Self-Orthogonal Convolutional Coding for the DPCM-AQB Speech Encoder

A fixed-tap differential pulse code modulation (DPCM) system with a robust backward-adaptive Jayant quantizer is investigated for speech encoding at 16-40 kbits/s using binary phase shift keying over an additive white Gaussian noise channel. The performance of this system becomes unacceptable as the channel bit error rate(Pb)approaches 10-2. Using high-rate, long constraint length, self-orthogonal convolutional codes, the DPCM system performance is much-improved for10⁻⁴ < Pb < 10⁻²depending on the transmitted data rate. The use of high-rate(n - 1)/n, n = 2,3,4,, and 5 codes minimizes the number of bits allocated to channel coding, and decoding complexity is reduced by employing self-orthogonal codes which admit threshold decoding. Subjectively, while there is additional quantization noise with channel coding, the irritating popping and squeaking sounds due to channel errors are eliminated.