Apparatus and quality enhancement algorithm for mixed excitation linear predictive (MELP) and other speech coders

Abstract

A system and method for enhancing the speech quality of the mixed-excitation linear predictive (MELP) coder and other low bit-rate speech coders are disclosed. The system includes a robust pitch-detection algorithm, which adjusts or slides a pitch-analysis window to provide the speech coder with more reliable pitch information. In addition, the system is shown to be compatible with the existing MELP coder in terms of the bit stream.

Therefore, having thus described the invention, we claim:  
     
         1 . A method of enhancing the speech quality of a speech coder comprising the steps of: 
 digitally sampling speech to create a speech waveform over a multiplicity of frames;    using a sliding-sample window to locate a frame position with the highest pitch correlation; and    formulating at least one synthesized voice parameter in response to the speech waveform within the located frame position.    
     
     
         2 . The method of  claim 1 , wherein the frame position with the highest pitch correlation is determined by performing a recurrence calculation on the autocorrelation of the pitch over multiple frame positions defined by the sliding-sample window.  
     
     
         3 . The method of  claim 1 , wherein the frame position with the highest pitch correlation is determined by performing a recurrence calculation on the autocorrelation of the pitch for a fixed-length sliding-sample window.  
     
     
         4 . The method of  claim 1 , wherein the frame position with the highest pitch correlation is determined by performing a recurrence calculation on the autocorrelation of the pitch for up to a predetermined number of frames.  
     
     
         5 . The method of  claim 1 , wherein the step of formulating comprises estimating a frame pitch in response to the signal contained within the located frame position.  
     
     
         6 . The method of  claim 5 , further comprising the step of: 
 estimating linear predictive coding (LPC) coefficients in response to the signal contained within the located frame position.    
     
     
         7 . The method of  claim 5 , further comprising the step of: 
 estimating gain in response to the signal contained within the located frame position.    
     
     
         8 . The method of  claim 5 , further comprising the step of: 
 estimating a voicing decision in response to the signal contained within the located frame position.    
     
     
         9 . The method of  claim 5 , further comprising the step of: 
 estimating a fractional pitch in response to the signal contained within the located frame position.    
     
     
         10 . A speech coder comprising: 
 means for sampling a speech waveform to generate a discrete representation of the speech waveform over a multiplicity of frames; and    means for locating a pitch-analysis window over that frame position with the highest pitch correlation.    
     
     
         11 . The coder of  claim 10 , wherein the means for locating a frame position with the highest pitch correlation compares pitch analysis results associated with multiple frames.  
     
     
         12 . The coder of  claim 10 , wherein the means for locating a frame position with the highest pitch correlation performs a recurrence calculation on the autocorrelation of the pitch for multiple frame positions defined by the sliding-sample window.  
     
     
         13 . The coder of  claim 10 , wherein the means for locating a pitch-analysis window comprises a fixed-length window.  
     
     
         14 . The coder of  claim 13 , wherein the frame position with the highest pitch correlation is determined by performing a recurrence calculation on the autocorrelation of pitch results from multiple frame positions defined by the fixed-length window.  
     
     
         15 . The coder of  claim 10 , wherein the frame position with the highest pitch correlation is determined by performing a recurrence calculation on the autocorrelation of the pitch from up to a predetermined number of frames defined by the sliding-sample window.  
     
     
         16 . The coder of  claim 10 , further comprising: 
 means for estimating a plurality of speech parameters in response to the signal contained within the located frame position.    
     
     
         17 . The coder of  claim 16 , wherein the means for estimating comprises at least one digital signal processor in the mixed-excitation linear predictive (MELP) coder.  
     
     
         18 . The coder of  claim 16 , wherein the means for estimating comprises at least one algorithm stored within the mixed-excitation linear predictive (MELP) coder.