Efficient vector quantization of lpc parameters for harmonic speech coding
V. Cuperman, Bhaskar Bhattacharya · 1996
The present thesis deals with the problem of efficient (in bit rate and computational complexity) quantization of Linear Prediction Coding (LPC) parameters for low bit rate speech coding. The thesis introduces a new LPC quantization technique based on the Multi-Stage Vector Quantization (MSVQ) combined with a multi-candidate M-L search. The resulting procedure is assessed by evaluating the quantization spectral distortion on a speech data-base and by evaluating the subjective speech quality of a low-rate speech coder which employs the MSVQ LPC quantization. The general structure of MSVQ is described along with a geometrical interpretation to provide insight into the structure of the reproduction alphabet in MSVQ. In particular, it is shown that MSVQ codevectors provide a tiling of the sample space with repetitive patterns. Two tree-search techniques are suggested and and one of them, the M-L search technique is studied in more detail. The experimental results obtained with MSVQ indicate that trasparent quantization of LSFs (Line Spectral Frequencies--an efficient LPC representation) can be achieved with just 22 bits/vector with computational complexity comparable to the Split VQ at 24 bits/vector. Alternatively, transparent quantization of LSFs can be done using 24 bits/vector (as is done using Split VQ) at a much lower computational complexity. Several results relating performance and complexity trade-offs are reported showing that MSVQ is a very flexible approach which provides a wide range of performance-complexity trade-offs and good robustness. The performance of MSVQ codes have been studied under channel error conditions and codebook ordering using pseudo-Gray coding. It is shown that while VQ based systems have lower average spectral distortion and a lower percentage of 2-4 dB outliers even with transmission errors, scalar quantization may lead to a lower percentage of 4 dB outliers particularly at high error rates. Performance of the MSVQ codes have also been studied for effects of language and input spectral shape. It has been shown that MSVQ codes become more robust as the number of stages are increased. Finally, one of the MSVQ codes developed here has been used to implement a 1800 bps speech coder using a harmonic coding of excitation and very coarse 0-bit quantization of harmonic spectral shape. The speech quality of the 1800 bps coder was better than the 2400 bps LPC-10c coder.