Abstract:
A sound reproducing device including a signal processing device including a first signal processing circuit supplied with an input audio signal of at least one channel to convert the input audio signal to a 2-channel audio signal with which an acoustic image is located at a predetermined position when the input audio signal is reproduced substantially by a 2-channel speaker system, and a second signal processing circuit supplied with the 2-channel audio signal to subject the 2-channel audio signal to signal processing which is equivalent to a transfer function of the 2-channel speaker system to both the ears of a listener, thereby converting and outputting the input audio signal to a 2-channel audio signal. Also included is an earphone device including a pair of electro-acoustic transducers supplied with the 2-channel audio signal from the second signal processing circuit, and a detector for detecting movement of the head of the listener, wherein the second signal processing circuit performs the processing corresponding to an alteration of the transfer function in accordance with the output of the detector to control the location position of the acoustic image which is perceived by the listener.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a sound reproducing device, an earphone device and signal processing device therefor with which multi-channel audio signals are reproduced. 
     Audio signals which are annexed to pictures such as movies or the like are multi-channeled and recorded on the assumption that these signals are reproduced from speakers located at both the left and right sides of a screen and speakers located at both the left and right rear sides of a listener or both the left and right sides of the listener, whereby the position of a sound source in pictures is made coincident with the position of an acoustic image which is actually heard by a listener and thus a sound field having more natural breadth is established. 
     However, when such audio signals are appreciated with a headphone, an earphone or the like, the acoustic image is fixedly located (positioned) in the head of the listener, and the direction (position) of the sound source in the pictures is not coincident with the locating position of the acoustic image thereof, so that the location (orientation) of the acoustic image is extremely unnatural. 
     This also occurs in such a case that a listener appreciates a music piece not accompanied by a picture. That is, unlike the case of the reproduction using speakers, the sound is heard from the inside of the head, and this also results in reproduction of an unnatural sound field. 
     Therefore, there has been considered a method of beforehand measuring or calculating a head transfer function (impulse response) from a speaker located in front of a listener to both the ears of the listener, convoluting the head transfer function thus measured (calculated) into an audio signal with a digital filter and then supplying the audio signal thus obtained to the headphone or the like. According to this method, the acoustic image is located at the outside of the head, and a sound field near to that obtained the speaker reproduction system can be achieved. 
     This method enables the acoustic image to be located outside of the head. However, when the listener changes his/her head position, the locating position of the acoustic image is also displaced together with the movement of the head. Therefore, when the audio signals accompany pictures, there occurs a displacement between the direction of the sound source in the pictures and the direction of the acoustic image, and thus the location of the acoustic image is unnatural. 
     In order to overcome such a disadvantage, there has been considered a method of detecting the movement of the head of a listener and renewing the coefficient of a digital filter in accordance with the movement of the head to fix the location of the acoustic image with respect to a listening environment. According to this method, the acoustic image is not fixedly located (positionally fixed) in the head, and also the acoustic image is not displaced even when the head is moved. Therefore, the substantially same acoustic image as achieved by speakers can be obtained. 
     In such a case that two persons appreciate a movie reproduced by a DVD player or the like, the motions of the heads of the two persons are not necessarily coincident with each other. Therefore, when the same sound field as achieved by the speaker reproduction system is required to be implemented by the above reproducing circuit, two sets of reproducing circuits must be prepared for the two persons, and the coefficient of the digital filter must be individually controlled in each of the reproducing circuit. 
     However, when the coefficient of the digital filter is renewed in accordance with the movement of the head, the coefficient of the digital filter must be renewed immediately every time the head is moved irrespective of a slight movement of the head. Accordingly, a large number of high-speed sum-of-products operating circuits and memories are needed. If the reproduction circuits whose number is equal to the number of audience are required, the price of the system would be extremely high. 
     On the other hand, when a music piece not accompanied by a picture is appreciated, if the acoustic image is located out of the head, there is little problem even when the acoustic image is moved together with the head of the listener. However, a headphone cord connecting an audio device and a headphone gets in the way. 
     SUMMARY OF THE INVENTION 
     The present invention has been implemented in view of the above situation, and has an object to provide a sound reproducing device, an earphone device and a signal processing device with which the same reproduction sound field as achieved by a speaker reproduction system (in which multi-channel audio signals are supplied to the corresponding speakers to reproduce sounds) can be achieved, and also even when a listener moves his/her head at that time, the locating position of the acoustic image can be fixed with respect to a listening environment. 
