Speech coding system utilizing a recursive computation technique for improvement in processing speed

Abstract

This invention provides a novel speech coding system which recursively executes a filter-applied "Toeplitz characteristic" by causing a drive signal (i.e., an excitation signal) to be converted into a "Toeplitz matrix" when detecting a pitch period in which distortion of the input vector and the vector subsequent to the application of filter-applied computation to the drive signal vector in the pitch forecast called either "closed loop" or "compatible code book" is minimized. The vector quantization method substantially making up the speech coding system of the invention is characteristically used by the system.

What is claimed is: 
     
       1. A speech coding system, comprising: means for receiving an input speech signal and outputting said input speech signal in the form of an input speech vector having one frame of unit;   analyzing means for analyzing said input speech vector by means of a linear predictive coding method and extracting a predictive parameter .[.rom.]. .Iadd.from .Iaddend.said input speech vector;   weighting means for weighting said input speech vector with said predictive parameter from said analyzing means, and for outputting a first weighted input speech vector;   a first synthesis filter for outputting a zero-input speech vector;   a first substraction means for producing a difference between said first weighted input speech vector and said zero-input speech vector;   a means for preventing influence of a last frame and influence of a pitch from said first weighted input speech vector;   an excitation signal vector generating means for generating a first excitation signal vector when a target pitch period exceeds a predetermined value, and for generating a second excitation signal vector when said target pitch period is below said predetermined value;   a computing means for recursively executing one or more operations using a drive signal matrix using one of said first and second excitation signal vectors in the form of a first Toeplitz matrix when executing said one or more operations to determine an optimal pitch period at which an error between said first weighted input speech vector and .Iadd.a synthesized vector obtained by using .Iaddend.said one of said first and second excitation signal vectors is a minimum;   a second synthesis filter for generating a synthesis speech vector corresponding to said optimal pitch period;   a third synthesis filter;   a codebook for generating a code vector for input to said third synthesis filter, said code vector being expressible in terms of a second Toeplitz matrix;   a second subtraction means for producing a difference between the output of said first subtraction means and said synthesis speech vector corresponding to said optimal pitch period;   a third subtraction means for producing a difference between the output of said second subtraction means and said second synthesis filter; and   a selection means for selecting from said codebook an optimal code vector used to provide stable quality vector quatization such that said difference between the output from said third synthesis filter and said second weighted input speech vector is minimized.   
     
     
       2. The speech coding system according to claim 1, wherein said excitation signal vector generating means includes: a delay circuit and a waveform coupling means which synthesize a predetermined speech waveform and speech waveforms preliminarily stored in a storage means for storing a previous speech waveform; and   wherein said excitation signal vector generating means is connected to a switching means which, in accordance with a predetermined condition, switches the destination of the excitation signal vector delivered from said excitation signal vector generating means either to said delay circuit or to said waveform coupling means.   
     
     
       3. The speech coding system according to claim 2, wherein, if said optimal pitch period exceeds a dimensional number of said code vector, said switching means provides an excitation signal vector from said excitation signal vector generating means to said delay circuit, whereas if said pitch period is less than the dimensional number of said code vector, said switching means provides an excitation signal vector from said excitation signal vector generating means to said waveform coupling means; wherein said delay circuit delays said pitch period by a predetermined amount and said waveform coupling means couples a zero-vector with a previous excitation signal vector so as to produce a new excitation signal vector.   
     
     
       4. The speech coding system according to claim 2, further comprising a pitch analyzing means which is connected to said analyzing means for executing pitch analysis for implementing long-term speech forecast by applying a forecast parameter extracted rom said analyzing means and also applying a forecast residual signal vector designating a predictive error, and wherein said pitch analyzing means extracts a pitch period resulting from said pitch analysis and an optimal gain parameter suited for said pitch period, and outputs the value of said optimal gain parameter to said waveform coupling means. 
     
     
       5. A speech coding system, comprising: an input speech means which, upon receipt of an input speech signal, generates an input speech vector;   a weighting means which weights the input speech vector by means of a predetermined parameter and generates a weighted input speech vector;   an excitation signal vector generating means which extracts and generates an excitation signal vector from a filter excitation signal for driving a linear predictive coding check filter;   a computing means for recursively executing operations by using a drive signal matrix having the excitation signal vector represented by a Toeplitz matrix when executing the operations to determine an optical pitch period at which an error between the weighted input speech vector and the excitation signal vector is at a minimum; and   output generating means for outputting a speech vector corresponding to the optimal pitch period.   
     
     
       6. The speech coding system according to claim 5, wherein said excitation signal vector generating means includes means for generating the excitation signal vector including a first excitation signal vector generated when a pitch period exceeds a predetermined value and a second excitation signal vector produced when the pitch period is below the predetermined value. .Iadd. 
     
     
       7.  A speech coding system, comprising: an input speech means which upon receipt of an input speech signal, generates an input speech vector;   a weighting means which weights the input speech vector by means of a predetermined parameter and generates a weighted input speech vector;   an excitation signal vector generating means which extracts and generates an excitation signal vector from a filter excitation signal for driving a linear predictive coding filter which outputs a synthesized vector;   a computing means for recursively executing operations by using a drive signal matrix having the excitation signal vector represented by a Toeplitz matrix when executing the operations to determine an optimal code vector such that an error between the weighted input speech vector and the synthesized vector is at a minimum; and   output generating means for outputting a speech vector corresponding to the optimal code vector. .Iaddend.