Speech coding and decoding apparatus

Abstract

A speech signal is input to an excitation signal generating section, a prediction filter and a prediction parameter calculator. The prediction parameter calculator calculates a predetermined number of prediction parameters (LPC parameter or reflection coefficient) by an autocorrelation method or covariance method, and supplies the acquired prediction parameters to a prediction parameter coder. The codes of the prediction parameters are sent to a decoder and a multiplexer. The decoder sends decoded values of the codes of the prediction parameters to the prediction filter and the excitation signal generating section. The prediction filter calculates a prediction residual signal, which is the difference between the input speech signal and the decoded prediction parameter, and sends it to the excitation signal generating section. The excitation signal generating section calculates the pulse interval and amplitude for each of a predetermined number of subframes based on the input speech signal, the prediction residual signal and the quantized value of the prediction parameter, and sends them to the multiplexer. The multiplexer combines these codes and the codes of the prediction parameters, and send the results as an output signal of a coding apparatus to a transmission path or the like.

We claim: 
     
       1. A speech coding apparatus, comprising: prediction filter means for producing a prediction residual signal in accordance with a prediction parameter and an input speech signal;   means for generating excitation pulses;   synthesis filter means for outputting a synthesized input speech signal based on the excitation pulses and the prediction parameter;   means for coding an amplitude and an interval of the excitation pulses and the prediction parameter;   in which said excitation pulse generating means comprises:   means for obtaining an error signal between the input speech signal and the synthesized input speech signal;   means for dividing a frame of the prediction residual signal for a predetermined time interval into subframes of the prediction residual signal, the time interval of the subframe being shorter than the time interval of the frame;   means for calculating a square sum of the prediction residual signal for each subframe; and   means for calculating a square sum of the error signal for each subframe; and   means for controlling said excitation pulse generating means such that the interval of the excitation pulses in each subframe is in accordance with the square sum of the prediction residual signal and the amplitude of the excitation pulses in the subframe is set so as to minimize the square sum of the error signal.   
     
     
       2. A speech coding apparatus according to claim 1, wherein said controlling means comprises mean for setting a short interval of the excitation pulses in the subframe if the square sum of the prediction residual signal has a large value and for setting a large interval of the excitation pulses in the subframe if the square sum of the prediction residual signal has a small value. 
     
     
       3. A speech coding apparatus according to claim 1, wherein said coding means comprises means for coding a pattern of the intervals of the excitation pulses for one frame. 
     
     
       4. A speech coding apparatus according to claim 1, wherein said prediction filter means comprises a linear prediction filter for eliminating short term correlation. 
     
     
       5. A speech coding apparatus according to claim 1, wherein said prediction filter means comprises a cascade connection of a linear prediction filter for eliminating correlation and a pitch-prediction filter for eliminating long term correlation. 
     
     
       6. A speech coding apparatus according to claim 1, wherein said prediction filter means and synthesis filter means comprise a prediction filter of a full pole model. 
     
     
       7. A speech coding apparatus according to claim 1, wherein said prediction filter means and synthesis filter means comprise a prediction filter of a zero pole model. 
     
     
       8. A speech coding apparatus according to claim 1, wherein said prediction filter means and synthesis filter means comprise a cascade connection of a long-term prediction filter and a short-term prediction filter. 
     
     
       9. A speech decoding apparatus which is adapted for decoding a code which is output from the speech coding apparatus according to claim 1, comprising: means for decoding the amplitude and the interval of the excitation pulses and the prediction parameter;   means for generating the excitation pulses having the amplitude and the interval obtained by said decoding means; and   means for synthesizing an input speech signal based on the excitation pulses and the prediction parameter obtained by said decoding means.   
     
     
       10. A speech coding apparatus comprising: prediction filter means for producing a short-term prediction residual signal in accordance with a prediction parameter and an input speech signal;   means for generating excitation pulses;   synthesis filter means for outputting a synthesis input speech signal based on the excitation pulses and the prediction parameter; and   means for coding an amplitude and an interval of the excitation pulses and the prediction parameter;   in which said excitation pulse generating means comprises:   means for obtaining an error signal between the input speech signal and the synthesized input speech signal;   means for dividing a frame of the prediction residual signal for a predetermined time interval into subframes of the prediction residual signal, the time interval of the subframe being shorter than the time interval of the frame;   means for counting a zero-crossing number of the short-term prediction residual signal for each subframe;   means for calculating a square sum of the error signal for each subframe; and   means for controlling said excitation pulse generating means such that the interval of the excitation pulses in each subframe is in accordance with the zero-crossing number of the short-term prediction residual signal and the amplitude of the excitation pulses in the subframe is set so as to minimize the square sum of the error signal. .[.   
     
