Abstract:
A signal processor with a small circuit size, a simplified arrangement, and reduced cost and which can generate the same sound field as that in a case of listening to a reproduced sound using speakers to a location outside of a head of the listener has FIR filters and reverberation adding circuits. At an addition point provided to a tap midpoint of one FIR filter, a delay output signal of a tap end from another FIR filter is multiplied by a predetermined multiplication coefficient and then added. Output of one FIR filter is subjected to reverberation adding processing by the reverberation adding circuits and then output of yet another FIR filter is added to the above output to obtain an output signal. Therefore, even if the FIR filters with short tap lengths are used, long impulse response can be reproduced and a scale of signal processing can be reduced substantially.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a signal processor for reproducing an impulse response representing a transmission characteristic between two points of a transmitting system calculated in advance by measurement or calculation and a headphone device and a speaker device for which the signal processor is used. 
     2. Description of the Related Art 
     Conventionally, there is proposed an out-of-head audio image localizing headphone device in which an audio image formed by respective input audio signals from a left front sound source LF, a right front sound source RF, a left back sound source LB, and a right back sound source RB with respect to a listener is localized on an outside of the listener head. 
     FIG. 6 shows an arrangement of a prior-art out-of-head audio image localizing headphone device. In FIG. 6, the out-of-head audio image localizing headphone device is formed of input terminals  101  to  104  to which four-channel audio signals of the above left front sound source LF, right front sound source RF, left back sound source LB, and right back sound source RB are supplied, A/D (Analog-Digital) converters (AD 1  to AD 4 )  105  to  108  for converting input audio signals into digital signals, a signal processor  109  for applying predetermined signal processing to the input audio signals converted into the digital signals, D/A (Digital-Analog) converters (DA 1  and DA 2 )  110  and  111  for respectively converting outputs of the signal processor  109  into analog signals, amplifiers (AM 1  and AM 2 )  112  and  113  for respectively amplifying the audio signals converted into the analog signals, and a headphone  114  to which the amplified audio signals are respectively supplied. 
     FIG. 7 is a block diagram showing an arrangement of a prior-art signal processor. In FIG. 7, the signal processor  109  is formed of digital FIR filters (F 1  to F 8 )  125  to  132  for convoluting respective impulse responses from the above four-channel sound sources to a left ear and a right ear of the listener calculated in advance by measurement or calculation to input audio signals DLF, DRF, DLB, and DRB converted into the digital signals and supplied to input terminals  121  to  124 , a pair of adding circuits  133  to  138  for respectively adding outputs of left channels and right channels of the digital FIR filters (F 1  to F 8 )  125  to  132 , and output terminals  139  and  140  for outputting outputs of the adding circuits  133  to  138  as output signals. 
     By the above prior-art headphone device, the same sound field as that in a case of listening to a reproduced sound obtained by reproducing four-channel input audio signals by a speaker was intended to be generated on an outside of a head of the listener. 
     However, according to the above-described prior-art headphone device, if the same sound field as that in the case of listening to a reproduced sound obtained by reproducing by a speaker was intended to be generated on an outside of a head of the listener, it was necessary to enormously increase a length of the impulse response convoluted by each of the digital FIR filters within the headphone device. Therefore, if each of the digital FIR filters was formed by using a digital signal processing IC (integrated circuit), the digital signal processing IC mounted with a high-speed product-sum operating circuit and a large number of memories was necessary. Thus, a size of the circuit increased and the system became extremely expensive. 
     It is an object of the invention to provide a signal processor in which the above problems are solved, a size of the circuit is reduced, an arrangement is simplified, a cost is reduced, and the same sound field as that in a case of listening to a reproduced sound obtained by reproducing by a speaker can be generated on an outside of a head of a listener and to provide a headphone device and a speaker device for which the signal processor is used. 
     Specifically, it is an object to provide a digital signal processor in which FIR filters for convoluting impulse responses are formed and which can perform convolution of the impulse responses having approximate lengths and characteristics to those of actual impulse responses by using the FIR filters with relatively short tap lengths. 
