Patent Publication Number: US-7917236-B1

Title: Virtual sound source device and acoustic device comprising the same

Description:
TECHNICAL FIELD 
     This invention relates to a virtual sound source device for generating virtual sound source and a sound apparatus and a sound equipment using such a virtual sound source device, and more particularly to a virtual sound source reproducing device comprising first signal processing means for carrying out signal processing in accordance with an impulse response portion which can perceive or sense position of virtual sound source of impulse responses of transfer functions of audio signals from the virtual sound source until to ears, second signal processing means for carrying out signal processing in accordance with impulse response portion which can perceive or sense only distance of the virtual sound source, and response characteristic control means for detecting change or movement of positions of both ears to control and correct response characteristic of the first signal processing means, and a sound apparatus and a sound equipment using such a virtual sound source device. 
     BACKGROUND ART 
     As the sound source followed by image of cinema, etc., audio signals of plural channels are used. Such audio signal channels of plural channels are recorded on the assumption that they are respectively delivered to speakers disposed at both sides and the center of screen onto which image is projected and speakers disposed at the rear side or both lateral sides of listener, etc. so that they are reproduced. As stated above, by reproducing audio signals of plural channels by three-dimensionally disposed speakers, sound position followed by image and position of sound image actually heard can be in correspondence with each other. Thus, sound field having natural spreading of sound can be established. 
     Meanwhile, when listener attempts to appreciate (listen) sound from sound source consisting of audio signals of plural channels by using headphone device of the head portion mounting type, sound image by audio signals to both ears is localized within the head. As a result, position of the sound source and localized position of sound image are not in correspondence with each other. Thus, there results extremely unnatural sound image localization. Further, in reproduction using headphone device, localization positions of sound images of respective sound sources cannot be separately and independently reproduced. Even in the case where audio signals of left and right two channels are reproduced, the headphone device is adapted so that sound image is localized within the head unlike reproduction by speaker. As a result, sound is heard from one portion within the head. Thus, localization position of sound image cannot be separated, and it is possible to only generate extremely unnatural sound field. 
     In order to eliminate such problems and to obtain sound image equivalent to the case where reproduction is carried out by speaker even in the case where listener listens to audio signals of plural channels by using headphone device, there are used headphone devices of the stereo out-of-head sound image localization type adapted to measure or calculate transfer functions or impulse responses from speakers provided so as to respectively reproduce audio signals of respective channels in advance until both ears of listener to convolute them onto audio signals from sound source by digital filter, etc. thereafter to listen to it by headphone device to thereby position (localize) sound image outside the head. 
     As the stereo out-of-head sound image localization type headphone device of this type, there is proposed headphone device constituted as shown in  FIG. 1 . This headphone device  8  localizes reproduction sound image by audio signal to the left ear and audio signal to the right ear at the outside of the head. 
     The operation of the stereo out-of-head sound image localization type headphone device  8  will now be described. 
     First, prior to the operation of the stereo out-of-head sound image localization type headphone device  8 , the case where audio signals are heard by two speakers installed or provided at the positions spaced from listener will be described. Audio signals are transmitted to left ear YL and right ear YR of listener M from sound source SL of the left side through paths having transfer functions of HLL, HLR, respectively. In addition, audio signals are transmitted to the left ear YL and the right year YR of listener from sound source SR of the right side through paths having transfer functions of HRL, HRR, respectively. 
     In order to reproduce the state where audio signals from left and right sound sources are reproduced by using two speakers by using headphone attached on the head portion shown in  FIG. 1 , audio signal Sal from sound source SL of the left side is caused to be through filter for realizing transfer function HLL so that audio signal Sbll for left ear is obtained and audio signal Sal is caused to be through filter for realizing transfer function HLR so that audio signal for right ear is obtained. In addition, audio signal Sar of sound source SR of the right side is caused to be through filter for realizing transfer function HRL so that audio signal Sbrl for left ear is obtained and audio signal Sar is caused to be through filter for realizing transfer function HRR so that audio signal Sbrr for right ear is obtained. 
     Then, left ear synthetic audio signal Sbl=(Sbll+Sbrl) and right ear synthetic signal Sbr=(Sblr+Sbrr) are obtained. Such an approach is employed to drive left and right headphone elements  6   a ,  6   b  of headphone  6  by these left and right synthetic audio signals Sbll, Sblr. Thus, listener can perceive or sense sound image as if sound source is disposed at sound sources SL and SR. 
     In this case, a more practical configuration of stereo out-of-head sound image localization type headphone device  8  conventionally proposed will now be described with reference to  FIG. 1 . This headphone device  8  comprises a first input terminal  1 L supplied with audio signal Sal, a second input terminal  1 R supplied with audio signal Sar, a second input terminal  1 R supplied with audio signal Sar, A/D converters  2 L,  2 R for respectively converting respective audio signals Sal, Sar into corresponding digital signals, signal processing circuits  3 L,  3 R for implementing filtering (processing) to the respective audio signals Sal, Sar converted into digital signals, adders  7 L,  7 R for adding outputs of respective two systems, D/A converters  4 L,  4 R for converting added outputs of 2 systems into analog signals, and amplifiers  5 L,  5 R for amplifying analog audio signals outputted from the respective D/A converters  4 L,  4 R to supply them to left and right headphone elements  6   a ,  6   b  of the headphone  6 . 
     In this example, one signal processing circuit  3 L is constituted by two digital filters  10 ,  11  as shown in  FIG. 3 , wherein one digital filter  10  carries out convolution of impulse response of transfer function HLL with respect to audio signal Sal inputted through an input terminal  12  to form a left ear audio signal Sbll to output it from an output terminal  13 , and the other digital filter  11  carries out convolution of impulse response of transfer function HLR with respect to audio signal Sal inputted through the input terminal  12  to form a right ear audio signal Sblr to output it from an output terminal  14 . 
     The other signal processing circuit  3 R is similarly constituted by two digital filters  10 , 11  as shown in  FIG. 3 , wherein one digital filter  10  carries out convolution of impulse response for realizing transfer function HRL with respect to an audio signal Sar inputted through the input terminal  12  to form a left ear audio signal Sbrl to output it from the output terminal  13 , and the other digital filter  11  carries out convolution of impulse response for realizing transfer function HRR with respect to an audio signal Sar inputted through the input terminal  12  to form a right ear audio signal Sbrr to output it from the output terminal  14 . 
     The above-mentioned impulse response has characteristic as shown in  FIG. 4 . To realize this, the respective digital filters  10 ,  11  are constituted by FIR type digital filter  15  as shown in  FIG. 5 , for example. This FIR type digital filter  15  comprises, as shown in  FIG. 5 , plural delay elements  16  cascade-connected having a predetermined delay quantity, plural coefficient multipliers  17  for multiplying inputted audio signal and audio signals delayed by respective delay elements  16  by coefficients for carrying out convolution of impulse response, and plural adders  18  for adding audio signals outputted from the respective coefficient multipliers  17 . 
