Sound image localization apparatus, stereophonic sound image enhancement apparatus, and sound image control system

A stereophonic sound image enhancement apparatus subtracts a right input signal from a left input signal and amplifies the subtracted signal to produce a difference signal, and also subtracts a right crosstalk signal from a left low-frequency-range enhanced signal obtained by filtering the left input signal. Then, the difference signal is added to this subtracted signal, and the added signal is outputted as a left output signal. Also, the left crosstalk signal is subtracted from a right low-frequency-range enhanced signal obtained by filtering the right input signal, and the difference signal is subtracted from the subtracted signal to output the subtraction result as a right output signal. Furthermore, a sound image control apparatus applies a left-channel head related transfer function to a monophonic input signal to output the resultant signal as a left input signal "Lin", and also applies a right-channel head related transfer function to this monophonic input signal "Min" to output the resultant signal as a right input signal "Rin". The sound image control apparatus performs a crosstalk canceling process operation with respect to these left input signal "Lin" and right input signal "Rin", and further mixes the left input signal "Lin" and the right input signal "Rin" with signals having phases reverse to those of the left/right input signals. Finally, the signals processed by the crosstalk canceling process and the mixing process are mixed with each other.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to a sound image localization 
apparatus, a stereophonic sound image enhancement apparatus, and a sound 
image control system suitably used in various acoustic devices, for 
instance, electronic musical instruments, game machines, and sound mixers. 
More specifically, the present invention is directed to a sound image 
localization apparatus capable of realizing sound image localization by a 
loudspeaker by employing a simple analog circuit, to a stereophonic sound 
image enhancement apparatus capable of enhancing a sound image in response 
to a stereophonic sound signal in a two-channel loudspeaker reproduction, 
and to a sound image control apparatus for localizing a sound image to an 
arbitrary position in a three-dimensional space in response to a 
monophonic sound signal. 
2. Description of the Related Art 
Conventionally, such a technical idea is known in the field that 2-channel 
stereophonic signals are produced, and these stereophonic signals are 
supplied to right/left loudspeakers so as to simultaneously produce 
stereophonic sounds, so that sound images may be localized. In accordance 
with this sound image localization technique, the sound images are 
localized by changing the balance in the right/left sound volume, so that 
the sound images could be localized only between the right/left 
loudspeakers. 
To the contrary, very recently, several techniques have been developed by 
which sound images can be localized outside the right/left loudspeakers. 
That is, in the first prior art to reproduce the sounds by way of the two 
right/left loudspeakers, the sound having the reverse phase with respect 
to the phase of the right-channel sound is mixed with the left-channel 
sound, and also the sound having the reverse phase with respect to the 
phase of the left-channel sound is mixed with the right-channel sound. As 
a result, the sound image may be localized outside the left/right 
loudspeakers. This sort of conventional technique is disclosed in, for 
instance, WO94/16538 (PCT/US93/12688) entitled "SOUND IMAGE MANIPULATION 
APATUS AND METHOD FOR SOUND IMAGE ENHANCEMENT." 
Concretely speaking, in this first conventional technique, the difference 
signal between the left-channel left input signal and the right-channel 
right input signal is produced. This difference signal is supplied to the 
band-pass filter while the amplitude of this difference signal is properly 
controlled. Then, the difference signal derived from the band-pass filter 
is added to a one-channel input signal, so that the output signal for this 
channel is produced. Similarly, the difference signal derived from the 
band-pass filter is subtracted from the other-channel input signal, so 
that the output signal for this channel is produced. The respect output 
signals are supplied to the right/left loudspeakers. In accordance with 
this first conventional technique, since the sound image can be localized 
outside the right/left loudspeakers, the sound stage can be greatly 
enhanced. 
Also, as the second conventional technique, the sound image localization 
technique called as "Schroeder" system is widely known in the field. In 
this Schroeder system, the sounds which are produced from the left 
loudspeaker and then reach the right ear of the audience, and also the 
sounds which are produced from the right loudspeaker and then reach the 
left ear (will be referred to as "crosstalk sounds" hereinafter) are 
canceled, so that the sound listening conditions through the headphone are 
established. As a consequence, the sound images can be localized not only 
between the right/left loudspeakers, but also arbitrary positions, for 
example, a position on one side of the audience. 
Furthermore, as the third conventional technique, such a technique is known 
that since the head related transfer function (head related acoustic 
transfer function) is added and the crosstalk canceling process is 
performed by way of the convolution calculation, the sound image can be 
localized at an arbitrary position (for instance, see "As to RSS" by 
Roland K. K. JAPAN, Japanese Acoustic Society volume 48, No. 9). 
However, the above-described first conventional technique has the following 
problem. That is, when the sounds are heard at a position apart from the 
loudspeakers, the audience hears the sound images which are localized 
outside the right/left loudspeakers. To the contrary, when the sounds are 
heard at a position close to the loudspeakers, the audience cannot cleary 
discern where the sound images are localized. 
Also, the first conventional technique has another problem. That is, when 
the mixing ratio of the other-channel sound having the reverse phase to 
one-channel sound is increased by controlling the magnitude of the 
difference signal in order to increase the sound stage enhancement effect, 
the sound quality deteriorates. This quality deterioration is caused 
because the comb filter characteristic is formed by the monophonic 
components of the input signal. This sound quality deterioration will 
appear as such a phenomenon that the audience hears the sounds from which 
the low frequency components are mainly cut off. When the sound quality 
deterioration becomes extreme, it becomes difficult to reproduce an input 
source. 
Also, the second conventional technique has a problem that the audience 
hear the sound image which is close by, resulting in unnatural sounds. In 
addition, if the above-described crosstalk canceling theory by the 
Schroeder system is strictly applied to constitute the sound image 
localization apparatus by employing the analog circuit, then a very large 
amount of hardware is necessarily required. On the other hand, if the 
sound image localization apparatus is constituted by employing the 
software of the digital processor (DSP) or the CPU, execution is very 
different. As a consequence, conventionally, the sound image localization 
apparatus with employment of the Schroeder system could be limitedly 
applied only to the high-grade electronic musical instruments and the 
high-grade acoustic appliances. 
Also, the above-explained third conventional technique owns another 
problem. That is, when the head related transfer function is added and the 
crosstalk canceling process operation is carried out by way of the 
convolution calculation, a large number of convolution stages is required, 
so that the hardware scale is increased. Conversely, if a total number of 
the convolution stages is decreased, then a different problem occurs. That 
is, the low frequency components of the signals which are processed by the 
head related transfer function are reduced, and also the crosstalk sounds 
canceled by the crosstalk canceling process are reduced. 
SUMMARY OF THE INVENTION 
The present invention has been made to solve the above-explained problems, 
and therefore, an object is to provide a sound image localization 
apparatus capable of localizing a sound image at an arbitrary position by 
employing a simple and low-cost analog circuit in a two-channel 
loudspeaker reproducing mode. 
Another object of the present invention is to provide a stereophonic sound 
image enhancement apparatus capable of enhancing a stereophonic sound 
image with a simple and low-cost arrangement, and further having less 
deterioration of sound qualities in a two-channel loudspeaker reproducing 
mode even when a stereophonic sound image enhancement effect is 
emphasized. 
A further object of the present invention is to provide a stereophonic 
sound image enhancement apparatus such that even when an audience hears 
sounds at a position near loudspeakers in a two-channel loudspeaker 
reproducing mode, the audience can have such a clear feeling of a position 
where a sound image is localized, and unnatural feelings can be removed by 
localizing sound images at a position apart from the audience, and 
furthermore the audience can discern the sound fields. 
A still further object of the present invention is to provide a sound image 
control apparatus capable of increasing precision in a process operation 
of a head related transfer function and also a crosstalk canceling 
operation by employing a small scale of hardware without executing a 
convolution operation. 
As shown in FIG. 1, a sound image localization apparatus, according to a 
first aspect of the present invention, is featured by comprising: 
a first group delay equalizer for delaying a left input signal entered from 
a left input terminal; 
a first low-pass filter for filtering an output signal derived from the 
first delay equalizer to output a filtered signal as a left crosstalk 
signal; 
a second group delay equalizer for delaying a right input signal entered 
from a right input terminal; 
a second low-pass filter for filtering an output signal derived from the 
second delay equalizer to output a filtered signal as a right crosstalk 
signal; 
first calculating means for subtracting the right crosstalk signal from the 
left input signal to output a subtracted signal as a left output signal; 
and 
second calculating means for subtracting the left crosstalk signal from the 
right input signal to output a subtracted signal as a right output signal. 
Now, a basic operation idea of this sound image localization apparatus will 
be simply summarized. When sounds are heard by an audience by using a 
headphone, sounds to be produced from a left loudspeaker SPL reaches only 
a left ear of this audience, and sounds to be produced from a right 
loudspeaker SPR reaches only a right ear of this audience. However, when 
sounds are heard by an audience by using loudspeakers, as indicated in 
FIG. 2, sounds produced from a left loudspeaker SPL and sounds produced 
from a right loudspeaker SPR reach a left ear and a right ear of the 
audience, respectively. It should be understood that, as illustrated in 
FIG. 2, a sound which is produced from the left loudspeaker SPL and then 
directly reaches the left ear of the audience will be referred to as a 
"left direct sound AL"; a sound produced from the left loudspeaker SPL, 
which cross-reaches the right ear, will be referred to as a "left 
crosstalk sound BL"; a sound which is produced from the right loudspeaker 
SPR and then directly reaches the right ear of the audience will be 
referred to as a "right direct sound AR," and a sound produced from the 
right loudspeaker SPR, which cross-reaches the left ear, will be referred 
to as a "right crosstalk sound BR." 
This sound image localization apparatus according to the basic idea of the 
present invention is controlled in such a manner that only the left direct 
sound AL can reach the left ear of the audience and only the right direct 
sound AR can reach the right ear by canceling the above-described left 
crosstalk sound BL and right crosstalk sound BR. As a result, it is 
possible to establish similar listening conditions to that by using the 
headphone, so that the sound images can be localized outside the 
right/left loudspeakers SPR/SPL. 
To establish the above-described listening conditions, the sound having the 
reverse phase to that of the right crosstalk sound BR is mixed with the 
sound produced from the left loudspeaker SPL in this sound image 
localization apparatus. As a result, the right crosstalk sound BR is 
canceled. Similarly, the sound having the reverse phase to that of the 
left crosstalk sound BL is mixed with the sound produced from the right 
loudspeaker SPR in this sound image localization apparatus. As a result, 
the left crosstalk sound BL is canceled. 
Assuming now that a left input signal inputted to the sound image 
localization apparatus is expressed by "Lin," a right input signal 
inputted thereto is expressed by "Rin"; a left output signal outputted 
from the sound localization apparatus is expressed by "Lout," a right 
output signal outputted therefrom is expressed by "Rout"; first and second 
group delay equalizers are expressed by a function "G1(s)"; and also first 
and second low-pass filters are expressed by a product "aG2(s)" made by a 
coefficient "a" and another function "G2(s)," both the left output signal 
"Lout" and the right output signal "Rout" may be expressed by the 
below-mentioned formula (1) and formula (2): 
EQU Lout=Lin-aG1(s)G2(s)Rin formula (1) 
EQU Rout=Rin-aG1(s)G2(s)Lin formula (2) 
A symbol "aG1(s)G2(s)Rin" contained in the above-described formula (1) 
indicates a right crosstalk signal corresponding to the right crosstalk 
sound BR, whereas a symbol "aG1(s)G2(s)Lin" contained in the 
above-described formula (2) indicates a left crosstalk signal corresponds 
to the left crosstalk sound BL. As a result, the formula (1) represents 
that the left output signal "Lout" can be obtained by subtracting the 
right crosstalk signal from the left input signal "Lin," and the formula 
(2) represents that the right output signal "Rout" can be obtained by 
subtracting the left crosstalk signal from the right input signal "Rin." 
The above-described first and second group delay equalizers G1(s) may be 
constructed by an all-pass filter. This all-pass filter owns such a 
characteristic that although the phase of the output signal varies in 
response to a change in a frequency, the amplitude of this output signal 
is not varied. The first group delay equalizer G1(s) simulates left 
difference time indicative of a difference between the time during which 
the left direct sound AL reaches the left ear, and the time during which 
the right crosstalk sound BR reaches the left ear. Similarly, the second 
group delay equalizer G1(s) simulates right difference time indicative of 
a difference between the time during which the right direct sound AR 
reaches the right ear, and the time during which the left crosstalk sound 
BL reaches the right ear. 
As the function G1(s), as indicated by a broken line of FIG. 3, such a 
function having a group delay which does not depend upon the frequency is 
ideal. However in a group delay equalizer constructed by an analog 
circuit, the higher the frequency becomes, the more difficult the large 
group delay is obtained. On the other hand, in this sound image 
localization apparatus, the Inventors could confirm by their experiments 
that when the group delay was equalized up to, for example, on the order 
of 2 kHz, the sufficient effect could be achieved. As a consequence, the 
first and second group delay equalizers G1(s) may be realized by, for 
example, a group delay equalizer having group delay time of 180.mu. 
expressed by the below-mentioned formula (3): 
##EQU1## 
where symbol ".omega..sub.0 " indicates an angular frequency at which the 
phase becomes 180.degree., symbol ".zeta." shows an attenuation rate 
(.zeta.=1/2Q), and symbol "s" denotes a Laplace operator (j.omega.). 
