A Review of Speech Processing

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The paper reviews the signal theory of speech and the possibilities of bandwidth reduction in speech. The current techniques using Vocoders and other methods have been discussed. The channel Vocoders of Dudley and other claim a bandwidth reduction of 15: 1, the Formant tracking Vocoders 10: 1 and Time domain Vocoders give a reduction of 8: 1. Among recent developments, the high fidelity Vocoders compress a 100 Hz-10 KHz speech to a 330–2700 Hz band, suitable for transmission in telephone channels. The Time Domain speech compression equipment using CEPSTRUM technique, realizes a good quality speech at 2400 Hz bit/s, but the system is very complex. Using Adaptive A/D conversion techniques good quality speech has been realised at 9600 bit/s.For computerized signal processing, synthetic speech and speech from printed texts have been developed. Few such techniques have been discussed. The speech recognition problems, however, are still very complex and yet to be formalised.