WO1991020164A1 - Method for eliminating the precedence effect in stereophonic sound systems and recording made with said method - Google Patents
Method for eliminating the precedence effect in stereophonic sound systems and recording made with said method Download PDFInfo
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- WO1991020164A1 WO1991020164A1 PCT/US1991/004168 US9104168W WO9120164A1 WO 1991020164 A1 WO1991020164 A1 WO 1991020164A1 US 9104168 W US9104168 W US 9104168W WO 9120164 A1 WO9120164 A1 WO 9120164A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04S—STEREOPHONIC SYSTEMS
- H04S1/00—Two-channel systems
- H04S1/002—Non-adaptive circuits, e.g. manually adjustable or static, for enhancing the sound image or the spatial distribution
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B20/00—Signal processing not specific to the method of recording or reproducing; Circuits therefor
- G11B20/00992—Circuits for stereophonic or quadraphonic recording or reproducing
Definitions
- the present invention relates to the field of acoustic reproduction and, more particularly, to the stereophonic reproduction of an acoustic image through two or more speakers.
- the per ⁇ ceived locations of the various sound sources generated by the stereophonic signals create for the listener what is known as an acoustic image, i.e., a map of the imaginary physical locations of these sound sources.
- the apparent location of the sound source is largely determined by the difference in arrival time and the intensity of the relevant component signals generated in the left and right speakers.
- the component of the acoustic image created by that signal will appear to be located on a line centered between the two speakers. If that signal component arrives fractionally earlier from the left speaker than from the right and/or the intensity of the component from the left speaker is greater than that from the right speaker, its image component should appear to be located left of center. The apparent locations of a set of such image components makes up the composite acoustic image perceived by the listener.
- 0 ers report that the sound images they perceive are either shifted toward the loca ⁇ tion of the nearest loudspeaker or almost entirely located in the nearest loud ⁇ speaker, depending upon the delay in question.
- this limit depends upon the qualities of the sound source. The precedence effect is more pronounced for transient sound sources such as struck or plucked musical instruments than it is for continuous sound sources
- Figure 1 is a block diagram of an apparatus according to the present inven ⁇ tion for converting a pair of standard stereophonic input signals into a pair of stereophonic output signals. 5
- Figure 2 illustrates the improvement obtained with the present invention. -. Summary of the Invention
- the present invention comprises an apparatus for processing first and second input signals to produce first and second stereophonic sound output sig ⁇ nals.
- the apparatus includes a means for receiving said first input signal and a
- the first processing circuit for generating a sum signal having a value substantially equal to the sum of N. band-limited signals.
- the ith said band-limited signal has an amplitude substantially equal to that of the first input signal in a prede ⁇ termined frequency range ! f. ⁇ l ⁇ f i and a phase which differs from the phase of said input signal in said predetermined frequency range by an amount 1 ⁇ i . m m
- i runs from 1 to N., N. > 2 and the set of 1 ⁇ i are distributed between -x and +x.
- the first processing means also includes circuitry for generating the first stereophonic output signal from said sum signal.
- the apparatus also includes a means for receiving said second input sig- nal; and a second processing means for generating a sum signal having a value substantially equal to the sum of N 2 band-limited signals.
- the ith said band-limited signal has an amplitude substantially equal to that of said second input signal in a predetermined frequency range 2 f. ⁇ 2 ⁇ f. and a phase which differs from the phase of said input signal in said predetermined frequency range by an - amount 2 ⁇ ..
- i runs from 1 to N 2 ; N 2 >2, and the set of 2 . are distributed between -x and +x.
- the second processing means includes circuitry for gener ⁇ ating the second stereophonic output signal from the sum signal.
- l ⁇ . differs from 2 ⁇ . for at least one value of i.
- the present invention comprises a method, and the audio recordings made with said method.
- the method provides a means for providing improved ster ⁇ eophonic sound reproduction which is capable of generating an acoustic image 5 whose perceived location is substantially independent of the position of the lis ⁇ tener relative to the loudspeakers.
- the system of the present invention utilizes an apparatus which generates a pair of stereophonic output signals having a cross-correlation which is very close to zero from a pair of standard stereophon ⁇ ic input signals.
