Quality ratings of LPC vocoders: Effects of number of poles, quantization, and frame rate.

Abstract

Four values for number of poles (13, 11, 9, 8) were combined factorially with three values of step size for quantization of log area ratios (0.5, 1, 2 dB), and with four values of frame rate (100, 67, 50, 33 per second), to define 48 LPC vocoder systems with overall bit rates ranging from 8.7 down to 1.3 kbps. Subjects rated the DEGRADATION of signal quality by each vocoder, for each of seven sentence tokens, chosen to challenge LPC vocoders maximally. The results define the combination of LPC parameters yielding the best speech quality for any desired overall bit rate.