Quantization apparatus
BACKGROUND OF THE INVENTION 
     This invention relates to a quantization apparatus and, more particularly, to a quantization apparatus in which digitized stereo input signals are processed with quantization and word length limitation. 
     In certain prior-art apparatus for quantization, a dither addition circuit is provided for improving reproducibility by alleviating dropout of the information of weak intensity signals produced on quantization and word length limitation. 
     With such quantization apparatus, as disclosed in JP Patent Kokai (laid-Open) Patent Publication No. 05-145376 (1993), a dither addition circuit is provided upstream of a quantizer for adding dither signals to digital data in order to prevent failure in the waveform or level shifting and consequent deterioration in reproducibility due to word length limitation by rounding or half-adjustment during quantization of digital data by the quantizer and consequent dropout in the information contained in substantially sinusoidal pre-quantization weak-intensity signals. In this case, if, after quantization of the dither signals added to the digital data, a pre-set number of the lower bits are rounded or half-adjusted, the information proper to the minute or weak-intensity signals contained in the input signal is left in the quantized data for further alleviating the failure in the information of the minute weak-intensity signals induced by the word length limitation. 
     If, when the right-channel digital stereo signals and the left-channel digital stereo signals are supplied to a quantizer for the right channel and to a quantizer for the left channel, respectively, the same dither signals or dither signals not correlated with each other are supplied to left-channel and right-channel dither addition circuits provided upstream of the quantizers, the correlation between the left and right channels, proper to the stereo input signals, is deteriorated. 
     For example, if the same dither signals, having the cross-correlation coefficient equal to unity, are supplied to the left and right dither addition signals, the cross-correlation of signal components having inherently low left channel--right channel correlation is increased. Specifically, the ambience feeling created by the reverberating stereophonic components in music signals is not spread sufficiently towards left and right, but is collected towards a center position. 
     On the other hand, if the dither signals not correlated with each other, such as the dither signals having the cross-correlation coefficient equal to zero, are supplied to the left and right dither addition circuits, the cross-correlation of signal components having the left channel-right channel correlation coefficient equal to unity is decreased. Specifically, the sound image of the sound having a fixed center sound source position feeling becomes; blurred and spread toward left and right. 
     SUMMARY OF THE INVENTION 
     In view of the above-described status of the prior art, it is an object of the present invention to provide a quantization apparatus unsusceptible to deterioration of the cross-correlation in the stereophonic signals. 
     The present invention provides a quantization apparatus for quantizing and word length limiting digitized stereo input signals including a stereo dither signal generating unit for generating stereo dither signals synthesized from distinct dither signals of at least two channels not correlated to each other at an arbitrary ratio, a first addition unit for adding one of the stereo dither signals to one of the digital stereo input signals, a second addition unit for adding the other of the stereo dither signals to the other of the digital stereo input signals, a first quantization unit for quantizing and word length limiting an output signal of the first addition unit, and a second quantization unit for quantizing and word length limiting an output signal of the second addition unit. 
     The stereo dither signal generating circuit preferably has a dither signal generator dedicated to a left channel, a dither signal generator dedicated to a right channel and at least one dither signal generator common to both the left and right channels. 
     It is also possible for the stereo dither signal generator to calculate the cross-correlation of the stereo input signals at an arbitrary time interval and to adjust the mixing ratio of the non-correlated dither signals of at least three routes so that the stereo signal will have cross-correlation proportional to the cross-correlation value. 
     With the quantization apparatus of the present invention, quantization may be achieved while maintaining cross-correlation between left and right channels proper to the stereo input signals. 
     The stereo dither signal generator includes an analyzer for analyzing the cross-correlation coefficients of the stereo input signals at a pre-set time interval, and a coefficient calculator for calculating cross-correlation coefficients of the stereo dither signals based upon the cross-correlation coefficients of the stereo input signals obtained from the analysis unit. The stereo signal generating unit generates stereo dither signals having a cross-correlation coefficient equal to the cross-correlation coefficient of the stereo input signal or to an arbitrary number multiple of the cross-correlation coefficient of the stereo input signal. In this manner, quantization may be achieved while maintaining cross-correlation between left and right channels proper to the stereo input signals, and the failure in the information concerning the cross-correlation between left and right channels proper to the stereo input signals may be decreased. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block circuit diagram showing an arrangement of a first embodiment of the apparatus for quantization according to the present invention. 
     FIG. 2 is a block circuit diagram showing an arrangement of a stereo dither signal generating circuit in the embodiment shown in FIG. 1. 
