Apparatus for localization of sound image

A sound image localization apparatus comprises crosstalk canceling means and direction localizing means, wherein first the crosstalk canceling means first subject an input sound signal to crosstalk cancellation, and then, the direction localizing means subject the processed signal to directional localization, whereby both crosstalk cancellation and directional localization share a signal to be processed, so the necessary amount of a storage device to hold the signal is reduced. That is, a reduction in circuit scale and calculation load can provide a sound image localization apparatus with low cost and high processing precision.

FIELD OF THE INVENTION
 The present invention relates to an apparatus for localization of a sound
 image and, more particularly, to an apparatus for localization of a sound
 image which receives a sound signal, subjects the sound signal to signal
 processing, localizes a virtual sound image, and outputs a sound image
 localization signal.
 BACKGROUND OF THE INVENTION
 A conventional stereophonic system controls sound image localization using
 a plural of (generally two) loudspeakers, conferring a realistic sensation
 to the hearing of a listener. The conventional system usually includes two
 laterally spaced loudspeakers in front of the listener, so a sound image
 is localized between them. Outside the two loudspeakers no sound image is
 localized in the system. To obtain the effect that a sound image is
 localized outside the two loudspeakers, i.e., the surround of the
 listener, for instance, a sound from the back of the listener, the system
 sometimes includes loudspeakers at the rear as well as the two
 loudspeakers in front of the listener.
 The development of technology for digitizing audio and hardware for DSP
 (Digital Signal Processor) facilitates various signal processing. Owing to
 this, the system using two loudspeakers in front of the listener can
 localize a sound image at any position around the listener, such as the
 side and rear of the listener.
 Conventional sound image localization apparatus are disclosed in Japanese
 Patent Published Application Nos. Hei 3-270400 (1991); Hei 4-273800
 (1992). A description will be given of a typical, conventional sound image
 localization apparatus.
 FIGS. 19(a) and 19(b) are diagrams for explaining about sound image
 localization. FIG. 19(a) shows a sound image to be localized in a virtual
 way. FIG. 19(b) shows a system using two loudspeakers. In this case, it is
 assumed that the positions of virtually localized sound images, and the
 positions of the two loudspeakers are left-and-right symmetrical with
 respect to the listener.
 In the sound image localization apparatus, a direction of a virtual
 position is localized and crosstalk is canceled by signal processing using
 a head related transfer function indicating transfer characteristics of
 sound from a sound source to the listener's head or ear.
 Here, in case like FIG. 19(b), a crosstalk signal is a signal transferred
 from a left loudspeaker to a right ear, or from a right loudspeaker to
 left ear. A signal is generated for canceling the crosstalk signal.
 In the virtual environment achieved by this system as shown in FIG. 19(a),
 sound signals uL and uR are radiated from the positions of virtual sound
 images located laterally at the back of the listener. Reference numerals,
 yL1 and yR1, indicate sound pressures given to left and right ears,
 respectively. Because of the left-and-right symmetry, transfer of sound
 from the left virtual position to the left ear is the same as that from
 the right virtual position to the right ear. A head related transfer
 function showing this transfer characteristics is indicated by TM. The
 transfer of sound from the left virtual position to the right ear and that
 from the right virtual position to the left ear are represented by the
 same head related transfer function TC. The relation between the sound
 pressures and the functions are represented by
EQU yL1=TM.multidot.uL+TC.multidot.uR (1-1) and
EQU yR1=TC.multidot.uL+TM.multidot.uR (1-2).
 On the other hand, in a system shown in FIG. 19(b), left and right
 loudspeakers 1901a and 1901b radiate sound signals xL and xR,
 respectively. Sound pressures given to the left and right ears of the
 listener are yL2 and yR2, respectively. As they are left-and-right
 symmetrical, the transfer of sound from the left loudspeaker position to
 the left car and that from the right loudspeaker position to the right ear
 are represented by the same head related transfer function SM. The
 transfer of sound from the left loudspeaker position to the right ear and
 that from the right loudspeaker position to the left ear are also
 represented by the same head related transfer function SC. The relation
 between those sound pressures and those functions are
EQU yL2=SM.multidot.xL+SC.multidot.xR (2-1) and
 yR2=SC.multidot.xL+SM.multidot.xR (2-2).
 In this system, to localize the positions of the sound images shown in FIG.
 19(a) using acoustics output from the loudspeakers 1901a and 1901b, the
 following equations must be satisfied,
EQU yL1=yL2 (3-1) and
EQU yR1=yR2 (3-2).
 The equations 3-1, 1-1, and 2-1 lead to the following equation 4-1, and the
 equations is 3-2, 1-2, and 2-2 lead to the following equation 4-2,
EQU TM.multidot.uL+TC.multidot.uR=SM.multidot.xL+SC.multidot.xR (4-1) and
EQU TC.multidot.uL+TM.multidot.uR=SC.multidot.xL+SM.multidot.xR (4-2).
 The solution to xL and xR is obtained from the equations 4-1 and 4-2. If
 assumed that, the gain being represented by .dbd.*.dbd.,
EQU .dbd.(SC/SM).sup.2.dbd.&lt;&lt;1 (5),
 xL and xR are approximated by
EQU xL.about.(FM+FC.multidot.FX).multidot.uL+(FC+FM.multidot.FX).multidot.uR
 (6-1) and
EQU xR.about.(FC+FM.multidot.FX).multidot.uL+(FM+FC.multidot.FX).multidot.uR
 (6-2),
EQU where FM=TM/SM (7-1),
EQU FC=TC/SM (7-2), and
EQU FX=-SC/SM (7-3).
 Using the above relations, a conventional sound image localization
 apparatus is constructed, shown in FIG. 18(a) . The conventional sound
 image localization apparatus comprises a crosstalk canceling means 1801,
 direction localizing means 1802a and 1802b, and adders 1803a and 1803b.
 Sound signals are input through input terminals 1804a and 1804b. Signals
 resulting from subjecting the input sound signals to signal processing are
 output through output terminals 1805a and 1905b.
 The direction localizing means 1802a and 1802b process the sound signals
 input through the input terminals 1804a and 1804b to generate signals
 indicating the directions of sound image positions, respectively. The
 adders 1803a and 1803b add input signals. The crosstalk canceling means
 1801 removes a crosstalk component of an input signal.
 FIG. 18(b) is a diagram illustrating a detailed structure of an example of
 the conventional sound image localization apparatus. The crosstalk
 canceling means 1801 shown in FIG. 18(a) comprises crosstalk canceling
 signal generating filters 1806a and 1806b, and adders 1803c and 1803d. The
 direction localizing means 1802a and 1802b shown in FIG. 18(a) comprise
 main-path filters 1807a and 1807b, and crosstalk-path filters 1808a and
 1808b, respectively. The combination of the main-path filter and the
 crosstalk-path filter is sometimes called a direction localizing filter.
 The prior art sound image localization apparatus generates the outputs xL
 and xR according to the expressions 6-1 and 6-2. A description will be
 given of how the sound image localization apparatus works.
 Left and right input sound signals are input through the input terminals
 1804a and 1804b, respectively. The first input sound signal input through
 the input terminal 1804a is input to the main-path filter 1807a and the
 crosstalk-path filter 1808a. The main-path filter 1807a multiplies the
 input signal by the coefficient shown in the equation 7-1. The
 crosstalk-path filter 1808a multiplies the input signal by the coefficient
 shown in the equation 7-2. The outputs of the main-path filter 1807a and
 the crosstalk-path filter 1808a are input to the adders 1803a and 1803b,
 respectively.
