications of the stereo difference signals tlef~ channel minus right channel and/or right channel minus left channel) have long been known and are discussed both in ~he literature and in various prior art patents. For example~ United States Patent No. 3j697,692 to Hafler describes a method vf synthesizing 4-channel sound using rear speakers fed by a difference signal. This system was later made commercially available as the Dynacu QD~ Quadaptorn~ As a further ~xample, , ~
36~3~
Un i ted S ta t~ s Patent NoO 4,308,423 to Cohen describes an electronic device for cancelling interaural crosstalk and amplifying of-axis stereo images. This is accomplished by creatiny a difference signal, left minus right~ which is electronically delayed and mixed with the main left signal. The inverted difference signal right minus left is delayed electronically and mixed with the main right signal. Cohen describes this technique as a method of cancelling interaural crosstalk without "muddying" the central region and without reducing bass output~ Cohen does not, however, present any detailed analysis of the effects of this system on the reproduction of recorded sound.
The present invention as shown in Figure 9 accomplishes many of the same ends as the Cohen U.S. Patent No. 4,30B,423 through purely acoustic means, and with some advantages over Cohen. That the present invention also produces a realistic treatment of recorded material will be seen from the following analysisO
In order to facilitate the analysis, consider the left and right signals as functions of ti~e Specifically, distances will be expressed as sound distances~ which correspond to the time it takes sound to travel the distance in question. As shown in Figure 9, the time required for sound from the main right speaker RMS to reach 30 the right ear Re is t. The signal at the right ear from this speaker will be designated R(t3.
The quantity ~t is the interaural time delay ; corresponding to the listening angle of the speakers relative to the listener as shown in Figure 9; and ~' is the delay of the difference signal, e.gO R-L, relative to the main signal~
e.g. R, as determined by the relative placement and orientation of the speakers and listener as shown in Figure 9. Using this notation, the signals arriving at the left and right ears would be:
Left Ear:
L(t) + L(t ~ ~t') - R(t ~ ~t') + R(t = ~ +
R(t ~ ~t ~ ~t') - L(t + ~t + ~t') (1) Riqht Ear:
R(t) ~ R(t ~ ~t') - Llt + ~t') ~ L(t ~ ~t) +
L(t + ~t + ~t') - R(t ~ ~t + ~tl) ~2) First, consider a source whose sound arrives at both microphones at the same time during recording. Since the left and right channel signals are the same, there will be no differencP signal. This is analogous to the situation shown and described with reference to Figure 3 where the listener, hearing the same signal in both ears at the same time, localizes an apparent sound source directly between the speakersO
As a second case consider a signal appearlng only in the left channel. The signals at each ear will reduce to the following:
Left Ear:
L~t~ ~ L~t ~ ~tl) - L(t ~ ~t + ~t') ~3) Riqht Ear:
~ L(t + ~t') ~ L(t ~ ~t) ~ L(t ~ ~t + ~t') (4) , If ~t is comparable to t' the right ear terms will largely cancel leaving only L(t ~ ~t ~ Qt') corresponding to the left channel main signal portion of the difference signal emanating from the left sub-speaker and delaye~ by both the inter-speaker time delay ~t' and the interaural time delay Qt. Due to the precedence effect, the left ear will mainly perceive only the first signal to arrive, L(t). Figure 10 illus-trates the apparent source that a listener wouldperceive in such a situation. Referring to Figure 10, hearing the main left signal in the left ear and the same signal delayed by t ~ t' in the right ear, the listener will perceive an apparent sound source with a listening angle outside the speakers corresponding to an interaural delay of t ~ ~t' as illustrated in Figure 10. Referring to Fi~ure 4, ambience information reflected from point Pl on wall Wl would appear first only in the left channel and sometime later ~roughly corres-pondinq to the microphones spacing for this specific case) would appear in the right channel.
Referring to Figure 10, the listener wou~d perceive an apparent source as shown in Figure 10 showing a good correspondence with the correct ambience information. A second apparent source on the right w~uld seem to be indicated at the time that the signal arrives at the right microphone, further away and at a lesser loudness. However, 30 it has been observed in experimen~s that the listener perceives only the first apparent source.
This is probably due to the ability of the auditory system to assign direction to ~he firs~c and loudest of similar sounds, as discussed 35 previously.
9~3 As the recorded source moves more towards the center of the recording microphones, the difference in arrival times at the microphones will become less. This means that the time that a signal will exist only in one or the other chann~l will become shorter~ and the question of the relative loudness of the signal in each channel becomes important in assi~ning a direction to the appaeent source. Consider a case where the same signal appears in both left and right channels but with the left channel twice as loud as the right channel. The respective ears w~uld receive the following signals, after combining like terms:
Left Ear:
lS L(t) + L/2(t + ~t') + L/2(t ~ ~t) -L/2(t + ~t ~ ~t') (5) Ri~ h t Ea r -L/2(t) + L(t ~ ~t) - L/2(t + ~t') +
L/2(t + ~t ~ ~t') (6) 20 If ~t equals ~t' these expressions will further ~: reduce to:
Left Earo L(t) + L(t + ~t) - L/2(t + ~t + ~t') (7) Riqh~ Ear:
L~2~t) ~ L/2~t ~ ~t) - L/2(t + ~t + ~t') (8) In this case the xight ear would hear the same signal at the same time as the left ear, but at half the ~trength, The listener will perceive the apparent sound source as slightly shifted to the left of center between the speakersO
,:
h~L5~86~3 ~ owever, if ~t~ is made slightly ~reater than ~t an important result is obt~ined. Referring back to the original terms with the terms being rearranged in order of arrival time at the ears, the following is obtained:
Left Earo L(t) ~ L/2(t + ~t) + L~2(t + Qt') -L/2(t + ~t + ~t') (9) Right Ear:
L/2(t) ~ L(t + ~t) ~ L/2(t + Qt') +
L/2(t + ~t ~ ~t') ~10) The left ear will perceive only the main signal,L(t), since the other signals are weaker and later The right ear how~ver, has a half strength signal which arrives first followed by a Full strength signal delayed by ~to The precedence effect does not fully mask the late arrival of the stronger signal so that the listener perceives, at least slightly, a direction cue placing the apparent sound source at a listening angle corresponding to an approximate interaural delay slightly less than ~t. This will place the apparent sound source nearly out to the left speaker. As ~he right channel signal is increased further~ relative to the left channel signal, the di fference signal is reduced gradually to zero as the channels become equal. The precedence effect gives increasing importance to the now louder first signal arrival at the right ear and the listener perceives a smooth shift of acoustic image towards the center between the speakers.
