Design of joint-source channel decoding algorithms for the 2400 bps melp speech coder
Thomas E. Fuja, T. Fazel · 1999
Mixed excitation linear predictive speech coding (MELP) is the new U.S. federal standard for 2400 bits/sec speech compression. Although, MELP's performance is much better that its predecessor, linear predictive coding (LPC) and code excited linear coding (CELP), it is not a perfect source coder. As a result there is redundancy in its output bit stream. In this dissertation, we consider the problem of reliable transmission MELP-encoded speech over noisy communication channels. The goal is to use this redundancy to improve the quality of reconstructed speech. Our objective is to design joint source-channel codes compatible with MELP bit stream. We begin by modeling some of the MELP parameters as one-step Markov chains. Then using this model, we quantify the residual redundancy of the MELP output bit stream. We find out that more than 20% of the bits in the multistage vector quantizer indices (MSVQ parameter) and pitch index and gain indices are redundant. We design a convolutional decoder to exploit the residual redundancy in the MELP bit stream using the Markov model parameters. Some simulation results are presented to compare the new source controlled decoder with an ordinary convolutional decoder. Our next contribution is the design of a source-controlled symbol (non-binary) turbo decoder compatible with the MELP bit stream. We introduce a new modified BCJR algorithm for source-controlled turbo decoding. Results indicates that in presence of source redundancy, the new decoder outperforms the regular turbo decoder. In the last chapter, we design a new source-controlled Generalized Minimum Distance (GMD) algorithm to exploit the redundancy of the pitch and gain and MSVQ bits. This new decoder can be used with a regular error/erasure decoder for block codes. We choose Reed-Solomon codes for our simulation and the results show that the new source-controlled GMD decoder is more successful in a very noisy channel than a regular GMD decoder.