     
       11.  A speech coding apparatus comprising: prediction filter means for producing a prediction residual signal in accordance with a prediction parameter and an input speech signal;   means for generating excitation pulses;   synthesis filter means for outputting a synthesized input speech signal based on the excitation pulses and the prediction parameter; and   means for coding an amplitude and an interval of the excitation pulses and the prediction parameter;   in which said excitation pulse generating means comprises:   means for obtaining an error signal between the input speech signal and the synthesized input speech signal;   means for dividing a frame of the prediction residual signal for a predetermined time interval into subframes of the prediction residual signal, the time interval of the subframe being shorter than the time interval of the frame;   means for calculating a pitch prediction residual signal;   means for calculating a square sum of the pitch prediction residual signal for each subframe;   means for calculating a square sum of the error signal for each subframe; and   means for controlling said excitation pulse generating means such that the interval of the excitation pulses in each subframe is in accordance with the zero-crossing number of the short-term prediction residual signal and the amplitude of the excitation pulses in the subframe is set so as to minimize the square sum of the error signal..].   
     
     
       12. A speech coding apparatus comprising: prediction filter means for producing a short-term prediction residual signal in accordance with a prediction parameter and an input speech signal;   means for generating excitation pulses;   synthesis filter means for outputting a synthesized input speech signal based on the excitation pulses and the prediction parameter; and   means for coding an amplitude and an interval of the excitation pulses and the prediction parameter;   in which said excitation pulse generating means comprises:   means for obtaining an error signal between the input speech signal and the synthesized input speech signal;   means for dividing a frame of the prediction residual signal for a predetermined time interval into subframes of the prediction residual signal, the time interval of the subframe being shorter than the time interval of the frame;   means for calculating a pitch prediction residual signal; means for counting a zero-crossing number of the pitch prediction residual prediction residual signal for each subframe;   means for calculating a square sum of the error signal for each subframe; and   means for controlling said excitation pulse generating means such that the interval of the excitation pulses in each subframe is in accordance with the zero-crossing number of the short-term prediction residual signal and the amplitude of the excitation pulses in the subframe is set so as to minimize the square sum of the error signal. .Iadd.   
     
     
       13.  A speech coding apparatus, comprising: prediction filter means for producing a prediction residual signal in accordance with a prediction parameter and an input speech signal;   excitation pulse generating means;   synthesis filter means for outputting a synthesized input speech signal based on the excitation pulses and the prediction parameter; and   means for coding an amplitude and a density of the excitation pulses and the prediction parameter, in which said excitation pulse generating means comprises:   means for obtaining an error signal between the input speech signal and the synthesized input speech signal;   means for dividing a frame of the prediction residual signal having a predetermined time into subframes of the prediction residual signal, each of the subframes of the prediction residual signal having a subframe time interval, the subframe time interval of each of the subframes being shorter than the time interval of the frame;   means for calculating a square sum of the prediction residual signal for each subframe; and   means for calculating a square sum of the error signal for each subframe; and   means for controlling said excitation pulse generating means such that the density of the excitation pulses in each subframe is in accordance with the square sum of the prediction residual signal and the amplitude of the excitation pulses in the subframe is set so as to minimize the square sum of the error signal. .Iaddend..Iadd.   
     