Conversely, if the right signal is reduced further Erom the L/2 relative loudness, the exact opposite will occurl The difference signals will become louder and the listener will perceive a smooth shift of acoustic image outward to the perimeter of the lS0 stereo field.
In order for a smooth image transition to occur, the inter-speaker delay ~t' between the respective main and sub-speakers along the listen-ing angle between the speakers and the listening location must be greater than the interaural delay ~t as shown in Figure 9 along the listening angle of the listening location with respect to the speaker locations by enough to insure the desired function of the precedence effect as outlined above. In Pxperiments, it has been found that if ~t equals ~t' the effect is not unpleasant, it is just that the optimum ambience information is not present in the reproduced sound field.
Although in accordance with a preferred embodiment ~t' is greater than ~t, in order to obtain the best image quality outside the listening angle of the speakers~ ~t' should be close enough to at such that a substantial cancellation of interaural crosstalk occurs. In practice, but with no intention to limit the inventi.on to such a particular spacing~ it has been found that values of ~' about 1.2 times greater than ~t provide a suitable compromise and provide a realistic ambient field and acoustic image.
As shown in Figure 9, in accordance with one specific embodiment of the invention the left and right main and sub-speakers are located at respective main and sub-speaker locations arranged on a speaker ax;s which is parallel to an ear axis of a 11 stener in a normal listening pos ition along a listening axis e~uidistant from the tWD sets of ,., speakers. It should be understood, however, that any arrangement of m~in and sub-speakers giving the proper inter speaker delay at~ will suffice.
The arrangement of Figure 9 where both the main and sub-speakers are located on an axis parallel to the ear axis of a listener does, however, have advantages in allowing greater flexibility in listener position. That is, exact listener positioning is more critical when the sub-speakers are not on the same axis as the main speakers, or if the sub-speakers are not parallel to the main speakers.
It is possible that some modifications of the frequency or phase response of the main or sub-speakers may be desirable. One example might be the attenuation of bass response in the sub-speakersr This w~uld be desirable since very little difference information exists between the channels at low frequencies other than turnt~ble rumble or other spurious signals. In addition, it is desirable that the main and sub-speakers be very similar~ if not identical, in construction.
This will assure that differences in acoustic position of dissimilar drive units or differences in phase shift o dissimilar cross-over networks will not occur and hence not degradP the performance of the system.
Additionally, it should be understood th~t in order to obtain the best performance from the system that there are some limitations on the placement of the speakers relative to the listener. If it is desired to obtain the best performance, the sum of ~t ~ ~t' (Figure 9) should never exceed the maximum possible interaural time delay ~tma~ corresponding to a distance along the 6~3 ear a~is. For an average person, the spacing between the ears is on the order of 6.5 inches, so that the ~tmaX corresponds to the time it takes sound to travel such a distance.
Referring to Figure 11, the condition that the sum of ~t and ~t' should not e~ceed the maximum possible interaural time delay QtmaX can be met in practice if the distance between the left and right main speakers D along the speaker axis is always less than the perpendicular distance from the listening location along the listening axis D' with respect to the speaker axis. In practice, it has been found that good results are obtained i the spacing D between the main 5peakers is on the order of .7 to .9 times as large as the distance D'. In experiments, it has been observed that as D gets very close to D~, the realistic ambient field and enhanced acoustic image that is otherwise obtained begins to disappear~
In accordance with one preferred embodi-ment of the invention, and as illustrated in Figure 11, the left main speaker and the left sub-speaker may be commonly mounted in a single ~nclosure LE, and the right main speaker and right sub-speaker are commonly mounted in a common enclosure RE. This has the advantages of fixing the inter-speaker delay t', and offers the advantage that only two speaker enclosures are required.
In accordance with a spPcific embodiment, a spacin~ between the main and sub-speakers of eight inches, with the main and sub-speakers being identical two-way loudspeakers each having a six inch woofer and a one inch tweeter, was found to ~, 2~
work well. With a main to sub-speaker spacing of eight inches, and assuming an ear spacing between the left and right ears of approximately 6~5 inches, this yields a value of ~t' approximately 1~2 times ~reater than ~t, as discussed herein before as a suitable compromise.
The difference signals left channel minus right channel and right channel minus left channel which have been referred to throughout this d~scription are easily obtained in practice by connecting the sub-speakers across the left plus and right plus terminals of a stereophonic amplifier's outputs. Connecting left plus to the plus speaker terminal of the left sub-speaker and right plus to the sub-speaker common or normal ground terminal will give a signal corresponding to the left channel minus right channel~ Reversing this connection will give a signal to the right sub-speaker corresponding to the right channel minus t.he left channel.
As discussed before, the known techniques for cancelling interaural crosstalk, if successful in their stated aim, create an unnatural impres-sion when reproducing sounds, particularly ambient sounds, far off the equidistant axis of two microphones placed farther apart than ear spacing.
