KENDALL GARY S (US)
MARTENS WILLIAM M (US)
| US4121059A | 1978-10-17 | |||
| US4972489A | 1990-11-20 | |||
| US4359601A | 1982-11-16 | |||
| US4186643A | 1980-02-05 |
| 1. | In a sound reproduction system in which program material consisting of first and second channels is played through first and second transducers, a method for creating an acoustical image of a predetermined width and distance comprising the step of replacing said second channel with a new second channel having substantially the same intensity as a function of frequency as said first sound track but differing in phase from said first sound track by p(f) which varies with the frequency, f, of said second channel. |
| 2. | The method of Claim 1 wherein said p(f) varies from P to P+δP, wherein P, and δP are determined so as to provide the desired image width and distance. |
| 3. | The method of Claim 2 wherein said step of replacing said second channel comprises generating a signal which is substantially equal to the sum of M bandlimited signals, the ith said bandlimited signal having an amplitude substantially equal to that of said second channel in a predetermined frequency range f ± δf and a phase which differs from the phase of said input signal in said predetermined frequency range by an amount p., i running from 1 to M, wherein M>2 and p. is chosen between P and P+δP. |
| 4. | The method of Claim 3 wherein said p; comprise a random sequence. |
Background of the Invention
The present invention relates to systems for the reproduction of sound, and more particularly, to a m.ethod for controlling the perceived width and distance of a sound source.
The goal of many sound systems is to provide the listener with the illusion that the sound is emanating from an object of a predetermined width located at a predetermined distance relative to the loudspeakers through which the sound material is played. The perceived locations of the various sound sources gener¬ ated by 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 direction of the sound source is determined largely by the differ¬ ence in arrival time and the intensity of the relevant component signals generat¬ ed in the left and right speakers.
With prior art stereophonic sound systems, the illusion of a broad sound source of any specific size is difficult to generate. Some prior art systems uti¬ lize reverberation to broaden the sound image. Others utilize 180° phase shifts. For example, Shimada (U.S. Patent 3,892,624) and Doi, et al. (U.S. Patent 4,069,394) describe a stereophonic reproduction system in which portions of the input signals are scaled by a constant, k, and cross-fed in 180° out-of-phase relationships. That is, given left and right input signals a.(t) and a r (t), left and right output signals L=a,(t)-ka r (t) and R=a r (t)-ka,(t) are generated. When the L and R signals are presented over two loudspeakers, a listener located between the loudspeakers perceives a broadened sound image.
These types of systems are problematic in that they often alter the timbral quality of the program material. The summation of the signals used to provide the output signals results in constructive and destructive interference. This
interference alters the perceived timbre of the sound. In addition, the acoustical images created often appear broken and are highly dependent on the listener's location relative to the loudspeakers. The magnitude of these problems depends critically upon the program material; hence, it is impossible to compensate for the distortions through further processing of the resulting signals. As a result, listeners at different locations hear quite different effects in timbre, image width, and image location.
In addition, the apparent distance of the sound source is limited to loca¬ tions on a line between the speakers. For example, the illusion of a sound source located between the speakers and the listener can not be produced with¬ out utilizing additional speakers closer to the listener.
Broadly, it an object of the present invention to provide an improved sound processing method.
It is a further object of the present invention to provide a sound processing method in which the perceived image width and distance can be controlled.
It is yet another object of the present invention to provide a sound process¬ ing method which is less sensitive to the program material than prior art sound processing methods.
It is a still further object of the present invention to provide a sound proc¬ essing system in which the apparent width and distance of the sound source can be controlled without adding reverberation.
It is yet another object of the present invention to provide a sound process¬ ing system in which the apparent width and distance of the sound source can be controlled without changing the timbre of the sound source.
It is another object of the present invention to provide a sound processing
system in which the apparent width and distance of the sound source can be controlled while utilizing only two loudspeakers.
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.
Brief Description of the Drawings
Figure 1 is a block diagram of a phase-processing circuit suitable for use in practicing the present invention.