     
       14.  A speech coding apparatus according to claim 13, wherein said controlling means comprises means for causing the density of the excitation pulses in the subframe to be dense if the square sum of the prediction residual signal has a large value and for causing the density of the excitation pulses in the subframe to be sparse if the square sum of the prediction residual signal has a small value. .Iaddend..Iadd.15. A speech coding apparatus according to claim 13, wherein said coding means comprises means for coding a pattern of the densities of the excitation pulses for one frame. .Iaddend..Iadd.16. A speech coding apparatus according to claim 13, wherein said prediction filter means comprises a linear prediction filter for eliminating short term correlation. .Iaddend..Iadd.17. A speech coding apparatus according to claim 13, wherein said prediction filter means comprises a cascade connection of a linear prediction filter for eliminating correlation and a pitch-prediction filter for eliminating long term correlation. .Iaddend..Iadd.18. A speech coding apparatus according to claim 13, wherein said prediction filter means and said synthesis filter means comprise a prediction filter of a full pole model. .Iaddend..Iadd.19. A speech coding apparatus according to claim 13, wherein said prediction filter means and said synthesis filter means comprise a prediction filter of a zero pole model. .Iaddend..Iadd.20. A speech coding apparatus according to claim 13, wherein said prediction filter means and said synthesis filter means comprise a cascade connection of a long-term prediction filter and a short-term prediction filter. .Iaddend..Iadd.21. A speech decoding apparatus which is adapted for decoding a code which is output from the speech coding apparatus according to claim 13, comprising: means for decoding the amplitude and the density of the excitation pulses and the prediction parameter;   means for generating excitation pulses having the amplitude and the density obtained by said decoding means; and   means for synthesizing an input speech signal based on the excitation pulses and the prediction parameter obtained by said decoding means. .Iaddend..Iadd.22. A speech coding apparatus comprising:   prediction filter means for producing a short-term prediction residual signal in accordance with a prediction parameter and an input speech signal;   excitation pulse generating means;   synthesis filter means for outputting a synthesized input speech signal based on the excitation pulses and the prediction parameter; and   means for coding an amplitude and a density of the excitation pulses and the prediction parameter, in which said excitation pulse generating mean comprises:   means for obtaining an error signal between the input speech signal and the synthesized input speech signal;   means for dividing a frame of the prediction residual signal having a predetermined time into subframes of the prediction residual signal, each of the subframes of the prediction residual signal having a subframe time interval, the subframe time interval of each of the subframes being shorter than the time interval of the frame:   means for counting a number of zero-crossings of the short-term prediction residual signal for each subframe;   means for calculating a square sum of the error signal for each subframe; and   means for controlling said excitation pulse generating means such that the density of the excitation pulses in each subframe is in accordance with the number of zero-crossings of the short-term prediction residual signal and the amplitude of the excitation pulses in the subframe is set so as to minimize the square sum of the error signal. .Iaddend..Iadd.23. A speech coding apparatus comprising:   prediction filter means for producing a prediction residual signal in accordance with a prediction parameter and an input speech signal;   excitation pulse generating means;   synthesis filter means, for outputting a synthesized input speech signal based on the excitation pulses and the prediction parameter; and   means for coding an amplitude and a density of the excitation pulses and the prediction parameter, in which said excitation pulse generating means comprises:   means for obtaining an error signal between the input speech signal and the synthesized input speech signal;   means for dividing a frame of the prediction residual signal having a predetermined time into subframes of the prediction residual signal, each of the subframes of the prediction residual signal having a subframe time interval, the subframe time interval of each of the subframes being shorter than the time interval of the frame;   means for calculating a pitch prediction residual signal;   means for calculating a square sum of the pitch prediction residual signal for each subframe;   means for calculating a squire sum of the error signal for each subframe; and   means for controlling said excitation pulse generating means such that the density of the excitation pulses in each subframe is in accordance with the square sum of the ditch prediction residual signal and the amplitude of the excitation pulses in the subframe is set so as to minimize the square sum of the error signal. .Iaddend..Iadd.24. A speech coding apparatus comprising:   prediction filter means for producing a short-term prediction residual signal in accordance with a prediction parameter and an input speech signal;   excitation pulse generating means;   synthesis filter means for