Digital speech coding circuit

Abstract

A digital speech coding circuit makes use of linear predictive coding, vector quantization and difference, Huffman coding, and excitation estimation to produce digital representations of human speech having bit rates low enough to be transmitted over such channels as telephone lines and at the same time being capable of being synthesized in the receiver portion of the circuit to produce analog speech of high intelligibility and quality. The transmitter portion of the circuit comprises a series connection of a low pass filter, analog to digital converter, linear predictive coding module comprising five resonators for establishing five center frequencies and bandwidths of the analog speech, vector quantization module comprising binary representation of the likely combinations of resonances found in human speech, Huffman coding module, a variable bit rate to fixed bit rate converter, and optionally, an encryption module. Another branch of the transmitter circuit extends from the output of the analog to digital converter to the bit rate converter and comprises a series combination of an inverse filter and an excitation estimation module having parallel outputs respectively representative of a voiced/unvoiced signal, the excitation amplitude, and the excitation pulse position. The receiver portion of the circuit comprises a series connection of a fixed bit rate to variable bit rate converter, a bit unmapping module which produces separate outputs representative of the reflection coefficients and excitation of the speech, a synthesis filter which receives these outputs and produces a digital signal representative of the analog speech, a digital to analog converter, and a low pass filter.