Apparatus for localization of sound image

[0002] 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.

[0003] 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] 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.

[0008] 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.

[0009] 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

[0010] yL1=TM·uL+TC·uR (1-1) and

[0011] yR1=TC·uL+TM·uR (1-2).

[0012] On the other hand, in a system shown in FIG. 19(b), left and right loudspeakers 1901aand 1901bradiate 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

[0013] yL2=SM·xL+SC·xR (2-1) and

[0014] yR2=SC·xL+SM·xR (2-2).

[0015] In this system, to localize the positions of the sound images shown in FIG. 19(a) using acoustics output from the loudspeakers 1901aand 1901b, the following equations must be satisfied,

[0016] yL1=yL2 (3-1) and

[0017] yR1=yR2 (3-2).

[0018] 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,

[0019] TM·uL+TC·uR=SM·xL+SC·xR (4-1) and

[0020] TC·uL+TM·uR=SC·xL+SM·xR (4-2).

[0021] The solution to xL and xR is obtained from the equations 4-1 and 4-2. If assumed that, the gain being represented by ═*═,

[0022] ═(SC/SM)²═<<1   (5),

[0023] xL and xR are approximated by

[0024] xL˜(FM+FC·FX)·uL+(FC+FM·FX)·uR (6-1) and

[0025] xR˜(FC+FM·FX)·uL+(FM+FC·FX)·uR (6-2),

[0026] where FM=TM/SM (7-1),

[0027] FC=TC/SM (7-2), and

[0028] FX=−SC/SM (7-3).

[0029] 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 1802aand 1802b, and adders 1803aand 1803b. Sound signals are input through input terminals 1804aand 1804b. Signals resulting from subjecting the input sound signals to signal processing are output through output terminals 1805aand 1905b.

[0030] The direction localizing means 1802aand 1802bprocess the sound signals input through the input terminals 1804aand 1804bto generate signals indicating the directions of sound image positions, respectively. The adders 1803aand 1803badd input signals. The crosstalk canceling means 1801 removes a crosstalk component of an input signal.

[0031] 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 1806aand 1806b, and adders 1803cand 1803d. The direction localizing means 1802aand 1802bshown in FIG. 18(a) comprise main-path filters 1807aand 1807b, and crosstalk-path filters 1808aand 1808b, respectively. The combination of the main-path filter and the crosstalk-path filter is sometimes called a direction localizing filter.

[0032] 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.

[0033] Left and right input sound signals are input through the input terminals 1804aand 1804b, respectively. The first input sound signal input through the input terminal 1804ais input to the main-path filter 1807aand the crosstalk-path filter 1808a. The main-path filter 1807amultiplies the input signal by the coefficient shown in the equation 7-1. The crosstalk-path filter 1808amultiplies the input signal by the coefficient shown in the equation 7-2. The outputs of the main-path filter 1807aand the crosstalk-path filter 1808aare input to the adders 1803aand 1803b, respectively.

[0034] Similarly, the second input sound signal input through the input terminal 1804bis input to the main-path filter 1807band 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 1807band the crosstalk-path filter 1808bare input to the adders 1803band 1803a, respectively.

[0035] The adders 1803aand 1803beach add input signals. The adder 1803aoutputs a result of the addition to the adder 1803cand the crosstalk canceling signal generating filter 1806a. The crosstalk canceling signal generating filter 1806amultiplies 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.

[0036] Similarly, the adder 1803boutputs a result of the addition to the adder 1803dand the crosstalk canceling signal generating filter 1806b. The crosstalk canceling signal generating filter 1806bmultiplies 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.

[0037] The adders 1803cand 1803deach add results of addition by the adders 1803aand 1803bto 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 1805aand 1805b, respectively.

[0038] 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 1803don the side having the output terminal 1805b). This structure is called feedforward.

[0039] 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.

[0040] 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.