     For example, delay element  16  of the first stage of the digital filter  10  ( 11 ) in the signal processing circuit  3 L delays, e.g., by one sampling period, audio signal Sal (or Sar) inputted through the input terminal  12 , and delay element  16  of the i (i=2, 3, . . . )-th stage delays, similarly by one sampling period, delayed audio signal outputted from the preceding stage (i−1-th stage) to deliver it to delay element  16  of the succeeding stage (i+1-th stage). The coefficient multipliers  17  of respective stages respectively multiply input audio signal Sal and audio signals delayed in sequence at delay elements  16  of respective stages by coefficients for carrying out convolution of impulse response to deliver them to adders  18  of corresponding stages. The adders  18  of respective stages add, to outputs of adders  18  of preceding stages, outputs of coefficient multipliers  17  of corresponding stages to deliver them to adders  18  of succeeding stages. Namely, the adder  18  of the final stage carries out convolution of impulse response of transfer function HLL (HLR) with respect to audio signal Sal (Sar) inputted through the input terminal  12  to form left ear audio signal Sbll (right ear audio signal Sblr) to output it through the output terminal  13  ( 14 ). 
     Similarly, the adder  18  of the final stage of the digital filter  10  ( 11 ) in the signal processing circuit  3 R carries out convolution of impulse response of transfer function HRL (HRR) with respect to audio signal Sal (Sar) inputted through the input terminal  12  to form left ear audio signal Sbrl (right ear audio signal Sbrr) to output it through the output terminal  13  ( 14 ). 
     In the case where the previously described two digital filters  10 ,  11  shown in  FIG. 3  are realized by FIR type digital filter, they are collectively represented as shown in  FIG. 6 . The FIR type digital filter  20  shown in  FIG. 6  is constituted as a single filter block by sharing cascade-connected plural delay elements  16  in obtaining outputs of two systems by using two FIR type digital filters  15  shown in  FIG. 5 . As stated above, as compared to the fact that two FIR type digital filters  15  as shown in  FIG. 5  are prepared, FIR type digital filter  20  is constituted as shown in  FIG. 6 . Thus, the number of delay elements is reduced to one half so that the circuit scale becomes compact and signal processing operation quantity is reduced. 
     The above-described left ear audio signals Sbll, Sbrl outputted from signal processing circuits  3 L,  3 R of the headphone device  8  shown in  FIG. 1  are added by one adder  7 L so that left ear synthetic audio signal Sbl is obtained, and the above-described right ear audio signals Sblr, Sbrr outputted from signal processing circuits  3 L,  3 R are added by one adder  7 L so that right ear synthetic audio signal Sbr is obtained. The left ear synthetic audio signal Sbl and the right ear synthetic audio signal Sbr obtained in this way are respectively converted into analog signals at D/A converters  4 L,  4 R. The left ear synthetic audio signal Sbl and the right ear synthetic audio signal Sbr converted into analog signals are respectively amplified by amplifiers  5 L,  5 R. The signals thus amplified are respectively delivered to left and right headphone elements ba,  6   b  of the headphone  6  so that they are reproduced. As the result of the fact that the left ear synthetic audio signal Sbl and the right ear synthetic audio signal Sbr are reproduced in this way, listener M who has attached the headphone  6  perceives (senses) as if left and right two sound sources SL, SR are actually exist as shown in  FIG. 2 , thus making it possible to respectively localize reproduction sound images by the left ear synthetic audio signal Sbl and the right ear synthetic audio signal Sbr at the outside of the head. 
     On the other hand, in reproduction of audio signals using speaker, there are also instances where arrangement of speakers within the room is restricted. As a result, there are cases where it is difficult to dispose a large number of speakers within listening room. In view of the above, there has been proposed an example where a lesser number of speakers, eg., two speakers are used to constitute a large number of reproduction sound sources around listener as virtual sound source. 
     The example where these two speakers are used to constitute a large number of virtual speaker sound sources will be described with reference to  FIGS. 7 and 8 . 
     First, the principle of speaker unit  30  shown in  FIG. 7  will be described with reference to  FIG. 8 . 
     In order to virtually reproduce sound source SO by using sound source SL and sound source SR, when it is assumed that transfer functions of audio signals from the sound source SL up to left ear YL and right ear TR of listener M are respectively HLL, HLR, transfer functions of audio signals from the sound source SR up to left ear YL, right ear YL of listener M are respectively HRL, HRR, and transfer functions of audio signals from sound source SO up to left ear YL, right ear YR of listener M are respectively HOL, HOR, the transfer (transmission) relationship between the sound source SL and the sound source SO is represented by the formula (1) described below and the transfer relationship between the sound source SR and the sound source SD is represented by the formula (2) described below.
 
 SL ={( HOL×HRR−HOR×HRL )/( HLL×HRR−HLR×HRL )}× SO   (1)
 
 SR ={( HOR×HLL−HOL×HLR )/( HLL×HRR−HLR×HRL )}× SO   (2)
 
     Accordingly, such an approach is employed to allow audio signal Sao of sound source SO to be through filter which realizes transfer function portion of the formula (1) to thereby obtain left ear synthetic audio signal Sbl, and to allow the audio signal Sao to be through filter which realizes transfer function portion of the formula (2) to thereby obtain right ear synthetic audio signal Sbr to drive two speakers disposed at positions of sound sources SL, SR by these left ear and right ear synthetic audio signals Sbl, Sbr, thereby making it possible to localize virtual sound source as if audio signal Sao is generated from the position of sound source SO. 
     Speaker unit  30  for reproducing virtual sound source SO as described above can localize sound image of input signal inputted from two speakers to both ears at an ordinary position as shown in  FIG. 7 . This speaker unit  30  comprises an input terminal  21  supplied with audio signal Sao, an A/D converter  22  for converting the audio signal Sao into digital signal, and a signal processing section (unit)  23  for implementing filtering (processing) to the audio signal Sao converted into digital signal. The signal processing unit  23  is constituted by the previously described two digital filters  10 ,  11  as shown in  FIG. 3 , wherein one digital filter  10  convolutes impulse response corresponding to transfer function portion of the above-described formula (1) with respect to audio signal Sao to form left ear synthetic audio signal Sbl, and the other digital filter  11  convolutes impulse response corresponding to transfer function portion of the above-described formula (2) with respect to audio signal Sao to form right ear synthetic audio signal Sbr. With respect to digital filters  10 ,  11  for realizing transfer function, e.g., the previously described FIR type digital filter  15  shown in  FIG. 5  or the previously described FIR type digital filter  20  shown in  FIG. 6  is used, thereby making it possible to reduce the circuit scale. 
     The left ear and right ear synthetic audio signals Sbl, Sbr are respectively converted into analog signals by D/A converters  24 L,  24 R, and the left and right ear synthetic audio signals Sbl, Sbr of analog signal are respectively amplified by amplifiers  25 L,  25 R and are delivered to a left speaker  26 L and a right speaker  26 R. The left and right speakers  26 L,  26 R are respectively disposed at positions of sound sources SL, SR with respect to listener M. 
     As stated above, reproduction sound image by audio signal Sao can be localized at position of virtual sound source SO. Further, with respect to a larger number of sound sources, it is sufficient to carry out the above-described processing by the number of sound sources. Since a larger number of virtual speaker sound sources can be constituted from a lesser number of speaker sound sources by this method, the number of speakers can be reduced. 
     In the above-described headphone device and speaker unit, it is necessary for obtaining sufficient distance feeling with respect to virtual sound source to reproduce impulse response from respective sound sources to both ears obtained by measurement within reverberation room. Since such impulse response is vast digital quantity having long reverberation time, there is the problem that in the case where such a system is constituted by digital filter, its operation (computing) quantity and the scale become extremely large. 