In FIG. 3, there is shown by a solid line, a group delay time 
characteristic of the first and second group delay equalizers when 
.omega..sub.0 =3386 Hz and .zeta.=1 are set in the above-described formula 
(3). As apparent from FIG. 3, this group delay time characteristic 
represents a quasi-idea group delay time characteristic near 2 kHz. 
The output signal from the first group delay equalizer is supplied to the 
first low-pass filter, and the output signal from the second group delay 
equalizer is supplied to the second low-pass filter. As to the function 
aG2(s) indicative of the first and second low-pass filters, the 
coefficient "a" denotes lowering of sound volumes based on the 
above-described left difference time and right difference time. This 
coefficient "a" may be selected to be on the order of 0.5 to 1.0. 
The crosstalk sound becomes indistinct sounds effected by the head related 
transfer function, namely softer sounds than direct sounds. The first and 
second low-pass filters own a function capable of obtaining such a sound 
similar to this crosstalk sound by equalizing the characteristic of such a 
crosstalk sound. In other words, these first and second low-pass filters 
may approximate the head related transfer function. 
The function G2(s) represents, for instance, a first-order low-pass filter 
having a cut-off frequency of 1 to 2 kHz. This function G2(s) may be 
expressed by, for example, the following formula (4): 
##EQU2## 
where symbol ".omega..sub.0 " indicates a cut-off angular frequency, 
symbol "R" denotes an attenuation suppressing value, and symbol "s" 
represents a Laplace operator. 
FIG. 4 represents a frequency characteristic of the function G2(s) when the 
cut-off angular frequency=1,700 Hz and the attenuation suppressing 
value=0.5. As apparent from FIG. 4, the characteristic of the first and 
second low-pass filters are different from the characteristic of the 
general-purpose low-pass filter. That is, the attenuation of the amplitude 
of the signal in the high frequency range is suppressed to gently descend, 
and then is saturated near -6 dB. The left crosstalk signal derived from 
the first low-pass filter is supplied to the second calculating means, and 
the right crosstalk signal derived from the second low-pass filter is 
supplied to the first calculating means. 
The first and second calculating means may be arranged by adders, 
respectively. The first calculating means produces the left output signal 
"Lout" by subtracting the right crosstalk signal from the left input 
signal "Lin". Similarly, the second calculating means produces the right 
output signal "Rout" by subtracting the left crosstalk signal from the 
right input signal "Rin". 
If the left output signal Lout and the right output signal Rout are 
supplied to the left loudspeaker and the right loudspeaker, respectively, 
then the right crosstalk sound BR derived from the right loudspeaker and 
the left crosstalk sound BL derived from the left loudspeaker can be 
canceled. Accordingly, since only the direct sounds from the right 
loudspeaker reach the right ear of the audience and the only the direct 
sounds from the left loudspeaker reach the left ear of the audience, it is 
possible to establish such a listening condition similar to that by the 
headphone. As a consequence, the sound images can be localized not only 
between the right/left loudspeakers, but also in the broad range around 
the audience. 
The sound image localization apparatus, according to the fist aspect of the 
present invention, may be arranged by further comprising: distributing 
means for distributing a monophonic input signal to the left input signal 
and the right input signal, and wherein the left input signal derived from 
the distributing means is supplied to the left input terminal, and the 
right input signal derived from the distributing means is supplied to the 
right input terminal. 
This distributing means may be arranged by, for example, a balancer 
constructed of a variable resistor. Since the distribution ratio of the 
right input signal to the left input signal is changed by this 
distributing means, the localization position of the sound image can be 
varied. In accordance with this sound image localization apparatus, the 
stereophonic sound image can be enhanced. It should be noted that if this 
sound image localization apparatus is provided in each of plural parts and 
the left output signals and the right output signals derived from the 
respective sound image localization apparatuses are mixed with each other 
to output the mixed output signals, then it is possible to realize such a 
sound image localization apparatus capable of controlling a plurality of 
sound images. 
In this sound image localization apparatus, the first group delay 
equalizer, the second group delay equalizer, the first low-pass filter, 
the second low-pass filter, the first calculating means, the second 
calculating means, and the distributing means are constituted by an analog 
circuit element. As the analog circuit element, for example, an 
operational amplifier, a resistor, a variable register and a capacitor may 
be utilized. As a result, the sound image localization apparatus can be 
made simple and in low cost. 
As previously described, in accordance with the sound image localization 
apparatus according to the first aspect of the present invention, the 
sound images can be localized at an arbitrary position by employing the 
simple and low-cost analog circuit in the two-channel loudspeaker 
reproducing mode. 
As represented in FIG. 5A, a stereophonic sound image enhancement 
apparatus, according to a second aspect of the present invention, is 
featured by comprising: 
a first group delay equalizer for delaying a left input signal entered from 
a left input terminal; 
a first high-pass filter for filtering an output signal derived from the 
first group delay equalizer to output a filtered signal as a left 
crosstalk signal; 
a first low-pass filter for filtering the left input signal to output a 
filtered signal as a left low-frequency-range enhanced signal; 
a second group delay equalizer for delaying a right input signal entered 
from a right input terminal; 
a second high-pass filter for filtering an output signal derived from the 
second group delay equalizer to output a filtered signal as a right 
crosstalk signal; 
a second low-pass filter for filtering the right input signal to output a 
filtered signal as a right low-frequency-range enhanced signal; 
subtracting means for subtracting the right input signal from the left 
input signal; 
amplifying means for amplifying an output signal derived from the 
subtracting means to output an amplified signal as a difference signal; 
first calculating means for subtracting the right crosstalk signal from the 
left low-frequency-range enhanced signal, and also for adding a 
subtraction result to the difference signal to thereby output an addition 
result as a left output signal; and 
second calculating means for subtracting the left crosstalk signal from the 
right low-frequency-range enhanced signal, and also for subtracting the 
difference signal from a subtracted signal to thereby output a subtraction 
result as a right output signal. 
In this stereophonic sound image enhancement apparatus, a difference signal 
is firstly produced by subtracting the right input signal from the left 
input signal. Next, this difference signal is added to the left input 
signal to thereby produce the left output signal. Similarly, this 
difference signal is subtracted from the right input signal to thereby 
produce the right output signal. When this left output signal is supplied 
to the left loudspeaker and this right output signal is supplied to the 
right loudspeaker, the sound image can be localized outside the right/left 
loudspeakers. 
The subtracting means subtracts the right input signal Rin from the left 
input signal Lin. As a result, the difference signal (Lin-Rin) is 
produced. This difference signal is multiplied by the coefficient "b" by 
the amplifying means, and the amplified signal is supplied to the first 
calculating means and the second calculating means. The coefficient "b" 
defines a degree of broad feelings, and may be selected to be 0.0 to 1.0. 
Assuming now that the first and second group delay equalizers are expressed 
by a product "cG3(s)" between a coefficient "c" and a function G3(s); the 
first and second low-pass filters are expressed by a function "G4'(s)"; 
and the first and second high-pass filters are expressed by another 
function "G4(s)", the left output signal "Lout" may be expressed by the 
following formula (5) and the right output signal "Rout" may be expressed 
by the following formula (6): 
EQU Lout(s)=LinG4'(s)+b(Lin-Rin)-aG3(s)G4(s)Rin formula (5) 
EQU Rout(s)=RinG4'(s)+b(Rin-Lin)-aG3(s)G4(s)Lin formula (6) 
In this case, the reason why the first and second high-pass filters G4(s) 
are employed is given as follows. That is, an audience does not have the 
capability of recognizing a localization direction with respect to such a 
sound image which is formed by low frequencies such as a bass guitar, and 
a bass drum. Generally speaking, accordingly, a stereophonic signal is 
produced in such a manner that sounds of low frequencies may be heard from 
a center position between right/left loudspeakers. In other words, the 
signals corresponding to the low frequency components are contained in the 
left-channel signal and the right-channel signal in the ratio of 50% to 
50%, and the same phase are produced by the left-channel signal and the 
right-channel signal. Therefore, there are few phase differences between 
the signal corresponding to the low frequency component contained in the 
left-channel signal and the signal corresponding to the low frequency 
component contained in the right-channel signal. As a consequence, when 
such a signal produced by executing a predetermined process operation to 
the right input signal Rin (namely, signal delayed by second group delay 
equalizer) is subtracted from the left input signal Lin, since the sounds 
with the low frequencies are canceled by each other, the bass sounds are 
decreased. This phenomenon is similarly applied to such a case that a 
signal produced by executing a predetermined process operation to the left 
input signal Lin (namely, signal delayed by first group delay equalizer) 
is subtracted from the right input signal Rin. 
As a consequence, in order to remove the low frequency components from the 
respective output signals derived from the first and second group delay 
equalizers, the first and second high-pass filters are employed. The 
cut-off frequency "f1" of the function G4(s) employed in these first and 
second high-pass filters may be selected to be on the order of 100 Hz. 
Under this condition, such a frequency characteristic of G4(s) indicated 
by a solid line is represented in FIG. 6. 
However, even when the first and second high-pass filters are provided, 
there is no clear recognition that lowering of the bass sounds can be 
sufficiently suppressed. Accordingly, first and second low-pass filters 
having such a frequency characteristic as shown by a broken line of FIG. 6 
are employed. The first and second low-pass filters produce a left 
low-frequency-image emphasized signal and a right low-frequency-range 
emphasized signal. In the left low-frequency-range emphasized signal, the 
bass sound range of the left input signal Lin is emphasized. In the right 
low-frequency-range emphasized signal, the bass sound range of the right 
input signal Rin is emphasized. Then, both the left low-frequency-range 
emphasized signal and the right crosstalk signal derived from the second 
high-pass filter are calculated to thereby produce the left output signal 
Lout. Similarly, both the right low-frequency-range emphasized signal and 
the left crosstalk signal derived from the first high-pass filter are 
calculated to thereby produce the right output signal Rout. As a result, 
it is possible to obtain the sounds with better qualities from the bass 
sound range to the treble sound range. 
In the above-described formula (5), symbol "cG3(s)G4(s)Rin" indicates the 
right crosstalk signal, whereas in the formula (6), symbol 
"cG3(s)G4(s)Lin" indicates the left crosstalk signal. Also, symbol 
"b(Rin-Lin)" indicates such a signal obtained by multiplying the following 
difference signal by the coefficient "b". This difference signal is 
obtained by subtracting the right input signal Rin from the left input 
signal Lin. This signal "b(Rin-Lin)" may apply the broad feelings. 
The function G4'(s) indicative of the first and second low-pass filters 
owns a characteristic approximated to the reverse characteristic of the 
function G4(s). As a consequence, the formula (5) indicates that the left 
output signal Lout is produced by adding "b(Rin-Lin)" to such a signal 
obtained by subtracting the right crosstalk signal from the signal 
produced by filtering the left input signal Lin by the first low-pass 
filter. Similarly, the formula (6) indicates that the right output signal 
Rout is produced by subtracting "b(Rin-Lin)" from such a signal obtained 
by further subtracting the left crosstalk signal from the signal produced 
by filtering the right input signal Rin by the second low-pass filter. 
As apparent from FIG. 6, it should be understood that when the cut-off 
frequency of the function G4'(s) is lower than the cut-off frequency of 
the function G4(s), the frequency range in which no broadening control is 
carried out will be produced. However, this frequency range is the bass 
sound range, and since the audiences cannot sense the direction of the 
sound image with respect to the low frequencies, there is no practical 
problem. 
The first and second group delay equalizers may be realized by employing 
those in the above-described sound image localization apparatus. The 
output signals derived from these first and second group delay equalizers 
are supplied to the first and second high-pass filters. The coefficient 
"c" of the first and second group delay equalizers may be selected to be 
on the order of 0.5 to 1.0. 
As indicated by a broken line of FIG. 6, the first and second low-pass 
filters G4'(s) may be constructed by having a characteristic approximated 
to the reverse characteristic of G4(s). The left low-frequency-range 
emphasized signal derived from the first low-pass filter is supplied to 
the first calculating means, and the right low-frequency-range emphasized 
signal derived from the second low-pass filter is supplied to the second 
calculating means. 
The first and second calculating means may be arranged by a calculating 
circuit having, for example, an operational amplifier. The first 
calculating means subtracts the right crosstalk signal from the left 
low-frequency-range emphasized signal, and then adds the difference signal 
to this subtraction result so as to output the addition result as the left 
output signal Lout. Similarly, the second calculating means subtracts the 
left crosstalk signal from the right low-frequency-range emphasized 
signal, and then subtracts the difference signal from this subtraction 
result so as to output the subtraction result as the right output signal 
Rout. 
As a consequence, if the left output signal Lout and the right output 
signal Rout are supplied to the left loudspeaker and the right 
loudspeaker, such sounds from which both the right crosstalk sound and the 
left crosstalk sound have been canceled may be produced. In addition, the 
difference signal is added to the left input signal Lin, and then the 
addition result is subtracted from the right input signal Rin, so that the 
sound images can be localized not only between the right and left 
loudspeakers, but also in the broad range around the audience. 
Furthermore, the stereophonic sound image can be greatly enhanced, as 
compared with the above-explained sound image localization apparatus. 