[0041] 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

[0042] (A) constraint on the amount of memory for storage of sound signal data,

[0043] (B) constraint on the amount of memory for storage of coefficients of a filter, and

[0044] (C) constraint on accessing time of a memory.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] It is an object of the present invention to provide a sound image localization apparatus achieving high sound quality by improving low-frequency characteristics.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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:

[0057] direction localizing means for localizing the direction of a virtual sound source position; and

[0058] 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.

[0059] As a result, the apparatus performs feedback processing by outputting a crosstalk canceling signal to the input side.

[0060] According to a second aspect of this invention, there is provided the sound image localization apparatus of the first aspect wherein

[0061] the crosstalk canceling means perform crosstalk cancellation to a signal generated by directional localization of the direction localizing means.

[0062] As a result, the apparatus performs feedback processing by outputting a crosstalk canceling signal to the input side.

[0063] According to a third aspect of this invention, there is provided the sound image localization apparatus of the first aspect wherein

[0064] the direction localizing means perform directional localization to a signal generated by crosstalk cancellation of the crosstalk canceling means.

[0065] 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.

[0066] According to a fourth aspect of this invention, there is provided the sound image localization apparatus of the third aspect wherein

[0067] 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;

[0068] 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.

[0069] As a result, a crosstalk canceling generating filter shares an input with a main-path filter and a crosstalk-path filter.

[0070] According to a fifth aspect of this invention, there is provided the sound image localization apparatus of the first aspect wherein

[0071] the crosstalk canceling means use a comb filter to generate the crosstalk canceling signal.

[0072] 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.

[0073] According to a sixth aspect of this invention, there is provided the sound image localization apparatus of the fifth aspect wherein

[0074] the apparatus further comprises a low-pass filter processing a signal input to or output from the crosstalk canceling means.

[0075] As a result, the apparatus performs crosstalk cancellation to a signal from which a high-frequency component is removed.

[0076] According to a seventh aspect of this invention, there is provided the sound image localization apparatus of the first aspect wherein

[0077] 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.

[0078] 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.

[0079] According to an eighth aspect of this invention, there is provided the sound image localization apparatus of the seventh aspect wherein

[0080] the apparatus further comprises a low-pass filter processing a signal input to or output from the crosstalk canceling means.

[0081] As a result, the apparatus performs crosstalk cancellation to a signal from which a high-frequency component is removed.

[0082] According to a ninth aspect of this invention, there is provided the sound image localization apparatus of the first aspect wherein

[0083] 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.

[0084] As a result, the apparatus switches feedback processing and feedforward processing.

[0085] According to a tenth aspect of this invention, there is provided the sound image localization apparatus of the first aspect wherein

[0086] 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.

[0087] As a result, the apparatus performs crosstalk cancellation by changing the amount of an initial delay.

[0088] According to an eleventh aspect of this invention, there is provided the sound image localization apparatus of the first aspect wherein

[0089] 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;

[0090] 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.

[0091] 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.

[0092] FIGS. 1(a) and 1(b) are block diagrams showing structures of a sound image localization apparatus in accordance with a first embodiment of this invention.

[0093] FIGS. 2(a) and 2(b) are block diagrams showing structures of a sound image localization apparatus in accordance with a second embodiment of this invention.

[0094] FIG. 3 is a diagram showing an example of a structure of a filter included in the sound image localization apparatus of the second embodiment.

[0095] FIG. 4 is a diagram showing an example of a structure of a filter included in the sound image localization apparatus of the second embodiment.

[0096] FIG. 5 is a block diagram showing a structure of an application example of the sound image localization apparatus of the second embodiment.

[0097] FIG. 6 is a block diagram showing a structure of a sound image localization apparatus in accordance with a third embodiment of this invention.

[0098] FIG. 7 is a block diagram showing a structure of an application example of the sound image localization apparatus of the third embodiment.

[0099] FIGS. 8(a) and 8(b) are graphs showing frequency characteristics of a filter used in the third embodiment to explain how the filter works.

