Phonetic Segmentation for Low Rate Speech Coding

Efforts to bridge the gap between waveform coders and vocoders has led to a new class of hybrid speech coders. These coders perform analysis-by-synthesis encoding of an excitation signal and reconstruct speech from the coded excitation signal and a quantized time-varying filter model of speech production. Most notable of these coders are those which use vector quantization to code the excitation signal as a sequence of vectors. The coding technique is called Code Excited Linear Prediction (CELP) [1], or Vector Excitation Coding (VXC) [2]. VXC coders result in coded speech with a waveform approximating the original and are able to achieve a satisfactory, natural-sounding quality at bit rates as low as 4.8 kb/s. When the bitrate is reduced below 4.8 kb/s, the quality of VXC coders degrades rapidly and becomes inferior to the synthetic quality of an LPC vocoder operating at 2.4 kb/s. There remains then the challenging problem to find an algorithm that at 2.4 kb/s (or even at 3.6 kb/s) will achieve the quality that VXC offers at 4.8 kb/s

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