Only the Iwahara Patent discussed previously addresses this problem, and requires that the input signal be recorded binaurally, by tw~
microphones at the ear spacing. In contrast, the present invention creates a realistic acoustic imag~ regardless of the position of the recorded source. In addition, this realistic ambient field and acoustic imag~ is created in accordance with the present inven~ion with commonly available , :
36~
- recorded material and does not require a specially recorded input signal.
As compared to the device described in the prior Cohen Patent referred to previously, the S present invention is a purely acoustic implementa-tion requiring no special electronic components and utilizing the unmodified output from a standard stereophonic high ~idelity system. In a~dition, the present invention recognizes the advantages of certain specific values of delay and sets forth a technique for fixing this value relative to the listener, i.e. incorporating the main and sub-speaker for each channel in a common enclosure, thereby offering increased simplifica-tion of set-up and operation to the user. Further, the performance of the present invention is not subject to the inevitable degradation caused by extra stages of electronic signal processing.
The invention described herein is a novel apparatus and method for creating a realistic impression of sounds reproduced from commonly available recorded material. It offers perfor-mance advantages over those techniques and apparatus described in the prior art, and i~
utterly straightforward and simple in its preferred embodiments. Although the invention has been described herein with respect to certain preferred embodiments, it is not intended to limit the invention to any specific details of those preferred embodiments. That is, it should be c~.ear that various mod i fications and changes can be made to those preferred embodiments without departing fr3m the true spirit and scope of the invention, which ~s intended to be set for~h in 35 the accompanying claims.
both s~ereo signals, Carver claims to effect a cancellation of interaural crosstalk and to create a ~dimensionalized effect."
Uni~ed States Pa~ent No. 4,199,Ç58 to Iwahara al~o discloses a technique for performin~
the interaural crosstalk cancellation. Iwahara uses a second pair of speakers tc) reproduce the cancellation signal, which is composed of a frequency and phase compensated version of the inverted main signal. This cancellation signal is fed to a speaker just outside the main speaker on the opposite side from which the cancellation signal was derived. The necessary delay is accomplished acoustically by the placement of the sub-spea~ers and detailed consideration is given to the phase and frequency compensation required to accomplish the cancellation. Additionally, a binaural signal input is specified. It will be seen later why a binaural input is essential to the correct function of an interaural crosstalk cancellation system.
Assuming that a method or technique is successful in cancelling the interaural crosstalk, it should be examined what effect this would have 25 on the listener's perception of the reproduced sound. Referring to Figure 2, if the interaural crosstalk cancellation were successful, paths a and b to the opposite ears would be eliminated.
This would help the localization of sources equi-distant from the recording microphones (Figures land 3)~ As the sources moved off-center, however, ~he difference in arrival times at the two micr~phones increases corresponding ~o larger values of interaural time.delay and hence greater an~les of incidence as illustrated in Figu~e 6~
Since the cross~alk paths from the speakers have been cancelled outl7 the speakers give no direc-tiesnal information abc)ut themselves. The perceived direction of the apparen~ sound sourc:e 5 will depend only s~n the difference in arrival times of ~he signal at the $wo recording microphones and to a much 1 esser deg ree the relative loudness. Figure 7, for example, shows an off axis source whose signal arrives at the 10 right microphone ~t later than at the left microphone. In this example ~t is equal to the maximum possible interaural time delay. When reproduced, with crosstalk cancelled, the right channel signal will arrive at the right ear ~t later than ~he left signal at the left ear.
Fiqure 8 shows the apparent source displaced far to the left of the listener, which it would appear to the listener in such a circumstance.
It should be clear that for microphones spaced far apart only a small displacement of f the equidistant axis will be required to create an arrival time difference at the microphone equal to the maximum possible interaural time delay. This will result in a rather dramatic expansion of a small p~rtion of the renter of the stereo stage.
For sound sources further displaced and corespond-ing to time delays greater than the maximum possible interaural time delay, which will include most of the ambience information, the listener will have diffi~ulty localizing any apparent source~ In effect, the listener will be forced to perceive sounds as if he had ears placed at the recording microphone spacing and may perceive apparent sound sources within his own head ~hen the microphone spacing is large~ An acurate prediction of the effects of this situation is beyond the current state of the art of psycho-aco~stics and beyond the scope of this disc~ssion.
It is precisely because ~f this potential difficulty that the U.S~ Patent No. 4,199,658 to Iwahara specifies a binaural signal inputO That is to say, that the recording has been made with a microphone spacing equal to the ear spac;ng.
However, recordings made in this manner are extremely rare. It is also possible that the problem outlined above accounts for the unspeci-fied "dimensionalized effect" referred to by Carver in Un;ted States Patent No. 4,218,505. Use of any of the above-mentioned crosstalk cancella-tion systems with commonly available recordings might well result in the effect described by Carver:
"The overall effect of this is a rather startling creation of the impression that the sound is 'totally dimensianalized', in that the hearer someho~ appears to be ~within the so~nd' or in s~me manner surro~nded by the various so~rces of the sound. n (U.S .
Patent No. 4,218,585, column 9~ lines 35_39) . n Although this effect that Carver describes may be an interesting a~ral e~fect, it is not believed to give a realistic impression of the original performance, particularly in the reproduction of ambient information which constitutes the majority of far-off axis si~nals.
DE~CRIPTION OF THE PREFERRED EMBODIMENTS
~eferrlng now to Figure 9, there is shown a diagram of one embodiment of a sound reproduc-,~ tion system in accordance with the present :L3 invention. A left main speaker LMS and a right main speaker RMS are disposed at left and right main speaker locations along a speaker axis and the left and right main speakers are equidistantly spaced from a listening location. The listening location is defined as the point common to a listening axis perpendicular to thP speaker axis and equidistantly spaced from the main speakers, and to the ear axis at a polnt midway between the left ear Le and right ear Re of a person P.