- the apparatus of the present invention operates by manipulation of the phase relationships of the output signals while maintaining a constant magnitude across frequency.
- the maintenance of a constant magnitude across frequency avoids changes in the colorization of the output signals.
- the manipulation of the phase relationships creates a phase incoherence which is sufficient to control the cross-correlation of the output signals. It has been found experimentally that both image shift and the precedence effect are substantially reduced when the cross-correlation measure defined below is between -0.5 and 0.5.
- cross-correlation of two signals y.(t) and y 2 (t) will be measured, for the purposes of this discussion, in terms of a cross-correlation measure which is defined to be the extreme value of the cross-correlation function ⁇ ( ⁇ ), where:
- the cross-correlation measure has a maximum possible value of 1 and a mini ⁇ mum possible value of -1. It has been found experimentally that both image shift and the precedence effect are substantially reduced when the cross-correla- tion measure has an absolute value less than 0.5.
- Figure 1 is a block diagram of an apparatus 100 for creating a pair of proc- essed stereophonic output signals, y t (t) and y 2 (t), from a pair of stereophonic input signals, X j (t) and x 2 (t).
- Output signal y-(t) is generated by a first channel 110
- output signal y 2 (t) is generated by a second channel 112.
- Each channel generates an output signal by dividing the appropriate input signal into a plurali ⁇ ty of components, each component representing the intensity of the signal in a specified frequency band.
- Apparatus 100 utilizes a plurality of band-pass fil- ters. Exemplary filters are shown at 120-125. The signal in the ith frequency band is then phase shifted by an amount ⁇ - utilizing a phase shifting network. 5 Exemplary phase shift networks are shown at 140-145. The phase-shifted sig ⁇ nals are then summed by signal adder 16 to form the output signal in question.
- the cross-correlation measure of the output signals, y ⁇ t) and y 2 (t), is determined by the phase shifts ⁇ - that are added to the various frequency
- the ⁇ - are chosen randomly between the specified limits of x and -x. It will be apparent to those skilled in the art that a sequency between P and P+2x, where P is any constant, will provide identical results.
- the manner in which the phase shifts ⁇ - are chosen between the specified limits is important in Q determining the quality of the output signals.
- the ⁇ are chosen by generating a sequence of random numbers between x and -x. Although the preferred embodiment of the present invention utilizes randomly selected phase shifts, other methods of selecting the phase shifts in question may be utilized without departing from the teachings of 5 ' the present invention.
- the important factor here is that the absolute value of the cross-correlation measure of y- and y 2 be made less than approximately 0.5.
- phase shifts added to the band-limited signals in the first channel 110 should be different than those added to the band-limited signals in the second Q channel 112. It will be apparent to those skilled in the art that the apparatus of the present invention will still function satisfactorily if a small percentage of the phase shifts in the first channel 110 are identical to the corresponding phase shifts in the second channel 112.
- the pre ⁇ ferred embodiment of the present invention satisfies additional constraints.
- the phase shifts selected should change rapidly from band to band. In other words, on average, the change in phase angle between adjacent pairs of 5 phase shifts, ⁇ -, ⁇ i+1 , and ⁇ i+2 , should not be monotonic; however, due to the statistics inherent in random selection, this requirement will not always be met.
- the size of the frequency bands utilized in the present invention also af ⁇ fects the performance of the invention. It is found experimentally that frequen- , Q cy bands as small as 50Hz perform satisfactorily. In fact, it is observed that smaller bandwidths perform better than larger bandwidths. The factors which determine the smallest bandwidth will discussed in more detail below.
- the transformation functions h j (t) and h 2 (t) provide a phase-shifting of the indi- 5 vidual frequency bands.
- the present invention preferably utilizes digital input signals. If the signal source consists of analog signals, they may be converted to digital form via a conventional analog-to-digital converters. In this case, each output signal con- sists of a sequence of digital values. The ith value for each output signal corre ⁇ sponds to the value of the output signal at time iT, where T is the time between digital samples. In this case, the convolution operation given in Eq. (2) reduces to:
- the filter coefficients, h m are calculated from:
- a similar convolution operation is used to generate the second output signal, y 2 (nT), with the exception that a different set of ⁇ j . are used.