Similar to the case of the above-described sound image localization 
apparatus, this sound image localization apparatus may be arranged, as 
shown in FIG. 5B, in such a way that the respective output signals derived 
from the first and second group delay equalizers are filtered by the third 
and fourth low-pass filters G.sub.LPF (S) indicated by the below-mentioned 
formula (7), and the filtered output signals are supplied to the first and 
second high-pass filters: 
##EQU3## 
where symbol ".omega..sub.0 " indicates a cut-off angular frequency, 
symbol "R" denotes an attenuation suppressing value, and symbol "s" shows 
a Laplace operator. These third and fourth low-pass filters are arranged 
by the same structures of the first and second low-pass filters employed 
in the above-described sound image localization apparatus, and own the 
same functions and effects as those of these first and second low-pass 
filters. 
In this stereophonic sound image enhancement apparatus, the first group 
delay equalizer, the first high-pass filter, the first low-pass filter, 
the second group delay equalizer, the second high-pass filter, the second 
low-pass filter, the subtracting means, the amplifying means, the first 
calculating means, the second calculating means, the third low-pass 
filter, and the fourth low-pass filter are constituted by an analog 
circuit element. As the analog circuit element, for instance, an 
operational amplifier, a resistor, a capacitor, and the like may be 
employed. As a consequence, the stereophonic sound image enhancement 
apparatus may be made simple and in low cost. 
It should be noted that although the above-described subtracting means is 
so arranged as to produce the difference signal by subtracting the right 
input signal Rin from the left input signal Lin, this difference signal 
may be produced by subtracting the left input signal Lin from the right 
input signal Rin. In this case, the first calculating means may be 
arranged in such a manner that the difference signal derived from the 
amplifying means is subtracted from the subtraction result obtained by 
subtracting the output signal derived from the second high-pass filter 
from the output signal derived from the first low-pass filter, and this 
subtraction result is outputted as the left output signal Lout to the left 
output terminal. Similarly, the second calculating means may be arranged 
in such a manner that the difference signal derived from the amplifying 
means is added to the subtraction result obtained by subtracting the 
output signal derived from the first high-pass filter from the output 
signal derived from the second low-pass filter, and this addition result 
is outputted as the right output signal Rout to the right output terminal. 
As previously explained, in accordance with the stereophonic sound image 
enhancement apparatus according to the second aspect of the present 
invention, the stereophonic image can be enhanced without any 
deterioration in the sound quality in the two-channel loudspeaker 
reproducing mode, and further with the simple and low-cost arrangement. 
Also, as indicated in FIG. 7, a stereophonic sound image enhancement 
apparatus, according to a third aspect of the present invention, is 
featured by comprising: 
crosstalk canceling means for producing a first left signal Lm1 formed by 
subtracting a right crosstalk signal from a left input signal Lin, and a 
first right signal Rm1 formed by subtracting a left crosstalk signal from 
a right input signal Rin; 
reverse-phase signal producing means for producing a second left signal Lm2 
formed by mixing a reverse-phase signal of the right input signal Rin with 
the left input signal Lin, and a second right signal Rm2 formed by mixing 
a reverse-phase signal of the left input signal Lin with the right input 
signal Rin; and 
mixing means for mixing the first left signal Lm1 derived from the 
crosstalk canceling means with the second left signal Lm2 derived from the 
reverse-phase signal producing means to thereby produce a left output 
signal Lout, and also for mixing the first right signal Rm1 derived from 
the crosstalk canceling means with the second right signal Rm2 derived 
from the reverse-phase signal producing means to thereby produce a right 
output signal Rout. 
The above-described crosstalk canceling means is effected to localize the 
sound image formed by the left input signal Lin and the right input signal 
Rin near the ears of the audience. Also, the reverse-phase signal 
producing means is effected to broaden the sound image formed by these 
left input signal Lin and right input signal Rin outside the right/left 
loudspeakers. Accordingly, the first left signal Lm1 and the second left 
signal Lm2 are mixed with each other at the proper mixing rate to form the 
left output signal Lout in the mixing means. Also, the first right signal 
Rm1 and the second right signal Rm2 are mixed with each other at the 
proper mixing rate to form the right output signal Rout. Then, when the 
sounds are produced based on these left output signal Lout and right 
output signal Rout, the sound images can be localized at the positions 
apart from the audience and along such a wide direction from the 
just-transverse direction of the audience to the front face direction 
thereof. 
As represented in FIG. 8, the above-described crosstalk canceling means may 
be constituted by: 
left crosstalk signal producing means for producing the left crosstalk 
signal based on the left input signal Lin; 
right crosstalk signal producing means for producing the right crosstalk 
signal based on the right input signal Rin; 
a left-channel crosstalk calculator for subtracting the right crosstalk 
signal from the left input signal Lin; 
a first left-channel crosstalk filter for filtering a signal derived from 
the left-channel crosstalk calculator to produce the first left signal 
Lm1; 
a right-channel crosstalk calculator for subtracting the left crosstalk 
signal from the right input signal Rin; and 
a first right-channel crosstalk filter for filtering a signal derived from 
the right-channel crosstalk calculator to produce the first right signal 
Rm1. 
The above-explained left crosstalk signal producing means may be arranged 
by a left-channel crosstalk attenuator, a left-channel crosstalk delay 
device, and a second left-channel crosstalk filter. This left crosstalk 
signal producing means produces the left crosstalk signal in such a way 
that the left input signal Lin is attenuated by the left-channel crosstalk 
attenuator, this attenuated signal is delayed by the left-channel 
crosstalk delay device, and then this delayed signal is filtered by the 
second left-channel crosstalk filter. 
Similarly, the above-explained right crosstalk signal producing means may 
be arranged by a right-channel crosstalk attenuator, a right-channel 
crosstalk delay device, and a second right-channel crosstalk filter. This 
right crosstalk signal producing means produces the right crosstalk signal 
in such a way that the right input signal Rin is attenuated by the 
right-channel crosstalk attenuator, this attenuated signal is delayed by 
the right-channel crosstalk delay device, and then this delayed signal is 
filtered by the second right-channel crosstalk filter. 
The respective gains of the above-described left-channel crosstalk 
attenuator and right-channel crosstalk attenuator are variable within a 
range between 0 and 1. Also, the above-explained second left-channel 
crosstalk filter and second right-channel crosstalk filter may be arranged 
by a primary IIR type filter, respectively. Furthermore, the respective 
delay amounts of the left-channel crosstalk delay device and the 
right-channel crosstalk delay device may be set to approximately 8 
sampling points in such a case that the analog signal is sampled at the 
frequency of 48 kHz to thereby produce the digital signal. 
Also, the filter coefficients of the first left-channel crosstalk filter 
and the first right-channel crosstalk filter may be formed based on the 
gain of either the left-channel crosstalk attenuator, or the right-channel 
crosstalk attenuator, and also the filter coefficient of either the second 
left-channel crosstalk filter or the second right-channel crosstalk 
filter. 
As shown in FIG. 9, the above-described reverse-phase signal producing 
means is constituted by: 
a left-channel reverse-phase attenuator for attenuating the left input 
signal Lin; 
a left-channel reverse-phase delay device for delaying an output signal 
derived from the left-channel reverse-phase attenuator; 
a right-channel reverse-phase attenuator for attenuating the right input 
signal Rin; 
a right-channel reverse-phase delay device for delaying an output signal 
derived from the right-channel reverse-phase attenuator; 
a left-channel reverse-phase calculator for subtracting the output signal 
derived from the right-channel reverse-phase delay device from the output 
signal derived from the left-channel reverse-phase attenuator to thereby 
produce the second left signal Lm2; and 
a right-channel reverse-phase calculator for subtracting the output signal 
derived from the left-channel reverse-phase delay device from the output 
signal derived from the right-channel reverse-phase attenuator to thereby 
produce the second right signal Rm2. 
The left-channel reverse-phase attenuator attenuates the left input signal 
Lin and then supplies the attenuated left input signal to the left-channel 
reverse-phase calculator and the left-channel reverse-phase delay device. 
The left-channel reverse-phase delay device delays the output signal 
derived from the left-channel reverse-phase attenuator by predetermined 
delay time and then supplies the delayed signal to the right-channel 
reverse-phase calculator. Similarly, the right-channel reverse-phase 
attenuator attenuates the right input signal Rin and then supplies the 
attenuated right input signal to the right-channel reverse-phase 
calculator and the right-channel reverse-phase delay device. The 
right-channel reverse-phase delay device delays the output signal derived 
from the right-channel reverse-phase attenuator by predetermined delay 
time and then supplies the delayed signal to the left-channel 
reverse-phase calculator. 
The left-channel reverse-phase calculator subtracts the output signal 
derived from the right-channel reverse-phase delay device from the output 
signal derived from the left-channel reverse-phase attenuator. This 
subtracted signal is equal to such a signal produced by mixing the signal 
having the reverse phase to that of the right input signal Rin with the 
left input signal Lin. The output signal derived from this left-channel 
reverse-phase calculator is supplied as the second left signal Lm2 to the 
above-explained mixing means. Similarly, the right-channel reverse-phase 
calculator subtracts the output signal derived from the left-channel 
reverse-phase delay device from the output signal derived from the 
right-channel reverse-phase attenuator. This subtracted signal is equal to 
such a signal produced by mixing the signal having the reverse phase to 
that of the left input signal Lin with the right input signal Rin. The 
output signal derived from this right-channel reverse-phase calculator is 
supplied as the second right signal Rm2 to the above-explained mixing 
means. 
The respective gains of the above-described left-channel reverse-phase 
attenuator and right-channel reverse-phase attenuator are variable within 
a range between 0 and 1. Furthermore, the respective delay amounts of the 
left-channel reverse-phase delay device and the right-channel 
reverse-phase delay device may be set to approximately 8 sampling points 
in such a case that the analog signal is sampled at the frequency of 48 
kHz to thereby produce the digital signal. 
As indicated in FIG. 10, the mixing means may be arranged by the 
left-channel mixer and the right-channel mixer. The left-channel mixer 
mixes the first left signal Lm1 derived from the crosstalk canceling means 
with the second left signal Lm2 derived from the reverse-phase signal 
producing means. As this left-channel mixer, an adder may be employed. The 
output signal derived from this left-channel mixer is externally outputted 
as the left output signal Lout. Similarly, the right-channel mixer mixes 
the first right signal Rm1 derived from the crosstalk canceling means with 
the second right-signal Rm2 derived from the reverse-phase signal 
producing means. As this right-channel mixer, an adder may be employed. 
The output signal derived from this right-channel mixer is externally 
outputted as the right output signal Rout. 
The above-explained crosstalk canceling means, reverse-phase signal 
producing means, and mixing means may be constituted by executing, for 
instance, the process operation by the digital signal processor (DSP). At 
this time, in order to secure the real-time characteristic of the signal 
processing operation, it is preferable to perform the time-divisional 
multiplexing process operation. 
In accordance with the stereophonic sound image enhancement apparatus with 
employment of the above-described arrangement, the sounds produced based 
on the left output signal Lout can be entered only to the left ear, and 
the sounds produced based on the right output signal Rout can be entered 
only to the right ear by way of the crosstalk canceling means. As a 
result, the audience can hears the panned sound images that are near the 
right/left ears. 
Also, the left-channel input sounds can be localized at the left-sided 
position apart from the left-sided loudspeaker, and the right-channel 
input sounds can be localized at the right-sided position apart from the 
right-sided loudspeaker by the reverse-phase signal producing means. As a 
consequence, the audience hears the panned sound images outside of the 
right/left loudspeakers. 
Also, the sound image localized near the right/left ears by the crosstalk 
canceling means is mixed with the sound image localized outside the 
right/left loudspeakers by the reverse-phase signal producing means by 
using the mixing means, so that it is possible to form the sound stage 
having the very broad feelings which could not be realized in the prior 
art system. 
Furthermore, while the mixing ratio is varied, the input signal is directly 
outputted, the input signal is processed only by the crosstalk canceling 
process to output the crosstalk-canceled input signal, the input signal is 
processed only by the reverse-phase signal mixing process to output the 
mixed input signal, or the input signal is processed by executing both the 
crosstalk canceling process and the reverse-phase signal mixing process to 
output the resultant signal. As a result, the magnitude of broadening 
feelings can be changed. Moreover, the listening point may be set to any 
points within the wide range by varying the delay amount of the crosstalk 
canceling means and the delay amount of the reverse-phase signal producing 
means. 
A stereophonic sound image enhancement apparatus, according to a fourth 
aspect of the present invention, is featured by such a stereophonic sound 
image enhancement apparatus for executing a predetermined process to a 
left input signal Lin and a right input signal Rin so as to output 
processed signals as a left output signal Lout and a right output signal 
Rout, comprising: 
left-channel mixing means for mixing the left input signal Lin, a signal 
obtained by multiplying the left input signal Lin by a gain "e", and a 
signal obtained by multiplying the right input signal Rin by the gain "e", 
by delaying the multiplied right input signal, and further by reversing a 
phase of the delayed signal with each other; 
right-channel mixing means for mixing the right input signal Rin, a signal 
obtained by multiplying the right input signal Rin by a gain "e", and a 
signal obtained by multiplying the left input signal Lin by the gain "e", 
by delaying the multiplied left input signal, and further by reversing a 
phase of the delayed signal with each other; 
left-channel sound quality correcting means for adding a signal obtained by 
multiplying an output signal derived from the left-channel mixing means by 
a gain (1-e) to another signal obtained by filtering the output signal 
derived from the left-channel mixing means by way of a low-pass filter and 
by multiplying the filtered signal by the gain "e" to thereby output this 
adding result as the left output signal Lout; and 
right-channel sound quality correcting means for adding a signal obtained 
by multiplying an output signal derived from the right-channel mixing 
means by a gain (1-e) to another signal obtained by filtering the output 
signal derived from the right-channel mixing means by way of a low-pass 
filter and by multiplying the filtered signal by the gain "e" to thereby 
output this adding result as the right output signal Rout. 