 Similarly, the second input sound signal input through the input terminal
 1804b is input to the main-path filter 1807b and the crosstalk-path filter
 1808b, where the input signal is multiplied by the coefficients expressed
 by 7-1 and 7-2, respectively. The outputs of the main-path filter 1807b
 and the crosstalk-path filter 1808b are input to the adders 1803b and
 1803a, respectively.
 The adders 1803a and 1803b each add input signals. The adder 1803a outputs
 a result of the addition to the adder 1803c and the crosstalk canceling
 signal generating filter 1806a. The crosstalk canceling signal generating
 filter 1806a multiplies the input signal by the coefficient represented by
 the equation 7-3 to produce a crosstalk canceling signal signal, and
 outputs the signal to the adder 1803d.
 Similarly, the adder 1803b outputs a result of the addition to the adder
 1803d and the crosstalk canceling signal generating filter 1806b. The
 crosstalk canceling signal generating filter 1806b multiplies the input
 signal by the coefficient represented by the equation 7-3 to produce a
 crosstalk canceling signal, and outputs the signal to the adder 1803c.
 The adders 1803c and 1803d each add results of addition by the adders 1803a
 and 1803b to the crosstalk canceling signal having phase almost equivalent
 to the inversed phase of the result of the addition, respectively. Thus,
 signals represented by the expressions 6-1 and 6-2, of which crosstalk
 components are removed, are output through the output terminals 1805a and
 1805b, respectively.
 In the sound image localization apparatus having the structure shown in
 FIG. 18(b), the output of a crosstalk canceling signal generating filter
 on either channel (for example, 1806a) is output to the output side of the
 other channel (the adder 1803d on the side having the output terminal
 1805b). This structure is called feedforward.
 As described above, the conventional sound image localization apparatus can
 localize a sound image over a wide range by localization of a virtual
 sound image and compensation of a crosstalk component. However, when
 trying to realize the foregoing sound image localization apparatus by a
 computer system using a CPU and a DSP, the following several problems
 arise.
 The first problem is that because in this feedforward type sound image
 localization apparatus the crosstalk canceling signal is output to the
 output side of the whole apparatus, the canceling of crosstalk cannot be
 repeated, whereby the adverse effect of sound diffraction of low-frequency
 component becomes serious. Thus, it is difficult to improve low-frequency
 characteristics to make sound quality better.
 The second problem is about a memory used for temporary storage in
 operational processing. The amount and performance of a memory in a
 computer system limit operational processing. The main constraints on
 memory are
 (A) constraint on the amount of memory for storage of sound signal data,
 (B) constraint on the amount of memory for storage of coefficients of a
 filter, and
 (C) constraint on accessing time of a memory.
 As to (A) and (B), when the number of words showing the amount of memory is
 small, the number of taps indicating the order of a filter is limited to
 an insufficient size, resulting in a reduction in precision of operational
 processing.
 Furthermore, when the amount of a high-speed internal memory included in a
 computer system is limited, if a relatively low-speed external memory
 (RAM) assists to secure a required precision of operational processing,
 the problem (C) arises. Because frequent memory accesses occur in
 operational processing realizing the above-described digital filter
 performing directional localization and crosstalk cancellation, a simple
 supplement of the external memory having a low accessing speed hardly
 solves the constraint on the amount of memory.
 The third problem relates to a controller included in a computer system,
 such as DSP. The processing speed of the controller limits operational
 processing. When the processing speed is not sufficient, the order of a
 digital filter is limited, thereby reducing precision in operational
 processing.
 The fourth problem is that it is difficult for the conventional sound image
 localization apparatus to deal with changes in setting of an acoustic
 system using it. For example, when loudspeakers are rearranged in the
 acoustic system in such a way as that the angle the loudspeakers attain
 changes, the conventional sound image localization apparatus modifies all
 the parameters of the filter FX. Thus, to adapt to changes in setting of
 the acoustic system, parameters for each setting are required to be held.
 The requirement of storage of parameters increases the amount of a memory.
 As those problems indicate, the prior art sound image localization
 apparatus has a difficulty in improving low-frequency characteristics.
 Furthermore, when implemented in a computer system, the apparatus requires
 the large amount of memory and the high-speed of processing, thereby
 making it difficult to realize both precision of controlling sound image
 localization and a reduction in costs of the computer system.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide a sound image
 localization apparatus achieving high sound quality by improving
 low-frequency characteristics.
 It is another object of the present invention to provide a sound image
 localization, apparatus realizing sound image localization with good
 precision while limiting an increase in the circuit scale caused by
 requirement of the amount of memory.
 It is still another object of the present invention to provide a sound
 image localization apparatus realizing sound image localization with good
 precision by additionally exploiting an external memory when the amount of
 a high-speed internal memory is limited.
 It is yet another object of the present invention to provide a sound image
 localization apparatus realizing sound image localization with good
 precision by simplifying operational processing when the computer system
 does not include a high-performance DSP.
 It is a further object of the present invention to provide a sound image
 localization apparatus flexibly coping with changes in setting of the
 acoustic system, without increasing the circuit scale.
 Other objects and advantages of the present invention will become apparent
 from the detailed description desired hereinafter; it should be
 understood, however, that the detailed description and specific embodiment
 are desired by way of illustration only, since various changes and
 modifications within the scope of the invention will become apparent to
 those skilled in the art from this detailed description.
 According to a first aspect of this invention, there is provided a sound
 image localization apparatus receiving a sound signal, performing signal
 processing to the sound signal, localizing a virtual sound image, and
 outputting a sound image localization signal, the apparatus comprising:
 direction localizing means for localizing the direction of a virtual sound
 source position; and
 crosstalk canceling means for performing crosstalk cancellation by
 generating a crosstalk canceling signal, and outputting the crosstalk
 canceling signal toward the direction of where the sound signal is input.
 As a result, the apparatus performs feedback processing by outputting a
 crosstalk canceling signal to the input side.
 According to a second aspect of this invention, there is provided the sound
 image localization apparatus of the first aspect wherein
 the crosstalk canceling means perform crosstalk cancellation to a signal
 generated by directional localization of the direction localizing means.
 As a result, the apparatus performs feedback processing by outputting a
 crosstalk canceling signal to the input side.
 According to a third aspect of this invention, there is provided the sound
 image localization apparatus of the first aspect wherein
 the direction localizing means perform directional localization to a signal
 generated by crosstalk cancellation of the crosstalk canceling means.
 As a result, the targets of crosstalk cancellation and directional
 localization are shared, and the apparatus performs feedback processing by
 outputting a crosstalk canceling signal to the input side.
 According to a fourth aspect of this invention, there is provided the sound
 image localization apparatus of the third aspect wherein
 the crosstalk canceling means comprise first and second crosstalk canceling
 signal generating filters, and first and second adders, the first adder
 adding a first sound signal and a signal generated by the second crosstalk
 canceling signal generating filter, the second adder adding a second sound
 signal and a signal generated by the first crosstalk canceling signal
 generating filter;
 the direction localizing means comprise first and second main-path filters,
 first and second crosstalk-path filters, and first and second adders, the
 first adder adding a signal processed by the first main-path filter and a
 signal processed by the second crosstalk-path filter, the second adder
 adding a signal processed by the second main-path filter and a signal
 processed by the first crosstalk-path filter.
 As a result, a crosstalk canceling generating filter shares an input with a
 main-path filter and a crosstalk-path filter.
 According to a fifth aspect of this invention, there is provided the sound
 image localization apparatus of the first aspect wherein
 the crosstalk canceling means use a comb filter to generate the crosstalk
 canceling signal.
 As a result, the apparatus performs crosstalk cancellation using a signal
 generated by a crosstalk canceling signal generating filter including a
 comb filter of which the coefficients are the same.