     In order to attain the above object, according to a first aspect of the present invention, a sound reproducing device comprises: a signal processing device including a first signal processing circuit supplied with an input audio signal of at least one channel to convert the input audio signal to a 2-channel audio signal with which an acoustic image is located at a predetermined position when the input audio signal is reproduced substantially by a 2-channel speaker device, and a second signal processing circuit supplied with the 2-channel audio signal to subject the 2-channel audio signal to signal processing which is equivalent to a transfer function from the 2-channel speaker device to both the ears of a listener, thereby converting and outputting the input audio signal to a 2-channel audio signal; and at least one earphone device including a pair of electro-acoustic transducing means supplied with the 2-channel audio signal from the second signal processing circuit, and detection means for detecting the movement of the head of the listener, wherein the second signal processing circuit performs the processing corresponding to an alteration of the transfer function in accordance with the output of the detection means to control the location position of the acoustic image which is perceived by the listener. 
     According to a second aspect of the present invention, a sound reproducing device comprises: a signal processing device including a first signal processing circuit supplied with an input audio signal of at least one channel to convert the input audio signal to a 2-channel audio signal with which an acoustic image is located at a predetermined position when the input audio signal is reproduced substantially by a 2-channel speaker device, and a second signal processing circuit supplied with the 2-channel audio signal to subject the 2-channel audio signal to signal processing which is equivalent to a transfer function from the 2-channel speaker device to both the ears of a listener, thereby converting and outputting the input audio signal to a 2-channel audio signal; and an earphone device including a third signal processing circuit supplied with 2-channel audio signals from the second signal processing circuit, a pair of electro-acoustic transducing means supplied with the output signal from the third signal processing circuit, and detection means for detecting the movement of the head of the listener, wherein the third signal processing circuit performs the processing corresponding to an alteration of the transfer function in accordance with the output of the detection means to control the location position of the acoustic image which is perceived by the listener. 
     According to a third aspect of the present invention, an earphone device used in combination with a signal processing device which is supplied with an input audio signal of at least one channel to convert the input audio signal to a 2-channel audio signal with which an acoustic image is located at a predetermined position when the input audio signal is reproduced substantially by a 2-channel speaker device, subjects the 2-channel audio signal to signal processing equivalent to a transfer function from the 2-channel speaker device to both the ears of a listener, thereby converting and outputting the input audio signal to a 2-channel audio signal, comprises: a signal processing circuit supplied with the 2-channel audio signals from the signal processing circuit; a pair of electro-acoustic transducing means supplied with the output signal from the signal processing circuit; and detection means for detecting the movement of the head of the listener, wherein the signal processing circuit performs the processing corresponding to an alteration of the transfer function on the 2-channel audio signals in accordance with the output of the detection means to control the location position of the acoustic image which is perceived by the listener. 
     According to a fourth aspect of the present invention, a signal processing device for transmitting 2-channel audio signals to an earphone device having a pair of electro-acoustic transducing means under a wireless condition, includes: a first signal processing circuit which is supplied with an input audio signal of at least one channel to convert the input audio signal to 2-channel audio signals with which an acoustic image is located at a predetermined position when the input audio signal is reproduced substantially by a 2-channel speaker device; a second signal processing circuit which is supplied with the 2-channel audio signal output from the first signal processing circuit and subjects the 2-channel audio signals to the signal processing equivalent to a transfer function from the 2-channel speaker device to both the ears of a listener to convert and output the input audio signals to 2-channel audio signals; and transmission means for transmitting the 2-channel audio signals output from the second signal processing circuit under the wireless condition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a systematic diagram showing an embodiment of the present invention; 
     FIG. 2 is a plan view showing the present invention; 
     FIG. 3 is a systematic diagram showing an embodiment of a circuit usable in the present invention; 
     FIG. 4 is a plan view showing the present invention; 
     FIG. 5 is a systematic diagram showing an embodiment of the circuit usable in the present invention; 
     FIG. 6 is a systematic diagram showing a part of another embodiment of the present invention; 
     FIG. 7 is a systematic diagram showing a part of another embodiment of the present invention; 
     FIG. 8 is a systematic diagram showing an embodiment of the circuit usable in the present invention; 
     FIG. 9 is a characteristic diagram showing the present invention; 
     FIG. 10 is a characteristic diagram showing the present invention; 
     FIG. 11 is a systematic diagram showing another embodiment of the present invention; 
     FIG. 12 is a systematic diagram showing the present invention; and 
     FIG. 13 is a systematic diagram showing an embodiment of the circuit usable in the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Preferred embodiments according to the present invention will be described hereunder with reference to the accompanying drawings. 
     FIG. 1 shows an embodiment of a sound reproducing device according to the present invention. 
