A Novel Variable Rate Lpc Quantizer for High Performance Speech Coder

A highly e cient algorithm termed adaptive forward-backward vector quantization (AFBVQ) is developed for variable bit rate quantization of linear predictive coding (LPC) coe cients and integrated with the FS1016 Federal Standard Code Excited Linear Predictive (CELP) coder. This results in a high performance low bit rate speech coder called as AFBVQCELP which brings in two-fold bit rate reduction by backward LPC indexing and by forward LPC VQ. In AFBVQ, a previously decoded and temporally close speech signal is re-segmented into overlapping blocks. As the LPC coe cients calculated from one of those synthetic blocks are spectrally close to the current unquantized LPC coe cients, the backward LPC indexing is used to encode the current speech block; otherwise, the forward linear prediction is practised with the split vector quantization supported by a very e cient codebook initialization termed Mixture Gaussian Clustering (MGC) [1]. When compared to FS1016 CELP coder, AFBVQCELP reduces the LPC bit rate by 18 bit-per-frame (bpf) at the same spectral distortion. It means the overall bit rate is reduced from 4.8 kbps (FS1016 CELP) to 4.2 kbps. Furthermore, the proposed AFBVQ consistently outperforms the traditional forward LPC VQ by 3 bpf with the same spectral distortion. Subjective listening tests show that with AFBVQ-CELP the LPC bit rate can be further reduced to 8.4 bpf, resulting in 3.94 kbps overall bit rate without compromising the decoded speech quality. This work was supported in part by NSF Award IRI 95-02074 and by NASA Applied Information Systems Research award NAG-2573 and NASA Innovative Research award NAGW-4698 managed by Glenn Mucklow.