Also, this stereophonic sound image enhancement apparatus further 
comprises: 
crosstalk canceling means for outputting a left signal and a right signal, 
corresponding to sounds from which crosstalk sounds have been removed 
based on the left input signal Lin and the right input signal Rin, and 
wherein the left-channel mixing means mixes the left signal derived from 
the crosstalk canceling means, a signal formed by multiplying the left 
input signal Lin by a gain "e", and a signal formed by multiplying the 
right input signal Rin by the gain "e", by delaying the multiplied right 
signal, and by reversing the phase of the delayed/multiplied right signal 
with each other, and 
the right-channel mixing means mixes the right signal derived from the 
crosstalk canceling means, a signal formed by multiplying the right input 
signal Rin by a gain "e", and a signal formed by multiplying the left 
input signal Lin by the gain "e", by delaying the multiplied left signal, 
and by reversing the phase of the delayed/multiplied left signal with each 
other. 
In accordance with this stereophonic sound image enhancement apparatus, 
since this enhancement apparatus is arranged in such a manner that the 
filtering effects by the low-pass filters are emphasized in accordance 
with increasing of the mixing ratio of the reverse-phase signal, the 
stereophonic sound image enhancement effect can be emphasized without 
deteriorating the sound qualities. 
As illustrated in FIG. 11, a sound image control apparatus, according to a 
fifth aspect of the present invention, is featured by comprising: 
head related transfer function applying means including left-channel head 
related transfer function applying means for applying a left-channel head 
related transfer function to a monophonic input signal Min to thereby 
output the resultant signal as a left input signal Lin, and right-channel 
head related transfer function applying means for applying a right-channel 
head related transfer function to the monophonic input signal Min to 
thereby output the resultant signal as a right input signal Rin; 
crosstalk canceling means for producing a first left signal Lm1 formed by 
subtracting a right crosstalk signal from the left input signal Lin 
derived from the left-channel head related transfer function applying 
means, and also a first right signal Rm1 formed by subtracting a left 
crosstalk signal from the right input signal Rin derived from the 
right-channel head related transfer function applying means; 
reverse-phase signal producing means for producing a second left signal Lm2 
formed by mixing the left input signal Lin with a signal having a phase 
reverse to the phase of the right input signal Rin, and a second right 
signal Rm2 formed by mixing the right input signal Rin with a signal 
having a phase reverse to the phase of the left input signal Lin; and 
mixing means for producing a left output signal Lout by mixing the first 
left signal Lm1 derived from the crosstalk canceling means with the second 
left signal Lm2 derived from the reverse-phase signal producing means, and 
also a right output signal Lout by mixing the first right signal Rm1 
derived from the crosstalk canceling means with the second right signal 
Rm2 derived from the reverse-phase signal producing means. 
This sound image control apparatus is realized by employing the 
left-channel head related transfer function applying means for applying 
the left-channel head related transfer function to the monophonic input 
signal "Min", and also the right-channel head related transfer function 
applying means for applying the right-channel head related transfer 
function to this monophonic input signal "Min" on the input side of the 
above-described stereophonic sound image enhancement apparatus according 
to the fourth aspect. As a consequence, the crosstalk canceling means, the 
reverse-phase signal producing means, and the mixing means are similar to 
those of the stereophonic sound image enhancement apparatus according to 
the fourth aspect. 
As indicated in FIG. 12, each of the above-described left-channel head 
related transfer function applying means and right-channel external ear 
transfer function applying means may be arranged by: 
a direct sound filter for filtering the monophonic input signal "Min"; 
a delay device for inter aural time difference, capable of delaying an 
output signal from this direct sound filter; 
a reflection sound filter for filtering this monophonic input signal "Min"; 
a plurality of delay devices D.sub.1 to D.sub.n for delaying an output 
signal derived from this reflection sound filter; 
a plurality of amplifiers G.sub.1 to G.sub.n for amplifying an output 
signal derived from each of these delay devices D.sub.1 to D.sub.n ; 
a reflection sound adder for adding output signals derived from the plural 
amplifiers G.sub.1 to G.sub.n together; and 
an adder for adding an output signal derived from this delay device for 
inter aural time difference to the output signal derived from the 
reflection sound adder. 
The direct sound filter simulates the frequency characteristics of the head 
related transfer functions of the sounds which directly reach from the 
sound source (e.g., loudspeaker) to the ears of the audience. The delay 
device for inter aural time difference simulates the time differences of 
the sounds at the right/left ears, which directly reach from the sound 
source to the ears of the audience. The reflection sound filter simulates 
the changes in the frequencies caused by the reflections occurred in the 
room. The delay devices D.sub.1 to D.sub.n, the amplifier units G.sub.1 to 
G.sub.n, and the reflection sound adder may simulate the reach times of 
the reflection sounds to the right/left ears in the room, and the levels 
of the reflection sounds when reached the right/left ears. Then, the adder 
adds the signals corresponding to the direct sounds derived from the delay 
device for inter aural time difference to the signals corresponding to the 
reflection sounds derived from the reflection sound adder. The adder 
outputs the summation result as either the left input signal Lin or the 
right input signal Rin. 
The above-described left-channel head related transfer function applying 
means and right-channel head related transfer function applying means may 
be arranged by, for instance, the DSP processing operation. In this case, 
in order to secure the real-time characteristic of the signal processing 
operation, it is preferable to execute the time-divisional multiplexing 
process operations. 
As described above, since the head related transfer function is applied to 
the monophonic input signal "Min" by way of the left-channel external 
transfer function applying means and the right-channel external transfer 
function applying means, the sound images can be localized at any 
arbitrary positions while the sounds are heard by using the headphone. 
Also, the sounds produced based on the left-channel signal can be entered 
only to the left ear, and also the sounds produced based on the 
right-channel signal can be entered only to the right ear by employing the 
crosstalk canceling means. As a consequence, since the audience may be 
positioned under similar listening conditions to the headphone listening 
conditions, the sound images can be localized at any arbitrary positions 
in the loudspeaker reproducing mode. 
Since the reverse-phase signal producing means is employed in this sound 
image control apparatus, the sound images can be localized at the 
positions apart from the audience, as compared with the sound image 
localization by the sound image control apparatus arranged by the 
crosstalk canceling means and the head related transfer function applying 
means. 
Also, the respective signals produced from the crosstalk canceling means 
and the reverse-phase signal producing means can be weighted by using the 
mixing means. In the case that the signal derived from the crosstalk 
canceling means is mixed with the signal derived from the reverse-phase 
signal producing means in a preselected mixing ratio, the sound image can 
be localized at the farmost position from the audience in the two-channel 
loudspeaker reproducing mode. In such a case that the weight given to the 
signal derived from the reverse-phase signal producing means is set to 
zero, it is possible to execute the logically correct sound image control 
process. Furthermore, in such a case that the weights given to the 
respective signals derived from the crosstalk canceling means and the 
reverse-phase signal producing means are set to zero, it is possible to 
perform the sound image control in the headphone reproducing mode. 
Also, the sound image control apparatus, according to the fifth aspect of 
the present invention, is featured by further comprising: 
direction instructing means for instructing a direction along which a sound 
image is moved, wherein 
the head related transfer function applying means comprises: 
first left-channel head related transfer function applying means for 
applying a left-channel head related transfer function corresponding to a 
first direction; 
first right-channel head related transfer function applying means for 
applying a right-channel head related transfer function corresponding to 
the first direction; 
second left-channel head related transfer function applying means for 
applying a left-channel head related transfer function corresponding to a 
second direction; 
second right-channel head related transfer function applying means for 
applying a right-channel head related transfer function corresponding to 
the second direction; 
first weighting means for applying a weighting factor ".alpha." to the 
respective signals derived from the first left-channel head related 
transfer function applying means and from the first right-channel head 
related transfer function applying means in response to the instruction 
issued from the direction instructing means; 
second weighting means for applying a weighting factor "1-.alpha." to the 
respective signals derived from the second left-channel head related 
transfer function applying means and from the second right-channel head 
related transfer function applying means in response to the instruction 
issued from the direction instructing means; 
left mixing means for mixing a signal derived from the first left-channel 
head related transfer function applying means to which the weighting 
factor ".alpha." has been applied by the first weighting means with 
another signal derived from the second left-channel head related transfer 
function applying means to which the weighting factor "1-.alpha." has been 
applied by the second weighting means to thereby produce the left input 
signal; and 
right mixing means for mixing a signal derived from the first right-channel 
head related transfer function applying means to which the weighting 
factor ".alpha." has been applied by the first weighting means with 
another signal derived from the second right-channel head related transfer 
function applying means to which the weighting factor "1-.alpha."has been 
applied by the second weighting means to thereby produce the right input 
signal; under which 0.ltoreq..alpha..ltoreq.1. 
As the above-explained direction instructing means, an apparatus capable of 
entering continuous values may be employed, e.g., a joy stick, a 
slide-type variable resistor, and a rotary variable resistor. The 
instruction issued by this direction instructing means may be reflected on 
the weighting factor ".alpha.". 
It is now assumed that a sound image is localized along a predetermined 
direction, to which the head related transfer functions have been applied 
by the first left-channel head related transfer function applying means 
and the first right-channel head related transfer function applying means. 
In this case, the weighting factor ".alpha." of the first weighting means 
is equal to "1", whereas the weighting factor ".alpha." of the second 
weighting means is equal to "0". When a new direction is instructed by the 
direction instructing means in this condition, the second left-channel 
head related transfer function applying means and the second right-channel 
head related transfer function applying means are set to apply the head 
related transfer functions corresponding to this newly instructed 
direction. Then, the weighting factor ".alpha." is sequentially changed 
from "0" to "1". As a result, the respective signals derived from the 
first left-channel head related transfer function applying means and from 
the first right-channel head related transfer function applying means are 
mixed in a cross-fade mode with the respective signals derived from the 
second left-channel head related transfer function applying means and from 
the second right-channel head related transfer function applying means, 
and then, the cross-fade-mixed signal is outputted. At the time when the 
weighting factor ".alpha." becomes "1", only the respective signals are 
outputted from the second left-channel head related transfer function 
applying means and from the second right-channel head related transfer 
function applying means, so that the sound image can be localized in the 
new direction. As explained above, according to this sound image control 
apparatus, since the sound image is moved by way of the cross-fade mixing 
operation, the sound image smoothing movement can be achieved, and further 
occurrences of noise can be suppressed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to drawings, various embodiments of a sound image 
localization apparatus, a stereophonic sound image enhancement apparatus, 
and a sound image control apparatus, according to the present invention, 
will be described in detail. 
Embodiment 1 
FIG. 13 schematically shows an electronic circuit arrangement of an 
embodiment of a sound image localization apparatus according to a first 
aspect of the present invention. A DC power supply voltage Vcc is applied 
to this circuit from, for example, a cell (not shown) and the like. This 
power supply voltage Vcc is subdivided by resistors R1 and R2, and the 
subdivided power supply voltages are applied to various circuit portions 
as bias voltages "BIAS." 
As a balancer 10, for instance, a variable resistor constructed of a 
resistance element and a slider may be employed. A monophonic input signal 
"Min" is supplied to the slider of this balancer 10. This monophonic input 
signal "Min" is distributed as two sets of signals having amplitudes 
defined in accordance with a position of this slider. Then, one signal is 
outputted as a left input signal "Lin" from one output terminal (upper 
terminal shown in FIG. 13), and the other signal is outputted as a right 
input signal "Rin" from the other output terminal (lower terminal shown in 
FIG. 13). The respective amplitudes of the left input signal Lin and the 
right input signal Rin are varied by this balancer 10, so that a position 
used to localize a sound image can be adjusted. 
A circuit constructed of a capacitor C2, resistors R3, R4 and an 
operational amplifier OP1 may eliminate noise contained in the left input 
signal Lin, and also may reverse a phase of this left input signal Lin. 
The reason why the phase is reversed is to obtain an output signal having 
a normal phase from a first adder 13a (will be described later). 
A circuit constructed of a capacitor C3, resistors R5, R6 and an 
operational amplifier OP2 may eliminate noise contained in the right input 
signal Rin, and also may reverse a phase of this right input signal Rin. 
The reason why the phase is reversed is to obtain an output signal having 
a normal phase from a second adder 13b (which will be described later). An 
output signal derived from the operational amplifier OP1 is supplied to a 
first group delay equalizer 11a and the first adder 13a. Similarly, an 
output signal derived from the operational amplifier OP2 is supplied to a 
second group delay equalizer 11b and the second adder 13b. 
The first group delay equalizer 11a and the second group delay equalizer 
11b own the same circuit arrangement which is indicated in detail in FIG. 
14. The first and second group delay equalizers 11a and 11b are arranged 
by such that two stages of the substantially same circuits are combined 
with each other in a serial manner in order to achieve a sufficiently 
large delay amount. An output signal derived from the first group delay 
equalizer 11a is supplied to a first low-pass filter 12a, and an output 
signal derived from the second group delay equalizer 12b is supplied to a 
second low-pass filter 12b. 