Summary of the Invention
The present invention provides a means for creating an acoustical image having a predetermined width and distance relative to the loudspeakers used to play the program material. In a sound reproduction system in which program material consisting of first and second channels is played through first and second transducers, the present invention comprises a method for creating an acoustical image of a predetermined width and distance. The method utilizes a step in which said second channel is replaced with a new second channel having substantially the same intensity as a function of frequency as said first channel but differing in phase from said first channel by p(f) which varies with the frequency, f, of said second channel. The width and distance of the image are determined by the phase shift function p(f).
Detailed Description of the Invention
The present invention provides a means for creating an acoustical image having a predetermined width and distance relative to the loudspeakers used to play the program material. Unlike prior art processing systems, the present invention allows the user to select locations between the listener and the speak- ers. The manner in which the present invention achieves its results can be more
easily understood in the context of a stereophonic sound system utilizing only two loudspeakers. These loudspeakers will be referred to as the left and right speakers, respectively.
To further simplify the discussion, it will also be assumed that a single " acoustical object" is to be created and that the sound made by that object is in the form of a monaural channel. In this case, a conventional stereophonic system would create the illusion of an object at a specific location by amplifying in one loudspeaker so as to create the illusion that the object is located closer to that speaker. Hence, two signals are created, one for playback through each speaker, and one of these is typically amplified relative to the other.
The present invention operates by altering the phase relationships of the signals played from the left and right speakers. In the method of the present invention, a frequency dependent phase difference, p(f), is introduced between the signals played through the left and right speakers. This phase alteration results in changes in the cross-correlation of the left and right soundtracks; however, the intensity as a function of frequency of these signals is not altered.
In the preferred embodiment of the present invention, the frequency spec¬ trum in the range of the signals to be played to the right and left speaker is divided into M bands having frequencies between f. and f.+δf, where i runs from 1 to M. The manner in which M is selected will be discussed in more detail below.
For each such frequency band, a phase shift, p. is selected between P and P+δP. The p. are preferably a sequence of random numbers between these limits. The signal for one of the two channels is divided into the frequency bands described above and the signal in each of the frequency bands shifted by the p. corresponding to the band in question. The signals in the various fre¬ quency bands are then recombined to form a new signal for the channel in ques¬ tion. For the purposes of the following discussion, it will be assumed that the left channel is the one which is so processed. The resultant new signal will be
referred to as the new left channel.
c The manner in which these operations are performed in the preferred embodiment of the present invention are more easily understood with reference to Figure 1 which is a block diagram of a phase-processor 200 according to the present invention. Phase-processor 200 converts an input signal x(t) to an phase processed output signal y(t) by altering the phase of various frequency
10 components of x(t) while leaving the amplitude of the signal in the various components substantially unchanged.
The output signal is generated by dividing the input signal into M compo¬ nents, each component matching the intensity of the input signal in a specific
, 5 frequency band. Phase-processor 200 utilizes a plurality of band-pass filters 12 for this purpose. The signal in the ith frequency band is then phase-shifted by an amount p. utilizing a phase shifting network 14. The p ; are provided by controller 112. Different sets of phase shifts are stored in controller 112 and provided in response to instructions from a console 114 which includes a means Q for inputting P and δP.
It is important that each of the band-pass filters preserve the phase of the frequency component of x(t) selected by the filter in question. The phase- shifted signals are then summed by signal adder 16 to form output signal y(t). 5
The cross-correlation function of two signals, y .(f) and y 2 (t), is defined by
Ω(x)= lim 1/(2T) I y.(t)y 2 (t+x) dt (1)
T-> oo 0
The present invention is believed to derive its effects because of the ability of the above described phase processing to affect the locations of various peaks in the cross-correlation function without substantially affecting the frequency spec¬ e trum of the left and right channels. In the present application, y. and y 2 are the
new left channel and the right channel, respectively. Since, the old left and right channels are perfectly correlated, y. and y 2 could alternately be defined to 5 be the left channel and the new left channel.
