Efficient excitation modeling in a low bit-rate CELP coder

The author describes an efficient model of the excitation signal of a CELP (code excited linear prediction) coder, capable of reducing significantly the corresponding bit rate while maintaining a good subjective quality. This topic is of paramount importance since, typically, in CELP coders the greater part of the available bit rate is allocated to encode the excitation signal. The model has been improved by resorting to (1) a dual loop algorithm to compute pitch lags; (2) a codebook of excitation vectors designed by a covering method; and (3) a combined backward/forward adaptive estimator to track the excitation gain. Numerical results and listening tests show that this excitation model overcomes the typical deficiencies of CELP coders at low bit rates. In particular, the proposed approach makes it possible to reduce the excitation rate of a stochastic CELP coder by 20% while at the same time improving the speech quality in voiced speech