     The sound reproducing device of this embodiment comprises a headphone adapter  10 , and a headphone  80  which is supplied with the output signal of the headphone adapter  10 . Reference characters SLF, SRF, SLB, SRB represents 4-channel audio signals. When these signals SLF, SRF, SLB, SRB are respectively supplied to speakers located at the left front side, the right front side, the left rear side and the right rear side respectively, the reproduction sound field of 4-channel stereo is implemented. 
     In the headphone adapter  10 , the audio signals SLF to SRB are supplied to A/D converter circuits  21  to  24  through input terminals  11  to  14  to be subjected to A/D conversion, and the audio signals SLF to SRB after the A/D conversion are supplied to a digital processing circuit  3  constructed by DSP, for example. The details of the digital processing circuit  3  will be described later, and it serves to convert the audio signals SLF to SRB (4-channel signals) to audio signals SL 3 , SR 3  (2-channel signals) with which the location of a 4-channel stereo sound field can be achieved through two speakers. 
     That is, the digital processing circuit  3  aims to convert the signals SLF to SRB to the signals SL 3 , SR 3  so that a reproduction sound field having the same level as achieved when the signals SLF, SRF, SLB, SRB are supplied to the speakers located at the left front side, the right front side, the left rear side and the right rear side of a listener is implemented when the signals SL 3 , SR 3  are supplied to the speakers located at the left front side and the right front side of the listener respectively (at this time point, the audio signals SLF to SRB, SL 3 , SR 3  are digital signals, however, the description will be made on the assumption that they are analog signals in order to simplify the description). 
     The audio signals SL 3 , SR 3  are output to two output connectors  31 ,  32 , for example. 
     For example, the connector  31  is connected to the connector  40 , and the signals SL 3 , SR 3  output to the connector  31  are output from the connector  40  through a cable  4  to the digital processing circuit  5 . The details of the digital processing circuit  5  will be also described later. It is constructed by DSP, for example, and it serves to convert the audio signals SL 3 , SR 3  to audio signals SL, SR with which the location of the acoustic image is achieved out of the head when these signals are heard by a headphone. 
     That is, the digital processing circuit  4  serves to convert the signals SL 3 , SR 3  to the signals SL, SR so that when the signals SL, SR are supplied to the headphone, the same-level reproduction sound field as achieved when the signals SL 3 , SR 3  are supplied to the speakers located at the left front side and the right front side of the listener is implemented. 
     The audio signals SL, SR are supplied to D/A converter circuits  6 L,  6 R to be subjected to D/A conversion, and the audio signals SL, SR after the D/A conversion are supplied to left and right acoustic units (electric/acoustic conversion elements)  8 L,  8 R of the headphone  80  through headphone amplifiers  7 L,  7 R. The acoustic units  8 L,  8 R are linked to each other through a band  81  so that the acoustic units  8 L,  8 R are held at the left and right ear positions of the listener when the headphone  80  is put on the head. 
     In addition, a rotational angular speed sensor  91  is provided to the band  81  of the headphone  80  for example, and the output signal thereof is supplied to a detection circuit  92  to detect the angular speed of the head of the listener when the listener rotates his/her head. The detection signal is supplied to an A/D converter circuit  93  and A/D-converted to a digital detection signal S 92 , and then the detection signal S 92  after the A/D conversion is supplied to a microcomputer  94 . 
     In the microcomputer  94 , the detection signal S 92  is sampled every predetermined time and the sampled signals S 92  are integrated to be converted to angle data representing the orientation of the head of the listener. A signal S 94  of control data for actually locating (orientating) the acoustic image is generated on the basis of the angle data, and the signal S 94  thus generated is supplied as a control signal to the digital processing circuit  5 . 
     In this case, the circuit system extending from the processing circuit  5  to the amplifiers  7 L,  7 R and the circuit system extending from the detection means (rotational angular speed sensor)  91  to the microcomputer  94  are accommodated integrally in an unit such as a housing for accommodating acoustic units in a general headphone, and thus the headphone  80  is designed to have the same outlook of a general headphone. 
     Next, the processing of changing (converting) the number of channels by the digital processing circuit  3  will be described. In this case, the digital processing circuit  3  will be described on the assumption that it is constructed by a discrete circuit. 