     Further, in the above-described stereo out-of-head sound image localization type headphone device, in the case where positions of both ears of listener are changed during listening of virtual sound source, transfer function from electro-acoustic transducer (headphone element) which is reproduction sound source to both ears is not changed. Accordingly, sound is heard always from the same direction with respect to both ears irrespective of movement of both ears of listener. Thus, listener suffers from unnatural feeling such that directions in which sounds are heard are the same although he moves the head. 
     In addition, in the speaker unit, since transfer function from acoustic transducer (speaker) which is reproduction sound source to both ears is changed by change of positions of both ears of listener, primary position of virtual sound source is localized at unsuitable position. This always gives feeling of disagreement to listener. 
     DISCLOSURE OF THE INVENTION 
     This invention has been proposed in view of circumstances described above, and its object is to provide a virtual sound source reproducing device, and a sound apparatus and a sound equipment using such a device, wherein, in a sound apparatus such as headphone unit or speaker unit, etc. and a sound equipment used in combination therewith, it is possible to localize sound image with sufficient feeling of distance at an arbitrary position while suppressing operation quantity of impulse response of the previously described transfer function so that position of virtual sound source is changed in correspondence with changes of positions of both ears of listener. 
     This invention proposed for the purpose of attaining such objected relates to a virtual sound source reproducing device for reproducing virtual sound source, wherein such an approach is employed to carry out signal processing in accordance with transfer function or impulse response until respective audio signals Sa generated from one sound source or more, e.g., speaker disposed within space reaches both ears so that there result respective audio signals Sb to generate respective audio signals Sb to synthesize these audio signals Sb to generate two kinds of synthetic audio signals for both ears to input these two kinds of synthetic audio signals to the both ears to perceive (sense) as if one sound source or more are disposed within space. 
     The virtual sound source reproducing device according to this invention comprises first signal processing means for forming portions of impulse response contributing to perception (sense) of respective positions of one virtual sound source or more of impulse responses corresponding to respective transfer functions of respective audio signals Sa produced from one sound source or more to carry out signal processing of respective audio signals Sa in accordance with the respective portions of impulse response thus formed to obtain a pair of initial response signals, and to delay inputted audio signal by time corresponding to time length of each impulse response portion to obtain delayed output signals, second signal processing means for carrying out signal processing of the delayed output signals in accordance with impulse response portion contributing to perception (sense) of only distance of one virtual sound source or more of impulse responses corresponding to respective transfer functions to obtain a pair of reflected response signal, and synthesizing means for adding the pair of initial response signals and the pair of reflection response signals with respect to respective both ears to form outputs to the both ears. 
     Further, the virtual sound source reproducing device according to this invention may comprise virtual sound source transfer characteristic correcting means for correcting transfer characteristic of the first signal processing means, wherein changes of positions of both ears also correspond to virtual sound source, the correcting means being operative in order to perceive (sense) as if positions of both ears are changed with respect to one sound source or more disposed within space. 
     Furthermore, the virtual sound source reproducing device, and sound apparatus and sound equipment using such a device according to this invention has a configuration described below. 
     Namely, the first signal processing means is of configuration in which there are provided FLR type digital filters every transfer functions corresponding to respective audio signals Sa in which plural delay elements having a predetermined delay quantity are cascade-connected to synthesize respective outputs of junctions of respective delay elements after undergone weighting, wherein FIR type digital filter corresponding to transfer function HL until one audio signal Sal is transferred (transmitted) to the left ear and FIR type digital filter corresponding to transfer function HR until one audio signal Sal is transferred (transmitted) to the right ear are provided with cascade-connected delay elements being as common element. In this first signal processing means, with respect to one audio signal Sal, there are obtained audio signal Sald in which this audio signal Sal is delayed, audio signal Sbll caused to undergo signal processing in accordance with transfer function HL and audio signal Sblr caused to undergo signal processing by transfer function HR. With respect to plural audio signals, there are obtained delayed synthetic audio signal obtained by synthesizing audio signals Sald with each other with respect to respective audio signals Sa, initial response signal for left ear obtained by synthesizing audio signals Sbll with each other with respect to respective audio signals Sa, and initial response signal for right ear obtained by synthesizing audio signals Sblr with each other with respect to respective audio signals Sa. 
     The second signal processing means is FIR type digital filter in which plural delay elements having a predetermined delay quantity are cascade-connected and respective outputs of junctions of respective delay elements are synthesized after undergone weighting, wherein, in the FIR type digital filter, FIR type digital filter supplied with delayed synthetic audio signal from the first signal processing means, and corresponding to transfer function for right ear and FIR type digital filter supplied with delayed synthetic audio signal and corresponding to transfer function for left ear are constituted with the cascade-connected delay elements being common element, thus to output reflected response signal for left ear and reflected response signal for right ear. 
     The virtual sound source transfer characteristic correcting means is composed of displacement speed detecting means in which, positions of both ears from which virtual sound source reproduced with positions of both ears with respect to position of one sound source or more disposed in space being as reference is caused to be initial state, thus to detect displacement speeds of both ears from the positions of the both ears in the initial state, displacement quantity calculating means for calculating position change quantities of the both ears from the initial state on the basis of output of the displacement speed detecting means, and response characteristic control means for correcting response characteristic of the first signal processing means with respect to respective audio signals Sa by output of the displacement quantity calculating means. 
     This response characteristic control means directly controls parameters constituting the first signal processing means to correct change of response characteristic. 
     Further, the response characteristic control means controls time difference adding sections and level difference adding sections separately provided for both ears in order to constitute the first signal processing means to correct change of response characteristic. 
     In the virtual sound source reproducing device according to this invention having configuration as described above carries out signal processing of respective audio signals in accordance with impulse response portion contributing to perception (sensing) of positions of the respective sound sources of impulse responses corresponding to transfer functions of respective audio signals from the respective sound sources to both ears to independently synthesize such audio signals for both ears to obtain a pair of reflected response signals and delayed output signals, and carries out signal processing of the delayed output signals in accordance with the portion of impulse response contributing to perception (sensing) of only distances of respective sound sources by the second signal processing means to obtain a pair of reflected response signals corresponding to respective both ears to add, with respect to respective both ears, a pair of initial response signals and a pair of reflected response signals by synthesizing means to deliver such signals to the both ears by acoustic transducer (conversion) element such as headphone element, etc., thereby making it possible to reproduce virtual sound sources with respect to respective sound sources with sufficient feeling of distance and feeling of direction. Further, there is employed such a configuration to carry out signal processing in accordance with the portion of impulse response contributing to perception (sensing) of only distances of the respective sound sources collectively with respect to respective audio signals by the second processing means, thereby making it possible to reduce the scale of the signal processing means. 
     In addition, positions of both ears in which virtual sound source reproduced with positions of both ears with respect to one sound source or more disposed within space being as reference are caused to be initial state, wherein such an approach is employed to detect, by displacement speed detecting means, displacement speed of both ears from the positions of the both ears placed in this initial state, and to calculate, by displacement quantity calculating means, position change (displacement) quantity of both ears from the initial state to correct change of response characteristic. Thus, it becomes possible to eliminate the state where sounds are heard from the same direction at all times with respect to both ears although both ears are moved with respect to the virtual sound source, and/or unnatural listening of reproduction sound that listener feels such that when both ears are moved, sound image is localized at unsuitable position entirely different from the primary position of virtual sound source. 