It has been found experimentally that P determines the image distance through the control of the ratio of the positive and negative peaks in the cross- correlation function. In loudspeaker reproduction, when P = O, the image is , 0 close to the loudspeakers. As P increases from O to x the image moves closer to the listener. At values near to x, the image will appear to be close to the head, inside the head, or behind the head. As the value of P increases from x to 2x or as it decreases from x to O, the image moves back toward the loudspeak¬ ers.
15
The effect of P is approximately symmetrical about x, but not entirely. For O < P < x, the positive peak in the cross-correlation function leads the negative peak. For x < P < 2x, the negative peaks leads the positive peak. Listeners report differences in the absolute distance of the sound source in these 0 two conditions.
It may also be shown that δP determines the magnitude of the positive and/or negative peaks in the cross-correlation function. When δP is O, the magnitude of the peaks in cross-correlation function are at their maximum (close _ to ± 1, but dependent on the value of P). As the value of δP increases from O to x, the magnitude of the peaks in the cross-correlation function decrease. When δP is equal to x, the magnitude of the peaks in the cross-correlation func- -* tion are at their minimum (close to zero regardless of the value of P).
0 It is found experimentally that δP determines the image width through control of the magnitude of the peaks in the cross-correlation function. In loudspeaker reproduction when δP = O, the image is narrow and tightly fo¬ cused. As δP increases from O to x the image becomes wider and more spatial¬ ly diffuse. At values near to x, the image will appear to be extending from far 5 on one side to the other side. When δP is close to x, the magnitude of P ceases
to have any substantial effect.
Hence, the present invention may be utilized to control both the image width and distance. P is selected in order to provide the desired image distance. δP is selected in order to provide the desired image width. This may be accom¬ plished by constructing a two-dimensional calibration curve for P as a function of image distance and δP. as a function of image width, wherein the choice of P and δP are also dependent on each other.
The manner in which the phase shifts p ; are chosen between the limits specified by P and δP is important in determining the quality of the output sig¬ nals. In the preferred embodiment of the present invention, the p. are chosen by generating a sequence of random numbers between the limits in question. Because of the finite number of frequency bands, it is found that different sets of random numbers produce slightly different effects. 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 selected.
Although the preferred embodiment of the present invention utilizes randomly selected phase shifts, other methods of choosing the phase shifts in question may be utilized without departing from the teachings of the present invention. Some of these methods are discussed below. In choosing a set of phase shifts within the range specified by P and δP, it is important that the phase shifts change direction frequently from band to band. Here, the phase shifts associated with two bands are said to change direction if the signal to the left speaker lags that to the right speaker in the first band while the signal to the left speaker leads that to the second speaker in the second band, or vice versa. As will be discussed in more detail below, this requirement is needed to prevent the perception of a "banded" or "broken" output signal. This requirement can be stated more precisely as follows. Consider three contiguous frequency bands having phase shifts p p i+1 , and p i+2 . On average, the change in phase shift should not be monotonic. That is, if p. > p. +1 then, on average, p. +1 < p. +2 .
Similarly, if p. < p. +1 then, on average, p i+1 > p i+2 . Clearly, because of the random manner in which the phase shifts are chosen, there will be cases for c which three consecutive phase shifts will be monotonic. However, on average this condition should be met.
To better understand the need for this requirement, consider the case in which one wishes to create the illusion of a physically broad sound source emit-
, 0 ting sound along its surface between the two speakers. A sound component having a positive phase shift will be perceived as originating from a source which is closer to one speaker. A sound component having a negative phase shift will be perceived as originating from a source which is closer to the other speaker. The exact position at which each of the components is perceived will
, 5 depend on the magnitude of the phase shift in question. Hence, the present invention produces a sound "image" that appears to emanate from a source that is made up of a collection of discrete sound components, each emitting sound in a specific frequency band and being located at a different position relative to the speakers. This requirement assures that, on average, signals from contiguous
20 frequency bands will be perceived as originating from non-contiguous sources between the speakers.