The first low-pass filter 12a and the second low-pass filter 12b own the 
same structure, and are constituted by a primary low-pass filter, 
respectively. FIG. 15 represents a detailed circuit arrangement for the 
first and second low-pass filters 12a and 12b. In FIG. 15, a resistor R31 
is used to determine an attenuation suppressing value R. Since this 
resistor R31 is added, such a frequency characteristic that is saturated 
near -6 dB, as shown in FIG. 4, can be obtained. An output signal derived 
from the first low-pass filter 12a is supplied to the second adder 13b, 
and an output signal derived from the second low-pass filter 12b is 
supplied to the first adder 13a. 
The first adder 13a and the second adder 13b are arranged by the same 
circuit which is indicated more in detail in FIG. 16. 
Considering now the first adder 13a for is producing a left output signal 
Lout, a signal having a phase reversed from the phase of the left input 
signal Lin is supplied from the operational amplifier OP1 to an input 
terminal IN3, and a right crosstalk signal having the normal phase is 
supplied from the second low-pass filter 12b to an input terminal IN4. 
These signals are mixed with each other through resistors R40 and R41. As 
a result, this mixed signal is equal to such a signal obtained by 
subtracting the left input signal Lin from the output signal derived from 
the second low-pass filter 12b. This mixed signal is supplied to an 
inverting input terminal (-) of the operational amplifier OP5. Then, the 
phase of this mixed signal is inverted by this operational amplifier OP5. 
As a result, a signal is outputted from the first adder 13a (operational 
amplifier OP5), which is produced by subtracting the right crosstalk 
signal from the left input signal Lin. Similarly, a signal is outputted 
from the second adder 13b, which is produced by subtracting the left 
crosstalk signal from the right input signal Rin. 
As indicated in FIG. 13, an output signal derived from the first adder 13a 
is externally outputted as a left output signal "Lout." An output signal 
derived from the second adder 13b is externally outputted as a right 
output signal "Rout." 
In response to one monophonic input signal Min, the above-described sound 
image localization apparatus may localize one sound image. When a 
plurality of such sound image localization apparatuses is employed, a 
plurality of sound images may be localized in response to a plurality of 
monophonic input signals. FIG. 17 schematically represents a block diagram 
of the above-explained plural sound image localization apparatuses. 
In FIG. 17, each of the sound image localization apparatuses 1 to 4 owns 
the same arrangement of the above-described sound image localization 
apparatus. A monophonic signal of a drum part is supplied to the sound 
image localization apparatus 1, a monophonic signal of a guitar part is 
supplied to the sound image localization apparatus 2, a monophonic signal 
of a base part is supplied to the sound image localization apparatus 3, 
and a monophonic signal of a vocal part is supplied to the sound image 
localization apparatus 4. The respective sound image localization 
apparatuses 1 to 4 process the inputted monophonic signals in such a 
manner that the sound images can be localized, and output the processed 
monophonic signals as stereophonic signals. These stereophonic signals of 
the respective parts are supplied to an analog mixer 5. Thus, these 
stereophonic signals are mixed in the analog mixer 5 as to the left 
channel and the right channel, and then the mixed stereophonic signals are 
supplied to a power amplifier 6. Then, both the right-channel and 
left-channel signals amplified by the power amplifier 6 are supplied to a 
left loudspeaker SPL and a right loudspeaker SPR. 
With employment of the above-described arrangements, as represented in, 
e.g., FIG. 18, since the sound images of the respective parts can be 
arranged to desirable positions around an audience by adjusting the 
balancers of the respective sound image localization apparatuses 1 to 4, 
it is possible to realize realistic musical play approximated to the 
actual musical play modes. 
Embodiment 2 
FIG. 19 schematically shows a circuit arrangement of an example of a 
stereophonic sound image enhancement apparatus according to a second 
aspect of the present invention. To this stereophonic sound image 
enhancement apparatus, a stereophonic signal (namely, left input signal 
Lin and right input signal Rin) is inputted. 
A circuit arranged by a capacitor C50, a resistor R50, and an operational 
amplifier OP6 is a buffer circuit for receiving the left input signal Lin. 
An output signal derived from the operational amplifier OP6 is supplied to 
a first group delay equalizer 20a, a first calculation circuit 22a, and a 
subtractor 23. 
A circuit arranged by a capacitor C51, a resistor R51, and an operational 
amplifier OP7 is a buffer circuit for receiving the right input signal 
Rin. An output signal derived from the operational amplifier OP7 is 
supplied to a second group delay equalizer 20b, a second calculation 
circuit 22b, and the subtractor 23. 
The first group delay equalizer 20a and the second group delay equalizer 
22b own the same circuit arrangement which is indicated in detail in FIG. 
20. The first and second group delay equalizers 20a and 20b are arranged 
by such that two stages of the substantially same circuits are combined 
with each other in a serial manner in order to achieve a sufficiently 
large delay amount. An output signal derived from the first group delay 
equalizer 20a is supplied to a first high-pass filter 21a, and an output 
signal derived from the second group delay equalizer 20b is supplied to a 
second high-pass filter 21b. 
The first high-pass filter 21a and the second high-pass filter 21b own the 
same circuit arrangement, which is indicated more in detail in FIG. 21. A 
left crosstalk signal derived from the first high-pass filter 21a is 
supplied to the second calculation circuit 22b, whereas a right crosstalk 
signal derived from the second high-pass filter 21b is supplied to the 
first calculation circuit 22a. 
It should be noted that first and second low-pass filters G4'(s) of this 
stereophonic sound image enhancement apparatus do not appear on the 
circuit of FIG. 19. The first and second low-pass filters G4'(s) may be 
formed by a portion of the elements for constituting the first and second 
high-pass filters, and a portion of the elements for constituting the 
first and second calculation circuits. 
That is, the output signal derived from the operational amplifier OP1 (see 
FIG. 19) is supplied via a resistor R90 of the first calculation circuit 
22a (see FIG. 23) to a non-inverting input terminal (+) of an operational 
amplifier OP12. Also, a terminal of the resistor R90 on the side of the 
operational amplifier OP12 is connected via a resistor R91 to the first 
and second high-pass filters (see FIG. 21). In the first and second 
high-pass filters (see FIG. 21), an output terminal OUT9 is connected via 
a parallel circuit of a capacitor C71 and the resistor R70 to one end of 
the capacitor C70. Then, the other end of this capacitor C70 is connected 
to the output terminal of the operational amplifier OP9. 
In this case, since an output impedance of the operational amplifier OP9 is 
low, it is conceivable that the other end of the capacitor C70 is 
grounded. As a result, it is conceivable that the low-pass filters are 
constituted by the resistors R90, R91, R70 and also the capacitors C70, 
C71. Accordingly, the signal entered to the non-inverting terminal of the 
operational amplifier OP12 is conceivable as either a left low-range 
emphasized signal or a right low-range emphasized signal, which are 
produced by such a manner that either the left input signal Lin or the 
right input signal Rin is filtered by the low-pass filter G4'(s) to 
emphasize a low range thereof. As previously described, in accordance with 
the above-explained arrangement, since the first and second low-pass 
filters G4'(s) may be formed by commonly employing a portion of the first 
and second high-pass filters, and a portion of the circuit elements of the 
first and second calculation circuits, a total amount of the hardware can 
be reduced. 
The subtractor 23 corresponds to the subtracting means and the amplifying 
means employed in the stereophonic sound image enhancement apparatus 
according to the second aspect of the present invention. A detailed 
circuit arrangement of this subtractor 23 is shown in FIG. 22. The 
subtractor 23 subtracts the right input signal Rin from the left input 
signal Lin (actually, signal passed through a buffer circuit is 
subtracted), and amplifies the subtraction result to produce a difference 
signal. This difference signal is supplied to the first and second 
calculation circuits 22a and 22b. More specifically, an output signal 
derived from an operational amplifier OP10 for constructing the subtractor 
23 is supplied to the second calculation circuit 22b, and also is supplied 
to an inverting circuit arranged by resistors R84, R85, and an operational 
amplifier OP11. Another difference signal having a reverse phase, which is 
outputted from this inverting circuit, is supplied to the first 
calculation circuit 22a. It should be noted that a coefficient "b" used in 
the amplifying means is determined by a resistance value of a feedback 
resistor R82 of the operational amplifier OP10. This coefficient "b" is 
used to increase and decrease a ratio of mixing the difference signal 
having the normal phase with the right input signal Rin, and is used to 
increase and decrease a ratio of mixing the difference signal having the 
reverse phase with the left input signal Lin. 
The first calculation circuit 22a and the second calculation circuit 22b 
own the same circuit arrangement which is indicated in FIG. 23. 
Considering now the first calculation circuit 22a for producing the left 
output signal, the left input signal derived from the operational 
amplifier OP6 is supplied to an input terminal IN9, and the right 
crosstalk signal having the reverse phase is supplied from the second 
high-pass filter 21b to an input terminal IN10. The left input signal and 
the right crosstalk signal are combined with each other via the resistors 
R90 and R91 so as to be mixed with each other. As a consequence, this 
mixed signal corresponds to a signal produced by subtracting the right 
crosstalk signal from the left low-range emphasized signal. This mixed 
signal is supplied to a non-inverting input terminal (+) of the 
operational amplifier OP12. On the other hand, the difference signal 
having the normal phase is supplied from the subtractor 23 (operational 
amplifier OP10) to the non-inverting input terminal (-) of the operational 
amplifier OP12. Accordingly, such a signal is outputted from the first 
calculation circuit 22a, which is obtained by subtracting the right 
crosstalk signal from the left low-range emphasized signal to further add 
the difference signal to the subtraction result. 
Considering similarly the second calculation circuit 22b for producing the 
right output signal, the right input signal derived from the operational 
amplifier OP7 is supplied to the input terminal IN9, and the left 
crosstalk a signal having the reverse phase is supplied from the first 
high-pass filter 21a to the input terminal IN10. The right input signal 
and the left crosstalk signal are combined with each other via the 
resistors R90 and R91 so as to be mixed with each other. As a consequence, 
this mixed signal corresponds to a signal produced by subtracting the left 
crosstalk signal from the right low-range emphasized signal. This mixed 
signal is supplied to a non-inverting input terminal (+) of the 
operational amplifier OP12. On the other hand, the difference signal 
having the reverse phase is supplied from the subtractor 23 (operational 
amplifier OP11) to the non-inverting input terminal (-) of the operational 
amplifier OP12. Accordingly, such a signal is outputted from the second 
calculation circuit 22b, which is obtained by subtracting the left 
crosstalk signal from the right low-range emphasized signal to further add 
the difference signal to the subtraction result. 
As indicated in FIG. 19, the output signal derived from the first 
calculation circuit 22a is externally outputted as the left output signal 
"Lout." The output signal derived from the second calculation circuit 22b 
is externally outputted as the right output signal "Rout." When the left 
output signal Lout and the right output signal Rout are supplied to two 
sets of right/left loudspeakers, the sound image can be localized not only 
between the right and left loudspeakers, but also over the broad range 
around the audience, and furthermore, the stereophonic sound image can be 
greatly enhanced, as compared with that of the sound image localization 
apparatus according to the first aspect of the present invention. 
Next, an example of a sound image enhancement system for utilizing the 
above-described stereophonic sound image enhancement apparatus will be 
explained with reference to FIG. 24. In FIG. 24, a computer 7 sends music 
data in the MIDI format to a sound source module 8. The sound source 
module 8 produces a left input signal Lin and a right input signal Rin in 
response to the music data in the MIDI format. These left input signal Lin 
and right input signal Rin are supplied to a stereophonic sound image 
enhancement apparatus 9. Then, since the above-described operation is 
carried out in this stereophonic sound image enhancement apparatus 9, a 
left output signal Lout and a right output signal Rout are produced. These 
left and right output signals Lout and Rout are supplied to a left 
loudspeaker SPL and a right loudspeaker SPR. The sound image formed by the 
sounds produced from these left and right loudspeakers SPL and SPR may be 
localized at outer sides of the left loudspeaker SPL and the right 
loudspeaker SPR, and the stereophonic sound image may be enhanced. 
It should be understood that although this sound image enhancement system 
is so arranged as to transmit the music data of the MIDI format to the 
sound source module 8, the present invention is not limited to the MIDI 
format, but various music data of the various formats may be employed. 
Alternatively, various apparatuses capable of recording the music data, 
for example, electronic musical instruments, and sequencers may be 
employed, instead of the computer. Also, there is no limitation in 
selection of the sound source module as the apparatus for producing the 
left input signal Lin and the right input signal Rin. For instance, an 
electronic musical instrument, a game machine, and an acoustic appliance 
may be employed as the sound source module. 
Embodiment 3 
Next, a description will now be made of an embodiment of a stereophonic 
sound image enhancement apparatus according to the third aspect of the 
present invention. To easily understand this invention, first, the known 
crosstalk canceling method of the Schroeder system will be explained. 
FIG. 25A represents a path along which a sound produced from a left 
loudspeaker SPL and a sound produced from a right loudspeaker SPR reach 
right/left ears of an audience. In FIG. 25A, symbol "S" indicates a 
transfer function of a direct sound, and symbol "A" shows a transfer 
function of a crosstalk sound. In a two-channel loudspeaker reproduction, 
the transfer characteristics such as "S" and "A" are added until the 
sounds produced from the loudspeaker reach the right/left ears of the 
audience. For example, when the sound is produced only from the left 
loudspeaker SPL based upon the left input signal Lin, "SLin" is entered 
into the left ear and also "ALin" is entered into the right ear. 