[0100] FIG. 9 is a block diagram showing a structure of an application example of the sound image localization apparatus of the third embodiment.

[0101] FIG. 10 is a block diagram showing a structure of an application example of the sound image localization apparatus of the third embodiment.

[0102] FIG. 11 is a block diagram showing a structure of an application example of the sound image localization apparatus of the third embodiment.

[0103] FIG. 12 is a block diagram showing a structure of a sound image localization apparatus in accordance with a fourth embodiment of this invention.

[0104] FIG. 13 is a block diagram showing a structure of a sound image localization apparatus in accordance with a fifth embodiment of this invention.

[0105] FIG. 14 is a block diagram showing a structure of an application example of the sound image localization apparatus of the fifth embodiment.

[0106] FIG. 15 is a block diagram showing a structure of an application example of the sound image localization apparatus of the fifth embodiment.

[0107] FIG. 16 is a block diagram showing a structure of a sound image localization apparatus in accordance which a sixth embodiment of this invention.

[0108] FIG. 17 is a block diagram showing a structure of a sound image localization apparatus in accordance with a seventh embodiment of this invention.

[0109] FIGS. 18(a) and 18(b) are block diagrams showing structures of a prior art sound image localization apparatus.

[0110] FIGS. 19(a) and 19(b) are diagrams for explaining sound image localization.

[0111] A sound image localization apparatus in accordance with a first embodiment of this invention improves low-frequency characteristics by feedback crosstalk cancellation.

[0112] 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 102aand 102b, and adders 103aand 103b. The apparatus receives input sound signals through input terminals 104aand 104b, and outputs signals resulting from signal processing through output terminals 105aand 105b.

[0113] The crosstalk canceling means 101 removes a crosstalk component from an input signal. The direction localizing means 102aand 102bprocess the input sound signals input through the input terminals 104aand 104bto produce signals indicating the directions of positions of sound images. The adders 103aand 103badd input signals.

[0114] 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.

[0115] xL=FM·uL+FC·uR+FX·xR  (8-1)

[0116] and

[0117] xR=FC·uL+FM·uR+FX·xL  (8-2)

[0118] 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.

[0119] The sound image localization apparatus of the first embodiment performs signal processing according to the equations 8-1 and 8-2.

[0120] 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 106aand 106b,and adders 103aand 103b.The direction localizing means 102aand 102bin FIG. 1(a) comprise main-path filters 107aand 107b,and crosstalk-path filters 108aand 108b,respectively. The adders 103aand 103bare the same as those in FIG. 1(a), and also part of the crosstalk canceling means 101.

[0121] 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.

[0122] Left and right input sound signals are input through the input terminals 104aand 104b,respectively. The first input sound signal input through the input terminal 104ais input to the main-path filter 107aand the crosstalk-path filter 108a.The main-path filter 107amultiplies the input signal by the coefficient represented by the equation 7-1, and outputs the result to the adder 103a.The crosstalk-path filter 108amultiplies 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 104bis input to the main-path filter 107band 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 103band 103a,respectively.

[0123] The adders 103aand 103beach add the input signals. The adder 103aoutputs a result of the addition to the crosstalk canceling signal generating filter 106a.The crosstalk canceling signal generating filter 106amultiplies 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 103boutputs a result of the addition to the crosstalk canceling signal generating filter 106b.The crosstalk canceling signal generating filter 106bmultiplies 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.

[0124] The adders 103aand 103badd 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 105aand 105b.

[0125] 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 106aand 106bare output nearer the input end of the apparatus than the crosstalk canceling signal generating filters (the adders 103aand 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.

[0126] 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.

[0127] 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 202aand 202b,adders 203aand 203b.The apparatus receives input sound signals through input terminals 204aand 204b,subjects the input signals to signal processing, and outputs the resulting signals through output terminals 205aand 205b.