- the number of frequency samples, N j or N 2 directly specified in the frequency domain and used to create the incoherent time domain signal is limit ⁇ ed by the number of points comprising the time domain signal. Typically, these points are linearly spaced across frequency.
- the filter coefficients that result from using the Fast Fourier Transform given in Eq. (5) will not be constant between the specified frequency points. As a result, timbral neutrality will be completely achieved only if this number is very large in the above described equations. There is a practical limit to the size of this number in commercially realizable apparatuses.
- the present invention minimizes the effects of this trade-off by selecting the particular random number sequence used in generating the phase shifts. It has been found experimentally that different sets of phase shifts, ' ⁇ -., produce different subjective effects on the listener. Hence, in the preferred embodiment of the present invention, a number of different sets of phase shifts are generated, and the set producing the best effect, as judged by listening to the output signals, is utilized.
- the output signals provided by the present invention may be played through conventional speakers. These signals may also be recorded onto con ⁇ ventional stereophonic recording media for subsequent playback through con ⁇ ventional stereophonic equipment. In other words, a pair of traditional stereo ⁇ phonic input signals may be converted to stereophonic output signals, using the
- Q apparatus of the present invention and these stereophonic output signals may then be recorded on the desired storage medium, to be reproduced later by playing the signals back through a conventional stereo sound reproduction system.
- a recording according to the present invention would have first and second channels in which the signal in the first channel had an intensity in each of a M predetermined frequency bands f- ⁇ ⁇ f- that was proportional to the intensity of the second channel in said predetermined frequency bands.
- i runs from 1 to M.
- the phase of the signal in the first channel in each Q of said predetermined frequency bands would differ from that of the second channel signal in the corresponding frequency band by ⁇ ; .
- the ⁇ - are preferably a random sequence between -x and +x.
- any sequence which results in the first and second channels having proportional amplitudes as a function of frequency and a cross-correlation measure less than 0.5 is acceptable. 5
- An apparatus may be installed before the loudspeaker in each channel of a conventional stereophonic sound reproduc ⁇ tion system to perform the phase conversion on-line. It will be apparent to those skilled in the art that such an apparatus converts the original signal in each Q channel to a new signal having an intensity in each of M predetermined frequen ⁇ cy bands f* ⁇ ⁇ f ; which is proportional to the original signal in said frequency bands.
- the phase of the new signal in each of said would differ from that of the orignial signal by ⁇ -, where i runs from 1 to M.
- the ⁇ are chosen such that the cross-correlation measure of the original signal and the new signal is less than °' 5 -
- the improvements obtained with the present invention are illustrated in Figure 2.
- Figure 2 compares the subjective response of a number of subjects to stereophonic program material being played through two speakers.
- the x-axis of the graph is the time delay between the signals from the two speakers.
- Each observer is asked to classify the sound source as coming from either one speaker or more than one speaker.
- the present inven ⁇ tion substantially reduces the precedence effect.
- each channel is assigned a separate set of phase shifts and the signals in that channel processed as described above.
- different sets of randomly chosen phase shifts are utilized.
- other sets of phase shifts may be utilized if the resultant output signals have pairwise cross-correlation measures which are less than 0.5.
- the processing circuitry in one of the channels may be replaced by a delay circuit having a delay equal to half the processing delay in the other channels.
- channel 112 shown in Figure 1 may be replaced by a delay circuit having a delay equal to one half the processing delay of channel 110. It has been found, experimentally, that replac ⁇ ing one channel by a delay circuit improves the perceived timbral fidelity of the output signals.
Abstract
A method for eliminating image shift and the precedence effect in stereophonic recordings is disclosed. The preferred embodiment of the invention utilizes random phase shifting of the signals (140-145) in various frequency bands to reduce the cross-correlation between the stereophonic signals.
Description
METHOD FOR ELIMINATING THE PRECEDENCE EFFECT IN STEREOPHONIC SOUND SYSTEMS AND RECORDING MADE WIT
SATO METHOD
Background of the Invention
The present invention relates to the field of acoustic reproduction and, more particularly, to the stereophonic reproduction of an acoustic image through two or more speakers.
