Method for improving speech quality in code excited linear predictive speech coding

A code excited linear predictive coder comprising:

• linear predictive code analysis means for receiving an input signal and generating from said input signal a set of linear predictive filter coefficients;
• weighting means for receiving said input sequence and said set of linear predictive filter coefficients for generating a weighted input sequence;
• codebook means for generating output codewords;
• first weighted linear predictive synthesis filter means responsive to said set of linear predictive filter coefficients and said codewords for generating synthesis filtered codewords;
• pitch filter means for generating pitch excitation sequences;
• second weighted linear predictive synthesis filter means responsive to said set of linear predictive filter coefficients and said pitch excitation sequences for generating synthesis filtered pitch excitation sequences;
• equation solving means receiving said weighted input sequence, said synthesis filtered codewords and said synthesis filtered pitch excitation sequences for computing a pitch predictor tap gain and a codeword excitation gain;
• first multiplying means for multiplying said codebook output sequences by said codeword excitation gain to produce a codebook excitation output signal;
• second multiplying means for multiplying said pitch excitation sequences by said pitch predictor tap gain to produce a pitch predictive excitation;
• and
• summing means for summing said codebook excitation output signal and said pitch predictive excitation to generate a combined excitation to be transmitted with said linear prediction coefficients.

A method of generating an excitation sequence for transmission with linear predictive coefficients of an input signal in a code excited linear predictive speech coder, comprising the steps of:

• computing a pitch lag by finding the location of a maximum cross-correlation between a weighted input sequence and synthesis-filtered contents of a pitch buffer of the coder;
• generating an unscaled pitch prediction sequence using the computed pitch lag and a pitch tap gain of 1.0;
• passing the unscaled pitch prediction sequence through a weighted linear predictive synthesis filter to produce an unscaled weighted synthesis pitch prediction sequence;
• computing a pitch prediction sequence variance from the unscaled weighted synthesis pitch prediction sequence and a cross-correlation between the weighted input sequence and unscaled weighted synthesis pitch prediction sequence;
• conducting an exhaustive Gaussian codebook search and, for each codeword output sequence obtained from said codebook, computing a codeword output sequence variance and a cross-correlation between the codeword output sequence and the weighted input sequence;
• determining optimal values for codeword gain and pitch tap gain from said computed variances and said cross-correlations;
• multiplying the pitch prediction sequence by the optimal value of pitch tap gain to arrive at a scaled pitch prediction sequence;
• multiplying the codeword output sequence by the optimal codeword gain to arrive at a scaled codeword sequence;
• and
• summing the scaled pitch and codeword sequences to generate parameters representing said excitation sequence.