[0128] The crosstalk canceling means 201 removes crosstalk components from the input signals input through the input terminals 204aand 204b.The direction localizing means 202aand 202bprocess input sound signals to produce signals indicating the directions of sound images. The adders 203aand 203badd input signals.

[0129] 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

[0130] xL=FM·vL+FC·vR  (9-1)

[0131] and

[0132] xR=FC·vL+FM·vR  (9-2).

[0133] The equation 9-1 is substituted to the equation 8-1, and 9-2 is substituted to 8-2, and then

[0134] FM·vL+FC·vR=FM·uL+FC·uR+FX·(FC·vL+FM·vR)  (10-1)

[0135] and

[0136] FC·vL+FM·vR=FC·uL+FM·uR+FX·(FM·vL+FC·vR)  (10-2)

[0137] are obtained. From 10-1 and 10-2, FM and FC are eliminated, and then

[0138] vL=uL+FX·vR  (11-1)

[0139] and

[0140] vR=uR+FX·vL  (11-2)

[0141] are obtained.

[0142] 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 202aand 202bon the output side.

[0143] 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 206aand 206b,and adders 203cand 203din FIG. 2(b). The direction localizing means 202aand 202bshown in FIG. 2(b) comprise main-path filters 207aand 207b,and crosstalk-path filters 208aand 208bin FIG. 2(b), respectively. An explanation will be given of the operation of the first example of the sound image localization apparatus.

[0144] Left and right input sound signals uL and uR are input through input terminals 204aand 204b.In FIG. 2(b), the input sound signal uL input through the input terminal 204ais input to the adder 203c.The second input sound signal uR input through the input terminal 204bis input to the adder 203d.Immediately after the sound image localization apparatus starts processing, the crosstalk canceling signal generating filters 206aand 206bdon't generate any signals to be output to the adders 203cand 203d,so the adders 203cand 203doutput input signals uL and uR as they are. The signals uL and uR are input to the crosstalk canceling signal generating filters 206aand 206bas signals vL and vR, respectively.

[0145] The crosstalk canceling signal generating filter 206amultiplies 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 206bperforms a similar processing to produce a crosstalk canceling signal, and outputs it to the adder 203c.

[0146] The adder 203cadds 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 207aand the crosstalk-path filter 208a.In a similar manner, the adder 203dgenerates the signal vR represented by 11-2, which is input to the main-path filter 207band the crosstalk-path filter 208b.

[0147] The main-path filter 207amultiplies the input signal by the coefficient represented by the equation 7-1, and outputs the result to the adder 203a.The crosstalk-path filter 208amultiplies 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 207ais represented by the first term on the right side of the equation 9-1. The output of the crosstalk-path filter 208ais represented by the second term on the right side of the equation 9-2.

[0148] Similarly, the adder 203dadds 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 207band 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 207band the crosstalk-path filter 208bare input to the adders 203band 203a,respectively. The output of the main-path filter 207bis represented by the first term on the right side of the equation 9-2. The output of the crosstalk-path filter 208ais represented by the second term on the right side of the equation 9-1.

[0149] The adders 203aand 203beach add input signals, and output results of the addition through the output terminals 205aand 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.

[0150] 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.

[0151] 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.

[0152] 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 206aand 206b,and the direction localizing filters 207a,207b,208a,and 208bare 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.

[0153] In FIG. 3, the crosstalk canceling signal generating filter 206aincluded in the first example (FIG. 2(b)) of the sound image localization apparatus, comprises delaying units 211aand 211cto 211f,multiplier 210x1 to 210x5, and an adder 203i.The crosstalk canceling signal generating filter 206bcomprises delaying units 211band 211gto 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 211cto 211f,show that the number of multipliers or delaying units is variable.