     Still further objects of this invention and more practical merits obtained by this invention will become more apparent from the explanation of the embodiment which will be given. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing outline of the configuration of stereo out-of-head sound image localization type headphone device. 
         FIG. 2  is an explanatory view for explaining transfer function of audio signal from sound source up to both ears. 
         FIG. 3  is a block diagram showing signal processing unit constituting the headphone device shown in  FIG. 1 . 
         FIG. 4  is an explanatory view for explaining impulse response of transfer function of audio signal from sound source up to both ears. 
         FIG. 5  is a diagram showing outline of the configuration of FIR type digital filter constituting signal processing unit of the headphone device shown in  FIG. 1 . 
         FIG. 6  is a diagram showing outline of the configuration of another FIR type digital filter constituting signal processing unit constituting the headphone device shown in  FIG. 1 . 
         FIG. 7  is a block diagram showing speaker unit for reproducing virtual sound source. 
         FIG. 8  is an explanatory view for explaining reproduction principle of virtual sound source by the speaker unit shown in  FIG. 7 . 
         FIG. 9  is a block diagram showing the example where virtual sound source reproducing device according to this invention is applied to headphone device. 
         FIG. 10  is a block diagram showing outline of the configuration of the virtual sound source reproducing device according to this invention. 
         FIG. 11  is an explanatory view for explaining impulse response of transfer function of audio signal from sound source up to both ears. 
         FIG. 12  is a block diagram of first signal processing means constituting the virtual sound source reproducing device according to this invention. 
         FIG. 13  is a block diagram of second signal processing means constituting the virtual sound source reproducing device according to this invention 
         FIG. 14  is a block diagram of virtual sound source reproducing device according to this invention in the case where virtual sound source having one sound source is reproduced. 
         FIG. 15  is a block diagram of the virtual sound source reproducing device according to this invention in the case where virtual sound source in which sound sources are bilaterally symmetric is reproduced. 
         FIG. 16  is a block diagram showing virtual sound source reproducing device for realizing transfer function up to both ears in the case where sound sources are bilaterally symmetric. 
         FIG. 17  is a block diagram showing virtual sound source transfer characteristic correcting means constituting the virtual sound source reproducing device according to this invention. 
         FIG. 18  is a block diagram showing another embodiment of virtual sound source transfer characteristic correcting means constituting the virtual sound source reproducing device according to this invention. 
         FIG. 19  is an explanatory view for explaining operation of time difference adding unit used in virtual sound source transfer characteristic correcting means shown in  FIG. 18 . 
         FIG. 20  is an explanatory view for explaining operation of level difference adding unit used in the virtual sound source transfer characteristic correcting means shown in  FIG. 18 . 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     More practical examples of a virtual sound source reproducing device and a sound apparatus and a sound equipment comprising such a device will now be described with reference to the attached drawings. 
     First, the example where this invention is applied to a headphone device (unit)  40  comprising virtual sound source reproducing device will be described. This headphone unit  40  comprises, as shown in  FIG. 9 , a first input terminal  31 L supplied with audio signal Sal, a second input terminal  31 R supplied with audio signal Sar, A/D converters  32 L,  32 R for respectively converting respective audio signals Sal, Sar into digital signals, a virtual sound source reproducing section (unit)  50  for implementing a predetermined digital signal processing to the respective audio signals Sal, Sar converted into digital signals to output such digital signals in the state divided into two systems of synthetic audio signals Sbl, Sbr for left and right ears as stereo signal, D/A converters  34 L,  34 R for converting respective audio signals Sbl, Sbr outputted from this virtual sound source reproducing section  50  into analog signals, and amplifiers  35 L,  35 R for amplifying analog audio signals outputted from respective D/A converters  34 L,  34 R to deliver them to left and right headphone elements  36   a ,  36   b  of a headphone  36 . 
     In this headphone unit  40 , there is employed such an approach to convert, by two A/D converters  32 L,  32 R, respective audio signals Sal, Sar from virtual sound sources respectively inputted from the first and second input terminals  31 L,  31 R into digital signals to implement digital signal processing, by the virtual sound source reproducing section  50 , to the respective audio signals Sal, Sar converted into digital signals to output such digital signals in the state divided into two systems of synthetic audio signals Sbl, Sbr for left and right ears to convert these synthetic audio signals Sbl, Sbr into analog signals by D/A converters  34  L,  34 R to amplify such analog signals by amplifiers  35 L,  35 R to deliver them to left and right headphone elements  36   a ,  36   b  of the headphone  36  to reproduce them. Thus, it is possible to localize sound image of virtual sound source at a predetermined position outside the head of listener who has attached headphone  36 . 
     It is to be noted that the virtual sound source reproducing section  50  used here may be provided within sound unit such as headphone  36 , etc., or may be provided within another sound equipment. 
     The virtual sound source reproducing section  50  according to this invention used in the above-described headphone unit  40  is composed, as shown in  FIG. 10 , of first signal processing means  51  such that when sounds corresponding to synthetic audio signals Sbl, Sbr for left and right ears are heard by the headphone  36 , it implements digital signal processing such that head outside sound image localization of virtual sound source is obtained with respect to a predetermined direction, and second signal processing means  52  for carrying out such a processing to perceive (sense) distance of sound image localization. 
     Here, an example of impulse response corresponding to transfer function from sound source to be reproduced by the virtual sound source reproducing section  50  up to both ears is shown. Waveform corresponding to this impulse response consists, as shown in  FIG. 11 , of impulse response portion (a) contributing to perception (sense) of position of sound source, and impulse response portion (b) contributing to perception (sense) of only distance up to sound source, wherein (a) is impulse response portion mainly indicating head related transfer function and is called head related transfer function region, and (b) is mainly impulse response portion mainly indicating reflected sound and is called reflected sound region. In this example, the impulse response portion (a) is about 10 to 30 ms. 
     The first signal processing means  51  constituting the virtual sound source reproducing section  50  is constituted by, e.g., FIR type digital filter  45  as shown in  FIG. 12 . This digital filter  45  is of configuration in which there are combined two FIR type digital filters adapted to output input signal Sal inputted through a first input terminal  53  as output signals of two systems and to output input signal Sar inputted through a second input terminal  54  as output signals of two systems, and respective digital filters are constituted as one filter block with cascade-connected plural delay elements  56  being as common element. 
     The respective digital filters constituting FIR type digital filter  45  shown in  FIG. 12  comprise cascade-connected plural delay elements  56  having a predetermined delay quantity, respective plural coefficient multipliers  57 ,  58  for multiplying inputted audio signal and audio signals delayed at respective delay elements  57  by coefficients for carrying out convolution of impulse response, and respective plural adders  59 ,  60  for adding audio signals outputted from the respective coefficient multipliers  57 ,  58 . 