     It is now considered that sound sources SL, SR are located at the left front and right front sides of a listener M, and a sound source SX is equivalently reproduced at any position out of the head by the sound sources SL, SR as shown in FIG.  2 . Defining as follows: 
     HLL: transfer function going from the sound source SL to the left ear of the listener M, 
     HLR: transfer function going from the sound source SL to the right ear of the listener M, 
     HRL: transfer function going from the sound source SR to the left ear of the listener M, 
     HRR: transfer function going from the sound source SR to the right ear of the listener M, 
     HXL: transfer function going from the sound source SX to the left ear of the listener M, and 
     HXR: transfer function going from the sound source SX to the right ear of the listener M, the sound sources SL, SR are represented as follows: 
     
       
           SL =( HXL×HRR−HXR×HRL )/( HLL×HRR−HLR×HRL )× SX   (1) 
       
     
     
       
           SR =( HXR×HLL−HXL×HLR )/( HLL×HRR−HLR×HRL )× SX   (2) 
       
     
     Accordingly, if the input audio signal SX corresponding to the sound source SX is supplied to a speaker located at the sound source SL through a filter for implementing the transfer function part of the equation (1), and the signal SX is supplied to a speaker located at the position of the source SR through a filter for implementing the transfer function part of the equation (2), the acoustic image based on the audio signal SX could be located at the position of the sound source SX. 
     Therefore, for example, as shown in FIG. 3, the digital processing circuit  3  may be constructed by FIR type digital filters  31 L to  34 L,  31 R to  34 R and addition circuits  35 L,  35 R. That is, the audio signals SLF to SRB from the A/D converter circuits  21  to  24  are supplied to the addition circuit  35 L through the digital filters  31 L to  34 L, and also supplied to the addition circuit  35 R through the digital filters  31 R to  34 R. 
     At this time, the transfer functions of the digital filters  31 L to  34 L,  31 R to  34 R are set to predetermined values according to the above review, and impulse responses obtained by converting the same transfer functions as the transfer function parts of the equations (1) and (2) to the time axis are convoluted into the audio signals SLF to SRB. 
     Accordingly, from the addition circuits  35 L,  35 R can be output the audio signals SL 3 , SR 3  with which the same reproduction sound field as achieved when 4-channel audio signals SLF to SRB are reproduced by four speakers can be reproduced by two speakers. 
     Next, the digital processing circuit  5  will be described on the assumption that it is constructed by a discrete circuit. 
     Now, when a sound source SM is located in front of a listener M as shown in FIG. 4, it is defined that: 
     HML: transfer function going from the sound source SM to the left ear of the listener M, and 
     HMR: transfer function going from the sound source SM to the right ear of the listener M. In this case, the digital processing circuit  5  may implement the transfer functions HML, HMR. 
     Therefore, the digital processing circuit  5  may be constructed by FIR type digital filters  51 L,  52 L,  51 R,  52 R and addition circuits  55 L,  55 R as shown in FIG. 5, for example. 
     That is, the audio signals SL 3 , SR 3  from the digital processing circuit  3  are supplied to the addition circuit  55 L through the digital filters  51 L,  52 L, and supplied to the addition circuit  55 R through the digital filters  51 R,  52 R. Further, at this time, the transfer functions of the digital filters  51 L to  52 R are set to predetermined values, and impulse responses obtained by converting the transfer functions to the time axis are convoluted into the audio signals SL 3 , SR 3 . 
     Accordingly, the audio signal SL is output from the addition circuit  55 L, and the audio signal SR is output from the addition circuit  55 R. That is, the audio signals SL, SR with which the same reproduction sound field as achieved when the audio signals SL 3 , SR 3  are reproduced by speakers can be reproduced by a headphone can be output from the addition circuits  55 L,  55 R. 
     As described above, the digital processing circuit  3  converts the 4-channel audio signals SLF to SRB to the 2-channel audio signals SL 3 , SR 3  with which the same reproduction sound field as obtained when four speakers are used can be obtained by two speakers, and further the digital processing circuit  5  converts the signals SL 3 , SR 3  to the audio signals SL, SR with which the same reproduction sound field as obtained when two speakers are used can be obtained with a headphone. Accordingly, when the audio signals SL, SR are supplied to the acoustic units  8 L,  8 R, the same reproduction sound field as obtained in the case of four-speaker reproduction system can be reproduced. 
     However, by using only the above construction, the location of the acoustic image reproduced by the acoustic units  8 L,  8 R is fixed with respect to the listener M. Therefore, when the listener M moves his/her head, the acoustic image is also moved together. 
     Therefore, as described above, the means  91  to  94  are further provided, and in the digital processing circuit  5 , the transfer functions of the digital filters  51 L to  52 R are controlled by the signal S 94  from the microcomputer  94 . In this case, when a sound source is located in front of the listener M, the left ear is nearer to the sound source if the listener M turns to the right. Therefore, the time lag of sound wave incident to the left ear is adjusted to be reduced, and the level thereof is adjusted to be increased. Conversely, the time lag of sound wave incident to the right ear is adjusted to be increased and the level thereof is adjusted to be reduced. Therefore, the coefficients of the digital filters  51 L to  52 R are controlled by the signal S 94  so that the above variation of the transfer functions is implemented. 