FIG. 25B conceptually represents an arrangement of a Schroeder type 
crosstalk canceling apparatus. When the sound signal reaching the left ear 
(a left-ear signal: LS) and the sound signal reaching the right ear (a 
right-ear signal: Rs) are calculated in accordance with this arrangement, 
the below-mentioned formula (8) and formula (9) results are obtained: 
##EQU4## 
Based upon the above-described formula (8) and formula (9), it is 
understandable that in accordance with this Schroeder method, the sounds 
corresponding to the left input signal Lin and the right input signal Rin 
are directly entered to the left ear and the right ear, respectively, 
irrelevant to the transfer functions S and A, and then the crosstalk 
sounds are canceled. 
Concretely speaking, crosstalk canceling means indicated in FIG. 7 may be 
arranged as shown in FIG. 8. Referring now to FIG. 26A to FIG. 28, a first 
example of this crosstalk canceling means will be explained. FIG. 26A 
schematically represents a path along which the sounds produced from the 
left loudspeaker SPL and the right loudspeaker SPR reach the right/left 
ears of the audience. In this embodiment 3, it is assumed that the 
transfer function S is equal to "1", and the transfer function A is equal 
to "aZ.sup.-n L.sub.o." In this case, symbol "a" denotes a gain, symbol 
"n" represents delay time of a crosstalk sound with respect to the direct 
sound, and symbol "L.sub.o " shows a low-pass filter for simulating 
diffraction of a crosstalk sound caused by the head of the audience. Based 
on this assumption, since S=1 and A=aZ.sup.-n L.sub.o, symbol "C" may be 
expressed by the following formula (10): 
##EQU5## 
When this is applied to the Schroeder type crosstalk canceling apparatus 
shown in FIG. 25B, the arrangement of the crosstalk canceling means may be 
represented as in FIG. 26B. In this case, if the low-pass filter is 
arranged by a primary IIR type filter, then this low-pass filter "L.sub.o 
" may be expressed by the following formula (11): 
##EQU6## 
When this formula (11) is applied to FIG. 26B, the crosstalk canceling 
means may be represented as in FIG. 27. When this is indicated by a block 
diagram, this block diagram is shown in FIG. 28. The crosstalk canceling 
means indicated in FIG. 28 corresponds to the crosstalk canceling means 
shown in FIG. 8. The arrangement shown in FIG. 28 is constructed of an 
amplifier, a delay device, and an adder, which may be realized by the DPS 
processing. 
In FIG. 28, an L-channel crosstalk attenuator, an L-channel crosstalk delay 
device, and a second L-channel crosstalk filter, which form left crosstalk 
signal generating means, are constituted by an amplifier 110, a delay 
device 111, and an IIR type primary low-pass filter 112. Similarly, a 
R-channel crosstalk attenuator, a R-channel crosstalk delay device, and a 
second R-channel crosstalk filter, which form right crosstalk signal 
generating means, are constituted by an amplifier 120, a delay device 121, 
and an IIR type primary low-pass filter 122. It should be noted that since 
the primary low-pass filter 122 is constructed by employing the same 
arrangement as the above-described primary low-pass filter 112, this 
arrangement is omitted in the drawing. In FIG. 28, symbol "a" shows an 
amplification coefficient for the amplifiers 110 and 120; symbols "b" to 
"d" denote filter coefficients for the primary low-pass filters 112 and 
122; and symbol "n" is delay coefficients for the delay devices 111 and 
121. 
The L-channel crosstalk calculator and the R-channel crosstalk calculator, 
included in the crosstalk canceling means, are constituted by subtractors 
113 and 123, respectively. The subtractor 113 subtracts the right 
crosstalk signal from the left input signal Lin. An output signal from 
this subtractor 113 is supplied to the filter 114. Similarly, the 
subtractor 123 subtracts the left 15 crosstalk signal from the right input 
signal Rin. An output signal from this subtractor 123 is supplied to the 
filter 124. It should be noted that since the arrangement of the filter 
124 is identical to that of the filter 114, this arrangement is omitted in 
the drawing. An amplification coefficient and a delay coefficient of the 
filters 114 and 124 are formed based upon the above-described "a" to "d" 
and "n." 
Note that in the crosstalk canceling means, a secondary IIR type filter may 
be employed as a low-pass filter. Also in this case, assuming now that S=1 
and A=aZ.sup.-n L.sub.o, the low-pass filter L.sub.o is expressed by the 
following formula (12): 
##EQU7## 
This low-pass filter may also be realized by the DSP processing. 
Alternatively, thirdly, or more IIR type filters may be employed as the 
low-pass filter. Also, in this case, these low-pass filters may be 
realized by the DSP processing. 
As previously described, assuming now that the transfer function S defined 
from the loudspeaker to the ears on the same sides is equal to "1" and the 
transfer function A defined from the loudspeaker to the ears on the 
opposite sides is equal to "aZ.sup.-n L.sub.o," the crosstalk canceling 
means can simplify the Schroeder type crosstalk canceling apparatus. In 
accordance with this arrangement, it is possible to achieve the high 
precision crosstalk canceling operation, although the arrangement is made 
simple. 
Next, a second example of the crosstalk canceling means will now be 
explained with reference to FIG. 29A and FIG. 29B. FIG. 29A shows a path 
along which the sounds produced from the left loudspeaker SPL and the 
right loudspeaker SPR reach the right/left ears of the audience. In these 
drawings, symbol "H.sub.m " represents a frequency characteristic of the 
ears located on the same side of the loudspeaker. Also, symbol "aZ.sub.-n 
H.sup.s " shows a frequency characteristic of the ears located on the 
opposite side of the loudspeaker. In this case, "S" of the Schroeder 
system is expressed by "H.sub.m, " and "A" thereof is expressed by 
"aZ.sup.-n H.sub.s." As a consequence, symbol "C" may be expressed by the 
following formula (13): 
##EQU8## 
In this case, since "H.sub.s /H.sub.m " may be considered as the frequency 
characteristics of the ears located on the opposite side when the 
frequency characteristic of the ears located on the same side of the 
loudspeaker is "1", this may be expressed by the below-mentioned formula 
(14), similar to the cases of FIG. 26A and FIG. 26B. As a consequence, an 
item of 1/(1-C.sup.2) is the same as that of FIG. 26A and FIG. 26B. 
##EQU9## 
Since an item of 1/S is present at the final stage in the Schroeder method, 
1/H.sup.m is entered to the final stage in this second example. As a 
consequence, the crosstalk canceling means of this second example is 
arranged as shown in FIG. 29B. Now, when the characteristic of H.sub.m is 
realized by a primary filter, H.sub.m may be expressed by the 
below-mentioned formula (15): 
##EQU10## 
Accordingly, it becomes: 
##EQU11## 
Similar to the above-explained first example, the filter L.sub.o may be 
expressed by the below-mentioned formula (17): 
##EQU12## 
When this formula (17) is substituted for FIG. 29B, the arrangement of the 
crosstalk canceling means may be expressed as indicated in FIG. 30. When 
this is indicated by a block diagram, this block diagram is shown in FIG. 
31. The arrangement shown in FIG. 31 is constructed of an amplifier, a 
delay device, and an adder, and then can be realized by the DSP 
processing. Also, the filter L.sub.o of this second example may be 
constituted by secondary, or more higher filters. 
The second example of the crosstalk canceling means indicated in FIG. 31 
owns a different structure from the first example of the crosstalk 
canceling means such that a filter 115 is further provided on the output 
side of the filter 114 shown in FIG. 28, and another filter 125 is further 
provided on the output side of the filter 124. 
As previously described, assuming now that the frequency characteristic 
defined from the loudspeaker to the ears on the same sides is equal to 
"H.sub.m " and the frequency characteristic defined from the loudspeaker 
to the ears on the opposite sides is equal to "aZ.sup.-n H.sub.s," the 
crosstalk canceling means can simplify the Schroeder type crosstalk 
canceling apparatus. In accordance with this arrangement, it is possible 
to achieve the high precision crosstalk canceling operation with realistic 
better quality, although the arrangement is made simple. 
Now, reverse-phase signal producing means will be explained. FIG. 32 
schematically shows an arrangement of the reverse-phase signal producing 
means. The reverse-phase signal producing means shown in FIG. 32 
corresponds to the reverse-phase signal producing means indicated in FIG. 
9. In FIG. 32, an L-channel reverse-phase signal attenuator, an L-channel 
reverse-phase signal delay device, and an L-channel reverse-phase signal 
calculator are constructed of an amplifier 130, a delay device 131, and an 
adder 132, respectively. Similarly, an R-channel reverse-phase signal 
attenuator, an R-channel reverse-phase signal delay device, and an 
R-channel reverse-phase signal calculator are constructed of an amplifier 
140, a delay device 141, and an adder 142, respectively. 
In FIG. 32, symbol "e" denotes amplification coefficients for the 
amplifiers 130 and 140, and symbol "m" represents delay coefficients for 
the delay devices 131 and 141. Symbol "m" is a quantity of sampling points 
in the case that an analog signal is sampled at a frequency of 48 kHz to 
produce a digital signal, for instance, "m" is selected to be equal to 
approximately 6. 
Consider now that only the left input signal Lin is applied. At this time, 
a second left signal Lm2 may be expressed by "eLin-eZ.sup.-m Rin." 
Similarly, a second right signal Rm2 may be expressed by "eRin-eZ.sup.-m 
Lin." 
Assuming now that symbol "m" is equal to the delay time of the crosstalk 
sound with respect to the direct sound, for example, the crosstalk sound 
among the sounds outputted from the left loudspeaker SPL is entered into 
the right ear with a delay of "m." Thus, if this crosstalk sound is 
outputted from the right loudspeaker SPR in such a manner that this 
crosstalk sound is delayed by the delay time "m" and reversed, the 
first-mentioned crosstalk sound can be canceled by the reversed crosstalk 
sound around the right ear. As a result of this process operation, such a 
phenomenon approximated to the above-described crosstalk canceling process 
will occur. 
It should be understood that since no consideration is made of the 
frequency changes caused by the gain and the diffraction in this 
reverse-phase signal processing means, the crosstalk canceling operation 
cannot always be correctly performed. When the crosstalk canceling 
operation is correctly carried out, it feels that the sound images are 
stuck around the ears. To the contrary, in accordance with the imperfect 
crosstalk canceling operation by this reverse-phase signal producing 
means, it feels that the sound images are not localized around the ears, 
but are localized outside the loudspeakers. As described above, in 
accordance with this reverse-phase signal producing means, the sound 
images can be localized outside the right/left loudspeakers by the 
imperfect crosstalk canceling operation. 
However, this circuit arrangement shown in FIG. 32 still has a problem. 
That is, there is a lack of clearness in the sound images. This clearness 
problem may be solved by constituting reverse-phase signal producing means 
in a manner as shown in FIG. 33. In the reverse-phase signal producing 
means indicated in FIG. 33, a left input signal "Lin" is further mixed 
with the second left signal "Lm2" appearing in the reverse-phases signal 
producing means indicated in FIG. 32, and a right input signal "Rin" is 
further mixed with the second right signal Rm2. As a result, the sound 
image can be clearly localized outside the right/left loudspeakers. 
Next, a description will now be made of an embodiment of mixing means. As 
previously explained with reference to FIG. 10, the mixing means is 
arranged by the L-channel mixer and the R-channel mixer. As indicated in 
FIG. 34, the L-channel mixer may be arranged by an adder 150. The adder 
150 adds the first left signal Lm1 derived from the crosstalk canceling 
means to the second left signal Lm2 derived from the reverse-phase signal 
producing means, and then outputs the added value as a left output signal 
"Lout." Similarly, the R-channel mixer may be arranged by an adder 151. 
The adder 151 adds the first right signal Rm1 derived from the crosstalk 
canceling means to the second right signal Rm2 derived from the 
reverse-phase signal producing means, and then outputs the added value as 
a right output signal "Rout." 
A mixing ratio of the first left signal Lm1 derived from the crosstalk 
canceling means and the second left signal Lm2 derived from the 
reverse-phase signal producing means with respect to the output signal may 
be controlled based on the respective gains "a" of the amplifiers 110 and 
120 of the crosstalk canceling means, and the respective gains "e" of the 
amplifiers 130 and 140 of the reverse-phase signal producing means. 
Another mixing ratio of the first right signal Rm1 derived from the 
crosstalk canceling means and the second right signal Rm2 derived from the 
reverse-phase signal producing means may be controlled in a similar 
manner. In other words, the crosstalk canceling amount may be determined 
based on the value of the gain "a" whereas the mixing amount of the 
reverse-phase signals may be determined based upon the value of the gain 
"e." 
Depending upon the values of the gains "a" and "e," the following sound 
stages may be formed. 
1). In the case of "a"="e"=0, the left input signal Lin directly 
constitutes the left output signal Lout, and the right input signal Rin 
directly constitutes the right output signal Rout. It should be understood 
that since a=0 and thus the coefficient of the filter 114 becomes 
symmetrical with respect to that of the filter 124 (see FIG. 28 and FIG. 
