On improving voice periodicity prediction in codebook-excited LPC coders
Daniel Wei Lin · The Journal of the Acoustical Society of America · 1988
Codebook-excited predictive coders are able to synthesize high-quality speech at bit rates of 8 kbits/s and above. However, at lower bit rates (e.g. 4.8 kbits/s), speech enhancement techniques, such as adaptive post-filtering, are needed to improve subjective performance [cf. P. Kroon and B. S. Atal, Proc. ICASSP (1987)]. The main source of quality degradation at 4800 b/s is due to the reduced transmission bandwidth of the excitation parameters, which results in an inaccurate representation of the LPC innovation signal. In this paper, methods are proposed for improving the voiced speech excitation and prediction in the codebook-excited coder. The modification uses a composite (i.e., product) excitation codebook together with a close-loop pitch prediction filter that is optimized relative to the source codebook. Several optimization creiteria for the source and predictor are compared in terms of performance and computational complexity. The best result shows a segmental SNR improvement of nearly 2 dB over the conventional stochastically excited LPC coder.