When a pair of loudspeakers are driven by stereophonic signals, the per¬ ceived locations of the various sound sources generated by the stereophonic signals create for the listener what is known as an acoustic image, i.e., a map of the imaginary physical locations of these sound sources. The apparent location of the sound source is largely determined by the difference in arrival time and the intensity of the relevant component signals generated in the left and right speakers.
Thus, if a particular signal component is fed to both speakers with no relative delay and the same signal amplitude, the component of the acoustic image created by that signal will appear to be located on a line centered between the two speakers. If that signal component arrives fractionally earlier from the left speaker than from the right and/or the intensity of the component from the left speaker is greater than that from the right speaker, its image component should appear to be located left of center. The apparent locations of a set of such image components makes up the composite acoustic image perceived by the listener.
In typical listening environments such as living rooms or theaters, most listeners are located nearer to one loudspeaker than to the other(s). For the
purposes of the following discussion, it will be assumed that the acoustic image is being produced by only two loudspeakers. If the listener moves nearer to one
-. loudspeaker, the sound from that speaker is more intense and reaches the listen¬ er ahead of the sound generated at the same time in the other speaker. Hence, moving the listener closer to one speaker is equivalent to introducing an intensi¬ ty loss and a time delay into the material being reproduced in the other speaker. When very similar material is reproduced by two or more loudspeakers, listen-
, 0 ers report that the sound images they perceive are either shifted toward the loca¬ tion of the nearest loudspeaker or almost entirely located in the nearest loud¬ speaker, depending upon the delay in question.
It should be noted that when a listener moves nearer to one speaker, both
15 the intensity of the sound and the time delay are affected. However, it as been shown that the arrival time difference has a more pronounced and important influence than does the intensity difference.
If the time delay is less than approximately 1.0 msec., listeners describe hearing a single sound image located between the speakers, but shifted toward
20 the closer speaker. This effect is referred to as image shift. If the time delay is greater than approximately 1.0 msec but less than an upper limit discussed below, the listener perceives a single sound image that is located at the closer loudspeaker. The traditional explanation for this phenomenon is that the listen¬
25 er's auditory system has attempted to suppress the delayed signal. This phe¬ nomenon is often referred to as the precedence effect, the Haas effect, or the law of the first wavefront. In the following discussion, the effect will be re¬ ferred to as the precedence effect.
- . There is an upper limit to the time delay at which the precedence operates.
At time delays greater than this limit, the delayed sound is heard. The exact magnitude of this upper limit depends upon the qualities of the sound source. The precedence effect is more pronounced for transient sound sources such as struck or plucked musical instruments than it is for continuous sound sources
3_ such as blown or bowed musical instruments. The upper limit is found experi¬ mentally to vary from 8 to 70 msec with a typical limit being about 15 msec.
When the precedence effect releases, listeners report that the sound image 5 is located in two loudspeakers. When the loudspeakers are separated by a suffi¬ ciently great distance, listeners report hearing two sound images, one of which being echo-like. As the time delay from the difference in distances to the two loudspeakers increases further, the intensity difference also increases significant¬ ly. When the intensity difference is approximately 15 dB, the more distant , 0 loudspeaker becomes difficult to hear. At this point, listeners report that the sound image is located in one loudspeaker.
Broadly, it is an object of the present invention to provide an improved apparatus and method for stereophonic sound reproduction.
15
It is another object of the present invention to provide a stereophonic sound reproduction system which generates an acoustic image whose perceived location is independent of the position of the listener relative to the two loud¬ speakers.
20
It is a further object of the present invention to provide a stereophonic sound reproduction system which overcomes or eliminates image shift and the precedence effect.
„ _ These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
Q Brief Description of the Drawings
Figure 1 is a block diagram of an apparatus according to the present inven¬ tion for converting a pair of standard stereophonic input signals into a pair of stereophonic output signals. 5
Figure 2 illustrates the improvement obtained with the present invention.