31), both the filters 114 and 124 employed in the crosstalk canceling 
means may function as filters capable of directly outputting the input 
signal. 
2). In the case of "a" is not equal to 0 and "e"=0. only the crosstalk 
canceling means becomes effective, and such a sound stage is formed that 
the sound images can be heard around the ears. 
3). In the case of "a"=o and "e" is not equal to 0, only the reverse-phase 
signal producing means becomes effectively, and such a sound stage is 
formed that the sound images are localized outside the right/left 
loudspeakers. 
4). In the case of "a" is not equal to 0 and "e" is not equal to 0, the 
sound stage such that the sound images can be heard around the ears due to 
the crosstalk canceling operation are overlapped with the sound stage such 
that the sound images are localized outside the right/left loudspeakers by 
mixing the reverse-phase signals. As a consequence, it is possible to form 
such a sound stage that the sound images are localized along such a wide 
direction defined from an accurate transverse direction of an audience up 
to a front direction, and also at a position separated from the audience. 
In the above-described case 4), since the delay amount is changed in 
response to the opening angle of the loudspeakers during the crosstalk 
canceling operation, there is a problem that the clearness of the sound 
image localization when the audience listens to the sounds at the position 
near the loudspeakers is different from that when the audience listens to 
the sounds at the position apart from the loudspeakers. Accordingly, the 
delay amount "m" is selected to be a value different from "n" in the 
reverse-phase signal producing means, so that the clearness of the sound 
image localization in accordance with the opening angle between the 
loudspeakers can be ensured. As a consequence, the listening point can be 
selected to be such a broad range. For instance, the delay amounts "n" and 
"m" may be selected to be n=8 and m=6. Alternatively, the delay amounts 
"m" and "n" may be equal to each other. In this case, it is possible to 
determine the best listening point at a preselected opening angle between 
the right/left loudspeakers. 
Referring now to the drawings, an example of a stereophonic sound image 
enhancement system utilizing the above-described stereophonic sound image 
enhancing apparatus will be explained. FIG. 35 is an outer view of the 
stereophonic sound image enhancement system, as viewed from an upper 
surface thereof. In this stereophonic sound image enhancement system, both 
the left input signal Lin and the right input signal Rin are inputted into 
a line input terminal, and are processed by way of the crosstalk canceling 
process operations and the reverse-phase signal mixing process operation, 
and then the processed signals are outputted from a line output terminal 
as a left output signal Lout and a right output signal Rout. An input 
apparatus 170 includes a variable resistor 172a and 172b. A variable 
resistor 172a is used to instruct an amount of the crosstalk canceling 
operation. In response to a signal produced from the variable resistor 
172a, the gain "a" is determined. Also, another variable resistor 172b may 
be used to instruct a mixing amount of a reverse-phase signal. In response 
to the signal produced from this variable resistor 172b, the gain "e" is 
determined. 
FIG. 36 is a schematic block diagram for representing an arrangement of an 
electronic circuit of the above-described stereophonic sound image 
enhancement system. In FIG. 36, an A/D converter 160 converts the left 
input signal Lin and the right input signal Rin, corresponding to analog 
signals, into digital signals. The digital signals are supplied to a DSP 
161. The DSP 161 executes the above-described crosstalk canceling process 
operation and the reverse-phase signal mixing process operation in 
accordance with the control data supplied from the CPU 170. Then, the 
processed digital signals are supplied to a D/A converter 162. The D/A 
converter 162 converts the received digital signals into analog signals 
thereof which will be then outputted as the left output signal Lout and 
the right output signal Rout. 
To a CPU 170, a memory 171, an input apparatus 172, and a display apparatus 
173 are connected. Into the memory 171, various sorts of coefficients are 
stored other than a control program used to operate the CPU 170. The CPU 
170 reads out a filter coefficient and the like from this memory 171 and 
supplies the read filter coefficient as control data to the DSP 161. 
Also, the input apparatus 172 contains the above-described variable 
resistor 172a and variable resistor 172b. Then, the CPU 172 accepts 
signals indicative of the setting conditions of these variable resistors 
172a and 172b, and converts these setting condition signals into values 
representative of the gain "a" and the gain "e," which are supplied as 
control data to the DSP 161. The DSP 161 performs the above-explained 
crosstalk canceling operation and reverse-phase signal mixing process 
operation based upon the values indicative of the above-explained filter 
coefficient, and gain "a" and also gain "e." 
The display apparatus 173 may be constituted by, for instance, an LED 
(light-emitting diode) display device, and an LCD (liquid crystal display) 
display device. When the user observes this display apparatus 173, the 
user can recognize the conditions of this stereophonic sound image 
enhancement system, and other information thereof. 
It should also be noted that although this stereophonic sound image 
enhancement apparatus can be solely used, this stereophonic sound image 
enhancement apparatus may be assembled into other apparatus as a 
multi-effector, and a reverberation generator. As other apparatuses, there 
are a sound source module, an electronic piano, an electronic keyboard, a 
game machine, and acoustic appliances. 
Embodiment 4 
Referring now to a block diagram of FIG. 37, a description will be made of 
an embodiment of a stereophonic sound image enhancement apparatus 
according to a fourth aspect of the present invention. In FIG. 37, an 
amplifier 200 amplifies the left input signal Lin by a gain "e." The 
output signal from this amplifier 200 is supplied to a delay device 201 
and an adder 202. The delay device 201 delays the output signal supplied 
from the amplifier 200 by the time "m." The output signal from this delay 
device 201 is supplied to a adder 212. Similarly, an amplifier 210 
amplifies the right input signal Rin by a gain "e." The output signal from 
this amplifier 210 is supplied to a delay device 211 and the adder 212. 
The delay device 211 delays the output signal supplied from the amplifier 
210 by the time "m." The output signal from this delay device 211 is 
supplied to the adder 202. 
The adder 202 corresponds to L-channel mixing means of the stereophonic 
sound image enhancement apparatus according to the fourth aspect of the 
present invention. This adder 202 adds the left input signal Lin to the 
output signal derived from the amplifier 200 (namely, signal obtained by 
multiplying left input signal Lin by gain "e"), and subtracts the output 
signal derived from the delay device 211 from the summation result. As a 
result, the mixing operation is carried out for the left input signal Lin, 
the signal obtained by multiplying the left input signal Lin by the gain 
"e," and also such a signal obtained by multiplying the right input signal 
Rin by the gain "e" and by delaying the multiplied signal and by further 
reversing the phase of this delayed multiplied signal. 
Similarly, the adder 212 corresponds to R-channel mixing means. This adder 
212 adds the right input signal Rin to the output signal derived from the 
amplifier 210 (namely, signal obtained by multiplying right input signal 
Rin by gain "e"), and subtracts the output signal derived from the delay 
device 201 from the summation result. As a result, the mixing operation is 
carried out for the right input signal Rin, the signal obtained by 
multiplying the right input signal Rin by the gain "e," and also such a 
signal obtained by multiplying the left input signal Lin by the gain "e" 
and by delaying the multiplied signal and by further reversing the phase 
of this delayed multiplied signal. 
L-channel sound quality correcting means of the stereophonic sound image 
enhancement apparatus according to the fourth aspect of the present 
invention is arranged by an amplifier 203, a low-pass filter 204, an 
amplifier 205, and an adder 206. An output signal derived from the 
L-channel mixing means (adder 202) is amplified by a gain (1-e) in the 
adder 203, and the amplified signal is supplied to the adder 206. Also, 
the output signal derived from the L-channel mixing means (adder 202) is 
filtered out by the low-pass filter 204, and the filtered signal is 
supplied to the adder 205. Then, this filtered signal is amplified by the 
gain "e" in this amplifier 205, and the amplified signal is supplied to 
the adder 206. In the adder 206, the output signal from the amplifier 203 
is added to the output signal from the amplifier 205, and this added 
result is outputted therefrom as a left output signal Lout. 
Similarly, R-channel sound quality correcting means of the stereophonic 
sound image enhancement apparatus according to the fourth aspect of the 
present invention is arranged by an amplifier 213, a low-pass filter 214, 
an amplifier 215, and an adder 216. An output signal derived from the 
R-channel mixing means (adder 212) is amplified by a gain (1-e) in the 
adder 213, and the amplified signal is supplied to the adder 216. Also, 
the output signal derived from the R-channel mixing means (adder 212) is 
filtered out by the low-pass filter 214, and the filtered signal is 
supplied to the adder 215. Then, this filtered signal is amplified by the 
gain "e" in this amplifier 215, and the amplified signal is supplied to 
the adder 216. In the adder 216, the output signal from the amplifier 213 
is added to the output signal from the amplifier 215, and this added 
result is outputted therefrom as a right output signal Rout. 
The above-described "e" implies a gain used to control a magnitude of 
stereophonic sound image enhancement effects. Also, symbols "h" to "j" 
indicate filter coefficients of the low-pass filter 204 and 214. Further, 
symbol "m" indicates a delay amount corresponding to delay time of 
crosstalk sounds with respect to direct sounds. This delay amount "m" is 
expressed by the number of sampling points in such a case that an analog 
signal is sampled at a frequency of 48 kHz to produce a digital signal. 
FIG. 38 indicates a disturbance in a frequency characteristic of a 
monophonic signal component, which is caused by mixing a reverse-phase 
signal with the left input signal Lin and the right input signal Rin when 
neither the low-pass filter 204, nor the low-pass filter 214 is employed. 
As shown in FIG. 38, the monophonic signal components owns a comb filter 
characteristic, and reductions in a low frequency range are especially 
emphasized. As a result, the audience may hear sounds in such a manner 
that the higher the stereophonic sound image enhancement effect is 
increased (the larger, the value of "e" becomes). the softer the sound 
quality becomes. In this stereophonic sound image enhancement apparatus, 
the low-pass filters are provided at the output stage, and the sound 
quality is corrected in such a manner that the higher the stereophonic 
sound image enhancement effect is increased, the larger the filtering 
effects by the low-pass filters becomes, so that the above-described 
phenomenon can be suppressed. 
The gain "e" is variable in a range from 0 to 1. In the case of e=0, the 
left input signal Lin directly constitutes the left output signal Lout, 
and the right input signal Rin directly constitutes the right output 
signal Rout. In the case of e=1, the filtering effects by the low-pass 
filters 204 and 214 become maximum. 
FIG. 39 indicates a modification of this embodiment 4. In this 
modification, crosstalk canceling means is further employed. The adder 202 
adds the left signal derived from this crosstalk canceling means to the 
output signal derived from the amplifier 200 (namely, a signal obtained by 
multiplying left input signal Lin by gain "e"), and then subtracts the 
output signal derived from the delay device 211 from the summation result. 
The adder 212 adds the right signal derived from this crosstalk canceling 
means to the output signal derived from the amplifier 210 (namely, a 
signal obtained by multiplying right input signal Rin by gain "e"), and 
subtracts the output signal derived from the delay device 201 from this 
summation result. 
As the crosstalk canceling means, the above-explained crosstalk canceling 
means as described in the first example and the second example in the 
previous embodiment 3 may be utilized. Apparently, the conventional 
Schroeder type crosstalk canceling apparatus may be employed. 
In accordance with the stereophonic sound image enhancement apparatus of 
this modification, since the sound quality is corrected based on the 
magnitudes of the stereophonic sound image enhancement effects, even when 
the stereophonic sound image enhancement effect is increased, the sound 
quality is not deteriorated. Moreover, the audience easily hear the sound 
images that are clearly localized at the position apart from the audience, 
and also can have a very wide feeling. 
When both the left input signal Lin and the right input signal Rin are such 
binaural signals to which an head related transfer function has been 
added, the sound images can be controlled by the two-channel loudspeaker 
reproduction. Also, in this case, the higher the stereophonic sound image 
enhancement effect is increased, the softer the sound quality becomes. As 
a result, if the L-channel sound quality correcting means and the 
R-channel sound quality correcting means are employed at the output stage 
so as to correct the sound quality, then such a drawback that the sound 
quality becomes soft can be eliminated. 
Embodiment 5 
A sound image control apparatus according to a fifth aspect of the present 
invention will now be described. A head related transfer function (head 
related impulse response) in the sound image control apparatus of this 
embodiment 5 may be obtained by a measurement. As illustrated in FIG. 40, 
the measurement is carried out not in a non-reflection room, but in a room 
where a certain amount of reflection occurs. This is because a sound image 
can be clearly localized in such a room with reflection, rather than the 
non-reflection room. The measurement is performed by employing a dummy 
head while impulse sounds produced from a sound source (loudspeaker) are 
acquired. 
Sounds which directly reach from the loudspeaker to right/left ears of the 
dummy head correspond to direct sounds, which are indicated by a solid 
line in FIG. 40. Sounds which reach from the loudspeaker via walls 
(involving floors and ceilings) to the right/left ears correspond to 
reflection sounds, which are indicated by a broken line. When the 
above-described measurement is carried out while changing the position of 
the sound source, it is possible to acquire data about the head related 
transfer functions (head related impulse response) along various 
directions. It should be noted that the data about the head related 
transfer function may be produced by way of the TSP (time stretch pulse) 
method. 
In FIG. 41A and FIG. 41B, there are shown one example of measured head 
related impulse response. That is, FIG. 41A graphically indicates the head 
related impulse response of the left ear when the sound source is set at a 
left-inclined front position with an angle of 60 degrees. FIG. 41B 
graphically indicates the head related impulse response of the right ear 
when the sound source is set at a right-inclined front position with an 
angle of 60 degrees. When this head related impulse response is convoluted 
with the input sounds and the convoluted sounds are heard by using the 
headphone, the audience may realize that the position of the sound source 
corresponds to the left-inclined front position with the angle of 60 
degrees. 