     Accordingly, when the listener M turns the head, the transfer functions in the digital processing circuit  5  are varied in accordance with the turning of the head, and the acoustic image formed by the acoustic units  8 L,  8 R is located at a fixed position out of the head irrespective of the turning of the head. For example, even when the listener turns the head while hearing music of an orchestra, the listener is kept in a natural state as if the orchestra is not moved and the listener turns the head before the orchestra. Alternatively, even when the listener turns the head while reproducing a DVD player, the locating position (fixed position) of the acoustic image can be made coincident with the position of the sound source in pictures. 
     As described above, according to the headphone adapter  10  and the headphone  80 , the 4-channel reproduction sound field which are originally reproduced by four speakers can be reproduced by a headphone. In this case, if two headphones  80  are connected to the connectors  31 ,  32  of the adapter  10  respectively, two persons could enjoy sounds (pictures) simultaneously by the headphones. In this case, since the locating processing of the acoustic image in connection with the movement of the head of each person is performed independently in each of the headphones  80 , the location of the acoustic image for one person is not effected by the movement of the head of the other person, and the same acoustic image location or the same reproduction sound field as achieved when only one person enjoy sounds can be obtained. 
     In addition, the headphone adapter  10  is common to the two headphones  80 , and the overall price of the system can be reduced. 
     Further, the audio signals SL 3 , SR 3  output to the connectors  31 ,  32  are signals obtained by converting the 4-channel audio signals SLF to SRB so that the 4-channel reproduction can be also performed by even two speakers. Therefore, when no headphone  80  is used, if the output signals SL 3 , SR 3  of the connector  31  or  32  are supplied to two speakers through an amplifier, the 4-channel stereo reproduction can be performed by the two speakers. 
     FIGS. 6 and 7 show a case where the headphone adapter  10  is designed to be connectable to a multi-channel audio signal source and particularly the transmission of the signals between the headphone adapter  10  and the headphone  80  is wirelessly performed. 
     That is, in FIG. 6, reference numeral  100  represents a digital audio signal source, and in this embodiment the signal source  100  is a DVD player. A so-called 5.1-channel digital audio signal SDA in Dolby digital (AC- 3 ) is picked up from the DVD player  100 . 
     The digital audio signal SDA is a signal obtained by encoding into one serial data (bit stream) 6-channel digital audio signals SLF, SCF, SRF, SLB, SRB, SLOW for left front, center front, right front, left rear and right rear and in a low frequency band below 120 Hz. In general, this signal SDA is supplied to a special-purpose adapter to be decoded and D/A-converted to original 6-channel audio signals SLF to SLOW, and the signals SLF to SLOW are supplied to the respective speakers to form a reproduction sound field. 
     Such a signal SDA is supplied from the player  100  to the input terminal  15  of the headphone adapter  10  through a coaxial cable  101 , and further supplied to the decoder circuit  2  to be decoded to the audio signals SLF to SLOW, and these audio signals SLF to SLOW are supplied to the digital processing circuit  3 . 
     When the digital processing circuit  3  is constructed by a discrete circuit, it is constructed as shown in FIG.  8 . That is, an acoustic image reproduced by supplying the audio signal SCF of the center front channel to the center front speaker can be reproduced by the left front and right front speakers. Further, the audio signal SLOW of a low-band channel has a low frequency, and thus generally the acoustic image formed by the signal SLOW accompanies no sense of direction. 
     Therefore, in the processing circuit  3  shown in FIG. 8, the digital audio signals SLF, SRF from the decoder circuit  2  are supplied to the digital filters  31 L to  32 R through addition circuits  311 ,  312 , and also the digital audio signal SCF from the decoder circuit  2  is supplied to the addition circuits  311 ,  312  through an attenuating circuit  31 C, whereby the audio signal SCF is distributed to the audio signals SLF, SRF. 
     Further, the digital audio signals SLB, SRB from the decoder circuit  2  are supplied to the digital filters  33 L to  34 R through the addition circuits  313 ,  314 , and the digital audio signal SLOW from the decoder circuit  2  is supplied to the addition circuits  311  to  314  through an attenuation circuit  31 W to distribute the audio signal SLOW to the audio signals SLF to SRB. The rear stage from the filters  31 L to  34 R is designed in the same construction as shown in FIG.  2 . 
