Error correction scheme for telephone line transmission of RELP vocoder

This paper presents a scheme for encoding the output bits of an 8500 bps residual excited linear predictive coder (RELPC), in which the quality of the synthesized speech is more sensitive to errors in certain bits. Fifty-two of the 272 bits in a 32 ms voice frame were identified as most critical and in need of protection. A Burst-trapping code, using a (21,14) Single Error Correcting Double Error Detecting shortened Cyclic Hamming code as the basic block code was constructed. The code was selected with due consideration to computational load, delay in decoding and total bit rate requirement of less than 9.6 kbs. The paper also describes a Markov model for the decoding process and then determines the improvement in quality resulting from the use of the above scheme. In particular a bit error rate of 5 × 10³is reduced to 3 × 10⁶and a burst error rate of 5 × 10⁴is reduced to 6.8 × 10⁵. Some of the effects of errors on auditory perception which appear counter intuitive are briefly discussed.