-. Summary of the Invention
The present invention comprises an apparatus for processing first and second input signals to produce first and second stereophonic sound output sig¬ nals. The apparatus includes a means for receiving said first input signal and a
, 0 first processing circuit for generating a sum signal having a value substantially equal to the sum of N. band-limited signals. The ith said band-limited signal has an amplitude substantially equal to that of the first input signal in a prede¬ termined frequency range !f. ± lδfi and a phase which differs from the phase of said input signal in said predetermined frequency range by an amount 1φi. m m Here, i runs from 1 to N., N. > 2 and the set of 1φi are distributed between -x and +x. The first processing means also includes circuitry for generating the first stereophonic output signal from said sum signal.
The apparatus also includes a means for receiving said second input sig- nal; and a second processing means for generating a sum signal having a value substantially equal to the sum of N2 band-limited signals. The ith said band- limited signal has an amplitude substantially equal to that of said second input signal in a predetermined frequency range 2f. ± 2δf. and a phase which differs from the phase of said input signal in said predetermined frequency range by an - amount 2φ.. Here, i runs from 1 to N2; N2>2, and the set of 2 . are distributed between -x and +x. The second processing means includes circuitry for gener¬ ating the second stereophonic output signal from the sum signal. Here, lφ. differs from 2φ. for at least one value of i.
0 Detailed Description of the Invention
The present invention comprises a method, and the audio recordings made with said method. The method provides a means for providing improved ster¬ eophonic sound reproduction which is capable of generating an acoustic image 5 whose perceived location is substantially independent of the position of the lis¬ tener relative to the loudspeakers. The system of the present invention utilizes
an apparatus which generates a pair of stereophonic output signals having a cross-correlation which is very close to zero from a pair of standard stereophon¬ ic input signals.
The apparatus of the present invention operates by manipulation of the phase relationships of the output signals while maintaining a constant magnitude across frequency. The maintenance of a constant magnitude across frequency avoids changes in the colorization of the output signals. The manipulation of the phase relationships creates a phase incoherence which is sufficient to control the cross-correlation of the output signals. It has been found experimentally that both image shift and the precedence effect are substantially reduced when the cross-correlation measure defined below is between -0.5 and 0.5.
The cross-correlation of two signals, y.(t) and y2(t), will be measured, for the purposes of this discussion, in terms of a cross-correlation measure which is defined to be the extreme value of the cross-correlation function Ω(τ), where:
Ω(r) = lirn^ 1/(2T) J y-(t) y2(t+r) dt (1)
The cross-correlation measure has a maximum possible value of 1 and a mini¬ mum possible value of -1. It has been found experimentally that both image shift and the precedence effect are substantially reduced when the cross-correla- tion measure has an absolute value less than 0.5.
An apparatus for altering the correlation of two signals by adjusting the relative phase as a function of frequency of the two signals is shown in Figure 1. Figure 1 is a block diagram of an apparatus 100 for creating a pair of proc- essed stereophonic output signals, yt(t) and y2(t), from a pair of stereophonic input signals, Xj(t) and x2(t). Output signal y-(t) is generated by a first channel 110, and output signal y2(t) is generated by a second channel 112. Each channel generates an output signal by dividing the appropriate input signal into a plurali¬ ty of components, each component representing the intensity of the signal in a specified frequency band. Apparatus 100 utilizes a plurality of band-pass fil-
ters. Exemplary filters are shown at 120-125. The signal in the ith frequency band is then phase shifted by an amount φ- utilizing a phase shifting network. 5 Exemplary phase shift networks are shown at 140-145. The phase-shifted sig¬ nals are then summed by signal adder 16 to form the output signal in question.
The number of frequency bands into which each of the input signals is split is preferably the same. That is, M- =M-. However, it will be apparent to j Q those skilled in the art that different numbers of bands can be used in the differ¬ ent channels provided the frequency bands are chosen as described below.
The cross-correlation measure of the output signals, y^t) and y2(t), is determined by the phase shifts φ- that are added to the various frequency
15 components of x-(t) and x2(t). In the preferred embodiment of the present invention, the φ- are chosen randomly between the specified limits of x and -x. It will be apparent to those skilled in the art that a sequency between P and P+2x, where P is any constant, will provide identical results. The manner in which the phase shifts φ- are chosen between the specified limits is important in Q determining the quality of the output signals. In the preferred embodiment of the present invention, the φ; are chosen by generating a sequence of random numbers between x and -x. Although the preferred embodiment of the present invention utilizes randomly selected phase shifts, other methods of selecting the phase shifts in question may be utilized without departing from the teachings of 5 ' the present invention. The important factor here is that the absolute value of the cross-correlation measure of y- and y2 be made less than approximately 0.5.
