Spectral quantization of cepstral coefficients

Studies the cepstral coefficients as a suitable representation of the linear prediction filter for spectral coding purposes. Spectral coding methods in predictive speech coders are usually evaluated using the spectral distance measure. The average spectral distance combined with a measure of the percentage of spectra with high distortion are used to predict the perceptual quality when quantizing the prediction filter. The authors show that the spectral distance is equivalent to a squared error in the cepstral domain. Methods for spectral quantization using vector quantization of cepstral coefficients are analyzed. Better results than for quantization of line spectrum frequencies are reported for both single-stage VQ at 11-14 bits as well as 2-stage VQ at 18-22 bits. It is concluded that the cepstral coefficients are the right representation for LPC spectral coding purposes