     As described above, in the processing circuit  3  the signals SLF to SLOW are converted to the 2-channel audio signals SL 3 , SR 3  reproduced by two speakers with which the same reproduction sound field as that obtained when they are supplied to the speakers located at the left front side, the center front side, the right front side, the left rear side and the right rear side of a listener and speakers for low frequency band. 
     The audio signals SL 3 , SR 3  are supplied to the digital processing circuit  5  to be converted to the audio signals SL, SR. That is, in the processing circuit  5 , as described above, when the signals SL, SR are supplied to the headphone, the signals SL 3 , SR 3  are converted to the signals SL, SR so as to implement the same reproduction sound field as obtained when the signals SL 3 , SR 3  are supplied to the speakers located at the left front and right front sides of the listener. 
     In this case, the processing circuit  5  may be constructed as shown in FIG.  5 . However, the coefficients of the digital filters  51 L to  52 R are fixed to values when the listener M faces the front side, and thus the acoustic image is fixed to the locating position when the listener M faces the front side. 
     The audio signals SL, SR from the processing circuit  5  are supplied to the encoder circuit  41  to be converted to 1-channel serial data signal S 41 . For example, it is converted to a digital audio interface signal S 41  defined by EIAJ, which is used for the digital output of a CD player, etc. This signal S 41  is supplied to a transmission circuit  42  to be converted to a transmission signal of a predetermined format, and this transmission signal is supplied to an infrared ray LED  43  to be converted to infrared rays, and then transmitted to the headphone  80 . 
     At this time, in the headphone  80 , the infrared rays from the LED  43  is received by a photosensor  44 , and the output signal thereof is supplied to a reception circuit  45  to pick up the original signal S 41 . This signal S 41  is supplied to the decoder circuit  46  to be separated into the original 2-channel audio signals SL 5 , SRS. 
     The signals SL 5 , SR 5  thus separated are supplied to the D/A converter circuits  6 L,  6 R through additive circuits  56 L,  56 R having a time difference described later and additive circuits  57 L,  57 R having a level difference to be D/A-converted, and the audio signals after the D/A conversion are supplied to the right and left acoustic units  8 L,  8 R through the headphone amplifiers  7 L,  7 R. 
     Further, by the means  91  to  94 , the facing direction of the head of the listener M is detected to form the signal S 94 , and the signal S 94  is supplied as a control signal to the additive circuits  56 L to  57 R. 
     In this case, the circuits from the photosensor  44  to the amplifiers  7 L,  7 R and the circuits from the detection means  91  to the microcomputer  94  are accommodated integrally in a portion such as a housing for accommodating acoustic units in a general headphone, and thus the headphone  80  is designed to have the same outlook as a general headphone. 
     Accordingly, the digital processing circuit  3  converts the audio signals SLF to SRB to the audio signals SL 3 , SR 3  with which the same reproduction sound field as obtained by six speakers can be obtained by two speakers, and the digital processing circuit  5  further converts these signals SL 3 , SR 3  to the audio signals SL, SR with which the same reproduction sound field as obtained by two speakers can be obtained by a headphone. The signals SL, SR are supplied to the acoustic units  8 L,  8 R to reproduce the reproduction sound field as obtained by six speakers. 
     With only the above construction, the coefficients of the digital filters  31 L to  34 R in the processing circuit  5  are fixed, and thus the locating position of the acoustic image reproduced by the acoustic units  8 L,  8 R is fixed with respect to the listener M. Therefore, when the listener M moves the head, the acoustic image is moved together. 
     Therefore, the additive circuits  56 L to  57 R are provided as described above, and the time difference and the level difference added by the additive circuits  56 L to  57 R are controlled by the signal S 94  from the microcomputer  94 . That is, the additive circuit  56 L,  56 R is constructed by a variable delay circuit, and the additive circuit  57 L,  57 R is constructed by a variable gain circuit. 
     For example, when the sound source is located in front of the listener M, if the listener M turns to the right, the time delay of sound wave incident to the left ear is reduced, and the level thereof is increased. Therefore, the characteristic of the additive circuit  56 L is controlled as indicated by a broken line B in FIG. 9, and the characteristic of the additive circuit  57 L is controlled as indicated by a curved line C in FIG.  10 . The left ear and the right ear are in the opposite position, so that the characteristic of the additive circuit  56 R is controlled as indicated by a broken line A in FIG. 9 while the characteristic of the additive circuit  57 R is controlled as indicated by a curved line D in FIG.  10 . 
     Accordingly, when the listener M turns the head, the time difference and the level difference of the signals SL, SR are varied in accordance with the turning direction as shown in FIGS.  9  and  10 , so that the acoustic image formed by the acoustic units  8 L,  8 R is located at a fixed place in the outside irrespective of the turning of the head. 
