Optimal quantization of LSP parameters using delayed decisions

A previously published study by the authors (Proc. ICASSP, p.394-7, 1988) of optimal quantization of line spectral pair (LSP) parameters is extended by incorporating delayed decisions coding (in frequency). The A* algorithm is proposed for finding the best quantization bit pattern of LSP frequency differences. The best coding pattern is obtained efficiently without an exhaustive, hence prohibitive, search. The proposed search achieves a better rate-distortion performance than the best results obtained in the previous study. At 30 bits/frame, a net gain of 2 bits/frame over the previous results, the novel method achieves 1-dB average spectral distortion. Most importantly, the number of frames with large spectral distortions (>2 dB), which can be subjectively disturbing and degrade the perceived quality of a speech coder, is significantly reduced. The search complexity of the A* algorithm is moderate. While the peak load is comparable to a nonoptimal M-algorithm, the average load is about an order of magnitude lower