     For example, delay element  56  of the 1-st stage of digital filter supplied with audio signal Sal delays, by predetermined delay quantity, e.g., one sampling period audio signal Sal inputted through the input terminal  53 , and delay element  56  of the i (i=2, 3, . . . )-th stage delays, similarly by one sampling period, delayed audio signal outputted from the delay element  56  of the preceding stage (i−1-th stage) to deliver it to delay element  56  of succeeding stage (i+1-th stage). The respective stages of coefficient multipliers  57  ( 58 ) multiply input audio signal Sal and audio signals delayed in sequence at the respective stages of delay elements  56  by coefficients for carrying out convolution of impulse response to deliver them to adders  59  ( 60 ) of corresponding stages. The adders  59  ( 60 ) of respective stages add outputs of coefficient multipliers  57  ( 58 ) of corresponding stages to outputs of adders  59  ( 60 ) of the preceding stages to deliver them to adders  59  ( 60 ) of succeeding stages. Namely, the adder  59  ( 60 ) of the final stage carries out convolution of impulse response of transfer function HLL (HLR) with respect to audio signal Sal inputted through the input terminal  53  to form response signal Sb ll (Sb′lr) to deliver it to adder  61  ( 62 ). 
     Similarly, adder of the final stage of digital filter supplied with audio signal Sar carries out convolution of impulse response of transfer function HRL (HRR) with respect to audio signal Sar inputted through the input terminal  54  to form response signal Sb′rl (Sb′rr) to deliver it to adder  61  ( 62 ). 
     Namely, since this FIR type digital filter  45  constitutes filters of four systems by combining two digital filters to output signals of two systems, delay element  56  cascade-connected can be shared by filters of two systems and the number of delay elements  56  used can be reduced to one half. 
     Impulse response of the FIR type digital filter  45  shown in  FIG. 12  forms head related transfer function region corresponding to one portion of respective impulse responses corresponding to the previously described transfer functions HLL, HLR, HRL, HRR of four systems which has been explained with reference to  FIG. 2 , i.e., impulse response portion (a) mainly representing head related transfer function from the first (initial) response as shown in  FIG. 11 . In this region, impulse responses (a) different from each other are convoluted at four systems of FIR type digital filter  45 . By this calculation, signal processing of input signal is carried out in accordance with impulse response from virtual sound source up to both ends of listener to reproduce it. Thus, there can be obtained response signal contributing to perception (sensing) of position of virtual sound source for localizing reproduction sound image at position of virtual sound source. 
     Outputs of four systems of the FIR type digital filter  45  shown in  FIG. 12  are respectively synthesized by adders  61 ,  62 , and are respectively outputted from first and second output terminals  63 ,  64  as initial response signals Sb′l, Sb′r for left and right ears. Moreover, input signals Sal, Sar delayed by predetermined times by cascade-connected delay elements  56 ,  56  constituting respective digital filters are synthesized by an adder  65 . Thereafter, the input signals thus obtained is delayed by a delay element  66  and is outputted from an output terminal  67  as synthetic delayed output signal. In this example, the delay element  66  additionally connected to the output terminal  67  serves to provide delay for timing correction when there are synthesized initial response signal from the first signal processing means  51  constituted by the FIR type digital filter  45  shown in  FIG. 12  and reflected response signal from the second signal processing means  52  which will be described later. In the case where rigorous timing correction is unnecessary, such delay element may be omitted, and this delay element  66  may be additionally connected to input terminal  68  of the second signal processing means  52 . 
     Further, synthetic delay signal outputted from the output terminal  67  of the FIR type digital filter  45  shown in  FIG. 12  corresponds to synthetic delay output signal obtained by synthesizing delay output signals outputted from head related transfer function processing means supplied with plural audio signals, and is inputted to the input terminal  68  of the second signal processing means  52  constituted by FIR type digital filter  46  shown in  FIG. 13 . 
     FIR type digital filter  46  constituting second signal processing means  52  shown in  FIG. 13  is caused to be of configuration to output input signal inputted through the input terminal  68  as output signals of 2 systems, and is caused to be of configuration as single filter block with cascade-connected plural delay elements  76  being as common elements. In more practical sense, this FIR type digital filter  46  is composed of cascade-connected plural delay elements  76  having a predetermined delay quantity, respective plural coefficient multipliers  77 ,  78  for multiplying input signal and signals delayed by the respective delay elements  76  by coefficients for carrying out convolution of impulse response, and respective plural adders  79 ,  80  for adding signals outputted from the respective coefficient multipliers  77 ,  78 . 
     The delay element  76  of the first stage delays signal inputted through the input terminal  68  by predetermined delay quantity, e.g., one sampling period, and the delay element  76  of the i (i=2, 3, . . . )-th stage delays, similarly one sampling period, delayed signal outputted from the delay element  76  of the preceding stage (i−1-th stage) to deliver it to the delay element  76  of the succeeding stage (i+1-th stage). The coefficient multipliers  77  ( 78 ) of respective stages respectively multiply input signal and signals delayed in sequence at delay elements  76  of respective stages by coefficients for carrying out convolution of impulse response to deliver them to adders  79  ( 80 ) of corresponding stages. The adders  79  ( 80 ) of respective stages add, to outputs of adders  79  ( 80 ) of the preceding stages, outputs of coefficient multipliers  77  ( 78 ) of corresponding stages to deliver them to the adders  79  ( 80 ) of the succeeding stages. Namely, adder  79  ( 80 ) of the final stage carries out convolution of impulse response indicating initial response sound with respect to signal inputted through input terminal  53  to form reflected sound to output this reflected sound through output terminal  81  ( 82 ). 
     In the FIR type digital filter  46  shown in  FIG. 13  constituting the second signal processing means  52 , portions of different impulse responses of either two systems of impulse responses corresponding to the above-described transfer functions HLL, HLR, HRL, HRR of four systems are convoluted, and are outputted from first and second output terminals  81 ,  82 . Respective outputs outputted from these output terminals  81 ,  82  serve to form (b) reflected sound region which is impulse response portion mainly indicating the previously described reflected sound, and correspond to impulse response portion contributing to perception (sensing) of only distance from virtual sound source up to both ears of listener. 
     In the case where virtual sound images to be reproduced are disposed substantially at the same distance from listener, impulse response portions of respective reflected sound regions are similar to each other in either system. For this reason, as stated above, such an approach may be employed to use impulse response portion of reflected sound regions of any two systems as coefficients of FIR type digital filter, to synthesize impulse response portions of reflected sound regions of any two systems, and to determine impulse response portion of reflected sound region of impulse responses corresponding to transfer functions from virtual sound source position except for the above, e.g., the front center. 
     Output Sb′l from the first output terminal  63  and output Sb′r from the second terminal  64  of FIR type digital filter  45  shown in  FIG. 12  constituting the first signal processing means  51  correspond to a pair of initial response signals contributing to position of the virtual sound source, and output signals respectively outputted from first output terminal  81  and second output terminal  82  of FIR type digital filter  46  shown in  FIG. 13  constituting second signal processing means  52  correspond to a pair of reflected response signal contributing to perception (sensing) of only distance from virtual sound source up to both ears of listener. 
     A pair of initial response signals outputted from first signal processing means  51  and a pair of reflected response signals outputted from the second signal processing means  52  are added at respective adders  84 ,  85  constituting operation (computing) means  83  every signals corresponding to left and right signals in the case where stereo reproduction is carried out, and are respectively outputted from the first and second output terminals  86 ,  87  as left ear synthetic audio signal Sbl and right ear synthetic audio signal Sbr. The left ear synthetic signal Sbl and the right ear synthetic signal Sbr respectively outputted from these output terminals  86 ,  87  are returned (converted) into analog signals for a second time by D/A converters  34 L,  34 R of two systems shown in  FIG. 9  to deliver them to left and right headphone elements  36   a ,  36   b  of headphone  6  through amplifiers  35 L,  35 R to reproduce them, thereby making it possible to allow listener to listen to reproduction sound having optimum (suitable) out-of-head sound image localization. 