 According to a sixth aspect of this invention, there is provided the sound
 image localization apparatus of the fifth aspect wherein
 the apparatus further comprises a low-pass filter processing a signal input
 to or output from the crosstalk canceling means.
 As a result, the apparatus performs crosstalk cancellation to a signal from
 which a high-frequency component is removed.
 According to a seventh aspect of this invention, there is provided the
 sound image localization apparatus of the first aspect wherein
 the crosstalk canceling means hold the crosstalk canceling signal generated
 at a certain time, delay the crosstalk canceling signal held, hold the
 plurality of crosstalk canceling signals delayed, and multiply some of the
 plurality of crosstalk canceling signals held by a predetermined
 coefficient to generate the crosstalk canceling signal at a time following
 the certain time.
 As a result, the apparatus performs crosstalk cancellation using a signal
 generated a crosstalk canceling signal generating filter including a
 circuit replacing a comb filter, of which the processing load is reduced.
 According to an eighth aspect of this invention, there is provided the
 sound image localization apparatus of the seventh aspect wherein
 the apparatus further comprises a low-pass filter processing a signal input
 to or output from the crosstalk canceling means.
 As a result, the apparatus performs crosstalk cancellation to a signal from
 which a high-frequency component is removed.
 According to a ninth aspect of this invention, there is provided the sound
 image localization apparatus of the first aspect wherein
 the crosstalk canceling means further comprise a crosstalk canceling signal
 generating filter generating the crosstalk canceling signal, and a switch
 switching the crosstalk canceling signal generated by the crosstalk
 canceling signal generating filter to the output side of the crosstalk
 canceling signal generating filter in place of the input side of the
 crosstalk canceling signal generating filter.
 As a result, the apparatus switches feedback processing and feedforward
 processing.
 According to a tenth aspect of this invention, there is provided the sound
 image localization apparatus of the first aspect wherein
 the crosstalk canceling means further comprise a crosstalk canceling signal
 generating filter generating the crosstalk canceling signal, and a
 delaying unit delaying a signal input to or output from the crosstalk
 canceling signal generating filter by various times.
 As a result, the apparatus performs crosstalk cancellation by changing the
 amount of an initial delay.
 According to an eleventh aspect of this invention, there is provided the
 sound image localization apparatus of the first aspect wherein
 the apparatus processes an input sound signal to be localized in a first
 direction, and an input sound signal to be localized in a second
 direction;
 the crosstalk canceling means comprising a first filter having a certain
 number of taps, a second filter different from the first filter, and a
 switch switching first and second modes; in the first mode the first
 filter functioning as a filter generating the crosstalk canceling signal,
 and in the second mode the second filter functioning as a filter
 generating the crosstalk canceling signal while the first filter
 functioning as a filter localizing the second direction.
 As a result, a crosstalk canceling signal generating filter for localizing
 a sound image to be localized in a first direction, and a crosstalk
 canceling signal generating filter for localizing a sound image to be
 localized in a second direction, are switched.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Embodiment 1
 A sound image localization apparatus in accordance with a first embodiment
 of this invention improves low-frequency characteristics by feedback
 crosstalk cancellation.
 FIG. 1(a) is a block diagram illustrating a structure of the sound image
 localization apparatus of the first embodiment. As shown in the figure,
 the sound image localization apparatus comprises a crosstalk canceling
 means 101, direction localizing means 102a and 102b, and adders 103a and
 103b. The apparatus receives input sound signals through input terminals
 104a and 104b, and outputs signals resulting from signal processing
 through output terminals 105a and 105b.
 The crosstalk canceling means 101 removes a crosstalk component from an
 input signal. The direction localizing means 102a and 102b process the
 input sound signals input through the input terminals 104a and 104b to
 produce signals indicating the directions of positions of sound images.
 The adders 103a and 103b add input signals.
 The operational processing of the sound image localization apparatus will
 be explained. The other solution to xL and xR from the equations 4-1 and
 4-2 is possible, rather than the expressions 6-1 and 6-2 described in the
 BACKGROUND OF THE INVENTION section.
EQU xL=FM.multidot.uL+FC.multidot.uR+FX.multidot.xR (8-1)
 and
EQU xR=FC.multidot.uL+FM.multidot.uR+FX.multidot.xL (8-2)
 are obtained. In the equations 8-1 and 8-2, the first and second terms on
 the right side indicate the directions of sound images, that is, they
 localize the directions. The third term on the right side cancels a
 crosstalk component.
 The sound image localization apparatus of the first embodiment performs
 signal processing according to the equations 8-1 and 8-2.
 FIG. 1(b) is a diagram showing a detailed structure of the sound image
 localization apparatus. The crosstalk canceling means 101 in FIG. 1(a)
 comprises crosstalk canceling signal generating filters 106a and 106b, and
 adders 103a and 103b. The direction localizing means 102a and 102b in FIG.
 1(a) comprise main-path filters 107a and 107b, and crosstalk-path filters
 108a and 108b, respectively. The adders 103a and 103b are the same as
 those in FIG. 1(a), and also part of the crosstalk canceling means 101.
 The sound image localization apparatus shown in FIG. 1(b) generates outputs
 xL and xR according to the equations 8-1 and 8-2. With the different
 structure from that shown in FIG. 18(b), the sound image localization
 apparatus is called a feedback type, because a crosstalk canceling signal
 generating filter (for instance, 106a) on either channel outputs a signal
 to the input side on the other channel (the adder 103b). A description
 will be given of how the sound image localization apparatus operates.
 Left and right input sound signals are input through the input terminals
 104a and 104b, respectively. The first input sound signal input through
 the input terminal 104a is input to the main-path filter 107a and the
 crosstalk-path filter 108a. The main-path filter 107a multiplies the input
 signal by the coefficient represented by the equation 7-1, and outputs the
 result to the adder 103a. The crosstalk-path filter 108a multiplies the
 input signal by the coefficient represented by the equation 7-2, and
 outputs the result to the adder 103b. In a similar way, the second input
 sound signal input through the input terminal 104b is input to the
 main-path filter 107b and the crosstalk-path filter 108b, where the
 signals are multiplied by coefficients represented by the equations 7-1
 and 7-2, and the results are output to the adders 103b and 103a,
 respectively.
 The adders 103a and 103b each add the input signals. The adder 103a outputs
 a result of the addition to the crosstalk canceling signal generating
 filter 106a. The crosstalk canceling signal generating filter 106a
 multiplies the input signal by the coefficient represented by the equation
 7-3 to generate a crosstalk canceling signal, and outputs it to the adder
 103b. Similarly, the adder 103b outputs a result of the addition to the
 crosstalk canceling signal generating filter 106b. The crosstalk canceling
 signal generating filter 106b multiplies the input signal by the
 coefficient represented by the equation 7-3 to generate a crosstalk
 canceling signal, and outputs it to the adder 103a.
 The adders 103a and 103b add the outputs of the direction localizing
 filter, and further add a result of the addition to the crosstalk
 canceling signal having a sign opposite to the result of the addition, to
 remove a crosstalk component. Hence, signals represented by the equations
 8-1 and 8-2 are output through the output terminals 105a and 105b.
 As hereinbefore described, in the sound image localization apparatus in
 accordance with the first embodiment, as shown in FIG. 1(b), the crosstalk
 canceling signals generated by the crosstalk canceling signal generating
 filters 106a and 106b are output nearer the input end of the apparatus
 than the crosstalk canceling signal generating filters (the adders 103a
 and 103b), which makes the apparatus a feedback type. Thereby, multiple
 cancellation, in which the generation of a crosstalk canceling signal and
 the crosstalk cancellation using the generated signal are repeated,
 becomes possible. Compared with the prior art feedforward type apparatus
 shown in FIGS. 18(a) and 18(b), the adverse effect of sound diffraction of
 a low-frequency component of a sound signal is reduced, thereby solving
 the first problem of the prior art and improving low-frequency
 characteristics.