     SUMMARY OF INVENTION 
     A signal processor of the present invention is applied to a signal processor for carrying out predetermined signal processing by signal processing means for convoluting impulse responses from sound sources to a measurement point calculated in advance by measurement or calculation to input audio signals of a plurality of channels and outputting the signals. 
     The signal processing means divides the input audio signals into two lines for respective channels and has finite-tap-length FIR filters respectively for the plurality of channels and reverberation adding circuits respectively for the first line and the second line. At an addition point provided to a tap midpoint of the FIR filter of one channel of the first and second lines, a delay output signal of a tap end from the FIR filter of the other channel of the first and second lines is multiplied by a predetermined multiplication coefficient and then added. Output of the FIR filter of the one channel of the first and second lines is subjected to reverberation adding processing by the reverberation adding circuits of the first and second lines and then output of the FIR filter of the other channel of the first and second lines is added to the output of the FIR filter of the one channel to obtain addition output as an output signal of the first and second lines of the signal processing means. 
     A headphone device of the invention is a headphone device to which the above signal processor is applied, the headphone device driving a headphone by an output left audio signal and an output right audio signal to localize an audio image by input audio signals on an outside of a head. 
     A speaker device of the invention is a speaker device to which the above signal processor is applied, the speaker device driving the two speakers by the output left audio signal and the output right audio signal to localize an audio image by the predetermined audio signals in a position other than the two speakers. 
     The invention has the following operations. 
     By properly choosing a position of the tap of the digital FIR filter of the one channel of the first and second lines into which the delay output signal multiplied by the predetermined multiplication coefficient is input, a length of impulse response reproduced by the digital FIR filter of the one channel of the first and second lines, the digital FIR filter of the other channel of the first and second lines, and the reverberation adding circuits can be increased, even if a length of the impulse response convoluted in the digital FIR filter of the other channel of the first and second lines is smaller than a length of the impulse response convoluted in the digital FIR filter of the one channel of the first and second lines. 
     A main portion representing a characteristic of the impulse response to be reproduced is convoluted by the digital FIR filter of the other channel of the first and second lines and the output of the digital FIR filter of the other channel is added to the reverberation output of the reverberation adding circuit with respect to the digital FIR filter of the one channel of the first and second lines. Therefore, even if a characteristic of a signal component of a portion to be processed of the delay signal output from the tap end of the digital FIR filter of the other channel of the first and second lines is different from the characteristic of the impulse response to be reproduced, a reproduced characteristic of the impulse response can be substantially the same as the desired characteristic of the impulse response. 
     As a result, the same sound field as that in a case of listening to a sound obtained by reproducing the above input audio signals by speakers disposed in positions of the sound sources can be generated on an outside of a head of the listener. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing an arrangement of a headphone device of the present embodiment; 
     FIG. 2 is a block diagram showing an arrangement of a signal processor of the present embodiment; 
     FIG. 3 is a block diagram of digital FIR filters; 
     FIG. 4 is a block diagram of a reverberation circuit; 
     FIG. 5 is a diagram showing localization of sound sources; 
     FIG. 6 is a diagram showing an arrangement of a prior-art headphone device; and 
     FIG. 7 is a block diagram showing an arrangement of a prior-art signal processor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the present invention will be described bellow. 
     FIG. 1 shows an arrangement of the embodiment of the headphone device of the invention that is an out-of-head audio image localizing headphone device for localizing an audio image generated by respective input audio signals on an outside of a head, the signals being from a left front sound source LF, a right front sound source RF, a left back sound source LB, a right back sound source RB, and a central sound source CE with respect to a listener L as shown in FIG.  5 . 
     In FIG. 1, The headphone device of the present embodiment is formed of input terminals  1  to  5  to which five-channel audio signals of the above left front sound source LF, right front sound source RF, left back sound source LB, right back sound source RB, and central sound source CE are supplied, A/D (Analog-Digital) converters (AD 1  to AD 5 )  7  to  11  for converting input audio signals into digital signals, a signal processor  12  that is for applying predetermined signal processing to the input audio signals converted into the digital signals and that is formed of a DSP (Digital Signal Processor), for example, D/A (Digital-Analog) converters (DA 1  and DA 2 )  13  and  14  for respectively converting output signals of the signal processor  12  into analog signals, amplifiers (AM 1  and AM 2 )  15  and  16  for respectively amplifying the audio signals converted into the analog signals, and a headphone  17  to which the amplified audio signals are respectively supplied and which outputs a reproduced sound. 