A left sub-speaker LSS and a right sub-speaker RSS are also provided at left and right sub-speak2r locations which, in accordance with this one embodiment, are situated on the speaker axis. The left and right sub-speakers are also equi-distantly spaced with respect to the listening location.
As shown in Figure 9, the right and left main speakers are fed the right and ~eft channel stereo signals, respectively. The sub-speakers, positioned outside the left main speaker and outside the right main speaker are fed the difference siynals left channel minus rignt channel and right channel minus left channel, respectivelyO
Applications of the stereo difference signals tlef~ channel minus right channel and/or right channel minus left channel) have long been known and are discussed both in ~he literature and in various prior art patents. For example~ United States Patent No. 3j697,692 to Hafler describes a method vf synthesizing 4-channel sound using rear speakers fed by a difference signal. This system was later made commercially available as the Dynacu QD~ Quadaptorn~ As a further ~xample, , ~
36~3~
Un i ted S ta t~ s Patent NoO 4,308,423 to Cohen describes an electronic device for cancelling interaural crosstalk and amplifying of-axis stereo images. This is accomplished by creatiny a difference signal, left minus right~ which is electronically delayed and mixed with the main left signal. The inverted difference signal right minus left is delayed electronically and mixed with the main right signal. Cohen describes this technique as a method of cancelling interaural crosstalk without "muddying" the central region and without reducing bass output~ Cohen does not, however, present any detailed analysis of the effects of this system on the reproduction of recorded sound.
The present invention as shown in Figure 9 accomplishes many of the same ends as the Cohen U.S. Patent No. 4,30B,423 through purely acoustic means, and with some advantages over Cohen. That the present invention also produces a realistic treatment of recorded material will be seen from the following analysisO
In order to facilitate the analysis, consider the left and right signals as functions of ti~e Specifically, distances will be expressed as sound distances~ which correspond to the time it takes sound to travel the distance in question. As shown in Figure 9, the time required for sound from the main right speaker RMS to reach 30 the right ear Re is t. The signal at the right ear from this speaker will be designated R(t3.
The quantity ~t is the interaural time delay ; corresponding to the listening angle of the speakers relative to the listener as shown in Figure 9; and ~' is the delay of the difference signal, e.gO R-L, relative to the main signal~
e.g. R, as determined by the relative placement and orientation of the speakers and listener as shown in Figure 9. Using this notation, the signals arriving at the left and right ears would be:
Left Ear:
L(t) + L(t ~ ~t') - R(t ~ ~t') + R(t = ~ +
R(t ~ ~t ~ ~t') - L(t + ~t + ~t') (1) Riqht Ear:
R(t) ~ R(t ~ ~t') - Llt + ~t') ~ L(t ~ ~t) +
L(t + ~t + ~t') - R(t ~ ~t + ~tl) ~2) First, consider a source whose sound arrives at both microphones at the same time during recording. Since the left and right channel signals are the same, there will be no differencP signal. This is analogous to the situation shown and described with reference to Figure 3 where the listener, hearing the same signal in both ears at the same time, localizes an apparent sound source directly between the speakersO
As a second case consider a signal appearlng only in the left channel. The signals at each ear will reduce to the following:
Left Ear:
L~t~ ~ L~t ~ ~tl) - L(t ~ ~t + ~t') ~3) Riqht Ear:
~ L(t + ~t') ~ L(t ~ ~t) ~ L(t ~ ~t + ~t') (4) , If ~t is comparable to t' the right ear terms will largely cancel leaving only L(t ~ ~t ~ Qt') corresponding to the left channel main signal portion of the difference signal emanating from the left sub-speaker and delaye~ by both the inter-speaker time delay ~t' and the interaural time delay Qt. Due to the precedence effect, the left ear will mainly perceive only the first signal to arrive, L(t). Figure 10 illus-trates the apparent source that a listener wouldperceive in such a situation. Referring to Figure 10, hearing the main left signal in the left ear and the same signal delayed by t ~ t' in the right ear, the listener will perceive an apparent sound source with a listening angle outside the speakers corresponding to an interaural delay of t ~ ~t' as illustrated in Figure 10. Referring to Fi~ure 4, ambience information reflected from point Pl on wall Wl would appear first only in the left channel and sometime later ~roughly corres-pondinq to the microphones spacing for this specific case) would appear in the right channel.
Referring to Figure 10, the listener wou~d perceive an apparent source as shown in Figure 10 showing a good correspondence with the correct ambience information. A second apparent source on the right w~uld seem to be indicated at the time that the signal arrives at the right microphone, further away and at a lesser loudness. However, 30 it has been observed in experimen~s that the listener perceives only the first apparent source.
This is probably due to the ability of the auditory system to assign direction to ~he firs~c and loudest of similar sounds, as discussed 35 previously.