 Embodiment 2
 A sound image localization apparatus in accordance with a second embodiment
 of this invention reduces the necessary amount of memory by subjecting a
 signal to directional localization after performing crosstalk cancellation
 to the signal.
 FIG. 2(a) is a block diagram showing a structure of the sound image
 localization apparatus of the second embodiment. As shown in the figure,
 the sound image localization apparatus comprises a crosstalk canceling
 means 201, and direction localizing means 202a and 202b, adders 203a and
 203b. The apparatus receives input sound signals through input terminals
 204a and 204b, subjects the input signals to signal processing, and
 outputs the resulting signals through output terminals 205a and 205b.
 The crosstalk canceling means 201 removes crosstalk components from the
 input signals input through the input terminals 204a and 204b. The
 direction localizing means 202a and 202b process input sound signals to
 produce signals indicating the directions of sound images. The adders 203a
 and 203b add input signals.
 The operational processing of the sound image localization apparatus will
 be explained. Initially, in addition to the equations 1-1 to 8-2 shown in
 the BACKGROUND OF THE INVENTION and Embodiment 1 sections, vL and vR are
 defined by
EQU xL=FM.multidot.vL+FC.multidot.vR (9-1)
 and
 xR=FC.multidot.vL+FM.multidot.vR (9-2).
 The equation 9-1 is substituted to the equation 8-1, and 9-2 is substituted
 to 8-2, and then
EQU FM.multidot.vL+FC.multidot.vR=FM.multidot.uL+FC.multidot.uR+FX.multidot.(FC
 .multidot.vL+FM.multidot.vR) (10-1)
 and
EQU FC.multidot.vL+FM.multidot.vR=FC.multidot.uL+FM.multidot.uR+FX.multidot.(FM
 .multidot.vL+FC.multidot.vR) (10-2)
 are obtained. From 10-1 and 10-2, FM and FC are eliminated, and then
EQU vL=uL+FX.multidot.vR (11-1)
 and
EQU vR=uR+FX.multidot.vL (11-2)
 are obtained.
 The equations 11-1 and 11-2 mean that a crosstalk canceling means is
 required to be set up on the input side. The equations 9-1 and 9-2 mean
 that direction localizing means are required to be set up on the output
 side. Accordingly, the sound image localization apparatus of the second
 embodiment, as shown in FIG. 2(a), includes a crosstalk canceling means
 201 on the input side, and direction localizing means 202a and 202b on the
 output side.
 FIG. 2(b) is a diagram showing a detailed structure of a first example of
 the sound image localization apparatus of the second embodiment. The
 crosstalk canceling means 201 shown in FIG. 2(a) comprises crosstalk
 canceling signal generating filters 206a and 206b, and adders 203c and
 203d in FIG. 2(b). The direction localizing means 202a and 202b shown in
 FIG. 2(b) comprise main-path filters 207a and 207b, and crosstalk-path
 filters 208a and 208b in FIG. 2(b), respectively. An explanation will be
 given of the operation of the first example of the sound image
 localization apparatus.
 Left and right input sound signals uL and uR are input through input
 terminals 204a and 204b. In FIG. 2(b), the input sound signal uL input
 through the input terminal 204a is input to the adder 203c. The second
 input sound signal uR input through the input terminal 204b is input to
 the adder 203d. Immediately after the sound image localization apparatus
 starts processing, the crosstalk canceling signal generating filters 206a
 and 206b don't generate any signals to be output to the adders 203c and
 203d, so the adders 203c and 203d output input signals uL and uR as they
 are. The signals uL and uR are input to the crosstalk canceling signal
 generating filters 206a and 206b as signals vL and vR, respectively.
 The crosstalk canceling signal generating filter 206a multiplies the input
 signal by the coefficient having a negative sign represented by the
 equation 7-3 to produce a crosstalk canceling signal, and outputs it to
 the adder 203d. The crosstalk canceling signal generating filter 206b
 performs a similar processing to produce a crosstalk canceling signal, and
 outputs it to the adder 203c.
 The adder 203c adds the input sound signal uL and the crosstalk canceling
 signal to perform crosstalk cancellation, generating the signal vL
 represented by the equation 11-1. The generated signal vL is input to the
 main-path filter 207a and the crosstalk-path filter 208a. In a similar
 manner, the adder 203d generates the signal vR represented by 11-2, which
 is input to the main-path filter 207b and the crosstalk-path filter 208b.
 The main-path filter 207a multiplies the input signal by the coefficient
 represented by the equation 7-1, and outputs the result to the adder 203a.
 The crosstalk-path filter 208a multiplies the input signal by the
 coefficient represented by the equation 7-2, and outputs the result to the
 adder 203b. The output of the main-path filter 207a is represented by the
 first term on the right side of the equation 9-1. The output of the
 crosstalk-path filter 208a is represented by the second term on the right
 side of the equation 9-2.
 Similarly, the adder 203d adds the crosstalk canceling signal to the input
 sound signal uR to perform crosstalk cancellation. The resulting signal vR
 is input to the main-path filter 207b and the crosstalk-path filter 208b,
 where the signal is multiplied by the coefficients represented by the
 equations 7-1 and 7-2, respectively. The outputs of the main-path filter
 207b and the crosstalk-path filter 208b are input to the adders 203b and
 203a, respectively. The output of the main-path filter 207b is represented
 by the first term on the right side of the equation 9-2. The output of the
 crosstalk-path filter 208a is represented by the second term on the right
 side of the equation 9-1.
 The adders 203a and 203b each add input signals, and output results of the
 addition through the output terminals 205a and 205b, respectively. Thus,
 the sound image localization apparatus in accordance with the second
 embodiment outputs signals xL and xR processed by directional
 localization, represented by the equations 9-1 and 9-2.
 As described above, in the sound image localization apparatus in accordance
 with the second embodiment, because signals are subjected to crosstalk
 cancellation prior to directional localization, as shown in FIG. 2(b), the
 inputs of the crosstalk canceling signal generating filter (FX) and the
 direction localizing filter (FM and FC) are the same signal, vL or vR.
 Thus, for filtering, just those two signals are required to hold. Compared
 with the conventional sound image localization apparatus shown in FIGS.
 18(a) and 18(b), required to hold four kinds of signals, the amount of
 memory required to hold sound signals, described as the second problem in
 the BACKGROUND OF THE INVENTION section, can be reduced to a small size.
 To explain the required amount of memory in the apparatus of the second
 embodiment, each structure of filters for crosstalk cancellation and
 directional localization will be shown.
 There are two sorts of filters, FIR (Finite Impulse Response) accumulating
 input signals and IIR (Infinite Impulse Response) accumulating output
 signals as well as input signals. Either of the two kinds of filters can
 realize the sound image localization apparatus of the second embodiment.
 FIG. 3 is a diagram showing the first example of the apparatus in which
 the crosstalk canceling signal generating filters 206a and 206b, and the
 direction localizing filters 207a, 207b, 208a, and 208b are FIR filters.
 FIG. 4 shows another example in which each filter shown in FIG. 2(b) is
 the concatenation of an FIR filter and an IIR filter.
 In FIG. 3, the crosstalk canceling signal generating filter 206a included
 in the first example (FIG. 2(b)) of the sound image localization
 apparatus, comprises delaying units 211a and 211c to 211f, multiplier
 210x1 to 210x5, and an adder 203i. The crosstalk canceling signal
 generating filter 206b comprises delaying units 211b and 211g to 211j,
 multipliers 210x6 to 210x10, and an adder 203j. The parts in FIG. 3
 represented by the dashed lines, such as the multipliers 210x1 to 210x5
 and the delaying units 211c to 211f, show that the number of multipliers
 or delaying units is variable.
