Speech coder methods and systems

Abstract

Coding systems that provide a perceptually improved approximation of the short-term characteristics of speech signals compared to typical coding techniques such as linear predictive analysis while maintaining enhanced coding efficiency. The invention advantageously employs a non-linear transformation and/or a spectral warping process to enhance particular short-term spectral characteristic information for respective voiced intervals of a speech signal. The non-linear transformed and/or warped spectral characteristic information is then coded, such as by linear predictive analysis to produce a corresponding coded speech signal. The use of the non-linear transformation and/or spectral warping operation of the particular spectral information advantageously causes more coding resources to be used for those spectral components that contribute greater to the perceptible quality of the corresponding synthesized speech. It is possible to employ this coding technique in a variety of speech coding techniques including, for example, vocoder and analysis-by-synthesis coding systems.

1. A method for coding a speech signal to generate a coded signal comprising:
 generating a sequence of spectral magnitude values for a frame interval of said speech signal representing voiced speech, said spectral magnitude value sequence characterizing spectral components of a short-term frequency spectrum of said interval; 
 performing at least one of a non-linear transformation or spectral warping process on said sequence to produce an intermediate spectral value sequence having an enhanced characterization of at least one particular frequency range relative to another frequency range in the intermediate spectral sequence; and 
 coding said intermediate spectral value sequence to produce at least a portion of said coded signal for said interval of said speech signal. 
 
     
     
       2. The method of  claim 1  wherein said coding step codes said processed spectral value sequence based on linear predictive analysis. 
     
     
       3. The method of  claim 2  wherein said coding step comprises:
 inverse transforming said intermediate spectral values into a time domain representation signal; and 
 generating linear predictive codes for said time domain representation signal. 
 
     
     
       4. The method of  claim 1  wherein said step of performing non-linear transformation includes processing at least a portion of said spectral magnitude value sequence according to the expression [A(i)] N  [A(i)] N , where A(i) represents the respective values in said sequence portion and the value N is not 0 or 1. 
     
     
       5. The method of  claim 4  where the value N is a value less than 0 and not less than −1. 
     
     
       6. The method of  claim 1 , further comprising performing a spectral warping process on said sequence of spectral magnitude values, and wherein said coding step includes generating a warp code for said coded signal indicating a portion of said sequence warped by said warping process. 
     
     
       7. The method of  claim 6  wherein said warp code is an index of an entry in a warping function codebook. 
     
     
       8. The method of  claim 1  further comprising performing spectral warping on said sequence to produce an intermediate spectral value sequence having an enhanced characterization of at least one particular frequency range relative to another frequency range in the intermediate spectral sequence, wherein said step of performing spectral warping comprises increasing the number of values in a portion of said intermediate spectral value sequence characterizing a particular frequency range that would effect the perceptual quality of a correspond speech signal synthesized from said coded signal. 
     
     
       9. The method of  claim 8  wherein said step of performing spectral warping comprises decreasing the number of values in at least one other portion of said intermediate spectral value sequence characterizing another particular frequency range. 
     
     
       10. The method of  claim 1  wherein the particular operation performed for said non-linear transformation or spectral warping process is based on a property of said speech signal. 
     
     
       11. The method of  claim 10  wherein said property of said speech signal is a duration of a pitch period of said frame interval. 
     
     
       12. The method of  claim 1  wherein the particular frequency range represented in the spectral magnitude value sequence that is warped by said warping process is selected based on the value magnitudes representing the signal energy for such frequency range. 
     
     
       13. The method of  claim 1  wherein said coding step performs analysis-by-synthesis coding. 
     
     
       14. The method of  claim 13  wherein said analysis-by-synthesis coding is code-excited linear prediction analysis. 
     
     
       15. The method of  claim 1  wherein said step of generating said spectral magnitude value sequence characterizing said short-term frequency spectrum generates such sequence based on spectral components of at least one pitch period interval in said frame. 
     
     
       16. The method of  claim 15  wherein said step of generating the sequence of spectral magnitude values comprises:
 identifying a portion of said frame interval of said speech signal representing a pitch period; 
 performing a discrete Fourier transform of said identified portion of said frame interval to generate a sequence of spectral component values; and 
 determining respective magnitudes of said spectral component values to produce said spectral magnitude value sequence for said frame interval. 
 
     
     
       17. A method for decoding a coded speech signal, said coded signal including successive coded frame intervals of a speech signal, the decoding of a frame interval of said coded signal comprising the steps of:
 generating an intermediate spectral value sequence for at least a portion of said interval representing voiced speech, said intermediate spectral value sequence characterizing spectral components of a short-term frequency spectrum of said interval and further having an enhanced characterization of at least one particular frequency range relative to another frequency range; and 
 processing said intermediate spectral value sequence with at least one of an inverse non-linear transformation or inverse spectral warping process to produce a sequence of spectral magnitude values characterizing the short-term frequency spectrum for the voiced portion of said interval. 
 
     
     
       18. The method of  claim 17  wherein said short-term frequency spectrum represented in said intermediate spectral value sequence is a pitch period of voiced speech represented in said interval. 
     
     
       19. The method of  claim 17  wherein said step of processing by inverse non-linear transformation includes processing at least a portion of said spectral magnitude value sequence according to the expression [Ā′(i)] N  [Ā′(i)] N , where Ā″(i) Ā′(i) represents the respective values in said sequence portion and the value N is not 0 or 1, and wherein said expression performs an inverse transformation of a non-linear transformation used in coding said coded signal interval. 
     