[0154] The main-path filter 207acomprises delaying units 211cto 211f,multipliers 210m1 to 210m5, and an adder 203e.The main-path filter 207bcomprises delaying units 211gto 211j,multipliers 210m6 to 210m10, and an adder 203f.The crosstalk-path filter 208acomprises delaying units 211cto 211fand 211nto 211p,multipliers 210c1 to 210c5, and an adder 203g.The crosstalk-path filter 208bcomprises delaying units 211gto 211jand 211kto 211m,multipliers 210c6 to 210c10, and an adder 203h.

[0155] Multipliers 210a1 and 210a2 function as attenuators to prevent overflow in executing fixed point calculation. Delaying units 211kto 211pare employed to produce the time difference between both cars.

[0156] As the filters in FIG. 3 include the delaying units 211cto 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.

[0157] FIG. 4 shows the example using IIR filters. In this example, a crosstalk canceling signal generating filter comprises IIR filter FXIs 212aand 212b.A main-path filter comprises IIR filter FMIs 213aand 213b.A crosstalk-path filter comprises IIR filter FCIs 214aand 214b.Those IIR filters are concatenated with the FIR filters shown in FIG. 3.

[0158] 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

[0159] FM=FMF·FMI  (12-1),

[0160] FC=FCF·FCI  (12-2),

[0161] and

[0162] FX=FXF·FXI  (12-3).

[0163] 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.

[0164] 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 203ato 203d,crosstalk canceling signal generating filters 206aand 206b,main-path filters 207aand 207b,crosstalk-path filters 208aand 208b,high-frequency main-path filters 217aand 217b,subsampling circuits 215aand 215b,and band compositing circuits 216aand 216b.As in the first example shown in FIG. 2(b), input sound signals are input through the input terminals 204aand 204b,and subjected to signal processing, and the resulting signals are output through the output terminals 205aand 205b.

[0165] The subsampling circuits 215aand 215bsubject input signals to prescribed subsampling to produce a low-frequency component and a high-frequency component. The band compositing circuits 216aand 216bsubject input signals to prescribed composition to produce composite signals. The high-frequency main-path filters 217aand 217boperate in a similar way to the main-path filters 207aand 207b.The adders 203ato 203d,the crosstalk canceling signal generating filters 206aand 206b,main-path filters 207aand 207b,and the crosstalk-path filters 208aand 208bare similar to those in the first example.

[0166] The operation of the second example of the sound image localization apparatus of the second embodiment will be described.

[0167] Left and right input sound signals are input through the input terminals 204aand 204b.The first input sound signal input through the input terminal 204ais input to the subsampling circuit 215a.The subsampling circuit 215asubsamples 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 215boperates in a similar way.

[0168] The high-frequency main-path filters 217aand 217bmultiply the input high-frequency components by the coefficient represented by the equation 7-1, and output the resulting signals to the band compositing circuits 216aand 216b,respectively.

[0169] 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 215aand 215b,respectively. The band compositing circuits 215aand 215bcomposite 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 205aand 205b,respectively.

[0170] 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.

[0171] 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 202aand 202bon the output side. Thereby, each filter included in the crosstalk canceling means 201 and the direction localizing means 202aand 202bshares 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.

[0172] A sound image localization apparatus in accordance with a third embodiment of this invention employs a comb filter.

[0173] 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 607aand 607b,crosstalk-path filters 608aand 608b,delaying units 611ato 611j,and multipliers 610x1 to 610x10. Input sound signals are input through input terminals 604aand 604b,and subjected to signal processing, and the resulting signals are output through output terminals 605aand 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.

[0174] In FIG. 6, the crosstalk canceling signal generating filter 106ashown in FIG. 1(b) comprises the delaying units 611a,611cto 611f,the multipliers 610x1 to 610x5, and the adder 603e.The crosstalk canceling signal generating filter 106bshown in FIG. 1(b) comprises the delaying units 611b,611gto 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).

[0175] 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.

[0176] 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.

