On improving the performance of low bit-rate CELP-type speech coder

Abstract

Low bit-rate speech coding has wide applications in communication systems. The applications include narrow-band cellular radio system, memory-efficient systems for voice response and voice mail. This dissertation focuses on three major issues in a low bit-rate CELP-type speech coder: efficient encoding of spectral information and excitation codebook, robust speech decoding. The main contributions of the dissertation are as follows. First, we propose two dimensional multiple type frame segmentation (2D-MTFS) method which allows various types of two-dimensional segmentation of speech frames to save the transmission bits for LSP parameters without increasing the spectral distortion. The conventional coding method of LSP parameters generate redundant spectral information or spectral distortion by fixed update rate of LSP parameters independent of the order of coefficients and phonetic context. The intra-frame spectral distortion (IFSD) is defined to measure the spectral distortion of the reconstructed spectrum. Split VQ is designed for the quantization of LSP parameters. Second, we propose partial algebraic codebook (PAC) for the efficient encoding of excitation code. The ACELP coder which has been proposed for efficient computation of excitation codebook search consumes a large amount of bits to encode all non-zero excitation pulses separately. The VQ of pulse positions and signs which have some correlations with each other can reduce the bits required for encoding the excitation codevectors. This means using the PAC instead of the full set of algebraic codebook. The iterative training algorithm to obtain the optimal PAC is proposed. The PAC also makes it easy to vary the transmission bit rate of ACELP type coder dynamically. Finally, we propose the channel adaptive parameter estimation (CAPE) which matches the estimator to the non-stationary channel characteristics. We discussed the parameter estimation schemes using a posteriori probability and demonstrated ...