Low delay, middle bit rate speech coder

A method of speech encoding comprising the steps of:

• a) filtering a speech signal S(n), a reconstructed speech signal S'(n), and a reconstructed residual signal r'(n), using a set of tuning coefficients a.sub.i to produce a residual excitation signal r(n),
• b) coding and decoding an error signal e(n) equal to the difference between said residual excitation signal r(n) and a predictive residual excitation signal X(n), to produce a reconstructed error residual signal e'(n),
• c) applying linear analysis to said reconstructed residual signal r'(n), equal to the sum of said reconstructed error residual signal e'(n) and said predictive residual excitation signal X(n), and deriving therefrom said set of tuning coefficients a.sub.i, and
• d) generating said predictive residual excitation signal X(n) from said reconstructed residual signal r'(n) and said set of tuning coefficients a.sub.i.

A speech processing system comprising:

• a speech encoder circuit comprising:
• a perceptual weighting filter W(z) receiving a speech signal S(n), a reconstructed speech signal S'(n), a reconstructed residual signal r'(n), and a set of tuning coefficients a.sub.i, and outputting a residual excitation signal r(n),
• a coding/decoding circuit receiving an error signal e(n) equal to the difference between said residual excitation signal r(n) and a predictive residual excitation signal X(n), and outputting a reconstructed error signal e'(n), a codebook index signal k, and a gain parameter c,
• a Linear Predictive Coding (LPC) circuit receiving said reconstructed residual signal r'(n), equal to the sum of said reconstructed error signal e'(n) and said predictive residual excitation signal X(n), and outputting said set of tuning coefficients a.sub.i,
• a Short Term Predictive (STP) circuit receiving said reconstructed residual signal r'(n) and said set of tuning coefficients a.sub.i, and outputting said predictive residual excitation signal X(n), and
• an first inverse perceptual weighting filter W.sup.-1 (z) receiving said reconstructed residual signal r'(n) and said set of tuning coefficients
• a.sub.i, and outputting said reconstructed speech signal S'(n), and a speech decoder comprising:
• a second decoder circuit receiving said codebook index signal k and said gain parameter c, and outputting a second reconstructed error residual signal e'(n),
• a second Linear Predictive Coding (LPC) circuit receiving a second reconstructed residual signal r'(n), equal to the sum of said reconstructed error residual signal e'(n) and a second predictive residual excitation signal X(n), and outputting a second set of tuning coefficients a.sub.i,
• a second Short Term Predictive (STP) circuit also receiving said second reconstructed residual signal r'(n) and said second set of tuning coefficients a.sub.i, and outputting said second predictive residual excitation signal X(n), and
• an second inverse perceptual weighting filter W.sup.-1 (z) receiving said second reconstructed residual signal r'(n) and said second set of tuning coefficients a.sub.i, and outputting a second reconstructed speech signal S'(n).