     A second embodiment of the virtual sound source reproducing unit according to this invention will be described with reference to  FIG. 14 . 
     This virtual sound source reproducing unit  150  serves to obtain left ear synthetic audio signal Sbl and right ear synthetic audio signal Sbr respectively inputted from one sound source to both ears, and is adapted, for the purpose of localizing one sound source at arbitrary position, to realize, by digital filter, convolution of impulse response of two transfer functions from virtual sound source up to both ears. 
     In the virtual sound source unit  150  shown in  FIG. 14 , as initial response of the impulse response, i.e., impulse response of the head related transfer function region is formed from head related transfer function of transfer function HL from the virtual sound source to the left ear and transfer function HR from the virtual sound source to the right ear, respective coefficients are assigned to two FIR type digital filters to independently convolute them. This portion corresponds to first signal processing means  51  of the previously described virtual sound source unit  50  shown in  FIG. 10  of the first embodiment. 
     First signal processing means  151  of this virtual sound source unit  150  is FIR type digital filter constituted in order to output, as output signals of 2 systems, input signal Sa inputted through an input terminal  168 , and is constituted as one filter block with cascade-connected plural delay elements being commonly used. In more practical sense, this FIR type digital filter is composed of cascade-connected plural delay elements  176  having a predetermined delay quantity, respective plural coefficient multipliers  177 ,  178  for multiplying input signal and signals delayed by the respective delay elements  176  by coefficients for carrying out convolution of impulse response, and respective plural adders  179 ,  180  for adding signals outputted from the respective coefficient multipliers  177 ,  178 . 
     The delay element  176  of the first stage delays, by predetermined delay quantity, e.g., one sampling period, signal Sa inputted through the input terminal  168 , and delay element of the i (i=2, 3, . . . )-th stage delays, similarly by one sampling period, delayed signal outputted from delay element  176  of preceding stage (i−1-th stage) to deliver it to delay element  176  of the succeeding stage (i+1-th stage). Coefficient multipliers  177  ( 178 ) of respective stages respectively multiply input signal and signals delayed in sequence at delay elements  176  of respective stages by coefficients for carrying out convolution of impulse response to deliver them to adders  179  ( 180 ) of corresponding stages. The adders  179  ( 180 ) of the respective stages add, to outputs of adders  179  ( 180 ) of the preceding stages, outputs of coefficient multipliers  177  ( 178 ) of corresponding stages to deliver them to the adders  179  ( 180 ) of the succeeding stages. Namely, the adder  179  ( 180 ) of the final stage carries out convolution of impulse response indicating initial response sound with respect to signal inputted through input terminal  168  to form reflected sound to add reflected sounds at respective adders  184 ,  185  constituting calculating means  183  every signals corresponding to left and right signals in the case of carrying out stereo reproduction to output them from first and second output terminals  186 ,  187  as left ear synthetic audio signal Sbl and right synthetic audio signal Sbr. 
     In the impulse response of the latter portion, i.e., impulse response of the reflected sound region, output signals delayed by delay elements  176  of FIR type digital filter constituting the above-described first signal processing means  151  are convoluted by coefficients common to impulse responses of respective reflected sound regions. Thus, the number of coefficients can be reduced, i.e., the number of multipliers can be reduced. Scale of signal processing can be reduced. This portion corresponds to second signal processing means  52  of the previously described virtual sound source unit  50  in the first embodiment. 
     In this case, the second signal processing means  152  is composed of cascade-connected plural delay elements  116  having a predetermined delay quantity, plural coefficient multipliers  117  for multiplying output signal from the first signal processing means  151  inputted and signals delayed at respective delay elements  116  by coefficients for carrying out convolution of impulse response, and plural adders  118  for adding signals outputted from the respective coefficient multipliers  117 . 
     The output signals processed by the second signal processing means  152  are respectively added to left ear synthetic audio signal Sbl and right ear synthetic audio signal Sbr, and are outputted from first and second output terminals  186 ,  187  in the state where they are synthesized with left ear synthetic audio signal Sbl and right ear synthetic audio signal Sbr. 
     Since impulse response portions of respective reflected sound regions are similar response in either system as described above, impulse response portion of reflected sound region of any system may be used as coefficients of FIR type digital filter, impulse response portions of reflected sound regions of plural systems may be synthesized, and impulse response portion of reflected sound region of impulse response corresponding to transfer function from virtual sound source position except for the above, e.g., front and center may be determined to use it. 
     A third embodiment of the virtual sound source reproducing device (unit) according to this invention will now be described with reference to  FIG. 15 . 
     This virtual sound source reproducing unit  250  shows the example in the case where it is assumed that, as shown in  FIG. 15 , four sound sources are disposed substantially bilaterally symmetric with respect to listener and transfer characteristics to left and right ears of listener are substantially symmetric. 
     In this example, since transfer characteristics from two sound sources disposed symmetrically with respect to the front direction of listener to both ears of listener are symmetric, the relationship of the following formula (3) holds in the transfer function as has been described with reference to  FIG. 2 .
 
HLR=HRL
 
HLL=HRR  (3)
 
     The signal processing means shown in  FIG. 16  constituting virtual sound source reproducing unit  250  is constituted in a manner to directly obtain transfer function with respect to two virtual sound sources symmetrically disposed by the FIR type digital filters in accordance with the above-mentioned formula (3). A pair of input signals respectively inputted from a pair of input terminals  201 ,  202  are inputted to an adding/subtracting processing section  274 . Thus, respective sum and difference signals are formed by the adding/subtracting processing section  274 . These sum and difference signals respectively are caused to undergo signal processing by first and second FIR type digital filters  203 ,  204 . Thereafter, respective signals outputted from the first and second FIR type digital filters  203 ,  204  are caused to undergo adding/subtracting processing by the adding/subtracting processing section  275 . Thus, respective sum and difference signals are outputted from first and second output terminals  205 ,  206  as a pair of output signals. 
     The first and second FIR type digital filters  203 ,  204  used here are constituted similarly to the previously described digital filter shown in  FIG. 5 , and respective digital filters  203 ,  204  are composed, as shown in  FIG. 16 , of cascade-connected plural delay elements  216  having a predetermined delay quantity, plural coefficient multipliers  217  for multiplying inputted audio signal and audio signals delayed at respective delay elements  216  by coefficients for carrying out convolution of impulse response, and plural adders  218  for adding audio signals outputted from the respective coefficient multipliers  217 . 