9~3 As the recorded source moves more towards the center of the recording microphones, the difference in arrival times at the microphones will become less. This means that the time that a signal will exist only in one or the other chann~l will become shorter~ and the question of the relative loudness of the signal in each channel becomes important in assi~ning a direction to the appaeent source. Consider a case where the same signal appears in both left and right channels but with the left channel twice as loud as the right channel. The respective ears w~uld receive the following signals, after combining like terms:
Left Ear:
lS L(t) + L/2(t + ~t') + L/2(t ~ ~t) -L/2(t + ~t ~ ~t') (5) Ri~ h t Ea r -L/2(t) + L(t ~ ~t) - L/2(t + ~t') +
L/2(t + ~t ~ ~t') (6) 20 If ~t equals ~t' these expressions will further ~: reduce to:
Left Earo L(t) + L(t + ~t) - L/2(t + ~t + ~t') (7) Riqh~ Ear:
L~2~t) ~ L/2~t ~ ~t) - L/2(t + ~t + ~t') (8) In this case the xight ear would hear the same signal at the same time as the left ear, but at half the ~trength, The listener will perceive the apparent sound source as slightly shifted to the left of center between the speakersO
,:
h~L5~86~3 ~ owever, if ~t~ is made slightly ~reater than ~t an important result is obt~ined. Referring back to the original terms with the terms being rearranged in order of arrival time at the ears, the following is obtained:
Left Earo L(t) ~ L/2(t + ~t) + L~2(t + Qt') -L/2(t + ~t + ~t') (9) Right Ear:
L/2(t) ~ L(t + ~t) ~ L/2(t + Qt') +
L/2(t + ~t ~ ~t') ~10) The left ear will perceive only the main signal,L(t), since the other signals are weaker and later The right ear how~ver, has a half strength signal which arrives first followed by a Full strength signal delayed by ~to The precedence effect does not fully mask the late arrival of the stronger signal so that the listener perceives, at least slightly, a direction cue placing the apparent sound source at a listening angle corresponding to an approximate interaural delay slightly less than ~t. This will place the apparent sound source nearly out to the left speaker. As ~he right channel signal is increased further~ relative to the left channel signal, the di fference signal is reduced gradually to zero as the channels become equal. The precedence effect gives increasing importance to the now louder first signal arrival at the right ear and the listener perceives a smooth shift of acoustic image towards the center between the speakers.
Conversely, if the right signal is reduced further Erom the L/2 relative loudness, the exact opposite will occurl The difference signals will become louder and the listener will perceive a smooth shift of acoustic image outward to the perimeter of the lS0 stereo field.
In order for a smooth image transition to occur, the inter-speaker delay ~t' between the respective main and sub-speakers along the listen-ing angle between the speakers and the listening location must be greater than the interaural delay ~t as shown in Figure 9 along the listening angle of the listening location with respect to the speaker locations by enough to insure the desired function of the precedence effect as outlined above. In Pxperiments, it has been found that if ~t equals ~t' the effect is not unpleasant, it is just that the optimum ambience information is not present in the reproduced sound field.
Although in accordance with a preferred embodiment ~t' is greater than ~t, in order to obtain the best image quality outside the listening angle of the speakers~ ~t' should be close enough to at such that a substantial cancellation of interaural crosstalk occurs. In practice, but with no intention to limit the inventi.on to such a particular spacing~ it has been found that values of ~' about 1.2 times greater than ~t provide a suitable compromise and provide a realistic ambient field and acoustic image.
As shown in Figure 9, in accordance with one specific embodiment of the invention the left and right main and sub-speakers are located at respective main and sub-speaker locations arranged on a speaker ax;s which is parallel to an ear axis of a 11 stener in a normal listening pos ition along a listening axis e~uidistant from the tWD sets of ,., speakers. It should be understood, however, that any arrangement of m~in and sub-speakers giving the proper inter speaker delay at~ will suffice.
The arrangement of Figure 9 where both the main and sub-speakers are located on an axis parallel to the ear axis of a listener does, however, have advantages in allowing greater flexibility in listener position. That is, exact listener positioning is more critical when the sub-speakers are not on the same axis as the main speakers, or if the sub-speakers are not parallel to the main speakers.
It is possible that some modifications of the frequency or phase response of the main or sub-speakers may be desirable. One example might be the attenuation of bass response in the sub-speakersr This w~uld be desirable since very little difference information exists between the channels at low frequencies other than turnt~ble rumble or other spurious signals. In addition, it is desirable that the main and sub-speakers be very similar~ if not identical, in construction.
This will assure that differences in acoustic position of dissimilar drive units or differences in phase shift o dissimilar cross-over networks will not occur and hence not degradP the performance of the system.
Additionally, it should be understood th~t in order to obtain the best performance from the system that there are some limitations on the placement of the speakers relative to the listener. If it is desired to obtain the best performance, the sum of ~t ~ ~t' (Figure 9) should never exceed the maximum possible interaural time delay ~tma~ corresponding to a distance along the 6~3 ear a~is. For an average person, the spacing between the ears is on the order of 6.5 inches, so that the ~tmaX corresponds to the time it takes sound to travel such a distance.
Referring to Figure 11, the condition that the sum of ~t and ~t' should not e~ceed the maximum possible interaural time delay QtmaX can be met in practice if the distance between the left and right main speakers D along the speaker axis is always less than the perpendicular distance from the listening location along the listening axis D' with respect to the speaker axis. In practice, it has been found that good results are obtained i the spacing D between the main 5peakers is on the order of .7 to .9 times as large as the distance D'. In experiments, it has been observed that as D gets very close to D~, the realistic ambient field and enhanced acoustic image that is otherwise obtained begins to disappear~
In accordance with one preferred embodi-ment of the invention, and as illustrated in Figure 11, the left main speaker and the left sub-speaker may be commonly mounted in a single ~nclosure LE, and the right main speaker and right sub-speaker are commonly mounted in a common enclosure RE. This has the advantages of fixing the inter-speaker delay t', and offers the advantage that only two speaker enclosures are required.
In accordance with a spPcific embodiment, a spacin~ between the main and sub-speakers of eight inches, with the main and sub-speakers being identical two-way loudspeakers each having a six inch woofer and a one inch tweeter, was found to ~, 2~
work well. With a main to sub-speaker spacing of eight inches, and assuming an ear spacing between the left and right ears of approximately 6~5 inches, this yields a value of ~t' approximately 1~2 times ~reater than ~t, as discussed herein before as a suitable compromise.
The difference signals left channel minus right channel and right channel minus left channel which have been referred to throughout this d~scription are easily obtained in practice by connecting the sub-speakers across the left plus and right plus terminals of a stereophonic amplifier's outputs. Connecting left plus to the plus speaker terminal of the left sub-speaker and right plus to the sub-speaker common or normal ground terminal will give a signal corresponding to the left channel minus right channel~ Reversing this connection will give a signal to the right sub-speaker corresponding to the right channel minus t.he left channel.