The distribution of phase shifts will determine the spatial distribution of sound components. If the phase shift distribution is not uniform in phase, the 5 spatial distribution will not be uniform in space. A uniform spatial distribution is desired since it is found experimentally that such a distribution remains uni¬ form when the listener moves from the center line between the loudspeakers to a point off of the center line. For example, when a listener is located left of the center line, sound from the left loudspeaker arrives before sound for the right 0 loudspeaker which introduces a time delay in the arrival sound between the two ears. This time delay affects the phase difference at each frequency differently. A uniform distribution of phase provides the greatest assurance that that shape of the sound image is not altered by the time delay, since it results in another uniform distribution of phase. 5
The above discussion deals only with the phase shifts, p.. The manner in which the width of the bands is selected will now be discussed. If the bands are ς too broad, the listener will perceive a broken or banded image. However, if the bands are made too narrow, other problems are encountered.
It is known from psycho-acoustical research that there is a critical band¬ width below which the human ear can not discriminate. The critical bandwidth n depends on frequency, varying from approximately 100 Hz at low frequencies
(<2000 Hz) to approximately one seventh the center frequency of the band in question at high frequencies (>2000 Hz).
It is found experimentally, that the optimum bandwidth is approximately 5 two critical bandwidths. In addition, the optimal bandwidth is also dependent on the sound material being processed. In the case of periodic and quasi-period¬ ic tones such as speech and most musical instruments it is useful to organize the bands such that the harmonically related partials fall into separate bands. The fundamental will fall into a first band, the second harmonic into another, and so on. The limit to this rule is that bands not become smaller than a critical band, 0 since higher harmonics will naturally fall together into a signal critical band as the frequency increases. In the case of non-periodic or noise-like sounds, there is no fundamental. In this case, partials will likely fall into every adjacent band. It is useful that these bands be as small as possible and again that the phase of these adjacent bands shift rapidly. Experience has shown that the optimal 5 bandwidth for non-periodic sounds is two critical bands wide.
Table 1 provides a list of center frequencies for bands and indications for left/right leading phase shifts such that adjacent bands lead in different direc¬ tions and fundamental and second harmonics fall into non-adjacent bands leading in different directions up to the limit of critical band spacing. The left channel is defined to lead the right channel if (φ R - φ h ) > 0. In the case in which a phase-shifted output signal is generated from the input signal, one of the 's will be zero. Hence, this is equivalent to requiring that the phase-shifts added to the frequency bands be chosen such that no three adjacent frequency bands are
given phase shifts with the same sign.
The above described embodiments of the present invention utilize band- pass filters and phase shift circuits. The same result may be obtained, however, by convolving x(t) with a filter function h(t) to produce y(t). That is,
y(t) = f x(t-z)h(z)dz (2)
The transformation function h(z) provides the phase shifting of the individual frequency bands.
The present invention preferably utilizes a digital input signal. If the signal source consists of an analog signal, it may be converted to digital form via a conventional analog-to-digital converter. In this case, each output signal consists of a sequence of digital values. The ith value for each output signal corresponds to the value of the output signal at a time iT, where T is the time between digital samples. In this case, the convolution operation given in Eq. (2) reduces to
y ■> (nT) / where m runs from 0 to N-l. The filter coefficients, h are calculated from
h m = (1/N) ∑ k exp(kmw+ k ) (4)
Here, k runs from 0 to N-l, w=2x/N, and exp(α)=e iα , and N is the total number of frequency samples.
The above described embodiments of the present invention utilize loud¬ speakers for the reproduction of the program material. However, it will be apparent to those skilled in the art that other sound transducers such as ear¬ phones may be used.
The above description has utilized a single sound source. It will be appar¬ ent to those skilled in the art that the output channels of the present invention may be mixed with others to produce the final left and right channels that are actually played through the speakers. In this manner the present invention may be utilized to control the position and size of one or more acoustical "objects" in a complex acoustical image consisting of a large number of "objects".
There has been described herein a novel method for creating a sound image with a predetermined width located at a point between a pair of stereo¬ phonic speakers and a listener. Various modifications to the present invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.