Real-time implementation of a low delay low bit rate vocoder with a single ADAP-21020

Abstract

This paper presents a low delay vocoder operating at 4.8 kb/s with a modified code excited linear prediction (CELP) algorithm. The total speech coding delay is only 2.5 msec. The coder has also been real-time implemented using a single ADSP-21020 chip and provided a good output speech quality. In this low delay vocoder, backward LPC analysis is used to obtain the short term filter coefficients and a new approach based on a dual-decision scheme is adopted to calculate the long term prediction (LTP) coefficients. At the excitation part, a new excited model is exploited which contains a sinusoidal codebook and a stochastic codebook to excite the whole speech system. The synthesized speech of this vocoder has a comparable quality to that of the conventional CELP coder, i.e., the FS-1016, while having a delay constraint more than an order of magnitude smaller than that of the conventional coder.