 The main-path filter 207a comprises delaying units 211c to 211f,
 multipliers 210m1 to 210m5, and an adder 203e. The main-path filter 207b
 comprises delaying units 211g to 211j, multipliers 210m6 to 210m10, and an
 adder 203f. The crosstalk-path filter 208a comprises delaying units 211c
 to 211f and 211n to 211p, multipliers 210c1 to 210c5, and an adder 203g.
 The crosstalk-path filter 208b comprises delaying units 211g to 211j and
 211k to 211m, multipliers 210c6 to 210c10, and an adder 203h.
 Multipliers 210a1 and 210a2 function as attenuators to prevent overflow in
 executing fixed point calculation. Delaying units 211k to 211p are
 employed to produce the time difference between both cars.
 As the filters in FIG. 3 include the delaying units 211c to 211j, the
 crosstalk canceling signal generating filer and the direction localizing
 filter receive the same input signals, as signals vL or vR shown in FIG.
 2(b). Hence, compared with the case where the input of each filter is
 held, it is possible to reduce the amount of memory required to hold
 signals.
 FIG. 4 shows the example using IIR filters. In this example, a crosstalk
 canceling signal generating filter comprises IIR filter FXIs 212a and
 212b. A main-path filter comprises IIR filter FMIs 213a and 213b. A
 crosstalk-path filter comprises IIR filter FCIs 214a and 214b. Those IIR
 filters are concatenated with the FIR filters shown in FIG. 3.
 The portions of the main-path filter, the crosstalk-path filter, and the
 crosstalk canceling signal generating filter, constituted by FIR filters,
 are represented by FMF, FCF, and FXF, respectively. The FM, FC, and FX
 shown in the equations 7-1 to 7-3 are represented by
EQU FM=FMF.multidot.FMI (12-1),
EQU FC=FCF.multidot.FCI (12-2),
 and
EQU FX=FXF.multidot.FXI (12-3).
 Also in this case, similar to the structure shown in FIG. 3, the FIR filter
 portions share an input, thereby making it possible to reduce the required
 amount of memory. It should be noted that the reduction is not as much as
 that in the case where only the FIR filters are employed.
 FIG. 5 is a diagram showing a detailed structure of a second example of a
 sound image localization apparatus, shown in FIG. 2(a), in accordance with
 the second embodiment. As shown in the figure, the second example of the
 sound image localization apparatus comprises adders 203a to 203d,
 crosstalk canceling signal generating filters 206a and 206b, main-path
 filters 207a and 207b, crosstalk-path filters 208a and 208b,
 high-frequency main-path filters 217a and 217b, subsampling circuits 215a
 and 215b, and band compositing circuits 216a and 216b. As in the first
 example shown in FIG. 2(b), input sound signals are input through the
 input terminals 204a and 204b, and subjected to signal processing, and the
 resulting signals are output through the output terminals 205a and 205b.
 The subsampling circuits 215a and 215b subject input signals to prescribed
 subsampling to produce a low-frequency component and a high-frequency
 component. The band compositing circuits 216a and 216b subject input
 signals to prescribed composition to produce composite signals. The
 high-frequency main-path filters 217a and 217b operate in a similar way to
 the main-path filters 207a and 207b. The adders 203a to 203d, the
 crosstalk canceling signal generating filters 206a and 206b, main-path
 filters 207a and 207b, and the crosstalk-path filters 208a and 208b are
 similar to those in the first example.
 The operation of the second example of the sound image localization
 apparatus of the second embodiment will be described.
 Left and right input sound signals are input through the input terminals
 204a and 204b. The first input sound signal input through the input
 terminal 204a is input to the subsampling circuit 215a. The subsampling
 circuit 215a subsamples the first input sound signal to a high-frequency
 component and a low-frequency component, and outputs the high-frequency
 component to the high-frequency main-path filter 217a, and the
 low-frequency component to the adder 203c. The subsampling circuit 215b
 operates in a similar way.
 The high-frequency main-path filters 217a and 217b multiply the input
 high-frequency components by the coefficient represented by the equation
 7-1, and output the resulting signals to the band compositing circuits
 216a and 216b, respectively.
 The low-frequency component of the input sound signal is subjected to
 crosstalk cancellation and directional localization in a similar manner to
 the first example, and the resulting signals are input to the band
 compositing circuits 215a and 215b, respectively. The band compositing
 circuits 215a and 215b composite a signal resulting from processing the
 high-frequency component with the high-frequency filter, and a signal
 resulting from processing the low-frequency component by directional
 localization after crosstalk cancellation, and output the composite
 signals through the output terminals 205a and 205b, respectively.
 As is clear from the above, a second example of the sound image
 localization apparatus subjects only the low-frequency component of the
 input signal to crosstalk cancellation. In general, the high-frequency
 component of an input signal is seriously affected by a slight shift of
 the head of a listener and differences among individuals, so that the
 benefit of crosstalk cancellation is little for the high-frequency
 component. Therefore, a second example of the sound image localization
 apparatus processes the high-frequency component only with the main-path
 filter. Thus, because the target of crosstalk cancellation is only the
 low-frequency component, the number of sampling frequency can be reduced,
 thereby making it possible to make the sizes of filter circuits in FIGS. 3
 and 4 smaller without reducing the precision of sound image localization.
 As hereinbefore pointed out, the sound image localization apparatus in
 accordance with the second embodiment, as shown in FIG. 2(a), comprises a
 crosstalk canceling means 201 on the input side, and direction localizing
 means 202a and 202b on the output side. Thereby, each filter included in
 the crosstalk canceling means 201 and the direction localizing means 202a
 and 202b shares an input signal by using delaying units as shown in FIGS.
 3 and 4. As a result, the amount of memory required to hold a sound signal
 is reduced while sound image localization can be satisfactory.
 Embodiment 3
 A sound image localization apparatus in accordance with a third embodiment
 of this invention employs a comb filter.
 FIG. 6 is a block diagram showing a structure of a first example of the
 sound image localization apparatus of the third embodiment. The outline of
 the structure of the sound image localization apparatus is similar to the
 structure of the feedback type apparatus of the first embodiment shown in
 FIGS. 1(a) and 1(b). As shown in FIG. 6, the sound image localization
 apparatus comprises adders 603a, 603b, 603e, and 603f, main-path filters
 607a and 607b, crosstalk-path filters 608a and 608b, delaying units 611a
 to 611j, and multipliers 610x1 to 610x10. Input sound signals are input
 through input terminals 604a and 604b, and subjected to signal processing,
 and the resulting signals are output through output terminals 605a and
 605b. As in FIG. 3 and so on, dashed lines on rows of the delaying units
 and the multipliers represent an arbitrary number of the delaying units
 and the multipliers in FIG. 6.
 In FIG. 6, the crosstalk canceling signal generating filter 106a shown in
 FIG. 1(b) comprises the delaying units 611a, 611c to 611f, the multipliers
 610x1 to 610x5, and the adder 603e. The crosstalk canceling signal
 generating filter 106b shown in FIG. 1(b) comprises the delaying units
 611b, 611g to 611j, the multipliers 610x6 to 610x10, and the adder 603f.
 All the coefficients of the multipliers 610x1 to 610x10 are possible to be
 the same, which makes the filter a comb type. Therefore, when using a comb
 filter, it is possible to reduce the amount of memory required to hold the
 coefficient, described in the BACKGROUND IN THE INVENTION section, as the
 second problem (B).
 The operation of the sound image localization apparatus of the third
 embodiment is similar to that of the feedback type sound image
 localization apparatus of the first embodiment.
