Methods of improving vector-scalar quantization of linear predictive coding (LPC) coefficients with 20 to 30 bits per 20 ms speech frame are studied. The first approach used is to couple the vector and scalar quantization stages. The second innovation is the incorporation of a small adaptive codebook to the larger fixed codebook. Frame-to-frame correlation of the LPC coefficients is exploited at no extra cost in bits. It is shown that the performance of vector-scalar quantization of speech with the use of the two new techniques is better than that of scalar coding techniques currently used in LPC coders.
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