Abstract:
A device for providing a sound field by adjusting the listener&#39;s interaural correlation coefficient. A sound field adjustment filter is provided with a pair of filter circuit groups corresponding to a left and right channel. This filter divides a desired frequency band of the audio signal into multiple frequency bands and assigns prescribed transmission characteristics to generate a sound field adjustment signal for each frequency band such that a prescribed impulse response is configured cooperatively for the entire desired frequency band of the audio signal. The transmission characteristics adjust the audio signal for each split frequency band by the addition of time delay and gain. By means of operation output units, the filter adds the sound field adjustment signals generated by each of the filter circuit groups to the audio signal of the other channels, and outputs to stereo speakers.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a sound field adjustment filter, sound field adjustment apparatus and sound field adjustment method to provide a good listening feeling to a sound field reproduced by stereo speakers by imparting particular sound field characteristics to acoustic signals. 
       BACKGROUND OF THE ART 
       [0002]    Conventionally, a sound field generating device is proposed to provide a reproduced sound field where a better spread feeling can be perceived by quantitatively representing the spread feeling of musical sounds using the interaural correlation coefficient of a listener (see the patent literature reference 1). This sound field generating device comprises, first and second processing circuits for carrying out a reverberation process of an input signal, a first filter for imparting a particular amplitude characteristic to the signal outputted from the first processing circuit, a second filter for imparting a particular amplitude characteristic to the signal outputted from the second processing circuit, a first adder for adding the signal outputted from the first processing circuit, the signal outputted from the first filter and the inverted output signal of the second filter, a second adder for adding the signal outputted from the second processing circuit and the signals outputted from the first and second filters, and wherein a first speaker is driven by the added signal outputted from the first adder, and a second speaker is driven by the added signal outputted from the second adder. The amplitude characteristic for each of the first and second filters is adjusted so as to approximate the frequency characteristic of the interaural correlation coefficient for narrow band stationary random signal to that of a diffuse sound field (reverberant room) to expand the sound image limited between left and right speakers. 
       PRIOR ART DOCUMENTS 
     Patent Documents 
       [0003]    Patent Document 1: JP 08-130799 
       DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
       [0004]    According to the conventional sound field generating device described above, the control of the interaural correlation coefficient is performed by a reflected sound generation circuit and a correlation coefficient control circuit. However, the obtained sound field and sound quality are deviated from the desired ones since the interaural correlation coefficient is adjusted after the reflected sound is separately added to the original sound. Further, the load of the hardware becomes large in order to approximate the interaural correlation coefficient to the diffuse sound field in the entire frequency band, so that it is difficult to provide the device at a low cost. In addition, in the case of a panel speaker provided with a common diaphragm, with regard to acoustic signals in the mid-high range frequency band, as shown in  FIG. 7 , there is a certain frequency range A where a reverse phenomenon that the sound pressure level outputted from one speaker of one of the channels based on the acoustic signal inputted to the one channel increases than the sound pressure level outputted from other speaker of other of the channels based on the acoustic signal inputted to the other channel occurs, thereby the sound field generated and outputted from the panel speaker is disturbed. 
         [0005]    Accordingly, the object of the present invention is to provide a simple and economical device and method for processing sound wherein the interaural correlation coefficient of a listener can be easily approximated to the value corresponding to a sound field in accordance with a preference, and applied to a panel speaker. 
       Means for Solving the Problems 
       [0006]    In order to solve the above problems, the present invention provides a sound field adjustment filter  5  for adding sound field adjustment signals for imparting particular sound field characteristics to acoustic signals outputted to a pair of left and right stereo speakers  2 , which comprises a pair of left and right filter circuit groups  5 L,  5 R for receiving left and right channel acoustic signals, respectively, dividing a predetermined frequency band of the acoustic signal into a plurality of frequency bands, adjusting a time delay and level of each divided frequency band such that a desired impulse responses is configured cooperatively for the entire frequency band of the acoustic signals to thereby generate the sound field adjustment signals corresponding to the left and right channels and respectively outputting to the other of the channels with each other. By adjusting the pass characteristic of each frequency band, a listener can perceive the virtual speakers  2 ,  2 ′ in which the opening angles thereof are changed with respect to a pair of speakers fixed in position, so that the interaural correlation coefficient is approximated for capable of perceiving a listening feeling in accordance with a preference of the listener. 