outputting a synthesized input speech signal based on the excitation pulses and the prediction parameter; and   means for coding an amplitude and a density of the excitation pulses and the prediction parameter, in which said excitation pulse generating means comprises:   means for obtaining an error signal between the input speech signal and the synthesized input speech signal;   means for dividing a frame of the prediction residual signal having a predetermined time into subframes of the prediction residual signal, each of the subframes of the prediction residual signal having a subframe time interval, the subframe time interval of each of the subframes being shorter than the time interval of the frame;   means for calculating a pitch prediction residual signal;   means for counting a number of zero-crossings of the pitch prediction residual signal for each subframe;   means for calculating a square sum of the error signal for each subframe; and   means for controlling said excitation pulse generating means such that the density of the excitation pulses in each subframe is in accordance with the number of zero crossings of the short-term prediction residual signal and the amplitude of the excitation pulses in the subframe is set so as to minimize the square sum of the error signal. .Iaddend..Iadd.25. A speech coding apparatus comprising:   a prediction parameter calculator for generating a prediction parameter based on an input speech;   an excitation pulse generator for generating excitation pulses;   a synthesis filter for generating a synthesized input speech based on the excitation pulses and the prediction parameter;   a subtracter for calculating a difference between the input speech and the synthesized input speech;   a controller for controlling said excitation pulse generator based on the input speech and said difference, such that density of the excitation pulses is varied in accordance with an amount of information contained in the input speech and an amplitude of the excitation pulses is set so as to minimize the difference; and   means for coding the excitation pulses. .Iaddend..Iadd.26. A speech coding apparatus according to claim 25, wherein said controller comprises:   a prediction filter for producing a prediction residual signal in accordance with the prediction parameter and the input speech;   a divider for dividing a frame of the prediction residual signal having a predetermined time interval into subframes of the prediction residual signal, each of the subframes of the prediction residual signal having a subframe time interval, the subframe time interval of each of the subframes being shorter than the time interval of the frame;   a calculator for calculating a square sum of the prediction residual signal for each subframe; and   means for controlling the density of the excitation pulses in each subframe in accordance with the square sum of the prediction residual signal. .Iaddend..Iadd.27. A speech coding apparatus according to claim 25, wherein said controller comprises:   a prediction filter for producing a short-term prediction residual signal in accordance with the prediction parameter and the input speech;   a divider for dividing a frame of the prediction residual signal for a predetermined time interval into subframes of the prediction residual signal, each of the subframes of the prediction residual signal having a subframe time interval, the time interval of each of the subframes being shorter than the time interval of the frame;   a counter for counting a number zero crossings of the short-term prediction residual signal for each subframe:   means for controlling the density of the excitation pulses in each subframe in accordance with the number of zero crossings of the short-term prediction residual signal. .Iaddend..Iadd.28. A speech coding apparatus according to claim 25, wherein said controller comprises:   a prediction filter for producing a prediction residual signal in accordance with the prediction parameter and the input speech;   a divider for dividing a frame of the prediction residual signal for a predetermined time into subframes of the prediction residual signal, each of the subframes of the prediction residual signal having a subframe time interval, the subframe time interval of each of the subframes being shorter than the time interval of the frame;   a first calculator for calculating a pitch prediction residual signal;   a second calculator for calculating a square sum of the pitch prediction residual signal: and   means for controlling the density of the excitation pulses in each subframe in accordance with the square sum of the pitch prediction residual signal. .Iaddend..Iadd.29. A speech coding apparatus according to claim 25, wherein said controller comprises:   a prediction filter for producing a short-term prediction residual signal in accordance with the prediction parameter and the input speech;   a divider for dividing a frame of the prediction residual signal for a predetermined time interval into subframes of the prediction residual signal, the time interval of the subframe being shorter than the time interval of the frame;   a calculator for calculating a pitch prediction residual signal;   a counter for counting a number of zero crossings of the pitch prediction residual signal for each subframe; and   means for controlling the density of the excitation pulses in each subframe in accordance with the number of zero crossings of the short-term   
     