     
       20. The method of  claim 17  further comprises the step of claim 17, further comprising processing said intermediate spectral value sequence with an inverse spectral warping process, and receiving a warp code for said coded signal interval indicating a portion of said intermediate spectral value sequence warped during said coded signal interval. 
     
     
       21. The method of  claim 20  wherein said warp code is an index of an entry in a warping function codebook. 
     
     
       22. The method of  claim 17  further comprising processing said intermediate spectral value sequence with an inverse spectral warping process to produce a sequence of spectral magnitude values characterizing the short-term frequency spectrum for the voiced portion of said interval, wherein said step of processing by inverse warping said intermediate spectral value sequence comprises adjusting a number of spectral values in the intermediate spectral value sequence characterizing at least one particular frequency range in producing said spectral magnitude value sequence and wherein said spectral value adjustment corresponds to inverse warping used in coding said coded signal interval. 
     
     
       23. The method of  claim 17  wherein the particular operation performed for said inverse non-linear transformation or spectral warping process is based on a property of said coded speech signal. 
     
     
       24. The method of  claim 23  wherein said property of said speech signal is a duration of a pitch period in said coded speech signal interval. 
     
     
       25. The method of  claim 17  wherein said generating step includes analysis-by-synthesis decoding. 
     
     
       26. The method of  claim 25  wherein said analysis-by-synthesis decoding is based on code-excited linear prediction analysis and comprises receiving codes identifying a respective excitation codebook entry corresponding to said interval. 
     
     
       27. A coder for generating a coded signal based on a speech signal comprising:
 a spectral transformer for generating a sequence of spectral magnitude values for a frame interval of said speech signal representing voiced speech, said spectral magnitude value sequence characterizing spectral components of a short-term frequency spectrum of said frame interval; 
 an encoder coupled to said spectral processor, said encoder for performing at least one of a non-linear transformation or a spectral warping process on said sequence to produce an intermediate spectral value sequence having an enhanced characterization of at least one particular frequency range relative to another frequency range in the intermediate spectral sequence; and 
 a spectral coder coupled to said encoder, said spectral coder for coding said intermediate spectral value sequence to produce at least a portion of said coded signal for said interval of said speech signal. 
 
     
     
       28. The coder of  claim 27  wherein said spectral coder comprises:
 an inverse transformer for inverse transforming said spectral parameters processed by said spectral processor into a time domain representation signal; and 
 a linear predictive code generator for generating linear predictive coefficients for said coded signal based on said time domain representation signal for said interval of said speech signal. 
 
     
     
       29. The coder of  claim 27  wherein said spectral coder includes a vocoder. 
     
     
       30. The coder of  claim 27  wherein said spectral coder includes an analysis-by-synthesis coder. 
     
     
       31. The coder of  claim 30  wherein said analysis-by-synthesis coder is a code-excited linear prediction coder. 
     
     
       32. The coder of  claim 27  wherein said spectral transformer for generating said spectral magnitude value sequence characterizing spectral components of a short-term frequency spectrum performs a transformation based on at least one pitch period represented in said interval. 
     
     
       33. The coder of  claim 32  wherein said spectral transformer comprises:
 a window processor and pitch detector for identifying an interval in said frame interval of said speech signal representing a pitch period; and 
 a discrete Fourier transformer coupled to said window processor, said discrete Fourier transformer for generating said spectral magnitude value sequence for said interval. 
 
     
     
       34. A coder for generating a coded signal from a speech signal comprising:
 means for generating a sequence of spectral magnitude values for a frame interval of said speech signal representing voiced speech, said spectral magnitude value sequence characterizing spectral components of a short-term frequency spectrum of said interval; 
 means for performing at least one of a non-linear transformation or spectral warping process on said sequence to produce an intermediate spectral value sequence having an enhanced characterization of at least one particular frequency range relative to another frequency range in the intermediate spectral sequence; and 
 means for coding said intermediate spectral value sequence to produce at least a portion of said coded signal for said interval of said speech signal. 
 
     
     
       35. A decoder for decoding a coded speech signal, said coded signal including successive coded frame intervals of a speech signal, said decoder comprising:
 a spectral decoder, said spectral decoder for generating an intermediate spectral value sequence for voiced speech represented in said frame interval of the coded signal, said intermediate spectral value sequence characterizing spectral components of a short-term frequency spectrum of said voiced speech and further having an enhanced characterization of at least one particular frequency range relative to another frequency range; and 
 inverse processor coupled to said spectral decoder, said inverse processor for processing said intermediate spectral value sequence with at least one of an inverse non-linear transformation or inverse spectral warping process to produce a sequence of spectral magnitude values characterizing a short-term frequency spectrum for the voiced portion of said interval. 
 
     
     
       36. The decoder of  claim 35  wherein said spectral decoder includes an analysis-by-synthesis decoder. 
     
     
       37. The decoder of  claim 35  wherein said analysis-by-synthesis decoder performs code-excited linear prediction analysis. 
     
     
       38. A decoder for decoding a coded speech signal, said coded signal including successive coded frame intervals of a speech signal, said decoder comprising:
 means for generating an intermediate spectral value sequence for voiced speech represented in said frame interval of the coded signal, said intermediate spectral value sequence characterizing spectral components of a short-term speech spectrum of voiced speech represented in said interval and further having an enhanced characterization of at least one particular frequency range relative to another frequency range; and 
 means for processing said intermediate spectral value sequence with at least one of an inverse non-linear transformation or inverse spectral warping process to produce a sequence of spectral magnitude values characterizing said short-term frequency spectrum for the voiced portion of said interval.