         [0007]    With regard to a panel speaker  7  provided with a common diaphragm  7   a,  a pair of left and right separation filters  10 L,  10 R is provided, which, for acoustic signals in the mid-high range frequency hand, separate a particular frequency range A where the reverse phenomenon, that the sound pressure level outputted from one speaker  7  of one of the channels based on the acoustic signal inputted to the one channel increases than the sound pressure level outputted from other speaker  7  of other of the channels based on the acoustic signal inputted to the other channel, occurs and output to the other of the channels, and the filter circuit groups  5 L,  5 R generate sound field adjustment signals based on the acoustic signals outputted from the separation filters  10 L,  10 R. 
         [0008]    Reduction filters  16 L,  16 R are provided for reducing the level of a particular frequency range A where the reverse phenomenon occurs, and the level of the pass characteristics assigned to the filter circuit groups  5 L,  5 R is adjusted so as to restore the frequency range A outputted to a low level. 
         [0009]    Each of the filter circuit groups  5 L,  5 R has a plurality of filter circuits  50   a,    50   b  . . .  50   n.  In order to adjust the pass characteristic, the filter circuits  50   a,    50   b  . . .  50   n  have band pass filters  51   a,    51   b  . . .  51   n  for dividing into a plurality of frequency bands, time delay setting means  52   a,    52   b  . . .  52   n  for adding additional time delays for the divided frequency bands, and gain setting means  53   a,    53   b  . . .  53   n  for adding predetermined additional gains for the divided frequency band, respectively. 
         [0010]    The additional gain of the particular frequency range A in the reduction filters  16 L,  16 R is set to 1/k, i.e. the inverse of gain k of the particular frequency range A in the filter circuit groups  5 L,  5 R. 
         [0011]    The sound filed adjustment device  1  using the sound field adjustment filter  5  is provided with the delay circuits  6 L,  6 R for adding the delay time required to generate the sound field adjustment signal to the acoustic signal, and operational output units  4 L,  4 R for adding the sound field adjustment signal to the acoustic signal and outputting the result to stereo speakers  2 ,  2 . 
       Effect of the Invention 
       [0012]    In accordance with the present invention, the interaural correlation coefficient of the listener to the stereo speakers is easily adjusted only by changing the opening angle of the virtual speaker, and accordingly, it is possible to generate the diffuse sound field and further to approximate to the value for perceiving the sound field matched to a preference of the listener. Therefore, the load in hardware and software is small to thereby simple and economical device can be provided. 
         [0013]    In addition, further effect is obtained that the adjustment is possible without the sound field being disturbed by the reverse phenomenon of the sound pressure level peculiar to the panel speaker provided with a common diaphragm for left and right channels. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    [ FIG. 1 ] A block diagram of a sound field adjustment apparatus according to the present invention. 
           [0015]    [ FIG. 2 ] A conceptual diagram of a sound field adjustment filter. 
           [0016]    [ FIG. 3 ] An explanatory diagram of the sound field adjustment. 
           [0017]    [ FIG. 4 ] A correspondence table of an opening angle of a speaker with respect to the frequency. 
           [0018]    [ FIG. 5 ] A block diagram of a sound field adjustment device according to another embodiment. 
           [0019]    [ FIG. 6 ] A block diagram of a sound field adjustment device according to further embodiment. 
           [0020]    [ FIG. 7 ] A graph showing frequency characteristic of a panel speaker. 
           [0021]    [ FIG. 8 ] A graph showing changes in interaural correlation coefficient with respect to the frequency. 
