Predictive coding of speech signals and subjective error criteria

Predictive coding methods attempt to minimize the r.m.s. error in the coded signal. However, the human ear does not perceive signal distortion on the basis of r.m.s. error regardless of its spectral shape relative to the signal spectrum. Specifically, for speech signals, the locations of the formant frequencies and their rates of change with time influence the audibility, and thus the subjective distortion of any quantizing noise. In this paper, methods for reducing the subjective distortion in predictive coders for speech siganls are described and evaluated. Improved speech quality is obtained a) by efficient removal of formant and pitch related redundant structure of speech before quantizing and b) by effective masking of the quantizer noise by the speech signal.