As discussed before, the known techniques for cancelling interaural crosstalk, if successful in their stated aim, create an unnatural impres-sion when reproducing sounds, particularly ambient sounds, far off the equidistant axis of two microphones placed farther apart than ear spacing.
Only the Iwahara Patent discussed previously addresses this problem, and requires that the input signal be recorded binaurally, by tw~
microphones at the ear spacing. In contrast, the present invention creates a realistic acoustic imag~ regardless of the position of the recorded source. In addition, this realistic ambient field and acoustic imag~ is created in accordance with the present inven~ion with commonly available , :
36~
- recorded material and does not require a specially recorded input signal.
As compared to the device described in the prior Cohen Patent referred to previously, the S present invention is a purely acoustic implementa-tion requiring no special electronic components and utilizing the unmodified output from a standard stereophonic high ~idelity system. In a~dition, the present invention recognizes the advantages of certain specific values of delay and sets forth a technique for fixing this value relative to the listener, i.e. incorporating the main and sub-speaker for each channel in a common enclosure, thereby offering increased simplifica-tion of set-up and operation to the user. Further, the performance of the present invention is not subject to the inevitable degradation caused by extra stages of electronic signal processing.
The invention described herein is a novel apparatus and method for creating a realistic impression of sounds reproduced from commonly available recorded material. It offers perfor-mance advantages over those techniques and apparatus described in the prior art, and i~
utterly straightforward and simple in its preferred embodiments. Although the invention has been described herein with respect to certain preferred embodiments, it is not intended to limit the invention to any specific details of those preferred embodiments. That is, it should be c~.ear that various mod i fications and changes can be made to those preferred embodiments without departing fr3m the true spirit and scope of the invention, which ~s intended to be set for~h in 35 the accompanying claims.
Claims (14)
1. In a stereophonic sound reproduction system having a left channel output and a right channel output, apparatus for reproducing sound having a realistic ambient field and acoustic image comprising:
a right main speaker and a left main speaker disposed respectively at right and left main speaker locations equidistantly spaced from a listening location, the listening location being a place in space for accommodating a listener's head facing the main speakers and having a right ear location and a left ear location along an ear axis, with the right and left ear locations separated along the ear axis by a maximum inter-aural sound distance of .DELTA.tmax, and the listening location being defined as the point on the ear axis equidistant to the right and left ears;
a right sub-speaker and a left sub-speaker disposed respectively at right and left sub-speaker locations equidistantly spaced from the listening location;
the right main speaker being separated from the right ear location by a sound distance t and being separated from the left ear by a sound distance t + .DELTA.t where .DELTA.t is the interaural sound distance of the right and left ear locations with respect to the right main speaker;
the right sub-speaker being separated from the right ear location by a sound distance t + .DELTA.t' where .DELTA.t' is the sound distance spacing with respect to the right ear location between the right main speaker location and right sub-speaker location;
the left main speaker being separated from the left ear location by a sound distance t and being separated from the right ear location by a sound distance t + .DELTA.t where .DELTA.t is the interaural sound distance between the left and right ear locations with respect to the left main speaker;
the left sub-speaker being separated from the left ear location by a sound distance t + .DELTA.t' where .DELTA.t' is the sound distance spacing with respect to the left ear location between the left main speaker location and left sub-speaker location;
the main speaker locations and sub-speaker locations being spaced from the listening location in a manner such that .DELTA.t + .DELTA.t' is ? tmax;
means coupling the right and left channel outputs, respectively, to said right and left main speakers;
means connected to the right and left channel outputs for developing a left channel minus right channel signal and a right channel minus left channel signal;
means coupling said left channel minus right channel signal to said left sub-speaker and said right channel minus left channel signal to said right sub-speaker;
whereby sound reproduced by said apparatus as perceived by a listener whose head is located generally at the listening location has a realistic acoustic field and enhanced acoustic image.
a right main speaker and a left main speaker disposed respectively at right and left main speaker locations equidistantly spaced from a listening location, the listening location being a place in space for accommodating a listener's head facing the main speakers and having a right ear location and a left ear location along an ear axis, with the right and left ear locations separated along the ear axis by a maximum inter-aural sound distance of .DELTA.tmax, and the listening location being defined as the point on the ear axis equidistant to the right and left ears;
a right sub-speaker and a left sub-speaker disposed respectively at right and left sub-speaker locations equidistantly spaced from the listening location;
the right main speaker being separated from the right ear location by a sound distance t and being separated from the left ear by a sound distance t + .DELTA.t where .DELTA.t is the interaural sound distance of the right and left ear locations with respect to the right main speaker;
the right sub-speaker being separated from the right ear location by a sound distance t + .DELTA.t' where .DELTA.t' is the sound distance spacing with respect to the right ear location between the right main speaker location and right sub-speaker location;
the left main speaker being separated from the left ear location by a sound distance t and being separated from the right ear location by a sound distance t + .DELTA.t where .DELTA.t is the interaural sound distance between the left and right ear locations with respect to the left main speaker;
the left sub-speaker being separated from the left ear location by a sound distance t + .DELTA.t' where .DELTA.t' is the sound distance spacing with respect to the left ear location between the left main speaker location and left sub-speaker location;
the main speaker locations and sub-speaker locations being spaced from the listening location in a manner such that .DELTA.t + .DELTA.t' is ? tmax;
means coupling the right and left channel outputs, respectively, to said right and left main speakers;
means connected to the right and left channel outputs for developing a left channel minus right channel signal and a right channel minus left channel signal;
means coupling said left channel minus right channel signal to said left sub-speaker and said right channel minus left channel signal to said right sub-speaker;
whereby sound reproduced by said apparatus as perceived by a listener whose head is located generally at the listening location has a realistic acoustic field and enhanced acoustic image.