However, the point number of the impulse responses shown in FIG. 41A and 
FIG. 41B is 150 points (at sampling frequency of 48 kHz), so that a 
large-scaled hardware is necessarily required to perform the convolution 
process operation. Therefore, as will be explained later, since the 
L-channel/R-channel head related transfer function applying means employed 
in the sound image control apparatus according to the fifth aspect of the 
present invention are constructed of a portion for simulating a direct 
sound, and also a portion for simulating a reflection sound, the overall 
hardware of this sound image control apparatus can be made simple. 
In other words, as shown in FIG. 12, each of the L-channel/R-channel head 
related transfer function applying means is arranged by a portion for 
simulating direct sounds, another portion for simulating reflection 
sounds, and an adding unit for adding them. 
The portion for simulating the direct sounds in the L-channel head related 
transfer function applying means is arranged by a direct-sound filter 300 
for filtering a monophonic input signal Min, and a delay device 301 for 
inter aural time difference, which delays the output signal from this 
direct-sound filter 300. Also, the portion for simulating the reflection 
sounds is arranged by a reflection-sound filter 302 for filtering the 
monophonic input signal Min; a delay unit 303 constructed of a plurality 
of delay devices D.sub.1 to D.sub.n for delaying the output signal from 
this reflecting-sound filter 302; an amplifier unit 304 constructed of a 
plurality of amplifiers G.sub.1 to G.sub.n for amplifying the output 
signals from the respective delay devices D.sub.1 to D.sub.n ; and also a 
reflection-sound adder 305 for adding the output signals from a plurality 
of amplifiers G.sub.1 to G.sub.n. 
The above-described adding unit is arranged by an adder 306 for adding the 
output signal from the delay device 301 for inter aural time difference to 
the output signal from the reflection-sound adder 305. The output signal 
derived from this adder 306 is supplied as the left input signal Lin to 
the crosstalk canceling means and the reverse-phase signal producing means 
(see FIG. 11). 
Similarly, the portion for simulating the direct sounds in the R-channel 
head related transfer function applying means is arranged by a 
direct-sound filter 310 for filtering the monophonic input signal Min, and 
a delay device 311 for inter aural time difference, which delays the 
output signal from this direct-sound filter 310. Also, the portion for 
simulating the reflection sounds is arranged by a reflection-sound filter 
312 for filtering the monophonic input signal Min; a delay unit 313 
constructed of a plurality of delay devices D.sub.1 to D.sub.n for 
delaying the output signal from this reflecting-sound filter 312; an 
amplifier unit 314 constructed of a plurality of amplifiers G.sub.1 to 
G.sub.n for amplifying the output signals from the respective delay 
devices D.sub.1 to D.sub.n ; and also a reflection-sound adder 315 for 
adding the output signals from a plurality of amplifiers G.sub.1 to 
G.sub.n. 
The above-described adding unit is arranged by an adder 316 for adding the 
output signal from the delay device 311 for inter aural time difference to 
the output signal from the reflection-sound adder 315. The output signal 
derived from this adder 316 is supplied as the right input signal Rin to 
the crosstalk canceling means and the reverse-phase signal producing 
means. 
Both the direct-sound filters 300 and 310 simulate the frequency 
characteristics of the head related transfer functions of the sounds which 
directly reach from the sound source (e.g., loudspeaker) to the ears of 
the audience. The delay devices 301 and 311 for inter aural time 
difference simulate the time differences of the sounds at the right/left 
ears, which directly reach from the sound source to the ears of the 
audience. The reflection-sound filters 302 and 312 simulate the changes in 
the frequencies caused by the reflections occurred in the room. The delay 
units 303 and 313, the amplifier units 304 and 314, and the 
reflection-sound adders 305 and 315 may realize the reach times of the 
reflection sounds to the right/left ears in the room, and the levels of 
the reflection sounds when reached the right/left ears. Then, the adders 
306 and 316 add the signals corresponding to the direct sounds, derived 
from the delay devices 301 and 311 for inter aural time difference to the 
signals corresponding to the reflection sounds, derived from the 
reflection-sound adders 305 and 315. These adders 306 and 316 outputs the 
summation result as either the left input signal Lin or the right input 
signal Rin. 
FIG. 42A and FIG. 42B graphically show impulse responses simulated by the 
head related transfer function applying means. The portion for simulating 
the direct sounds is equally made by employing an IIR filter and a delay 
device, whereas the portion for simulating the reflection sounds is 
equally made by employing an IIR filter, a multiplier, and a delay device. 
The impulse response shown in FIG. 42A and FIG. 42B are obtained from such 
an example that the direct-sound filter is arranged by a sixth-order IIR 
filter, and the reflection-sound filter is arranged by a third-order IIR 
filter, and further a number of taps on the delay device included in the 
portion for simulating the reflection sounds are selected to be 8. 
Apparently, the L-channel/R-channel head related transfer function 
applying means are not limited those arrangement. 
As previously explained, since the head related transfer function are 
applied to the monophonic input signal Min by the L-channel head related 
transfer function applying means and the R-channel head related transfer 
function applying means, the sound images can be localized at any 
arbitrary positions while the audience hears the sounds by using the 
headphone. 
Next, a method for moving a sound image in this sound image control 
apparatus according to the embodiment 5 will now be explained. As 
indicated in FIG. 43, when a front direction is set to 0 degree and a rear 
direction is set to 180 degrees, considering now such a case that the 
sound image is moved every 10 degrees on the horizontal plane by using the 
data about the head related transfer function. 
For instance, in such a case that the values of the filter coefficient and 
the delay amount are varied from the direction of 60 degrees to the 
direction of 70 degrees in the head related transfer function applying 
means shown in FIG. 12, if the input signal is continued, then noise will 
occur due to waveform distortions. To suppress this noise, the sound image 
is moved by replacing the 60-degree data by the 70-degree data in such a 
manner that these degree data are mutually cross-faded. As a consequence, 
the sound image localization process operation along the two directions is 
continuously performed, and the data along the different directions are 
replaced with each other in such a way that these data are cross-faded 
based upon the direction changing information. The direction changing 
information may be manually entered by the user, or may be automatically 
entered by the processing apparatus such as the CPU. 
FIG. 44 is a schematic block diagram of an arrangement of this sound image 
control apparatus capable of smoothly moving the sound image along the 
adjacent directions while cross-fading these degree data. In FIG. 44, 
first L-channel head related transfer function applying means 400 is used 
to apply an L-channel head related transfer function corresponding to a 
first direction to the monophonic input signal Min. First R-channel head 
related transfer function applying means 401 is used to apply an R-channel 
head related transfer function corresponding to the first direction to the 
monophonic input signal Min. Similarly, second L-channel head related 
transfer function applying means 410 is used to apply an L-channel head 
related transfer function corresponding to a second direction to the 
monophonic input signal Min. Second R-channel head related transfer 
function applying means 411 is used to apply an R-channel head related 
transfer function corresponding to the second direction to the monophonic 
input signal Min. 
A first series gain control circuit 402 corresponds to first weighting 
means employed in the sound image control apparatus according to the fifth 
aspect of the present invention. This first series gain control circuit 
402 applies a weight ".alpha." to the respective signals derived from the 
first L-channel head related transfer function applying means and the 
first R-channel head related transfer function applying means in response 
to an instruction issued from direction instructing means. In this case, 
0.ltoreq..alpha..ltoreq.1. This first series gain control circuit 402 may 
be constituted by a multiplier for multiplying the respective signals 
derived from the first L-channel head related transfer function applying 
means 400 and the first R-channel head related transfer function applying 
means 401 by the weighting coefficient ".alpha.". 
Similarly, a first series gain control circuit 412 corresponds to second 
weighting means employed in the sound image control apparatus according to 
the fifth aspect of the present invention. This second series gain control 
circuit 412 applies a weight "1-.alpha." to the respective signals derived 
from the second L-channel head related transfer function applying means 
and the second R-channel head related transfer function applying means in 
response to an instruction issued from the direction instructing means. 
This second series gain control circuit 412 may be constituted by a 
multiplier for multiplying the respective signals derived from the second 
L-channel head related transfer function applying means 410 and the second 
R-channel head related transfer function applying means 411 by the 
weighting coefficient "1-.alpha.". 
An adder 403 corresponds to left mixing means employed in the sound image 
control apparatus according to the fifth aspect of the present invention. 
This adder 403 mixes the signal derived from the first L-channel head 
related transfer function applying means, to which the weight ".alpha." 
has been applied by the first series gain control circuit 402, with the 
signal derived from the second L-channel head related transfer function 
applying means, to which the weight "1-.alpha." has been applied by the 
second series gain control circuit 412. Then, this adder 403 produces the 
left input signal accordingly. 
Similarly, an adder 413 corresponds to right mixing means employed in the 
sound image control apparatus according to the fifth aspect of the present 
invention. This adder 413 mixes the signal derived from the first 
R-channel head related transfer function applying means, to which the 
weight "a" has been applied by the first series gain control circuit 402, 
with the signal derived from the second R-channel head related transfer 
function applying means, to which the weight "1-.alpha." has been applied 
by the second series gain control circuit 412. Then, this adder 413 
produces the right input signal accordingly. 
In the above-described arrangement, while setting the data about the 
60-degree direction to the first L-channel head related transfer function 
applying means 400 and the first R-channel head related transfer function 
applying means 401, and also setting the data about the 70-degree 
direction to the second L-channel external transfer function applying 
means 410 and the second R-channel head related transfer function applying 
means 411, the sound image is moved by changing the weighting factor 
".alpha.". When the present data is replaced by the data about the new 
direction, while the gain of this series is set to "0", if the gain is 
gradually increased in accordance with the time elapse, then the noise 
caused by replacing these data can be eliminated. 
It should be understood that the direction determined by the head related 
transfer function may be selected from any planes other than the 
horizontal plane, for instance, the upper/lower/right/left directions, and 
the near distance up to the far distance. As previously described, when 
the audience hears the sounds by using the headphone, the sound images can 
be smoothly moved to any places within the three-dimensional space by way 
of the head related transfer function applying means. 
Subsequently, an example of a sound image control system for utilizing the 
above-explained sound image control apparatus will now be described with 
reference to drawings. It should be noted that an electronic circuit of 
this sound image control system is identical to the electronic circuit 
(see FIG. 36) of the stereophonic sound image enhancement apparatus 
according to the third aspect of the present invention. However, this 
electronic circuit owns such a different arrangement that a variable 
resistor 172c and a joy stick 172d are additionally provided with the 
input apparatus 172. 
FIG. 45 is an outer view of this sound image control system, as viewed from 
an upper surface thereof. In this sound image control system, the process 
operation is carried out by inputting the monophonic input signal Min from 
a line input terminal and by applying the head related transfer function 
to this monophonic input signal, and both the crosstalk canceling process 
operation and the reverse-phase signal mixing process operation are 
performed. In addition, the processed signals are outputted as the left 
output signal Lout and the right output signal Rout from the line output 
terminal. 
The variable resistor 172a of the input apparatus 172 is used to instruct 
an amount of crosstalk canceling. A gain "a" is determined in response to 
a signal derived from this variable resistor 172a. Another variable 
resistor 172b is used to instruct a mixing amount of a reverse-phase 
signal. Another gain "e" is determined based upon a signal derived from 
this variable resistor 172b. Another variable resistor 172c is used to 
instruct upper/lower directions of a sound image. The joy stick 172d of 
the input apparatus 172 is employed so as to determine an angle (0 to 360 
degrees) of the sound image along the horizontal direction, and a distance 
measured from the audience. A weighting factor ".alpha." is determined in 
response to the signal from this variable resistor 172c and the signal 
from the joy stick 172d. 
FIG. 36 is a schematic block diagram for representing an arrangement of an 
electronic circuit of the sound image control system. A DSP 161 is 
operated in a different manner from that of the stereophonic sound image 
enhancement apparatus according to the third aspect of the present 
invention. That is, both the crosstalk canceling process operation and the 
reverse-phase signal mixing process operation are carried out in 
accordance with the control data supplied from a CPU 170, and also the 
head related transfer function is applied to the monophonic input signal 
Min in the above-described manner. 
The CPU 170 reads the filter coefficient from a memory 171 and then 
supplies the read filter coefficient as the control data to the DSP 161. 
The CPU 170 receives the signals produced from the variable resistors 172a 
to 172c and the joy stick 172d, and converts these signals into values 
indicative of the weighting factor ".alpha.", the gain a and the gain "e". 
Then, this CPU 170 supplies the converted values as the control data to 
the DSP 161. In response to the data indicative of the weighting factor 
".alpha.", the gains "a" and "e", and further the filter coefficient, the 
DSP 161 executes the above-described head related transfer function 
applying process operation, the crosstalk canceling process operation, and 
the reverse-phase signal mixing process operation. 
It should also be noted that this sound image control apparatus may be 
solely used, but alternatively, may be assembled into other apparatuses as 
a multi-effector, and reverberation generator. As these other apparatuses, 
for instance, a sound source module, an electronic piano, an electronic 
keyboard, a game machine, and an acoustic appliance may be employed.