The phase shifts added to the band-limited signals in the first channel 110 should be different than those added to the band-limited signals in the second Q channel 112. It will be apparent to those skilled in the art that the apparatus of the present invention will still function satisfactorily if a small percentage of the phase shifts in the first channel 110 are identical to the corresponding phase shifts in the second channel 112.
5 In order to avoid the perception of a banded or broken image, the pre¬ ferred embodiment of the present invention satisfies additional constraints.
First, the phase shifts selected should change rapidly from band to band. In other words, on average, the change in phase angle between adjacent pairs of 5 phase shifts, φ-, φi+1, and φi+2, should not be monotonic; however, due to the statistics inherent in random selection, this requirement will not always be met.
The size of the frequency bands utilized in the present invention also af¬ fects the performance of the invention. It is found experimentally that frequen- , Q cy bands as small as 50Hz perform satisfactorily. In fact, it is observed that smaller bandwidths perform better than larger bandwidths. The factors which determine the smallest bandwidth will discussed in more detail below.
The above described embodiments of the present invention utilize band- 5 pass filters and phase-shift circuits. The same results may be obtained, howev¬ er, by convolving x-(t) and x2(t) with a filter functions h-(t) and h2(t) to produce y-(t) and y2(t). That is:
y,(t) m. j χ.(t-z)h-(z)dz (2) 0 y2(t) = { x2(t-z)h2(z)dz (3)
The transformation functions hj(t) and h2(t) provide a phase-shifting of the indi- 5 vidual frequency bands.
The present invention preferably utilizes digital input signals. If the signal source consists of analog signals, they may be converted to digital form via a conventional analog-to-digital converters. In this case, each output signal con- sists of a sequence of digital values. The ith value for each output signal corre¬ sponds to the value of the output signal at time iT, where T is the time between digital samples. In this case, the convolution operation given in Eq. (2) reduces to:
y.(nT) = yn = ∑m χn-m *h-. (4)
where m runs from 0 to N-l. The filter coefficients, hm are calculated from:
ihm = (1/N-) ∑k expdαnw + *pk) (5)
Here, k runs from 0 to N--1, w-=2*x/N-, exp(z) =e>z, and N- is the total number of frequency samples. A similar convolution operation is used to generate the second output signal, y2(nT), with the exception that a different set of ^j. are used.
The number of frequency samples, Nj or N2 directly specified in the frequency domain and used to create the incoherent time domain signal is limit¬ ed by the number of points comprising the time domain signal. Typically, these points are linearly spaced across frequency. The filter coefficients that result from using the Fast Fourier Transform given in Eq. (5) will not be constant between the specified frequency points. As a result, timbral neutrality will be completely achieved only if this number is very large in the above described equations. There is a practical limit to the size of this number in commercially realizable apparatuses.
In addition, for complete timbral neutrality, the integrals given in Eqs. (1) and (2) must be performed from -oo to + ∞ . However, in practice, the maximum acceptable convolution time is of the order to 20 msec. If longer times are chosen, transient properties of the input signal are smeared in time. Hence, for any given sampling rate, there is a trade-off between timbral neutral¬ ity and the effect at low frequencies. Hence, the smallest frequency bands into which each stereophonic input signal can be satisfactorily broken is of the order of 50HZ.
The present invention minimizes the effects of this trade-off by selecting the particular random number sequence used in generating the phase shifts. It has been found experimentally that different sets of phase shifts, 'φ-., produce different subjective effects on the listener. Hence, in the preferred embodiment of the present invention, a number of different sets of phase shifts are generated, and the set producing the best effect, as judged by listening to the output signals,
is utilized.
c The output signals provided by the present invention may be played through conventional speakers. These signals may also be recorded onto con¬ ventional stereophonic recording media for subsequent playback through con¬ ventional stereophonic equipment. In other words, a pair of traditional stereo¬ phonic input signals may be converted to stereophonic output signals, using the
, Q apparatus of the present invention, and these stereophonic output signals may then be recorded on the desired storage medium, to be reproduced later by playing the signals back through a conventional stereo sound reproduction system.