2. Apparatus in accordance with Claim 1 wherein the spacing of the main speakers and sub-speakers with respect to the listening location is such that the sound distance .DELTA.t' is approximately 1.2 times the sound distance .DELTA.t.
3. Apparatus in accordance with Claim 1 including a left speaker enclosure wherein the left main speaker and right main speaker are incorporated to fix the spacing therebetween, and a right speaker enclosure wherein the right main speaker and right sub-speaker are incorporated to fix the spacing therebetween.
4. Apparatus in accordance with Claim 1 wherein the spacing of the main speakers with respect to each other and with respect to the listening location is such that the distance along a main speaker axis between right and left main speakers is less than the perpendicular distance from the listening location to the main speaker axis.
5. In a stereophonic sound reproduction system having a left channel output and a right channel output, apparatus for reproducing sound having a realistic ambient field and acoustic image comprising:
a right main speaker and a left main speaker disposed respectively at right and left main speaker locations spaced apart along a speaker axis, with a listening location located generally along a listening axis perpendicular to the speaker axis and intersecting the speaker axis at a point midway between the right and left main speaker locations;
means coupling the right and left channel outputs, respectively, to said right and left main speakers;
a right sub-speaker positioned on the speaker axis at a right sub-speaker location spaced a predetermined distance from the right main speaker location and further from the listening axis than said right main speaker location;
a left sub-speaker positioned on the speaker axis at a left sub-speaker location spaced a predetermined distance from the right main speaker location and further from the listening axis than said left main speaker location;
means connected to the right and left channel outputs for developing a left channel minus right channel signal and a right channel minus left channel signal;
means coupling said left channel minus right channel signal to said left sub-speaker and said right channel minus left channel signal to said right sub-speaker;
whereby sound reproduced by said apparatus as perceived by a listener located generally along the listening axis has a realistic acoustic field at enhanced acoustic image.
a right main speaker and a left main speaker disposed respectively at right and left main speaker locations spaced apart along a speaker axis, with a listening location located generally along a listening axis perpendicular to the speaker axis and intersecting the speaker axis at a point midway between the right and left main speaker locations;
means coupling the right and left channel outputs, respectively, to said right and left main speakers;
a right sub-speaker positioned on the speaker axis at a right sub-speaker location spaced a predetermined distance from the right main speaker location and further from the listening axis than said right main speaker location;
a left sub-speaker positioned on the speaker axis at a left sub-speaker location spaced a predetermined distance from the right main speaker location and further from the listening axis than said left main speaker location;
means connected to the right and left channel outputs for developing a left channel minus right channel signal and a right channel minus left channel signal;
means coupling said left channel minus right channel signal to said left sub-speaker and said right channel minus left channel signal to said right sub-speaker;
whereby sound reproduced by said apparatus as perceived by a listener located generally along the listening axis has a realistic acoustic field at enhanced acoustic image.
6. Apparatus in accordance with Claim 5 wherein the listening location has a right and left ear location on an ear axis parallel to the speaker axis, with the right and left ear locations spaced along the ear axis by a sound distance .DELTA.tmax' wherein the right and left ear locations are differentially spaced from each of the main speakers by an interaural sound distance .DELTA.t, wherein the respective main and sub-speakers are differentially spaced from a corresponding ear location by a sound distance .DELTA.t', and wherein .DELTA.t + .DELTA.t' is ? to .DELTA.tmax.
7. Apparatus in accordance with Claim 6 wherein the left and right main speakers are separated from each other by a distance D, and wherein the listening location is spaced along the listening axis at a distance D' from the speaker axis and wherein D is < than D'.
8. Apparatus in accordance with Claim 7 including a right channel speaker enclosure wherein the right main speaker and right sub-speaker are commonly mounted to fix the spacing therebetween, and including a left channel enclosure wherein the left main speaker and left sub speaker are commonly mounted to fix the spacing therebetween.
9. A method for reproducing sound from a stereophonic source having a left channel output and a right channel output in which the reproduced sound has a realistic ambient field and acoustic image comprising the steps of:
disposing a right main speaker and a left main speaker at right and left main speaker locations equidistantly spaced from a listening location, the listening location being a place in space for accommodating a listener's head facing the main speakers and having a right ear location and a left ear location along an ear axis, with the right and left ear locations separated along the ear axis by a maximum interaural sound distance of .DELTA.tmax, and the listening location being defined as the point on the ear axis equidistant to the right and left ears;
disposing a right sub-speaker and a left sub-speaker respectively at right and left sub-speaker locations equidistantly spaced from the listening location;
selecting the right main speaker location as separated from the right ear location by a sound distance t and separated from the left ear by a sound distance t plus .DELTA.t, where .DELTA.t is the interaural sound distance of the right and left ear locations with respect to the right main speaker location;
selecting the right sub-speaker location as separated from the right ear location by a sound distance t plus .DELTA.t', where .DELTA.t' is the sound distance spacing with respect to the right ear location between the right main speaker location and right sub-speaker location;
selecting the left main speaker location as separated from the left ear location by a sound distance t and separated from the right ear loca-tion by a sound distance t plus .DELTA.t, where .DELTA.t is the interaural sound distance between the left and right ear locations with respect to the left main speaker;
selecting the left sub-speaker location as separated from the left ear location by a sound distance t plus .DELTA.t' where .DELTA.t' is the sound distance spacing with respect to the left ear location between the left main speaker location and left sub-speaker location;
selecting the main speaker locations and sub-speaker locations wtih respect to the listening location in a manner such that .DELTA.t plus .DELTA.t' is ? .DELTA.tmax;
coupling the right and left channel outputs, respectively, to the right and left main speakers;
developing from the right and left channel outputs a left channel minus right channel signal and a right channel minus left channel signal; and coupling the left channel minus right channel signal to the left sub-speaker and the right channel minus left channel signal to the right sub-speaker;
whereby sound reproduced by the method as described above as perceived by a listener whose head is located generally at the listening loca-tion has a realistic acoustic field and enhanced acoustic image.