 FIGS. 8(a) and 8(b) are graphs for explaining frequency characteristics of
 a filter. FIG. 8(a) shows amplitude characteristics. FIG. 8(b) indicates
 phase characteristics. In either figure, a solid line represents
 characteristics of the comb filter used in the third embodiment, and a
 dashed line represent characteristics obtained from the ratio of head
 related transfer functions. In general, a comb filter has a linear phase
 type low-pass characteristics. As is apparent from the figure, both the
 characteristics are similar to each other in a low-frequency range of the
 amplitude and phase characteristics. As described in the second
 embodiment, cancellation is particularly effective in a low-frequency
 range of a sound signal. Because the characteristics of the comb filter is
 approximate to that obtained from the head related transfer function in
 the low-frequency range, the comb filter operates well for the
 low-frequency range. For a high-frequency range in which the two
 characteristics differ, crosstalk cancellation is hardly effective, so the
 influence of differences between the two characteristics is little.
 FIG. 7 is a block diagram showing a structure of a second example of the
 sound image localization apparatus of the third embodiment. As shown in
 FIG. 7, this example includes a first example of the sound image
 localization apparatus, and further comprises low-pass filters 720a and
 720b. The low-pass filter 720a comprises an adder 703c, multipliers 710f1
 and 710f2, and a delaying unit 711a. The low-pass filter 720b comprises an
 adder 703d, multipliers 710f3 and 710f4, and a delaying unit 711b.
 As to the operation of the sound image localization apparatus, the
 high-frequency components of signals input to the crosstalk canceling
 signal generating filters 106a and 106b shown in FIG. 1(b) are removed,
 and the other operation is similar to that of the first example. As
 hereinbefore pointed out, in generating a crosstalk canceling signal, the
 high-frequency component of a sound signal is not necessarily taken into
 consideration. In this example, the high-frequency component is not the
 target of processing, thereby making it possible to improve the precision
 of sound localization better than the first example. Note that the scale
 of the circuit of the second example becomes slightly larger than that of
 the first example by the low-pass filter.
 Although in the second example the low-pass filter is disposed in front of
 the crosstalk canceling signal generating filter, i.e., on the input side,
 the low-pass filter can be disposed at the rear of the crosstalk canceling
 signal generating filter, i.e., on the output side, thereby making
 possible the same effect.
 FIG. 9 is a diagram showing a structure of a third example of the sound
 image localization apparatus of the second embodiment. As shown in the
 figure, this example employs a comb filter, similar to that in the first
 example, but having FIRs of which the number of taps is small. In the
 structure shown in FIG. 9, the number of taps is two, and all the
 coefficients can be set to, for instance, -0.46. In this case, the filter
 becomes a filter having linear phased low-pass characteristics. This sound
 image localization apparatus operates in a similar way to the first
 example.
 In an acoustic system using the sound image localization apparatus, when
 the distance between two loudspeakers is set to be short, for example, the
 angle the loudspeakers attain is 10 to 20 degrees, the ratio of head
 related transfer functions shown in FIG. 19(b), i.e., SC/SM, becomes close
 to 1. Therefore, considering the stability of sound image localization and
 a reduction in a high-frequency component due to the sound diffraction of
 a sound signal, a filter having a small number of taps has good
 approximation in this case. In the case, the apparatus having the
 structure shown in FIG. 9 can reduces the amount of memory required to
 store the coefficient further than the first example shown in FIG. 6. As a
 result, the amount of data held by the delaying unit becomes small, and it
 is possible to make the scale of the circuit smaller.
 FIGS. 10 and 11 are diagrams showing a structure of a fourth example of the
 sound image localization of the third embodiment. As shown in FIG. 10,
 this example of the sound image localization apparatus includes a third
 example of the apparatus, and further comprises high-frequency main-path
 filters 1017a and 1017b, subsampling circuits 1015a and 1015b, and band
 compositing circuits 1016a and 1016b. These are similar to those shown in
 the second example of the second embodiment, i.e., the high-frequency
 main-path filters 217a and 217b, the subsampling circuits 215a and 215b,
 and the band compositing circuits 216a and 216b. The same with
 high-frequency main-path filters 1117a and 1117b, subsampling circuits
 1115a and 1115b, and band compositing circuits 1116a and 1116b, shown in
 FIG. 11.
 As to the operation of this example of the sound image localization
 apparatus, subsampling and band composition are similar to those in the
 second embodiment, and the other processes are similar to those in the
 third embodiment. Therefore, similar to the second example in the second
 embodiment and the third example in the second embodiment, this example of
 the sound image localization apparatus can reduce the required amount of
 memory and make the scale of the circuit smaller.
 The crosstalk canceling signal generating filter as the FIR filter having
 two taps similar to the third example is disposed between the direction
 localizing filter and the band compositing circuit in the structure shown
 in FIG. 10, while being disposed at the rear of the band compositing
 circuit, i.e., on the output side, in the structure shown in FIG. 11.
 However, the crosstalk canceling signal generating filter may be disposed
 in front of the subsampling circuit, i.e., on the input side, or between
 the subsampling circuit and the direction localizing filter, and may
 receive only the low-frequency component output from the subsampling
 circuit as the target of processing, resulting in the similar effect.
 As described above, the sound image localization apparatus in accordance
 with the third embodiment includes the comb filters in which the
 coefficients of the multipliers 610x1 to 610x10 shown in FIG. 6 are the
 same, whereby the operation using the filters requires only one parameter,
 i.e., the coefficient, and therefore, the amount of memory for holding the
 coefficient is reduced while making possible a high level of sound image
 localization.
 Although in the third embodiment the outline of the structure is the same
 as the feedback type sound image localization apparatus shown in FIGS.
 1(a) and 1(b), the feedforward type sound image localization apparatus
 shown in FIG. 18(b) may be used, or a comb filter can be used for the
 sound image localization apparatus of the second embodiment shown in FIG.
 2(b), resulting in the same effect.
 Embodiment 4
 A sound image localization apparatus in accordance with a fourth embodiment
 of this invention employs a circuit including delay buffers and
 accumulation registers (or memories) instead of comb filters of the third
 embodiment.
 FIG. 12 is a block diagram showing a structure of the sound image
 localization apparatus of the fourth embodiment. The outline of the
 structure of the sound image localization apparatus of the fourth
 embodiment include the same feedback structure as shown in FIGS. 1(a) and
 1(b), similar to the third embodiment. As shown in FIG. 12, the sound
 image localization apparatus comprises adders 1203a, 1203b, 1203c, and
 1203d, main-path filters 1207a and 1207b, crosstalk-path filters 1208a and
 1208b, delaying units 1211a to 1211j, and multipliers 1210f1 to 1210f4,
 1210x1, and 1210x5, 1210x6, and 1210x10. Input sound signal are input
 through input terminals 1204a and 1204b, and subjected to signal
 processing, and the resulting signals are output through output terminals
 1205a and 1205b. As in FIG. 3, dashed lines in the rows of the delaying
 units represent an arbitrary number of the delaying units.
 In the figure, the portion including the adder 1203c, the multipliers
 1210f1 and 1210f2, and the delaying unit 1211m, and the portion including
 the adder 1203d, the multipliers 1210f3 and 1210f4, and the delaying unit
 1211n constitute low-pass filters similar to that in the second example of
 the third embodiment. In place of the comb filters constituting the
 crosstalk canceling signal generating filters (106a and 106b in FIG.
 1(b)), the delaying units 1211a, 1211b, 1211c to 1211f, and 1211g to
 1211j, the multipliers 1210x1, 1210x5, 1210x6, and 1210x10, and the adders
 1203e to 1203h are included in the sound image localization apparatus of
 the fourth embodiment.