, - A recording according to the present invention would have first and second channels in which the signal in the first channel had an intensity in each of a M predetermined frequency bands f- ± δf- that was proportional to the intensity of the second channel in said predetermined frequency bands. Here, i runs from 1 to M. However, the phase of the signal in the first channel in each Q of said predetermined frequency bands would differ from that of the second channel signal in the corresponding frequency band by φ;. The φ- are preferably a random sequence between -x and +x. However, any sequence which results in the first and second channels having proportional amplitudes as a function of frequency and a cross-correlation measure less than 0.5 is acceptable. 5
An apparatus according to the present invention may be installed before the loudspeaker in each channel of a conventional stereophonic sound reproduc¬ tion system to perform the phase conversion on-line. It will be apparent to those skilled in the art that such an apparatus converts the original signal in each Q channel to a new signal having an intensity in each of M predetermined frequen¬ cy bands f* ± δf; which is proportional to the original signal in said frequency bands. The phase of the new signal in each of said would differ from that of the orignial signal by φ-, where i runs from 1 to M. The φ; are chosen such that the cross-correlation measure of the original signal and the new signal is less than °'5-
The improvements obtained with the present invention are illustrated in Figure 2. Figure 2 compares the subjective response of a number of subjects to stereophonic program material being played through two speakers. The x-axis of the graph is the time delay between the signals from the two speakers. Each observer is asked to classify the sound source as coming from either one speaker or more than one speaker. As can be seen from the Figure, the present inven¬ tion substantially reduces the precedence effect.
The above description has utilized a two channel stereophonic system for purposes of illustration. However, it will be apparent to those skilled in the art that the principles of the present invention may be applied to stereophonic systems having more than two channels. In this case, each channel is assigned a separate set of phase shifts and the signals in that channel processed as described above. In the preferred embodiment of the present invention, different sets of randomly chosen phase shifts are utilized. However, other sets of phase shifts may be utilized if the resultant output signals have pairwise cross-correlation measures which are less than 0.5.
It will also be apparent to those skilled in the art that not all of the chan¬ nels need be processed. In particular, the processing circuitry in one of the channels may be replaced by a delay circuit having a delay equal to half the processing delay in the other channels. For example, channel 112 shown in Figure 1 may be replaced by a delay circuit having a delay equal to one half the processing delay of channel 110. It has been found, experimentally, that replac¬ ing one channel by a delay circuit improves the perceived timbral fidelity of the output signals. The replacement of channel 112 by a delay circuit is functional¬ ly equivalent to setting each 2φ. to zero for i= 1 to M2.
There has been described herein a novel apparatus and method for defeat¬ ing precedence effects. Although the above embodiments of the present inven¬ tion have been described with reference to a two channel stereophonic system, it will be apparent to those skilled in the art that the principles described above may be utilized in systems having more than two channels. In this case, each channel is convolved with a different set of random phase shifts.
It will also be apparent to those skilled in the art that the stereophonic output signals produced by the present invention can be recorded on any suitable medium and then played back through a conventional stereophonic system.
Various modifications of the present invention will become apparent to those skilled in the art from the foregoing description and accompanying draw¬ ings. Accordingly, the present invention is to be limited solely by the scope of the following claims.
Claims
1. A method for reducing precedence effect in a sterophonic sound system comprising first and second sound tracks, said method comprising proc¬ essing said first sound track to provide a new first sound track, said new first sound track having an intensity as a function of frequency which is substantially proportional to said first sound track, wherein the absolute value of the cross-
10 correlation measure of said first sound track and said new first sound track is less than 0.5.
2. The method of Claim 1 wherein said processing step comprises gener¬ ating a sum signal having a value substantially equal to the sum of Mj band- , 5 limited signals, the ith said band-limited signal having an amplitude substantially equal to that of said first sound track in a predetermined frequency range f- ± δfj and a phase which differs from the phase of said first sound track in the ith said predetermined frequency range by an amount Φ-, i running from 1 to N
Q 3. The method of Claim 2 wherein said φ- are a random sequence of values between -x and +x.