     FIG. 3 is a graph showing the cross-correlation coefficients of the stereo dither signals. 
     FIG. 4 is a graph showing the cross-correlation coefficients of the stereo dither signals. 
     FIG. 5 is a block circuit diagram showing an arrangement of another stereo dither signal generating circuit in the embodiment shown in FIG. 1. 
     FIG. 6 is a block circuit diagram showing an arrangement of a second embodiment of the apparatus for quantization according to the present invention. 
     FIG. 7 is a block circuit diagram showing an arrangement of the stereo dither signal generating circuit in the embodiment shown in FIG. 6. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to the drawings, preferred embodiments of the quantization apparatus according to the present invention will be explained in detail. 
     Referring first to FIGS. 1 to 4, a first embodiment is explained. 
     One ST 1  of digitized stereo input signals, supplied via an input terminal 1, are supplied to a first dither addition circuit 3. The other ST 2  of the digitized stereo input signals, supplied via the input terminal 1, is supplied to a second dither addition circuit 4. 
     To the first dither addition circuit 3 and to the second dither addition circuit 4, stereo dither signals SDZ 1  and SDZ 2  are also supplied from a stereo dither signal generator 5. Thus the first dither addition circuit 3 sums the stereo dither signals SDZ 1  to the stereo input signal ST 1 . The second dither addition circuit 4 sums the stereo dither signals SDZ 2  to the stereo input signal ST 2 . 
     A sum output STD 1  of the first dither addition circuit 3 is supplied to a first quantizer 6, while a sum output STD 2  of the second dither addition circuit 4 is supplied to a second quantizer 7. The first quantizer 6 processes the sum output STD 1  with quantization and word length limitation and routes a quantized output Q 1  to an output terminal 8. The second quantizer 7 processes the sum output STD 2  with quantization and word length limitation and routes a quantized output Q 2  to an output terminal 8. 
     In the first embodiment, the stereo input signal is a 20 bit signal, while an output signal is a 16-bit signal and the stereo dither signal is a 4-bit signal. By adding the dither signal to the lower four bits of the stereo input signal, the information owned by minute weak-intensity signals of the stereo input signal is left in the output signal even after quantization from 20 bits to 16 bits. 
     Referring to FIG. 2, the stereo dither signal generator 5 includes three dither signal generators 11, 12 and 13 for generating non-correlated dither signals DZ 1 , DZ 2  and DZ 3  of three different routes, respectively, and multipliers 14, 15 and 16 for multiplying the dither signals DZ 1 , DZ 2  and DZ 3  from the dither signal generators 11 to 13 with optional multiplication coefficients K A , K B  and K C , respectively. In addition, the generator 5 includes an addition circuit 17 for adding multiplication outputs MDZ 2  and MDZ 1  among multiplication outputs MDZ 1 , MDZ 2  and MDZ 3  of the multipliers 14, 15 and 16, to each other, an addition circuit 18 for adding the multiplication outputs MDZ 2  and MDZ 3  to each other, and quantizers 19, 20 for quantizing and word length limiting sum outputs AD 1  and AD 2  from the addition circuits 17, 18. The stereo dither signals SDZ 1  and SDZ 2  are outputted by these quantizers 19, 20 so as to be supplied via output terminals 21, 22 to the first dither addition circuit 3 and to the second dither addition circuit 4, shown in FIG. 1, respectively. 
     The dither signal generator 11 generates dither signals for the left channel, the dither signal generator 13 generates dither signals for the right channel, and the dither signal generator 12 generates dither signals for the both the left and right channels. 
     Thus the stereo dither signal generator 5 adds to the product MDZ 1 , obtained by multiplying the left channel dither signal DZ 1 , generated by the dither signal generator 11, with the multiplication coefficient K A , and to the product MDZ 3 , obtained by multiplying the right channel dither signal DZ 3 , generated by the dither signal generator 13, with the multiplication coefficient K C , the product MDZ 2  obtained by multiplying the dither signal DZ 2  for both the lest and right channels, generated by the dither signal generator 12, with the multiplication coefficient K b , to produce the results of addition AD 1  an AD 2 , which are quantized and word length converted in order to produce stereo dither signals SDZ 1  and SDZ 2  having arbitrary cross-correlation coefficients. 
     The cross-correlation coefficients owned by the stereo dither signals SDZ 1  and SDZ 2 , that is the stereo dither cross-correlation coefficients, will be explained by referring to FIGS. 3 and 4. 
