Cascaded likelihood vector coding of the LPC information

Cascade likelihood vector quantization is proposed for coding LPC spectral parameters for application in low-rate code-excited linear prediction systems. The approach is based on representing the LPC all-pole filter as a cascade of two lower-order all-pole filters. The partitioning of the LPC polynomial is done in the root domain by clustering the roots into two distinct groups. The quantizer uses two codebooks to quantize each of the lower-order filters. However, the quantization process is done so as to optimize jointly the performances of the two subsystems. The likelihood-ratio distortion measure is used as a performance criterion in the design and coding processes. Splitting the LPC filter into two subsystems dramatically reduces the coding complexity while the efficiency of using vector quantization is essentially preserved. Experimental results show an average performance of 1.59, 1.41, 1.24, and 1.10 dB log-spectral distortion at the rates of 1.0, 1.1, 1.2, and 1.3 kb/s, respectively. These figures represent good quality at 1.0 kb/s up to essentially transparent quality at 1.3 kb/s