     In this case, only one cable  101  is sufficient to the connection between the DVD player  100  and the headphone adapter  10 , and thus the connection is simple. Further, the digital audio signal SDA reproduced by the DVD player  100  is not D/A-converted to an analog audio signal, but directly supplied to the headphone adapter  10  to implement the sound field reproduction. Therefore, deterioration of sound quality can be avoided. 
     Further, a wireless state is kept between the headphone adapter  10  and the headphone  80  with infrared rays, and thus a cumbersome work due to a cable connecting both the elements can be avoided. In addition, if headphones  80  whose number is equal to that of listeners are prepared, any persons can listen to DVD or the like simultaneously. 
     Further, when the coefficients of the digital filters  51 L to  52 R of the digital processing circuit  5  are renewed in accordance with the movement of the head, if the head is slightly moved, the coefficients of the digital filters  51 L to  52 R must be renewed every time, and thus a large number of high-speed sum-of-products operating circuits and memories are needed. However, in the headphone  80 , the variation of the coefficients of the digital filters  51 L to  52 R with respect to the movement of the head portion is substituted or simulated by the change of the time difference and the level difference of the audio signals SL, SR, so that the circuit scale can be greatly simplified. 
     Further, when the acoustic image is fixed to the locating position by the signal S 94  formed in accordance with the detection signal S 92  of the movement of the head, it is unnecessary to supply the signal S 94  from the headphone  80  to the headphone adapter  10  wirelessly, and thus the construction can be simplified. 
     FIG. 11 shows a case where the headphone adapter  10  is designed so that the same reproduction sound field as obtained by the speaker reproduction is obtained by using an existing infrared-ray type wireless headphone. That is, a signal line from the input terminal  15  to the digital processing circuit  5  is de signed in the same construction as the headphone adapter  10  of FIG. 6 to pick up the digital audio signals SL, SR from the digital processing circuit  5 , and the audio signals SL, SR are supplied to D/A converter circuits  71 L,  71 R to be D/A-converted to analog audio signals SL, SR. 
     The audio signals SL, SR after the D/A conversion are supplied to FM modulation circuits  72 L,  72 R to be converted to FM signals SLFM, SRFM in this case, as an example, the FM signals SLFM, SRFM are set as follows: 
     Carrier frequency of FM signal SLFM: 2.3 MHz 
     Carrier frequency of FM signal SRFM: 2.8 MHz 
     Maximum frequency shift of signals SLFM, SRFM: ±150 KHz 
     The FM signals SLFM, SFMR are supplied to an addition circuit  73  to pickup an addition signal S 73  of the signals SLFM, SRFM, and the signal S 73  is supplied to an infrared-ray emitting element, for example, an infrared-ray LED  75  through a drive amplifier  74 . Infrared radiation IR whose light amount is modulated in accordance with the signal S 73  is output from the LED  75 . 
     At this time, the audio signals SL, SR from the D/A converter circuits  71 L,  71 R are picked up to the output terminals  77 L,  77 R through amplifiers  76 L,  76 R. 
     Accordingly, if the infrared rays from the adapter  10  are received by the infrared ray type wireless headphone, the stereo reproduction sound could be obtained. In this case, a general infrared-ray type wireless headphone on the market may be used as the headphone. 
     That is, FIG. 12 shows an embodiment of the infrared-ray type wireless headphone  200  as described above. Infrared radiation IR from the headphone adapter  10  is photodetected by a photodetecting element such as a photodiode  201  to pick up the addition signal S 73 . 
     The output signal S 73  of the photodetecting element  201  is supplied to π-shaped band pass filters  203 L,  203 R through an amplifier  202  to pick up the FM signals SLFM, SRFM from the addition signal S 73 . The signals SLFM, SRFM are supplied to FM reception circuits  204 L,  204 R. The reception circuit  204 L,  204 R directly uses general one chip IC for an FM receiver, and it has elements from a high-frequency amplifier to an FM demodulation circuit. Accordingly, in the reception circuit  204 L,  204 R, the FM signal SLFM, SRFM is frequency-converted to an intermediate frequency signal having a frequency of 10.7 MHz, and the intermediate frequency signal is subjected to FM demodulation to pick up the analog audio signals SL, SR. 
     The pickup audio signals SL, SR are supplied through drive amplifiers  205 L,  205 R to acoustic units  206 L,  206 R of the headphone  200 . 
     Accordingly, according to the headphone adapter  10  of FIG. 11, the 6-channel stereo reproduction sound field as obtained in the case of the speaker reproduction can be implemented. In this case, the 6-channel stereo reproduction sound field can be implemented by using an infrared-ray type wireless headphone  200  on the market. 