     The delay element  216  of the first stage of these first and second digital filters  203 ,  204  delays, by, e.g., one sampling period, audio signal Sal (or Sar) inputted through input terminal  201  ( 202 ), and the delay element  216  of the i (i=2, 3 . . . )-th stage similarly delays, by, e.g., one sampling period, delayed audio signal outputted from the delay element  216  of the preceding stage ((i−1)-th stage) to deliver it to delay elements  216  of the succeeding stage ((i+1)-th stage). Coefficient multipliers  217  of respective stages respectively multiply input audio signal Sal and audio signals delayed in sequence at the delay elements  216  of respective stages by coefficients for carrying out convolution of impulse response to deliver them to adders  218  of corresponding stages. The adders  218  of respective stages add, to outputs of the adders of the preceding stages, outputs of coefficient multipliers  217  of the corresponding stages to deliver them to the adders  218  of the succeeding stages. Namely, the adder  218  of the final stage carries out convolution of impulse response of transfer function HLL (HLR) with respect to audio signal Sal (Sar) inputted through the input terminal  201  ( 202 ) to form left ear audio signal Sbll (right ear audio signal Sblr) to output it to an adding/subtracting processing section  275 . 
     In the case of four virtual sound sources substantially symmetrically disposed, it is sufficient to expand the case where the above-described two sound sources are symmetrically disposed. In the virtual sound source unit  250  shown in  FIG. 15 , impulse response of the signal processing means which has been explained with reference to  FIG. 16  in the case where plural pairs of symmetrical sound sources exist is divided into the portion of impulse response of the head related transfer function region and impulse response portion of reflected sound region succeeding thereto, signal processing corresponding to the head related transfer function region is carried out by independently constituting transfer functions from the sound source up to both ears as first signal processing means  251 , and signal processing corresponding to reflected sound region is carried out by giving a configuration such that common coefficients are convoluted by the FIR type digital filter as the second signal processing means  252 . 
     By constituting the virtual sound source unit  250  in a manner as shown in  FIG. 15 , with respect to the sound source assumed to be bilaterally symmetric, also in the signal processing for reproducing virtual sound source, similarly to the first signal processing means  51  and the second signal processing means  52  which have been described with reference to  FIG. 10  in the previously described first embodiment, scale of the signal processing means can be greatly reduced. While the case where the virtual sound source is reproduced when the number of sound sources is four in this example has been described, this invention may be applied to a larger number of sound sources generally bilaterally symmetrically disposed, thus making it possible to reproduce the virtual sound source. 
     It is to be noted that, with respect to such a signal to localize virtual sound source at the front and center, adding/subtracting processing section  274  may be provided for this signal to input both input signals in the state equally distributed, or to equally synthesize such signal with audio signal of other symmetric sound source to input it to adding/subtracting processing section  274 . 
     A fourth embodiment of a virtual sound source reproducing unit  350  according to this invention will now be described with reference to  FIG. 17 . 
     This headphone unit  300  using the virtual sound source reproducing unit  350  is provided with virtual sound source transfer characteristic correcting means  310  corresponding to the virtual sound source reproducing unit according to this invention. Virtual sound source reproducing unit for reproducing the previously described four bilaterally symmetrically disposed virtual sound sources shown in  FIG. 15  of the third embodiment is constituted as headphone unit  300 . This headphone unit  300  comprises adding/subtracting processing section  374  including two adding/subtracting processing circuits  374   a ,  374   b  for carrying out adding/subtracting processing of a pair of input signals respectively inputted from a pair of input terminals  311 ,  312 . The two adding/subtracting processing circuits  374   a ,  374   b  constituting this adding/subtracting processing section  374  carry out adding/subtracting processing of pairs of input signals respectively inputted to form respective sum and difference signals. 
     The virtual sound source transfer characteristic correcting means  310  constituting this headphone unit  300  comprises a rotational angle speed sensor  370  attached on a headphone  306  in order to detect rotational angle speed, i.e., displacement speed of the head portion of listener who has attached the headphone  306 , a band-limiting filter  371  for band-limiting output of this rotational angle speed sensor  370 , an A/D converter  372  for converting band-limited analog signal output into digital signal, and a microprocessor  373  having rotational movement angle calculation function for calculating rotational movement speed from the front direction of listener who has attached headphone  306 , i.e., position change quantity of both ears from digital signal output outputted from the A/D converter  372 . The rotational angle speed sensor  370  and the microprocessor  373  constitute characteristic change means comprising head portion rotational angle detecting means for detecting rotational angle speed of the head portion of listener, displacement quantity calculating means for calculating position change quantities of both ears of listener from this head portion rotational angle detecting means, and characteristic control means for changing response characteristic of the head related transfer function processing means in accordance with output of this displacement calculating means, and constitutes response characteristic control means  301 . As virtual sound source transfer correcting means  310  of this invention, if there is employed such means to detect rotation of head portion or change of positions of both ears of listener, etc. to carry out a predetermined control, this means is not limited to the above-described detecting means. 
     When listener who has attached the headphone  306  rotates both ears in left and right directions so that the headphone  306  is rotationally moved, rotational angle speed sensor  370  attached at the headphone  306  outputs, as detection output, voltage proportional to its angular speed. This output signal is band-limited by band-limiting filter  371 , and is then converted into digital signal at A/D converter  372 . The digital signal thus obtained is inputted to microprocessor  373 . The microprocessor  373  makes sampling, at fixed time interval, inputted output signal of the A/D converter  372  thereafter to integrate it to convert it into angular data to calculate rotational angle for rotating virtual sound source from the angular data to transfer corresponding response characteristic control data to first signal processing means  351 . 
     The first signal processing means  351  used here updates signal processing contents of audio signals reproduced from four virtual sound sources in accordance with response characteristic control data calculated at the microprocessor  373  to carry out digital signal processing for localizing sound images of virtual sound sources at suitable positions of the outside of the head and the front portion thereof, i.e., change of parameters (coefficient data of coefficient multipliers) of FIR type digital filter corresponding to head related transfer function region. Although not shown, digital circuit capable of changing coefficients of FIR type digital filter corresponding to the head related transfer function region is prepared within the first signal processing means  351 . Thereafter, output of the first signal processing means  351  is arithmetically processed together with output through signal processing corresponding to reflected sound region obtained by second signal processing means  352 . Thus, a pair of computed output signals are inputted to adding/subtracting processing section  375 . A pair of processing signals which have been caused to undergo adding/subtracting processing by this adding/subtracting processing section  375  are outputted as sum signal and difference signal. A pair of processing signals which have been caused to undergo adding/subtracting processing by the adding/subtracting processing section  375  are respectively outputted to two D/A converters  304 L,  304 R. Both ear synthetic audio signals Sbl, Sbr returned (converted) into analog signal for a second time by the two D/A converters  304 L,  304 R are respectively delivered to left and right headphone elements  306   a ,  306   b  of the headphone  306  through amplifiers  305 L,  305 R and are reproduced, thus providing optimum out-of-head localization feeling in correspondence with position change of both ears to listener who listens to it. 
     Respective FIR type digital filters constituting the first signal processing means  351  constitute head related transfer functions of transfer functions of HLL, HLR, HRL, HRR ranging from speakers to both ears of the listener which have been described with reference to  FIG. 2 , and these head related transfer functions are not fixed in practice, but change with movement of the head of listener. Change in synchronism with movement of the head of this head related transfer function constitutes cause for allowing listener to recognize position of sound image. Accordingly, it is known that precisely reproducing such operation contributes improvement in the quality of sound image localization. 
