Towards optimal LPC spectral parameters estimation in standard CELP 4800 b/s speech coder

The influence of nonuniform scalar quantization and the specific interpolation procedure of LSP parameters, as proposed in USA FED STD 1016 CELP 4800 b/s speech coder, to the LPC spectra of speech obtained by using four standard LPC methods: autocorrelation with Hamming window, covariance, modified covariance and lattice, is considered. Comparative experimental analysis is done referring to the results of three different spectral measures related to the RMS LOG spectral measure: likelihood ratio, cosh measure and cepstral distance. The experimental analysis shows that, in the sense of the global LPC spectral representation, the best results are obtained by using the modified covariance method. In this way, the orientation to the autocorrelation method with Hamming window, proposed in USA FED STD 1016 CELP coder, is not justified