     Details of an arrangement of the above signal processor  12  will be described later. In the present embodiment, the signal processor  12  applies signal processing such that the same sound field as that in a case of listening to a reproduced sound obtained by reproducing audio signals by speakers can be generated on an outside of a head of the listener when the audio signal converted by the signal processor  12  is reproduced by the headphone  17 . Furthermore, the arrangement of the above signal processor  12  is simplified, a signal processing speed increases, and thus a signal processing amount increases substantially. Therefore, it is possible to substantially increase lengths of impulse responses convoluted by respective digital FIR filters within the above headphone device as compared with prior art. As a result, it is possible to obtain the above signal processor  12  by using a low-priced digital signal processing IC. 
     FIG. 2 is a block diagram showing an arrangement of the signal processor of the embodiment. 
     In FIG. 2, the signal processor  12  is formed of digital FIR filters (F 1  to F 10 )  26  to  35  for convoluting respective impulse responses from the above five-channel sound sources to a left ear and a right ear of the listener calculated in advance by measurement or calculation to input audio signals DLF, DRF, DLB, DRB, and DCE converted into digital signals and supplied to input terminals  21  to  25 , a pair of adding circuits  36  to  39  for respectively adding outputs of left channels and right channels of the digital FIR filters (F 1  to F 10 )  26  to  35 , reverberation adding circuits  40  and  41  to which outputs from the adding circuits  36  and  37  are supplied as input signals, a pair of adding circuits  42  and  43  for respectively adding outputs of left channels and right channels of the reverberation adding circuits  40  and  41  and the adding circuits  38  and  39 , and output terminals  44  and  45  for outputting outputs of the adding circuits  42  and  43  as output signals. 
     In the signal processor with the above arrangement, to the FIR filters  26  to  35  and the reverberation adding circuits  40  and  41 , at addition points provided to tap midpoints of the FIR filters  26  to  29 , delay output signals at tap ends from the FIR filters  30  to  33  multiplied by a predetermined multiplication coefficient as described later are added. After reverberation adding processing is applied by the reverberation adding circuits  40  and  41  to outputs of left channels and right channels of the FIR filters  26  to  29 , outputs of the left channels and the right channels of the FIR filters  30  to  35  are respectively added to outputs of the reverberation adding circuits for the left channel and right channel to obtain output signals. 
     By such a headphone device, the same sound field as that in a case of listening to a reproduced sound obtained by reproducing the five-channel input audio signals by speakers is generated on an outside of a head of the listener. 
     Here, by reference to FIGS. 3 and 4, details of the arrangement of the signal processing of the above signal processor  12  will be described. 
     FIG. 3 is a block diagram of the digital FIR filters of the signal processor of the present embodiment. FIG. 3 shows a relationship between the digital FIR filter (F 1 )  26  and the digital FIR filter (F 5 )  30 . Relationships between the digital FIR filter (F 2 )  27  and the digital FIR filter (F 6 )  31 , between the digital FIR filter (F 3 )  28  and the digital FIR filter (F 7 )  32 , and between the digital FIR filter (F 4 )  29  and the digital FIR filter (F 8 )  33  are similar to the above relationship between the digital FIR filter (F 1 )  26  and the digital FIR filter (F 5 )  30 . Therefore, details of signal processing operation of the above signal processor will be described below by using the relationship between the digital FIR filter (F 1 )  26  and the digital FIR filter (F 5 )  30 . 
     In the above digital FIR filter (F 1 )  26 , a plurality of delay devices  52  ( 52 - 1  to  52 -n) are connected in series to an input terminal  51  to which the digital signal DLF is input. Tapped delay lines for delay processing of input audio signals of the digital signal DLF are provided to the respective delay devices  52 - 1  to  52 -n, a plurality of multipliers  53  ( 53 - 1  to  53 -n+1) for multiplying signals of respective taps of the delay lines by a coefficient are provided in parallel, adders  54  ( 54 - 1  to  54 -n) for successively adding output signals of the plurality of multipliers  53 - 1  to  53 -n+1 are provided to be connected in series, and an output signal is output from an output terminal  55 . 