     The graphs of FIGS. 3 and 4 show the relation between the mixing ratio of the dither signals and the stereo dither signals generated as described above, with the multiplication coefficients K a , K C  being both &#34;1&#34; and the multiplication coefficient K B  being increased from &#34;0&#34;, with the cross-correlation function of the stereo dither signals SDZ 1 , SDZ 2  being plotted on the vertical axis. FIGS. 3 and 4 illustrate the cases wherein the multiplication coefficient K B  on the horizontal axis is increased from &#34;0&#34; to &#34;3&#34; and from &#34;0&#34; to &#34;20&#34;, respectively. 
     In any of these cases, the cross-correlation coefficient is &#34;0&#34; for the multiplication coefficient K B  equal to &#34;0&#34; and becomes &#34;0.5&#34; for the multiplication coefficient K B  equal to &#34;1&#34;. With increase in the value of the multiplication coefficient K b , the cross-correlation coefficient becomes closer to &#34;1&#34;. 
     Thus the cross-correlation coeificient may be changed by changing the multiplication coefficient K B , such that stereo dither signals having an arbitrary cross-correlation coefficient may be generated. 
     If, when supplying the multiplication output MDZ 2 , obtained on changing the multiplication coefficient K B , to the addition circuits 17 and 18, the multiplication output MDZ 2  supplied to the addition circuit 17 or 18 is of a minus sign, it becomes possible to provide a left-channel and a right-channel stereo signal oppositely phased to each other in order to produce a cross-correlation coefficient of a minus sign. Such cross-correlation coefficient of the minus sign may be employed for generating special effects of producing an impression that the sound is being generated from outside the speaker. 
     With the above-described first embodiment of the quantization apparatus, the stereo dither signals STD 1 , STD 2  having an arbitrary cross-correlation, obtained by mixing and combining non-correlated dither signals of three different routes by the stereo dither signal generating circuit 5 at an arbitrary mixing ratio, are supplied to the first dither addition circuit 3 provided upstream of the first quantizer 6 and to the second dither addition circuit 4, provided upstream of the second quantizer 7, respectively, so that it becomes possible to maintain the cross-correlation between the stereo signals. 
     FIG. 3 shows a simplified arrangement of the stereo dither signal generator 5. The stereo dither signal generator shown in FIG. 5 includes dither signal generators 11, 13 for generating mutually non-correlated dither signals DZ 1 , DZ 2 , and multipliers 15a, 15b for multiplying the dither signals DZ 1 , DZ 2  from the dither signal generators 11, 13 with the multiplication coefficient K B  of a desired value. The stereo dither signal generator also includes multipliers 14, 16 for multiplying the dither signals DZ 1 , DZ 2  from the dither signal generators 11, 13 with the multiplication coefficients K A  and K B  of desired values and an addition circuit 17 for summing an output MDZ 2  &#39; of the multiplier 15b and an output MDZ 1  of the multiplier 14 together. The stereo dither signal generator also includes an addition circuit 18 for summing an output MDZ 2  &#34; of the multiplier 15a and an output MDZ 3  of the multiplier 16 and quantizers 19, 20 for quantizing and word length limiting the addition outputs AD 1 , AD 2  from these addition circuits 19, 20. The stereo dither signals SDZ 1 , SDZ 2  are outputted by these quantizers 19, 20 so as to be supplied via output terminals 21, 22 to the first and second dither addition circuits 3 and 4, shown in FIG. 1, respectively. 
     If two dither signal generators are employed a described above, the circuit construction may be simplified significantly, although the cross-correlation coefficient of the stereo dither signals cannot be set to &#34;0&#34; or &#34;1&#34; completely and can only be set to some intermediate value. 
     Referring to FIGS. 6 and 7, the second embodiment is explained. 
     The second embodiment is arranged as shown in FIG. 6. 
     That is, one ST 1  of stereo digital input signals, supplied via an input terminal 31, is inputted at a first dither signal addition circuit 33 and to a stereo dither signal generator 35 The other one ST 2  of stereo digital input signals, supplied via an input terminal 32, is inputted at a second dither signal addition circuit 34 and to a stereo signal dither generator 35. 
     The stereo dither signals SDZ 1 , SDZ 2  also enter the first dither signal addition circuit 33 and the second dither signal generator 34, from the stereo dither signal generator 35, respectively. Thus the first dither signal addition circuit 33 adds the stereo dither signal SDZ 1  to the stereo input signal ST 1 . The second dither signal addition node 34 adds the stereo dither signal SDZ 2  to the other stereo input signal ST 2 . 