     Further, a wireless state is established between the headphone adapter  10  and the headphone  200 , and thus disturbance due to a cable connecting both the elements is avoided. In addition, if headphones whose number is equal to that of listeners are prepared, any persons can listen to music at the same time. 
     The digital processing circuit  5  may be constructed as shown in FIG. 13, for example. That is, the audio signals SL 3 , SR 3  from the digital processing circuit  4  or the cable  4  are added in a predetermined rate in an addition circuit  58 L, and then supplied to the digital filter  51 . The audio signals SL 3 , SR 3  are subtracted in a predetermined rate in a subtraction circuit  58 R, and then supplied to the digital filter  52 . 
     The respective output signals of the digital filters  51 ,  52  are subtracted in a predetermined rate in a subtraction circuit  59 L to pick up the digital audio signal SL, and the respective output signals of the filters  51 ,  52  are added in a predetermined rate in an addition circuit  59 R to pick up the digital audio signal SR. 
     According to the above manner, the processing amount of data as the digital processing circuit  5  can be reduced, and it is particularly effective when the digital processing circuit  5  is constructed by a DSP. 
     Further, in the headphone adapter  10  and the headphone  80  of FIG. 1, the signals SL 3 , SR 3  can be transmitted from the headphone adapter  10  to the headphone  80  wirelessly as in the case of the headphone adapter  10  and the headphone  80  of FIGS. 6 and 7. 
     Further, in the headphone adapter  10  of FIGS. 6 and 11, the signal line between the terminal  15  and the decoder circuit  2  may be provided with a sampling rate converter circuit to convert the sampling rate of the digital audio signal SDA. Further, in FIG. 6, in place of the coaxial cable  101  and the terminal  15 , an optical cable and a photodetecting element (TOS link) may be used. 
     Further, the rotational angle sensor  91  for detecting the facing direction of the head of the listener M may be constructed by a piezoelectric vibrating gyro or a geomagnetic azimuth sensor. Alternatively, it may be adopted that light emitting means is located in front of or around the listener M, and at least two light intensity sensors are provided to the headphone  80  to calculate the rotational angle of the head of the listener M on the basis of the output ratio of these light intensity sensors. 
     Further, burst-shaped ultrasonic wave output from an ultrasonic oscillator in front of or around the listener M may be detected by ultrasonic sensors located at two places which are remote from each other on the headphone  80  to be converted to a reception signal, and then the rotational angle of the headphone  80  may be calculated on the basis of the time difference of the reception signal. 
     According to the present invention, the same reproduction sound field as obtained when the multi-channel audio signals are supplied to the corresponding speakers, can be implemented by a headphone, and also even when the listener moves his or her head at that time, the locating position of the acoustic image can be fixed with respect to the outside. 
     Further, even if a plurality of listeners listen to music at the same time when the locating position of the acoustic image is fixed with respect to the outside, the locating position of the acoustic image is not affected by the movement of the head of another person, and the same acoustic image locating position or the same reproduction sound field as obtained when one listener enjoys listening can be obtained. 
     In addition, at that time, the headphone adapter is common to a plurality of headphones, so that the price of the overall system can be reduced. Further, as compared with the case where all the processing is collectively performed, the circuit can be designed in a smaller scale, and also the cost can be more reduced. 
     Only one cable is sufficient for the connection with a digital audio signal source such as a DVD player or the like, and thus the connection is simple. In addition, the digital audio signal from the signal source can be directly supplied, and deterioration of sound quality can be avoided. 
     In the above embodiments, the digital audio signal is supplied from a player such as a DVD player to the input terminal  15  of the headphone adapter  10  through a coaxial cable, and further supplied to the decoder circuit  2  to be decoded to the audio signals SLF to SLOW. However, these audio signals may be transmitted from the player under a wireless condition by using infrared rays, and the decoder circuit may be designed to receive the audio signals transmitted under the wireless condition. 
     Further, when signal transmission between the headphone adapter and the headphone is performed wirelessly, disturbance due to a cable connecting both the elements is avoided, and if headphones whose number is equal to the number of listeners are prepared, any number of persons can listen to DVD or the like. 
     In the above-described headphone, when the variation of the coefficients of the digital filters with respect to the movement of the head portion is substituted or simulated by the change of the time difference and the level difference of the audio signals, the circuit scale can be greatly simplified. Further, when the locating position of the acoustic image is fixed by the signal formed in accordance with the detection signal of the movement of the head, it is unnecessary to supply the signal from the headphone to the headphone adapter, and thus the construction can be simplified.