           [0022]    [ FIG. 9 ] A graph comparing the presence and absence of a sound field adjustment signal processing with regard to changes of the interaural correlation coefficient with respect to the frequency. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    An embodiment of the present invention is described hereinafter with reference to the drawings. 
         [0024]    As shown in  FIG. 1 , a sound field adjustment device  1  for outputting acoustic signals to a pair of left and right independent stereo speakers  2 ,  2  comprises input terminals  3 L,  3 R corresponding to left and right channels, respectively, operational output units  4 L,  4 R, a sound field adjustment filter  5 , and delay circuits  6 L,  6 R. The digitized acoustic signals are inputted to the input terminals  3 L,  3 R. As an alternative, the acoustic signal processing device  1  is provided with a built-in AD converter, and is configured to convert the input analog signals into digital signals. 
         [0025]    The sound field adjustment filter  5  is comprised of a pair of filter circuit groups  5 L,  5 R corresponding to the left and right channels, respectively. The one filter circuit group  5 L generates a sound field adjustment signal from the acoustic signal inputted to the left channel input terminal  3 L to output to the right channel operational output unit  4 R. The operational output unit  4 R adds the right channel sound acoustic signal passed through the delay circuit  6 R and the sound field adjustment signal generated by the sound field adjustment filter  5 L and outputs the result. Similarly, the other filter circuits  5 R generates a sound field adjustment signal from the acoustic signal inputted to the right channel input terminal  3 R to output to the left channel operational output unit  4 L. The operational output unit  4 L adds the left channel sound acoustic signal passed through the delay circuit  6 L and the sound field adjustment signal generated by the sound field adjustment filter  5 R and outputs the result. The delay circuits  6 L,  6 R add the delay time due to the formation of the sound field adjustment signals at the sound field adjustment filters  5 L,  5 R to the acoustic signals. 
         [0026]    As shown in  FIG. 2 , each of the filter circuit groups  5 L,  5 R constituting the sound field adjustment filter  5  comprises a plurality of filter circuits  50   a,    50   b  . . .  50   n.    
         [0027]    These filter circuits  50   a,    50   b  . . .  50   n  comprise band pass filters  51   a,    51   b  . . . and  51   n  for covering the desired frequency band in combination, delay setting means  52   a,    52   b  . . .  52   n  for adding additional time delays τ 1 , τ 2  . . . τn to the band pass filters  51   a,    51   b  . . .  51   n,  respectively, and gain setting means  53   a,    53   b  . . .  53   n  for adding a predetermined additional gains K 1 , K 2  . . . Kn. The outputs of each of the frequency bands from the filter circuits  50   a,    50   b  . . .  50   n  are combined, and the phase of the combined output is reversed by an inverter  54 . Each impulse response of the filter circuit groups  5 L,  5 R is adjusted with respect to each frequency band of each of the band pass filters  51   a,    51   b  . . . and  51   n  by means of the delay setting means  52   a,    52   b  . . .  52   n  and the gain setting means  53   a,    53   b  . . .  53   n  and thereafter summed, so that it is possible to configure a desired pass characteristic. 
         [0028]    In this regard, FIR filters having the same impulse responses described above may be applied for the filter circuit groups  5 L,  5 R. 
         [0029]    By the way, the good spatial spread feeling of the sound field reproduced by stereo speakers can be implemented by approximating to the interaural correlation coefficient at the listening position of the listener in the reverberant room (diffuse sound field) having regulated acoustic environment. However, with regard to the interaural correlation coefficient, in the case of the ordinary reproduced sound field wherein each of the left and right speakers is arranged at opening angle (direction angle θ OP  of each speaker around the listener) 60°, as shown in the triangular points in  FIG. 9 , the interaural correlation coefficient of the reproduced sound field is greatly different especially in comparison with that of the diffuse sound field (broken line in  FIG. 9 ) in or above the middle range. Therefore, the frequency band is divided into narrow bands, and by individually adjusting the interaural correlation coefficient for each of the narrow bands, the interaural correlation coefficient of the diffuse sound field can be approximated (black dots in  FIG. 9 ). To adjust this interaural correlation coefficient, the speaker position corresponding to the interaural correlation coefficient is virtually modified. As shown in  FIG. 3 , this position of the virtual speaker is expressed as a physical quantity, such as the opening angles θ OP  of a pair of speakers with respect to the listener. 
