A 32-kbit/sec ADPCM coder robust to channel errors

This paper presents an ADPCM coder that has been specially designed to be very insensitive to channel errors. For this reason, the predictor at the receiving end has an all-zero transfer function ensuring unconditional stability. It eliminates the effect of transmission errors very quickly, thus avoiding transmitter and receiver desynchronisation and disturbance of the adaptation algorithm. In the absence of channel errors, the speech signal results are slightly better than those obtained with a 7-bit PCM system (33dB). Moreover, modem data up to 4800 bit/sec can be transmitted without errors over several trunks. Under noisy channel conditions, signal degradation goes nearly unnoticed at a 0.1 per cent error rate and the quality is only slightly impaired at 1 per cent. With data transmission, the error rate introduced at the modem output is always inferior to that observed in PCM transmission.