Finite state CELP for variable rate speech coding

Abstract

A finite-state code excited linear prediction (CELP) system is proposed for variable-rate speech coding. The encoding system consists of a number of CELP coders with different linear predictive coding parameter quantization patterns, code book sizes, and population densities. The selection of the encoding state for each input vector depends on the input signal characteristics, the desired bit rate/signal to quantized noise ratio, and the current state of the encoder. In CELP coders the greater part of the bit resources, about 70%, is used for encoding of the excitation signal. However, the excitation accuracy needed to encode a speech segment with a desired level of fidelity strongly depends on its short-term spectral characteristics. The use of a finite-state system involves some implicit clustering of speech and allows variable-rate coding of the excitation. The gain in compression that can be obtained from variable-rate coding of the excitation signal is investigated. Experiments with a four-state variable-rate CELP coder produce good-quality encoded speech at 5 kbit/s. >