disposing a right main speaker and a left main speaker at right and left main speaker locations equidistantly spaced from a listening location, the listening location being a place in space for accommodating a listener's head facing the main speakers and having a right ear location and a left ear location along an ear axis, with the right and left ear locations separated along the ear axis by a maximum interaural sound distance of .DELTA.tmax, and the listening location being defined as the point on the ear axis equidistant to the right and left ears;
disposing a right sub-speaker and a left sub-speaker respectively at right and left sub-speaker locations equidistantly spaced from the listening location;
selecting the right main speaker location as separated from the right ear location by a sound distance t and separated from the left ear by a sound distance t plus .DELTA.t, where .DELTA.t is the interaural sound distance of the right and left ear locations with respect to the right main speaker location;
selecting the right sub-speaker location as separated from the right ear location by a sound distance t plus .DELTA.t', where .DELTA.t' is the sound distance spacing with respect to the right ear location between the right main speaker location and right sub-speaker location;
selecting the left main speaker location as separated from the left ear location by a sound distance t and separated from the right ear loca-tion by a sound distance t plus .DELTA.t, where .DELTA.t is the interaural sound distance between the left and right ear locations with respect to the left main speaker;
selecting the left sub-speaker location as separated from the left ear location by a sound distance t plus .DELTA.t' where .DELTA.t' is the sound distance spacing with respect to the left ear location between the left main speaker location and left sub-speaker location;
selecting the main speaker locations and sub-speaker locations wtih respect to the listening location in a manner such that .DELTA.t plus .DELTA.t' is ? .DELTA.tmax;
coupling the right and left channel outputs, respectively, to the right and left main speakers;
developing from the right and left channel outputs a left channel minus right channel signal and a right channel minus left channel signal; and coupling the left channel minus right channel signal to the left sub-speaker and the right channel minus left channel signal to the right sub-speaker;
whereby sound reproduced by the method as described above as perceived by a listener whose head is located generally at the listening loca-tion has a realistic acoustic field and enhanced acoustic image.
10. A method in accordance with Claim 9 wherein the right and left main speakers and the right and left sub-speakers are disposed along a common axis.
11. A method in accordance with Claim 10 including the steps of incorporating the left main speaker and left sub-speaker in a common enclosure to fix the spacing therebetween, and incorporating the right main speaker and right sub-speaker in a common enclosure to fix the distance therebetween.
12. A method in accordance with Claim 11 wherein the spacing between the left and right main speakers is selected to be a distance D, and wherein the listening location is selected to be a distance D' in a perpendicular direction from the common axis on which the main speakers are disposed, with D selected to be < than D'.
13. In a stereophonic sound reproduction system having a left channel output and a right channel output, apparatus for reproducing sound having a realistic ambient field and acoustic image comprising:
a right and left channel speaker enclosure, each having a main speaker and a sub-speaker mounted therein which are spaced from each other by a predetermined distance, with the right and left speaker enclosures intended for situating such that the right and left main speakers are closer than the sub-speakers with respect to a listening location on a listening axis midway between the speaker enclosures;
means coupling the right and left channel outputs, respectively, to said right and left main speakers;
means connected to the right and left channel outputs for developing a left channel minus right channel signal and a right channel minus left channel signal;
means coupling said left channel minus right channel signal to said left sub-speaker and said right channel minus left channel signal to said right sub-speaker;
whereby sound reproduced by said apparatus as perceived by a listener whose head is located generally at the listening location has a realistic acoustic field and enhanced acoustic image.
a right and left channel speaker enclosure, each having a main speaker and a sub-speaker mounted therein which are spaced from each other by a predetermined distance, with the right and left speaker enclosures intended for situating such that the right and left main speakers are closer than the sub-speakers with respect to a listening location on a listening axis midway between the speaker enclosures;
means coupling the right and left channel outputs, respectively, to said right and left main speakers;
means connected to the right and left channel outputs for developing a left channel minus right channel signal and a right channel minus left channel signal;
means coupling said left channel minus right channel signal to said left sub-speaker and said right channel minus left channel signal to said right sub-speaker;
whereby sound reproduced by said apparatus as perceived by a listener whose head is located generally at the listening location has a realistic acoustic field and enhanced acoustic image.
14. Apparatus in accordance with Claim 13 wherein the left and right speaker enclosures are separated from each other by a distance D
along a speaker axis, and wherein the listening location is spaced along the listening axis at a distance D' from the speaker axis, and wherein D
is less than D'.
along a speaker axis, and wherein the listening location is spaced along the listening axis at a distance D' from the speaker axis, and wherein D
is less than D'.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000430384A CA1198683A (en) | 1983-06-14 | 1983-06-14 | Method and apparatus for reproducing sound having a realistic ambient field and acoustic image |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000430384A CA1198683A (en) | 1983-06-14 | 1983-06-14 | Method and apparatus for reproducing sound having a realistic ambient field and acoustic image |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1198683A true CA1198683A (en) | 1985-12-31 |
Family
ID=4125480
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000430384A Expired CA1198683A (en) | 1983-06-14 | 1983-06-14 | Method and apparatus for reproducing sound having a realistic ambient field and acoustic image |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1198683A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5184574A (en) * | 1990-12-20 | 1993-02-09 | Kirk Robert C | Cat litter box |
-
1983
- 1983-06-14 CA CA000430384A patent/CA1198683A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5184574A (en) * | 1990-12-20 | 1993-02-09 | Kirk Robert C | Cat litter box |
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