4. The method of Claim 2 further comprising the step of delaying said second sound track by a predetermined time to compensate for the time needed 5 to perform said processing step on said first sound track.
5. The method of Claim 2 wherein said first sound track comprises a sequence of digital values and wherein said processing step comprises convolv¬ ing said sequence of digital values with a set of filter coefficients. 0
6. A recording comprising first and second sound tracks wherein said first sound track intensity as a function of frequency is proportional to the inten¬ sity of said second sound tracks and said first and second sound tracks have an absolute value of the cross-correlation measure less than 0.5. 5
7. The recording of Claim 6 wherein said first sound track comprises a sum signal having a value substantially equal to the sum of M- band-limited signals, the ith said band-limited signal having an amplitude substantially equal 5 to that of said secondsound track in a predetermined frequency range f- ± δf- and a phase which differs from the phase of said second sound track in the ith said predetermined frequency range by an amount φ;, i running from 1 to M.
8. The recording of Claim 7 wherein said φ- comprises a random se- , 0 quence of values between -x and +x.
5
0
5
0
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US53854390A | 1990-06-15 | 1990-06-15 | |
| US538,543 | 1990-06-15 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| WO1991020164A1 true WO1991020164A1 (en) | 1991-12-26 |
Family
ID=24147346
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| PCT/US1991/004168 WO1991020164A1 (en) | 1990-06-15 | 1991-06-11 | Method for eliminating the precedence effect in stereophonic sound systems and recording made with said method |
Country Status (2)
| Country | Link |
|---|---|
| AU (1) | AU8053691A (en) |
| WO (1) | WO1991020164A1 (en) |
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| EP0553832A1 (en) * | 1992-01-30 | 1993-08-04 | Matsushita Electric Industrial Co., Ltd. | Sound field controller |
| WO2005086139A1 (en) * | 2004-03-01 | 2005-09-15 | Dolby Laboratories Licensing Corporation | Multichannel audio coding |
| US7461002B2 (en) | 2001-04-13 | 2008-12-02 | Dolby Laboratories Licensing Corporation | Method for time aligning audio signals using characterizations based on auditory events |
| US7508947B2 (en) | 2004-08-03 | 2009-03-24 | Dolby Laboratories Licensing Corporation | Method for combining audio signals using auditory scene analysis |
| US7610205B2 (en) | 2002-02-12 | 2009-10-27 | Dolby Laboratories Licensing Corporation | High quality time-scaling and pitch-scaling of audio signals |
| US7711123B2 (en) | 2001-04-13 | 2010-05-04 | Dolby Laboratories Licensing Corporation | Segmenting audio signals into auditory events |
| US8280743B2 (en) | 2005-06-03 | 2012-10-02 | Dolby Laboratories Licensing Corporation | Channel reconfiguration with side information |
| CN101552007B (en) * | 2004-03-01 | 2013-06-05 | 杜比实验室特许公司 | Method and device for decoding encoded audio channel and space parameter |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5381482A (en) * | 1992-01-30 | 1995-01-10 | Matsushita Electric Industrial Co., Ltd. | Sound field controller |
| EP0553832A1 (en) * | 1992-01-30 | 1993-08-04 | Matsushita Electric Industrial Co., Ltd. | Sound field controller |
| US8195472B2 (en) | 2001-04-13 | 2012-06-05 | Dolby Laboratories Licensing Corporation | High quality time-scaling and pitch-scaling of audio signals |
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| CN101552007B (en) * | 2004-03-01 | 2013-06-05 | 杜比实验室特许公司 | Method and device for decoding encoded audio channel and space parameter |
| US10796706B2 (en) | 2004-03-01 | 2020-10-06 | Dolby Laboratories Licensing Corporation | Methods and apparatus for reconstructing audio signals with decorrelation and differentially coded parameters |
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Also Published As
| Publication number | Publication date |
|---|---|
| AU8053691A (en) | 1992-01-07 |
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