     Similarly, in the digital FIR filter (F 5 )  30 , a plurality of delay devices  57  ( 57 - 1  to  57 -n) are connected in series to an input terminal  56  to which the digital signal DLB is input. Tapped delay lines for delay processing of input audio signals of the digital signal DLB are provided to the respective delay devices  57 - 1  to  57 -n, a plurality of multipliers  58  ( 58 - 1  to  58 -n+1) for multiplying signals of respective taps of the delay lines by a coefficient are provided in parallel, adders  59  ( 59 - 1  to  59 -n) for successively adding output signals of the plurality of multipliers  58 - 1  to  58 -n+1 are provided to be connected in series, and an output signal is output from an output terminal  61 . 
     FIG. 4 is a block diagram of the reverberation adding circuit of the signal processor of the present embodiment. 
     Because the reverberation adding circuit (R 1 )  40  and the reverberation adding circuit (R 2 )  41  have the same arrangement, only the arrangement of the reverberation adding circuit (R 1 )  40  will be described here. 
     In the above reverberation adding circuit (R 1 )  40 , a plurality of delay devices  72  ( 72 - 1  to  72 -n) are provided in parallel to an input terminal  71 , a plurality of multipliers  73  ( 73 - 1  to  73 -n) for multiplying delay outputs of the plurality of respective delay devices  72 - 1  to  72 -n by a predetermined coefficient are provided in parallel, an adder  74  for adding multiplication outputs of the plurality of multipliers  73 - 1  to  73 -n is provided, and an addition output of the adder  74  is supplied to an output terminal  75 . 
     Next, details of the signal processing operation of the above signal processor  12  will be described by reference to FIGS. 3 and 4. 
     In the digital FIR (Finite Impulse Response) filter (F 1 )  26  that is a finite-length digital filter and has the above arrangement, a so-called convolution operation in which the digital signal DLF is delayed by the delay devices  52 , multiplied by the predetermined coefficient by the multipliers  53 , and added by the adders  54  is performed. Thus, the digital signal to which a predetermined impulse response is convoluted is supplied to the reverberation adding circuit (R 1 )  40 . 
     In the reverberation adding circuit (R 1 )  40 , the signal is subjected to delay processing by the plurality of delay devices  72 . By choosing proper values for the number of delay elements of each of the delay devices  72  and the multiplication coefficient of each of the multipliers  73  in view of a position of the sound source of the input audio signal DLF, a length of the impulse response reproduced by the digital FIR filter (F 1 )  26  and the reverberation adding circuit (R 1 )  40  increases, even though a length of the impulse response convoluted in the digital FIR filter (F 1 )  26  is small. As a result, the same sound field as that in the case of listening to a reproduced sound obtained by reproducing the above input audio signals by speakers disposed in positions of the sound sources can be generated on an outside of a head of the listener. 
     Next, in the digital FIR filter (F 5 )  30 , a so-called convolution operation in which the input audio signal DLB is delayed by the delay devices  57 , multiplied by the predetermined coefficients by the multipliers  58 , and added by the adders  59  is performed similarly. 
     Here, a delay signal output from a delay device output terminal  60  connected to the tap of the delay line in a back stage of the delay device  57 -n of the digital FIR filter (F 5 )  30  is multiplied by a predetermined coefficient by a multiplier  62  and a multiplication output of the multiplier  62  is added to a delay signal supplied to an adder  63  provided to the tap of the delay line in a front stage of the delay device  52 -m (n&gt;m) in a predetermined position of the above digital FIR filter (F 1 )  26 . 
     An output from the output terminal  61  of the digital FIR filter (F 5 )  30  and an output of the reverberation adding circuit (R 1 )  40  are added by the adding circuit  42  and an output signal is output from the output terminal  44 . 