         [0030]    The interaural correlation coefficient ρ d  of the diffuse sound field is represented by ρ d =sin (kr)/kr. 
         [0031]    where, k=ω/c=2πf/c, k: wavelength constant, c: sound velocity, and r: equivalent interaural distance 31.5 cm. 
         [0032]    The interaural correlation coefficient ρ lr  at the listening position in a reproduced sound field of speakers is represented as follows. 
         [0000]    
       
         
           
             
               
                 
                   
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         [0033]    where, A and A′: amplitude value, τ lr : interaural time difference, Φ 11 (τ lr ): autocorrelation function of the signal, Φ 12 (τ lr ): cross-correlation function between the left and right signals. 
         [0034]    Here, since the correlation coefficients between signals are uncorrelated, the term (A′ 2 +A 2 )Φ 12 (0)=0, and the term 2AA′Φ 12 (τ lr )=0 in the above equation, and accordingly, ρ lr  is determined by Φ 11 (τ lr ), and the interaural time difference τ lr  is determined by the direction of the speaker. 
         [0035]    The above estimate values almost coincide with the actual measured values, and the opening angles of the speaker with respect to the frequencies are such as shown in the Table of  FIG. 4 . Therefore, by dividing into the frequency bands where the frequency in the Table being the band median value thereof, followed by adjusting the speaker position at the corresponding opening angle, it is possible to adjust to the interaural correlation coefficient of the diffuse sound field. 
         [0036]    On the premise of the above, the adjustment of the opening angle of the virtual speaker is controlled by the sound field adjustment filter. That is, the sound field adjustment filter  5  presets impulse responses of band-pass filters  51   a,    51   b  . . .  51   n  for each of the divided frequency bands, the sound field adjustment signals are generated by passing the acoustic signals of right and left channels through these filters, and by adding the sound field adjustment signal in reverse phase with the acoustic signal of the other of the channels, it is possible to provide the sound field of the virtual speakers in which the opening angles thereof are changed with respect to the pair of stereo speakers fixed in position relative to the listener, so that the interaural correlation coefficient of the diffuse sound field can be approximated. 
         [0037]    In the filter circuit groups  5 L,  5 R of the sound field adjustment filter  5 , the desired impulse responses can be configured by adjusting the pass characteristic of each of the band-pass filter  51   a,    51   b  . . .  51   n  for each band shown in  FIG. 2  by means of the additional gain delay τ 1  . . . τn of the time delay setting means  52   a,    52   b  . . .  52   n  and the additional gain K 1  . . . Kn of the gain setting means  53   a,    53   b  . . .  53   n,  thereby it is possible to adjust the opening angle θ OP  of the virtual speaker  2 ′ such that the virtual speaker  2 ′ is located at the predetermined position on an arc which being centered on the listener P and having the distance from the listener P to the speaker  2  as a radius, as shown in  FIG. 3 . Accordingly, the good spread feeling can be achieved as the sound field reproduced by the speakers  2 ,  2  wraps around the listener, and it is possible to change not only the diffuse sound field but also the listening feeling of the listener in accordance with the preference. 
         [0038]      FIG. 5  shows a sound processing apparatus according to another embodiment. In the following description, with regard to the same component in the above embodiment, the same reference number is used and the description thereof is omitted. 
