Low rate speech coding using contour quantization

Abstract

Vector quantization-based approaches to speech coding have generated new interest in very low bit rate speech coding, that is, speech coded to bit rates below 1200 bits/sec. To achieve such low bit rates, it is necessary to quantize the pitch and energy parameters at rates below 100 bits/sec. Contour quantization is introduced as a technique in which the contour of a given parameter is normalized by a nominal value and vector quantized. Contour quantization is shown to be extremely robust and efficient in encoding the pitch and energy parameters of the LPC vocoder. In this paper, a low rate speech coding system which uses contour quantization to encode the LPC excitation is presented. The system is a fixed bit rate system which is intended to operate at bit rates ranging from 400 bits/s to 800 bits/s. The overall system delay varies from 300 ms at 800 bits/s to 400 ms at 400 bits/s. At 800 bits/s, the system achieved a score of 89 on a three male speaker DRT, and a score of 81 on a three female speaker DRT.