     Here, by properly choosing the multiplication coefficient of the multiplier  62  and the position of the tap of the delay line in the front stage of the delay device  52 -m (n&gt;m) of the digital FIR filter (F 1 )  26  to which the delay signal to which the output of the multiplier  62  is added is input, a length of the impulse response reproduced by the digital FIR filter (F 5 )  30 , the digital FIR filter (F 1 )  26 , the reverberation adding circuit (R 1 )  40  and the adding circuit  42  can be increased, even though a length of the impulse response convoluted in the digital FIR filter (F 5 )  30  is smaller than a length of the impulse response convoluted in the digital FIR filter (F 1 )  26 . 
     A main portion representing a characteristic of the impulse response to be reproduced is convoluted by the digital FIR filter (F 5 )  30  and the output of the digital FIR filter (F 5 )  30  is added to the reverberation output of the reverberation adding circuit (R 1 )  40  with respect to the impulse response convoluted in the digital FIR filter (F 1 )  26  by the adding circuit  42  in the last stage. Therefore, even if a characteristic of a signal component of a portion to be processed of the delay signal output from the delay device output terminal  60  of the digital FIR filter (F 5 )  30  is different from the characteristic of the impulse response to be reproduced, a reproduced characteristic of the impulse response can be substantially the same as the desired characteristic of the impulse response. 
     As a result, the same sound field as that in a case in which the listener L is listening to a reproduced sound obtained by reproducing the above input audio signals by speakers disposed in positions of the left front sound source LF, right front sound source RF, left back sound source LB, right back sound source RB, and central sound source CE as shown in FIG. 5 can be generated on an outside of a head of the listener L. 
     By the operation of the digital FIR filters in the signal processor of the above embodiment, an initial reflected sound of a predetermined impulse response based on the input audio signal may be generated to generate primary reverberation sound and secondary reverberation sound by operation of the reverberation adding circuits. As a result, by increasing a length of the impulse response of the reproduced initial reflected sound, localization of the sound source can be achieved further effectively. By increasing lengths of impulse responses of the primary reverberation sound as well as the secondary reverberation sound, density of the impulse response can be increased. Therefore, it is possible to generate a reproduced sound with presence and high quality. 
     With regard to the position of the tap of the delay line in the back stage of the delay device  57 -n of the digital FIR filter (F 5 )  30  and the position of the tap of the delay line in the front stage of the delay device  52 -m (n&gt;m) of the digital FIR filter (F 1 )  26  in the signal processor of the embodiment, 100-tap output may be added to 50 tap output and 256-tap output may be added to 128-tap output, for example. As a result, the number of taps can be reduced and a processing amount can be reduced as compared with prior art that requires 4k taps, for example. 
     Furthermore, the reverberation adding processing, as a way of weighting, can be easily applied to the audio signal with a small processing amount by the reverberation adding circuit. Therefore, by adding artificial reverberation sound to a convergence portion of the impulse response by a simple processing, it is possible to cause the impulse response to approximate to the original impulse response. 
     Although an example in which the five-channel audio signals of the left front sound source LF, right front sound source RF, left back sound source LB, right back sound source RB, and central sound source CE are supplied is described in the above embodiment, the invention is not limited to this and the same effects can be obtained if the signal is slightly displaced from the five-channel positions or if a subchannel such as a superwoofer or a subwoofer is installed additionally to expand the channel to N-channel. 
     By the signal processing operation in the signal processor of the above embodiment, an amount of the convolution operation of the impulse responses by the digital FIR filters required to obtain the same sound field can be reduced substantially and an amount of the whole operation including the operational processing of the reverberation adding circuit in the back stage and the required number of the memories can be reduced as compared with the prior art. 
     Therefore, by applying the signal processor of the embodiment to the headphone device, signal processing for reproducing by reproduction by the headphone the same sound field as that in a case of listening to a reproduced sound obtained by reproducing the audio signals by speakers disposed in positions of sound sources can be achieved by using a low-priced and small-sized digital signal processing IC with low operation performance and a small number of installed memories, which was impossible by the prior art. 
     The invention is not limited to this and two speakers may be driven by an output left audio signal and an output right audio signal to localize an audio image formed by predetermined audio signals in a position other than the two speakers by applying the signal processor of the embodiment to a speaker device for reproducing multichannel audio signals by the two speakers disposed in front of the listener. 
     Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments and that various changes and modifications could be effected therein by one skilled in the art without departing from the spirit or scope of the invention as defined in the appended claims.