         [0039]    A sound field adjustment filter  9  in this sound processing device  8  is applied to a panel speaker  7  which is provided with a common diaphragm  7   a.  The sound field adjustment filter  9  comprises, in addition to a pair of filter circuit groups  5 L,  5 R, separation filters  10 L,  10 R and operational output units  11 L,  11 R corresponding to each channel. With regard to the acoustic signal, it is possible to set the additional time delay τ 1  . . . τn and the additional gain K 1  . . . Kn to the filter circuit groups  5 L,  5 R along the trend of change of the interaural correlation coefficient as in the previous embodiment in the mid-low frequency range band less than or equal to 1,000 Hz. However, in the mid-high range frequency band greater than or equal to 1,414 Hz, the interaural correlation coefficient tends to become negative, and the reverse phenomenon occurs so that the sound pressure level of the speaker of the channel of the other side each other to the left and right ears increases (region A in  FIG. 7 ). The separation filters  10 L,  10 R comprise extraction filters  12 L,  12 R for isolating a particular frequency range A where the reversal phenomenon occurs among the acoustic signal and outputting to other channel, and transmission filters  13 L,  13 R for passing the frequency band where no reversal phenomenon occurs to its channel. The operational output units  11 L,  11 R combine the acoustic signal in the particular frequency area A separated by the separation filters  10 R,  10 L of other channel with the acoustic signal for its channel, respectively. Further, the filter circuit groups  5 L,  5 R generate sound field adjustment signals based on the acoustic signals outputted from the operational output units  11 L,  11 R. That is, the acoustic signal of a particular frequency area A separated by the extraction filters  12 L,  12 R of the separation filters  10 L,  10 R is synthesized in other channel, and the sound field adjustment signal is generated on the basis of this synthesized acoustic signal. Thereby the reversal phenomenon of the speaker  7  is avoided, and as in the previous embodiment, it is possible to approximate the interaural correlation coefficient of the diffuse sound field by adjusting the opening angle θ OP  of the virtual speaker  7 ′. 
         [0040]      FIG. 6  shows a sound processing apparatus according to further embodiment. A sound field adjustment filter  15  in the sound processing apparatus  14  is applied to the panel speaker  7  which is provided with a common diaphragm  7   a.  The sound field adjustment filter  15  comprises reduction filters  16 L,  16 R corresponding to each of the channels in addition to a pair of filter circuit groups  5 L,  5 R. The reduction filters  16 L,  16 R reduce the level of only a particular frequency range A where reversal phenomenon occurs in the acoustic signals, and directly pass through the band except this frequency range. On the other hand, the filter circuit groups  5 L,  5 R are assigned a predetermined pass characteristics and further the level adjustment is performed so as to restore the acoustic signals in the frequency range A outputted to a low level. For example, the gain of the reduction filters  16 L,  16 R is set to k (0&lt;k&lt;1) for the acoustic signals in particular frequency range A where the reversal phenomenon occurs, and the additional gain in a particular frequency range A of the filter circuit groups  5 L,  5 R is set to reciprocal 1/k for recovering, to thereby generate the sound field adjustment signal. According to this, the reversal phenomenon of the speaker  14  is avoided, and as in the previous embodiment, it is possible to approximate the interaural correlation coefficient of the diffuse sound field by adjusting the opening angle θ OP  of the virtual speaker  7 ′. 
       REFERENCE NUMBER MEANING 
       [0000]    
       
           1  sound field adjustment apparatus 
           2  speaker 
           3 L input terminal 
           3 R input terminal 
           4 L operational output unit 
           4 R operational output unit 
           5  sound field adjustment filter 
           5 L filter circuit group 
           5 R filter circuit group 
           6 L delay circuit 
           6 R delay circuit 
           7  speaker 
           7   a  diaphragm 
           8  sound field adjustment device 
           9  sound field adjustment filter 
           10 L separation filter 
           10 R separation filter 
           11 L operational output unit 
           11 R operational output unit 
           12 L extraction filter 
           12 R extraction filter 
           13 L transmission filter 
           13 R transmission filter 
           14  sound field adjustment device 
           15  sound field adjustment filter 
           16 L reduction filter 
           16 R reduction filter 
           50   a,    50   b  . . .  50   n  filter circuit 
           51   a,    51   b  . . .  51   n  band pass filter 
           52   a,    52   b  . . .  52   n  time delay setting means 
           53   a,    53   b  . . .  53   n  gain setting means 
           54  inverter