     In the headphone unit  300  of this embodiment, in order to realize, in accordance with detected rotational angle of the head of listener, head related transfer function corresponding to that angle, the head related transfer function is realized as described above by updating, on the real time basis, coefficients of respective FIR type digital filters by the microprocessor  373 . As stated above, coefficients of the FIR type digital filter corresponding to the head related transfer function region contributing to perception (sensing) of position are updated on the real time basis, and coefficients of FIR type digital filter corresponding to reflected sound region contributing to perception (sensing) of distance remain to be fixed. Accordingly, as compared to the case where all coefficients of FIR type digital filter constituting the transfer function, coefficient memory capacity necessary for coefficient updating can be reduced to much degree. 
     A fifth embodiment of a virtual sound source reproducing unit  450  according to this invention will now be described with reference to  FIG. 18 . 
     This virtual sound source unit  450  serves to control response characteristic corresponding to the head related transfer function region by the virtual sound source transfer characteristic correcting means  310  shown in  FIG. 17 , and is related to first signal processing means  451  of the virtual sound source reproducing unit  450  according to this invention in connection with two virtual sound sources. 
     It is to be noted that reference numeral  402  in  FIG. 18  indicates only the portion for changing parameters related to virtual sound source position to be reproduced of the first signal processing means  451  of the virtual sound source transfer characteristic correcting means  310 , and the description of the response characteristic control means  301  will be omitted since the operation is similar to that which has been described in the fourth embodiment. 
     As first to fourth FIR type digital filters  90  to  93  constituting first signal processing means  451  shown in  FIG. 18 , there are used, e.g., digital filters similar to the digital filters which have been explained with reference to  FIG. 12  in the first embodiment. In this case, the respective FIR type digital filters  90  to  93  realize impulse response portions corresponding to the previously described head related transfer function regions of HLL, HLR, HRL, HRR up to both ears in the case where listener is directed in fixed direction, e.g., at front side from speaker which has been described with reference to  FIG. 2 . 
     In this example, the first and second FIR type digital filters  90 ,  91  are supplied with audio signal through first input terminal  411  from one sound source, and the third and fourth FIR type digital filters  92 ,  93  are supplied with audio signal through second input terminal  412  from the other one sound source. 
     Outputs of the first FIR type digital filter  90  and the third FIR type digital filter  92  are added, and outputs of the second FIR type digital filter  91  and the fourth FIR type digital filter  93  are added, and these outputs thus added are respectively outputted to time difference adding sections  484 ,  485 . Further, respective outputs are delivered to level difference adding sections  483 ,  486 . Their outputs are outputted as output signals of left and right two systems through first and second output terminals  480 ,  481  similarly to the case where explanation has been given with reference to  FIG. 10  in the first embodiment and are inputted to the second signal processing means. Thus, these output signals are added. In addition, output signal from the first signal processing means  451  is inputted to the second signal processing means  402  through third output terminal  482 . Processing similar to that at the previously described first embodiment is carried out. 
     Here, in the time difference adding sections  484 ,  485  and level difference adding sections  483 ,  486 , in the case where, e.g., listener rotates his head portion in a right direction, audio signal reached to the left ear becomes faster than audio signal reached to the right ear, and the left ear becomes close to the sound source and the right ear becomes away from the sound source. For this reason, level of audio signal which is reached to the left ear becomes higher than that of audio signal which is reached to the right ear. By paying attention to this fact, change of transfer function resulting from the fact that head of listener is moved is represented by time difference and level difference of audio signal reached to the both ears to control these differences reached to the both ears by microprocessor, thereby making it possible to simulate and simplify dynamic transfer function. 
       FIG. 19  shows delay time characteristic of the time difference adding sections  484 ,  485 , wherein delay time added at the time difference adding section  484  for the left side is represented by characteristic curve Ta of single dotted lines and delay time added at the time difference adding section  485  for right side is represented by characteristic curve Tb of broken lines. The characteristic curves Ta and Tb are curves having increase/decrease directions entirely opposite to each other with respect to rotational direction of head portion of listener M. 
     As stated above, by providing time difference with respect to arrival of audio signals to both ears by using the time difference adding sections  484 ,  485 , it is possible to realize change of time difference from sound source to both ears by using sound device such as headphone, etc. which is similar to the case where listener M listens to sound from sound source disposed within the range of forward 180 degrees while rotating his head in left and right directions as has been explained with reference to the previously mentioned  FIG. 2 . 
     Moreover,  FIG. 20  shows relative level characteristic of level difference adding sections  483 ,  486 . Level difference added at level difference adding section  483  for left side is indicated by characteristic curve La of single dotted lines and level difference added at level difference adding section  486  for right side is indicated by characteristic curve Lb of broken lines. In  FIG. 20 , there are indicated relative levels from the state where rotational position of the head is 0 degrees (forward front). 
     In  FIG. 20 , the characteristic curves La, Lb are curves having increase/decrease directions entirely opposite to each other with respect to rotational direction of the head portion of listener M. By providing level difference with respect to audio signal at both ears by using level difference adding sections  483 ,  486 , it is possible to realize, by using sound apparatus such as headphone, etc., change of level difference at both ears similarly to the case where listener M listens to sound from sound source placed within the range of forward 180 degrees while rotating the head in left and right directions shown in  FIG. 2 . 
     While the virtual sound source reproducing device of this invention has been described above, the virtual sound source reproducing device of this invention may be provided at sound apparatus such as headphone unit or speaker unit, etc., and may be provided at sound equipments for handling sound such as audio equipment, etc. Also in both cases, it is clear that the virtual sound source can be suitably formed and the scale of the configuration of the sound apparatus and sound equipment can also become compact. 
     INDUSTRIAL APPLICABILITY 
     In the virtual sound source reproducing device and the sound apparatus and the sound equipment provided with such a device according to this invention, the virtual sound source reproducing device is composed of first signal processing means and second signal processing means, wherein the first signal processing means is used to mainly represent the portion of head related transfer function to have ability to reproduce the portion of impulse response capable of perceiving (sensing) position of virtual sound source to localize sound image at the outside of head portion in a desired direction, and the second signal processing means is used to reproduce the portion of impulse response to perceive (sense) only distance of the virtual sound source to add reflected sound, etc. to thereby carry out signal processing to provide feeling of distance with respect to sound source, thus making it possible to localize, at the outside of the head, sound image having high reality as if sound source disposed within space actually exists, and to reduce to much degree operation quantity required for signal processing as compared to the case where transfer function from virtual sound source up to both ears is directly constituted by signal processing means. Accordingly, virtual sound source reproducing device of which configuration scale is compact can be advantageously realized. 
     Moreover, there is employed virtual sound source transfer function correcting means to detect displacement speeds of both ears by using rotational angular sensor or microprocessor to calculate change of positions of both ears for the purpose of coping with change of transfer function from virtual sound source to both ears resulting from the fact that listener moves his head in left and right directions to directly control coefficients of digital filter corresponding to the head related transfer function region of first signal processing means of the above-described virtual sound source reproducing device, or to indirectly control impulse response portion corresponding to the above-described head related transfer function region by using the time difference adding section and the level difference adding section added to the succeeding stage of the first signal processing means. Thus, even when listener moves his head in left and right directions, he can feel sound image having high reality as if he moves his head with respect to sound source disposed within space. 
     In addition, when the virtual sound source reproducing device according to this invention is provided at sound apparatus such as headphone unit or speaker unit, etc., or sound equipment such as audio unit, etc., miniaturization of the sound apparatus and the sound equipment and low power consumption can be realized.