     
        prediction residual signal. .Iaddend..Iadd.30.  A speech coding apparatus comprising: a prediction parameter calculator for generating a prediction parameter based on an input speech;   an excitation pulse generator for generating excitation pulses;   a synthesis filter for generating a synthesized input speech based on the excitation pulses and the prediction parameter;   a subtracter for calculating a difference between the input speech and the synthesized input speech;   a controller for controlling said excitation pulse generator based on the input speech and said difference, such that density of the excitation pulses is varied in accordance with an importance of information contained in the input speech and an amplitude of the excitation pulses is set so as to minimize the difference; and   means for coding the excitation pulses. .Iaddend..Iadd.31. A speech coding apparatus according to claim 30, wherein said controller comprises:   a prediction filter for producing a prediction residual signal in accordance with the prediction parameter and the input speech;   a divider for dividing a frame of the prediction residual signal for a predetermined time interval into subframes of the prediction residual signal, each of the subframes of the prediction residual signal having a subframe time interval, the subframe time interval of each of the subframe being shorter than the time interval of the frames;   a calculator for calculating a square sum of the prediction residual signal for each subframe; and   means for controlling the density of the excitation pulses in each subframe in accordance with the square sum of the prediction residual signal. .Iaddend..Iadd.32. A speech coding apparatus according to claim 30, wherein said controller comprises:   a prediction filter for producing a short-term prediction residual signal in accordance with the prediction parameter and the input speech;   a divider for dividing a frame of the prediction residual signal having a predetermined time interval into subframes of the prediction residual signal, each of the subframes of the prediction residual signal having a subframe time interval, the subframe time interval of each of the subframes being shorter than the time interval of the frame;   a counter for counting a number of zero crossings of the short-term prediction residual signal for each subframe;   means for controlling the density of the excitation pulses in each subframe in accordance with the number of zero crossings of the short-term prediction residual signal. .Iaddend..Iadd.33. A speech coding apparatus according to claim 30, wherein said controller comprises;   a prediction filter for producing a prediction residual signal in accordance with the prediction parameter and the input speech;   a divider for dividing a frame of the prediction residual signal for a predetermined time into subframes of the prediction residual signal, each of the subframes of the prediction residual signal having a subframe time interval, the subframe time interval of each of the subframes being shorter than the time interval of the frame;   a first calculator for calculating a pitch prediction residual signal;   a second calculator for calculating a square sum of the pitch prediction residual signal; and   means for controlling the density of the excitation pulses in each subframe in accordance with the square sum of the pitch prediction residual signal. .Iaddend..Iadd.34. A speech coding apparatus according to claim 30, wherein said controller comprises:   a prediction filter for producing a short-term prediction residual signal in accordance with the prediction parameter and the input speech;   a divider for dividing a frame of the prediction residual signal for a predetermined time interval into subframes of the prediction residual signal, each of the subframes of the prediction residual signal having a subframe time interval, the subframe time interval of each of the subframes being shorter than the time interval of the frame;   a calculator for calculating a pitch prediction residual signal;   a counter for counting a number of zero crossings of the pitch prediction residual signal for each subframe; and   means for controlling the density of the excitation pulses in each subframe in accordance with the number of zero crossings of the short-term prediction residual signal. .Iaddend..Iadd.35. A speech coding method comprising the following steps of:   generating a prediction parameter based on an input speech;   generating excitation pulses;   generating a synthesized input speech based on the excitation pulses and the prediction parameter;   calculating a difference between the input speech and the synthesized input speech;   controlling generation of said excitation pulses based on the input speech and said difference such that density of the excitation pulses is varied in accordance with an amount of information contained in the input speech and an amplitude of the excitation pulses is set so as to minimize the difference; and   coding the excitation pulses. .Iaddend..Iadd.36. A speech coding method comprising the following steps of:   generating a prediction parameter based on an input speech;   generating excitation pulses;   generating a synthesized input speech based on the excitation pulses and the prediction parameter;   calculating a difference between the input speech and the synthesized input speech;   controlling generation of said excitation pulses based on the input speech and said difference such that density of the excitation pulses is varied in accordance with an importance amount of information contained in the input speech and an amplitude of the excitation pulses is set so as to minimize the difference; and   coding the excitation pulses. .Iaddend..Iadd.37. A speech coding apparatus comprising:   prediction filter means, for producing a prediction residual signal in accordance with a prediction parameter and an input speech signal;   excitation pulse generating means;   synthesis filter means, for outputting a synthesized input speech signal based on the excitation pulses and the prediction parameter; and   means for coding an amplitude and an interval of the excitation pulses and the prediction parameter, in which said excitation pulse generating means comprises:   means for obtaining an error signal between the input speech signal and the synthesized input speech signal;   means for dividing a frame of the prediction residual signal having a predetermined time into subframes of the prediction residual signal, each of the subframes of the prediction residual signal having a subframe time interval, the subframe time interval of each of the subframes being shorter than the time interval of the frame;   means for calculating a pitch prediction residual signal;   means for calculating a square sum of the pitch prediction residual signal for each subframe:   means for calculating a square sum of the error signal for each subframe; and   means for controlling said excitation pulse generating means such that the interval of the excitation pulses in each subframe is in accordance with the square sum of the pitch prediction residual signal and the amplitude of the excitation pulses in the subframe is set so as to minimize the square sum of the error signal. .Iaddend.