 The comb filter included in the apparatus of the third embodiment shown in
 FIG. 6 performs the operation equivalent to calculating the average of
 data held in the delaying units 611c to 611f at a time so as to generate a
 crosstalk canceling signal at the time. Accordingly, based on the
 crosstalk canceling signal obtained at a certain time, the oldest among
 the data is reduced to one n-th, and one n-th of the newest data is added
 to the data. Thereby, a crosstalk canceling signal at a next time is
 obtained.
 In the sound image localization apparatus shown in FIG. 12, the delaying
 units 1211a and 1211b hold immediately previous signals. Among data held
 by the delaying units 1211c to 1211f and 1211g to 1211j, the oldest data,
 i.e., the data held in the delaying units 1211f and 1211j having maximum
 delay in FIG. 12, are multiplied by one n-th in the multipliers 1210x5 and
 1210x10, and the results are subtracted from the immediately previous
 signals by the adders 1203g and 1203h, respectively. Among the data held
 by the delaying units, the newest data, i.e., the data held in the
 delaying units 1211c and 1211g having minimum delay in FIG. 12, are
 multiplied by one n-th in the multipliers 1210x1 and 1210x6, and the
 results are added to the results of the subtraction by the adders 1203e
 and 1203f. The results of the addition are crosstalk canceling signals
 similar to that is obtained from the operation of the comb filter. The
 generated signals are held by the delaying units 1211a and 1211b to
 generate signals at a next time.
 In the sound image localization apparatus of the fourth embodiment, the
 data held in the delaying units 1211c to 1211f and 1211g to 1211j are
 accessed only when the oldest data are taken and when the newest data are
 written. Since the delaying unit included in the comb filter of the third
 embodiment is frequently accessed, a high-speed memory is required. In
 contrast, a relatively low-speed memory can be employed for the delaying
 unit included in the fourth embodiment. The amounts of multiplication and
 addition are further reduced in the fourth embodiment than in the third
 embodiment. Thus, the sound image localization apparatus in accordance
 with the fourth embodiment solves the access time problem of a memory,
 i.e., (C) of the second problem, and the processing speed problem, i.e.,
 the third problem.
 As explained above, the sound image localization apparatus of the fourth
 embodiment includes delay buffers (the delaying units 1211c to 1211f and
 1211g to 1211j in FIG. 12) and accumulation registers (the delaying units
 1211a and 1211b in FIG. 12) as filters for crosstalk cancellation in place
 of the comb filter. Thereby, the incidence of access to a memory, and the
 loads of addition and multiplication are reduced. As a result, in a
 computer system implementing the sound image localization apparatus, even
 when the amount of a high-speed memory and the processing speed of a
 processor are limited, a high level of sound image localization is
 possible.
 Similar to the second embodiment, the outline of the structure in the
 fourth embodiment is the same feedback type sound image localization
 apparatus as shown in FIGS. 1(a) and 1(b). However, the feedforward type
 apparatus shown in FIG. 18(b) is possible, and a circuit substituting the
 comb filter can be employed in the apparatus of the second embodiment
 shown in FIG. 2(b).
 Embodiment 5
 A sound image localization apparatus in accordance with a fifth embodiment
 of this invention can localize a sound image by switching the apparatus to
 feedforward or feedback.
 FIG. 13 is a diagram showing a structure of a first example of the sound
 image localization apparatus of the fifth embodiment. As shown in the
 figure, the sound image localization apparatus comprises the apparatus
 shown in FIGS. 1(a) and 1(b)and, further, adders 1303c and 1303d, and
 switches 1318a and 1318b.
 FIG. 13 shows a case where the switches 1318a and 1318b both turn to
 feedback (an FB side in the figure). In this situation, crosstalk
 canceling signals generated by crosstalk canceling signal generating
 filters 1306a and 1306b are input to the adders 1303a and 1303b. That is,
 the crosstalk canceling signal is output to the input side, so the
 apparatus is a feedback type, and is equivalent to the apparatus shown in
 FIGS. 1(a) and 1(b). In this case, the apparatus of the fifth embodiment
 operates in a similar way to the apparatus of the first embodiment.
 As opposed to this, when the switches 1318a and 1318b both turn to
 feedforward (an FF side in the figure), crosstalk canceling signals
 generated by crosstalk canceling signal generating filters 1306a and 1306b
 are input to the adders 1303c and 1303d. That is, the crosstalk canceling
 signal is output to the output side, so the apparatus is a feedforward
 type, and equivalent to the apparatus shown in FIG. 18(b). In this case,
 the apparatus of the fifth embodiment operates in a similar way to the
 apparatus in the prior art.
 The sound image localization apparatus of the first embodiment, which is a
 feedback type, improves the reproducibility of the low-frequency component
 compared with the feedforward type apparatus. However, when a loudspeaker
 included in an acoustic system using the sound image localization
 apparatus is small in diameter, the large energy of the low-frequency
 component causes sound distortion. To improve this point, it might be
 considered to use a filter cutting off the low-frequency component.
 However, the additional filter leads to an increase in circuit scale and
 cost.
 As opposed to this, the feedforward type apparatus has high-pass frequency
 characteristics which cut off the low-frequency component, and is suited
 to that system. Accordingly, the sound image localization apparatus of the
 fifth embodiment switches a feedback or feedforward type apparatus by the
 switches, so that when a loudspeaker with a large diameter is used, the
 apparatus operates as a feedback circuit so that good sound quality can be
 reproduced, while when a loudspeaker with a small diameter is used, the
 apparatus operates as a feedforward circuit so as to prevent sound
 distortion.
 Thus, the sound image localization apparatus of the fifth embodiment
 includes the switches 1318a and 1318b, thereby becoming suited to an
 acoustic system, to which the apparatus is applied, by switching feedback
 and feedforward.
 FIG. 14 is a diagram showing a structure of a second example of the sound
 image localization apparatus of the fifth embodiment. FIG. 15 is a diagram
 showing a structure of a third example of the sound image localization
 apparatus of the fifth embodiment. As shown in FIG. 14, the second example
 of the apparatus is the apparatus according to the second embodiment that
 crosstalk cancellation is performed on the input side, and further that
 switches are added. The third example of the apparatus shown in FIG. 15
 comprises the feedback type apparatus in FIGS. 1(a) and 1(b) and, further,
 switches, as the first example does. While in the first example the
 switches are disposed at the rear of the crosstalk canceling signal
 generating filter, i.e., on the output side, in the third example the
 switches are disposed in front of the filter, i.e., on the input side. The
 second and third examples of the sound image localization apparatus shown
 in FIGS. 14 and 15 can be suited to an acoustic system by switching
 feedback and feedforward.
 Embodiment 6
 A sound image localization apparatus in accordance with a sixth embodiment
 has capability of changing an initial delay in generating a crosstalk
 canceling signal.
 FIG. 16 is a diagram showing a structure of the sound image localization of
 the sixth embodiment. As shown in the figure, the sound image localization
 of the sixth embodiment is such that delaying units 1611a and 1611d and
 switches 1616a and 1618b are added to the feedback type apparatus shown in
 FIGS. 1(a) and 1(b).
 In the situation shown in FIG. 16, the switches 1618a and 1618b are set in
 a way that the crosstalk canceling signal generating filters 1606a and
 1606b output generated signals to the adders 1603b and 1603a without
 passing the signals through the delay units. In this situation, the sound
 image localization of the sixth embodiment is equivalent to the apparatus
 shown in FIGS. 1(a) and 1(b). The sound image localization apparatus of
 the sixth embodiment with this setting operates in a similar way to that
 described in the first embodiment.