[0177] 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 720aand 720b.The low-pass filter 720acomprises an adder 703c,multipliers 710f1 and 710f2, and a delaying unit 711a.The low-pass filter 720bcomprises an adder 703d,multipliers 710f3 and 710f4, and a delaying unit 711b.

[0178] As to the operation of the sound image localization apparatus, the high-frequency components of signals input to the crosstalk canceling signal generating filters 106aand 106bshown 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.

[0179] 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.

[0180] 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.

[0181] 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.

[0182] 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 1017aand 1017b,subsampling circuits 1015aand 1015b,and band compositing circuits 1016aand 1016b.These are similar to those shown in the second example of the second embodiment, i.e., the high-frequency main-path filters 217aand 217b,the subsampling circuits 215aand 215b,and the band compositing circuits 216aand 216b.The same with high-frequency main-path filters 1117aand 1117b,subsampling circuits 1115aand 1115b,and band compositing circuits 1116aand 1116b,shown in FIG. 11.

[0183] 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.

[0184] 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.

[0185] 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.

[0186] 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.

[0187] 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.

[0188] 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 1207aand 1207b,crosstalk-path filters 1208aand 1208b,delaying units 1211ato 1211j,and multipliers 1210f1 to 1210f4, 1210x1, and 1210x5, 1210x6, and 1210x10. Input sound signal are input through input terminals 1204aand 1204b,and subjected to signal processing, and the resulting signals are output through output terminals 1205aand 1205b.As in FIG. 3, dashed lines in the rows of the delaying units represent an arbitrary number of the delaying units.

[0189] 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 1211nconstitute 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 (106aand 106bin FIG. 1(b)), the delaying units 1211a,1211b,1211cto 1211f,and 1211gto 1211j,the multipliers 1210x1, 1210x5, 1210x6, and 1210x10, and the adders 1203eto 1203hare included in the sound image localization apparatus of the fourth embodiment.

[0190] 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 611cto 611fat 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.

[0191] In the sound image localization apparatus shown in FIG. 12, the delaying units 1211aand 1211bhold immediately previous signals. Among data held by the delaying units 1211cto 1211fand 1211gto 1211j,the oldest data, i.e., the data held in the delaying units 1211fand 1211jhaving 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 1203gand 1203h,respectively. Among the data held by the delaying units, the newest data, i.e., the data held in the delaying units 1211cand 1211ghaving 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 1203eand 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 1211aand 1211bto generate signals at a next time.

[0192] In the sound image localization apparatus of the fourth embodiment, the data held in the delaying units 1211cto 1211fand 1211gto 1211jare 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.

[0193] As explained above, the sound image localization apparatus of the fourth embodiment includes delay buffers (the delaying units 1211cto 1211fand 1211gto 1211jin FIG. 12) and accumulation registers (the delaying units 1211aand 1211bin 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.

[0194] 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).

[0195] 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.

[0196] 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 1303cand 1303d,and switches 1318aand 1318b.

[0197] FIG. 13 shows a case where the switches 1318aand 1318bboth turn to feedback (an FB side in the figure). In this situation, crosstalk canceling signals generated by crosstalk canceling signal generating filters 1306aand 1306bare input to the adders 1303aand 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.

[0198] As opposed to this, when the switches 1318aand 1318bboth turn to feedforward (an FF side in the figure), crosstalk canceling signals generated by crosstalk canceling signal generating filters 1306aand 1306bare input to the adders 1303cand 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.

[0199] 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.

[0200] 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.

[0201] Thus, the sound image localization apparatus of the fifth embodiment includes the switches 1318aand 1318b,thereby becoming suited to an acoustic system, to which the apparatus is applied, by switching feedback and feedforward.

[0202] 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.

[0203] A sound image localization apparatus in accordance with a sixth embodiment has capability of changing an initial delay in generating a crosstalk canceling signal.

[0204] 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 1611aand 1611dand switches 1616aand 1618bare added to the feedback type apparatus shown in FIGS. 1(a) and 1(b).