     The addition output STD 1  of the first dither addition circuit 33 is supplied to the first quantizer 36. The addition output STD 2  of the second dither addition node 34 is supplied to the second quantizer 37. The first quantizer 36 quantizes and word length limits the addition output STD 1  to route a quantized output Q 1  to an output terminal 38. The second quantizer 37 quantizes and word length limits the addition output STD 2  to route a quantized output Q 2  to an output terminal 39. 
     In the second embodiment, the stereo input signal is a 20 bit signal, while an output signal is a 16-bit signal and the stereo dither signal is a 4-bit signal. By adding the dither signal to the lower four bits of the stereo input signal, the information owned by minute signals of the stereo input signal is left in the output signal even after quantization from 20 bits to 16 bits. 
     The present second embodiment differs from the first embodiment in that the stereo dither signal generator 35 fetches the stereo input signal and analyzes the cross-correlation coefficients of the stereo input signal at an arbitrary time interval in order to generate the stereo dither signal having a cross-correlation coefficient which is the same as or an arbitrary number multiple of the cross-correlation coefficient of the stereo input signal. 
     Referring to FIG. 7, the stereo signal generator 35 includes three dither signal generators 41, 42 and 43 of three different routes for generating three non-correlated dither signals DZ 1 , DZ 2 , DZ 3  and an analyzer 55 for analyzing the cross-correlation coefficients ST 1 , ST 2  via input terminals 53, 54 at an arbitrary time interval. The stereo signal generator 35 also includes a coefficient calculator 56 for calculating the cross-correlation coefficients of stereo input signals based upon the cross-correlation coefficients of the stereo dither signals ST 1  and ST 2  obtained by analysis by the analyzer 55, and multipliers 44, 45 and 46 for multiplying the dither signals. DZ 1 , DZ 2 , DZ 3  by arbitrary number multiples using the multiplication coefficients K A , K B , K C  supplied from the coefficient calculator 56. The stereo signal generator also includes an addition circuit 47 for summing the multiplication outputs MDZ 1  and MDZ 2  among the multiplication outputs MDZ 1 , MDZ 2  and MDZ 3  of the multipliers 44 to 46, an addition circuit 48 for summing the multiplication outputs MDZ 2  and MDZ 3  among the multiplication outputs MDZ 1 , MDZ 2  and MDZ 3  and quantizers 49 and 50 for quantizing and word length limiting addition outputs AD 1  and AD 2  from the addition circuits 47 and 48. These quantizers 49 and 50 output stereo dither signals SDZ 1  and SDZ 2  which are supplied via output terminals 51 and 52 to the first dither addition node 93 and the second dither addition node 34, shown in FIG. 4, respectively. 
     The dither signal generators 41, 42 and 43 generate dither signals for the stereo left channel, dither signals for the stereo right channel and dither signals for both the stereo left and right channels. 
     Thus the stereo dither signal generator 35 adds to the product MDZ 1 , obtained by multiplying the left channel dither signal DZ 1 , generated by the dither signal generator 41, with the multiplication coefficient K A , obtained via ananalyzer 55 and a coefficient calculator 56, and to the product MDZ 3 , obtained by multiplying the right channel dithersignal DZ 3 , generated by the dither signal generator 43, with the multiplication coefficient K C , obtained via the analyzer 55 and the coefficient calculator 56, the product MDZ 2  obtained by multiplying the dither signal DZ 2  for both the lest and right channels, generated by the dither signal generator 42, with the multiplication coefficient K B , obtained by the analyzer 55 and the coefficient calculator 56, to produce the results of addition AD 1  an AD 2 , which are quantized and word length converted in order to produce stereo dither signals SDZ 1  and SDZ 2  having arbitrary cross-correlation coefficients. 
     In the present second embodiment, the cross-correlation coefficients owned by the stereo dither signals SDZ 1  and SDZ 2 , that is the stereo dither cross-coefficients, may be explained by referring to FIGS. 3 and 4. 
     In addition, the cross-correlation coefficients having the minus sign may be obtained, as in the first embodiment. 
     Besides, two stereo dither signal generators may be employed for constituting the stereo dither signal generator. 
     With the above-described second embodiment of the quantization device, the cross-correlation of the stereo input signals is calculated at an arbitrary time interval, and the mixing ratio of the dither signals is adjusted for a pre-set time division so that the stereo dither signal will have the cross-correlation proportional to the calculated value, so that the cross-correlation of the stereo signals may be maintained more completely, while dropout of the information concerning the cross-correlation inherently owned by the stereo input signals may be diminished. 
     The present invention is not limited to the above-described first and second embodiments. For example, it may be applied to stereo panpot employed in a digital mixer.