 The sound image localization apparatus can use delayed crosstalk canceling
 signals held in the delaying units 1611b and 1611d, or delayed crosstalk
 canceling signals held in the delaying units 1611a and 1611c, depending on
 the setting of the switches 1618a and 1618b, respectively. The sound image
 localization apparatus of the sixth embodiment with this setting operates
 in a similar way to that described in the first embodiment, except that
 the delayed crosstalk canceling signal is used for crosstalk cancellation.
 In calculation by the crosstalk canceling signal generating filter, the
 input signal is multiplied by the coefficient shown in the equation 7-3,
 representing the ratio of the head related transfer functions SC and SM
 shown in FIG. 19(b). As is apparent from FIG. 19(b), as the crosstalk path
 is longer than the main path, there occurs a difference in the times of
 arrivals of sound signals from two loudspeakers. When the angle of the two
 loudspeakers is small, the difference in the arrival time is small. When
 the angle is large, the difference in the arrival time is large. This must
 be taken into account for sound image localization. In the crosstalk
 canceling signal generating filter, the arrival time difference is
 equivalent to the amount of an initial delay. Therefore, in an acoustic
 system using a sound image localization apparatus, when the fixed amount
 of an initial delay is used, if the positions of setting up the
 loudspeakers are changed, crosstalk cancellation is not possibly
 satisfactory.
 In the crosstalk canceling signal generating filter, in cases except for
 initial delay, the frequency characteristics do not change to a large
 extent if the angle of two loudspeakers is around 30 to 60 degrees. The
 change in the angle can be coped with by switching initial delays. The
 sound image localization apparatus of the sixth embodiment can change the
 amount of an initial delay in a step-by-step manner by setting of the
 switches.
 As described above, the sound image localization apparatus in accordance
 with the sixth embodiment further includes the delaying units 1611a to
 1611d and the switches 1618a and 1618b, thereby performing a high level of
 sound image localization by coping with a case where the angle of two
 loudspeakers are changed in an acoustic system to which the apparatus is
 applied.
 Embodiment 7
 A sound image localization apparatus in accordance with a seventh
 embodiment changes a crosstalk canceling signal generating filter.
 FIG. 17 is a block diagram showing a structure of the sound image
 localization apparatus of the seventh embodiment. As shown in the figure,
 the sound image localization apparatus comprises main-path filters 1707a
 and 1707b, crosstalk-path filters 1708a and 1708b, adders 1703a to 1703f,
 crosstalk canceling signal generating filters 1706a and 1706b, delaying
 units 1711a to 1711d, multipliers 1710x1 to 1710x4, inverting circuits
 1731a and 1731b, and switches 1718a to 1718f. The apparatus receives input
 sound signals through input terminals 1704a to 1704d, and outputs
 processed signals through output terminals 1705a and 1705b.
 The delaying units 1711a and 1711b, the multipliers 1710x1 and 1710x2, and
 the adder 1703c constitute a first FIR filter having two taps. The
 delaying units 1711c and 1711d, the multipliers 1710x3 and 1710x4, and the
 adder 1703d constitute a second FIR filter having two taps. Either filter
 functions as a crosstalk canceling signal generating filter. The switches
 1718a to 1718f are switched depending on the distance between two
 loudspeakers of an acoustic system using the sound image localization
 apparatus.
 The main-path filters 1707a and 1707b, the crosstalk-path filters 1708a and
 1708b, the adders 1703a to 1703d, and the crosstalk canceling signal
 generating filters 1706a and 1706b are similar to those of the feedback
 type sound image localization apparatus shown in FIG. 1(a) and 1(b).
 The operation of the sound image localization apparatus of the seventh
 embodiment will be described as to when the distance between two
 loudspeakers is wide or narrow.
 At first, when the distance between two loudspeakers is wide, the switches
 1718a, and 1718b, 1718e, and 1718f are set to respective W sides, while
 the switches 1718c and 1718d are set to be released. This is the situation
 shown in the figure. In this case, sound signals input through the input
 terminals 1704c and 1704d are output to the output terminals 1705a and
 1706b, passing through the sound image localization apparatus of the
 seventh embodiment.
 Signals input through the input terminals 1704a and 1704b are subjected to
 directional localization, and then, input through the switches 1718a and
 1718b to the crosstalk canceling signal generating filters 1706a and
 1706b. Thereafter, signals output from the first and second FIR filters
 each having two taps are not used because the switches 1718c and 1718d are
 released. Therefore, the operation of the apparatus is equivalent to that
 of the feedback type sound image localization apparatus shown in FIGS.
 1(a) and 1(b).
 As opposed to this, when the distance between the two loudspeakers is
 narrow, the switches 1718a, 1718b, 1718e, and 1718f are set to N sides,
 while the switches 1718c and 1718d are closed. Thus, signals after
 subjected to directional localization are processed by the first and
 second FIR filters each having two taps, and then, input through the
 switches 1718c and 1718d to the adders 1703a and 1703b. That is, the first
 and second FIR filters are used for crosstalk cancellation.
 On the other hand, the phases of sound signals input through the input
 terminals 1704c and 1704d are inverted by the inverting circuits 1731a and
 1731b, and then, input through the switches 1718a and 1718b to the filters
 1706a and 1706b. The filters 1706a and 1706b generate signals based on the
 phase inverted signals, and output the generated signals to the adders
 1703a and 1703b.
 In this case, the channels to the adders 1703a and 1703b function as main
 paths due to the switches 1718e and 1718f, while the filters 1706a and
 1706b generate crosstalk canceling signals. This is effective processing
 when a sound image to be localized at an arbitrary position (at the side
 or the rear) coexist in a sound signal. When the distance between two
 loudspeakers is narrow, if a sound image to be localized at the front is
 extended further outward, stereophony increases.
 That is, in the apparatus of the seventh embodiment, a sound signal of the
 second image to be localized at the arbitrary position is input through
 the input terminals 1704a and 1704b, while sound a signal of the sound
 image to be localized at the front position is input through the input
 terminals 1704c and 1704d. When the distance between two loudspeakers is
 wide, the sound image to be localized at the front position is output as
 it is, while the sound image to be localized at the arbitrary position is
 subjected to crosstalk cancellation similar to that in the first
 embodiment. When the distance between the two loudspeakers is narrow, a
 crosstalk canceling signal is generated for the sound image to be
 localized at the front position to extend the sound image outward. On the
 other hand, for the sound image to be localized at the arbitrary position,
 the crosstalk canceling signal generating filter used for sound
 localization multiplies an input signal by the coefficient shown in the
 equation 7-3, representing the ratio of the head related transfer
 functions SC and SM shown in FIG. 19(b). Because the distance between the
 two loudspeakers is narrow, the ratio is small, so that it is possible to
 use a filter having a small number of taps. Therefore, the filter having
 two taps is used.
 As described above, the sound image localization apparatus of the seventh
 embodiment comprises the conventional feedback type sound image
 localization apparatus and, further, the FIR filters with two taps
 comprising the delaying units 1711a to 1711d, the multipliers 1710x1 to
 1710x4, and the adders 1703c and 1703d, the switches 1718a to 1718d, and
 the inverting circuits 1731a and 1731b, whereby when the distance between
 two loudspeakers is wide, the feedback sound localization similar to that
 in the first embodiment is performed, while when the distance between two
 loudspeakers is narrow, the outward extension of a sound image to be
 localized at the front is performed as well as the feedback sound
 localization.
 Note that although the apparatus of the seventh embodiment is based on the
 feedback type sound image localization apparatus shown in FIGS. 1(a) and
 1(b), the apparatus of the seventh embodiment can be based on the
 feedforward type apparatus shown in FIG. 18(b) or the apparatus of the
 second embodiment shown in FIG. 2(b).