[0205] In the situation shown in FIG. 16, the switches 1618aand 1618bare set in a way that the crosstalk canceling signal generating filters 1606aand 1606boutput generated signals to the adders 1603band 1603awithout 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.

[0206] The sound image localization apparatus can use delayed crosstalk canceling signals held in the delaying units 1611band 1611d,or delayed crosstalk canceling signals held in the delaying units 1611aand 1611c,depending on the setting of the switches 1618aand 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.

[0207] 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.

[0208] 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.

[0209] As described above, the sound image localization apparatus in accordance with the sixth embodiment further includes the delaying units 1611ato 1611dand the switches 1618aand 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.

[0210] A sound image localization apparatus in accordance with a seventh embodiment changes a crosstalk canceling signal generating filter.

[0211] 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 1707aand 1707b,crosstalk-path filters 1708aand 1708b,adders 1703ato 1703f,crosstalk canceling signal generating filters 1706aand 1706b,delaying units 1711ato 1711d,multipliers 1710x1 to 1710x4, inverting circuits 1731aand 1731b,and switches 1718ato 1718f.The apparatus receives input sound signals through input terminals 1704ato 1704d,and outputs processed signals through output terminals 1705aand 1705b.

[0212] The delaying units 1711aand 1711b,the multipliers 1710x1 and 1710x2, and the adder 1703cconstitute a first FIR filter having two taps. The delaying units 1711cand 1711d,the multipliers 1710x3 and 1710x4, and the adder 1703dconstitute a second FIR filter having two taps. Either filter functions as a crosstalk canceling signal generating filter. The switches 1718ato 1718fare switched depending on the distance between two loudspeakers of an acoustic system using the sound image localization apparatus.

[0213] The main-path filters 1707aand 1707b,the crosstalk-path filters 1708aand 1708b,the adders 1703ato 1703d,and the crosstalk canceling signal generating filters 1706aand 1706bare similar to those of the feedback type sound image localization apparatus shown in FIG. 1(a) and 1(b).

[0214] 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.

[0215] At first, when the distance between two loudspeakers is wide, the switches 1718a,and 1718b,1718e,and 1718fare set to respective W sides, while the switches 1718cand 1718dare set to be released. This is the situation shown in the figure. In this case, sound signals input through the input terminals 1704cand 1704dare output to the output terminals 1705aand 1706b,passing through the sound image localization apparatus of the seventh embodiment.

[0216] Signals input through the input terminals 1704aand 1704bare subjected to directional localization, and then, input through the switches 1718aand 1718bto the crosstalk canceling signal generating filters 1706aand 1706b.Thereafter, signals output from the first and second FIR filters each having two taps are not used because the switches 1718cand 1718dare 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).

[0217] As opposed to this, when the distance between the two loudspeakers is narrow, the switches 1718a,1718b,1718e,and 1718fare set to N sides, while the switches 1718cand 1718dare 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 1718cand 1718dto the adders 1703aand 1703b.That is, the first and second FIR filters are used for crosstalk cancellation.

[0218] On the other hand, the phases of sound signals input through the input terminals 1704cand 1704dare inverted by the inverting circuits 1731aand 1731b,and then, input through the switches 1718aand 1718bto the filters 1706aand 1706b.The filters 1706aand 1706bgenerate signals based on the phase inverted signals, and output the generated signals to the adders 1703aand 1703b.

[0219] In this case, the channels to the adders 1703aand 1703bfunction as main paths due to the switches 1718eand 1718f,while the filters 1706aand 1706bgenerate 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.

[0220] 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 1704aand 1704b,while sound a signal of the sound image to be localized at the front position is input through the input terminals 1704cand 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.

[0221] 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 1711ato 1711d,the multipliers 1710x1 to 1710x4, and the adders 1703cand 1703d,the switches 1718ato 1718d,and the inverting circuits 1731aand 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.

[0222] 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).