Spectral dynamics is more important than spectral distortion

Linear prediction coefficients are used to describe the power-spectrum envelope in the majority of low-bit-rate coders. The performance of quantizers for the linear-prediction coefficients is generally evaluated in terms of spectral distortion. This paper shows that the audible distortion in low-bit-rate coders is often more a function of the dynamics of the power-spectrum envelope than of the spectral distortion as usually evaluated. Smoothing the evolution of the power-spectrum envelope over time increases the reconstructed speech quality. A reasonable objective is to find the smoothest path that keeps the quantized parameters within the Voronoi regions associated with the transmitted quantization index. We demonstrate increased quantizer performance by such smoothing of the line-spectral frequencies