Voice transmission at a very low bit rate on a noisy channel: 800 bps vocoder with error protection to 1200 bps

A codec was studied at 800 b/s source coding and total bit rate of 1200 b/s after error protection. After an LPC 10 analysis, the 800-b/s coder encodes several frames together in order to use the intercorrelation between successive frames. The coder chooses from eight different spectrum coding schemes the one which best corresponds to the speech on the superframe, viz. the energy is quantized by a VQ of the evolution of the power on the superframe and a scalar quantization of a reference energy. The redundancy of 3/2 (1200/800) is filled up with block codes. Comparison between, on the one hand, BCH(63,36) codes and, on the other hand, a combination of Golay codes and short BCH gives, after mathematical simulations and listening tests, an advantage to the latter protection scheme. The vocoder remains nearly unaltered up to 3% BER on the 1200 b/s channel and is intelligible up to 5% BER