Enhancement of differentially coded speech signals by means of quantization error correction

A method to increase the Signal-to-Noise Ratio (SNR) of speech codecs is presented. It has been applied to Adaptive Delta Modulation (ADM) according to Jayant´s algorithm. Observations indicate that bit streams resulting from these coders and quantization errors are correlated to some extend. Based on this experience a Quantization Error Correction method (QEC) has been developed that improves the SNR of the output signal. With this method the decoder output is updated with a correction signal derived from the received bit sequence. An expression for the optimal correction level is derived. Application of the method by means of computer simulation and hardware implementation gave a 5 to 6 dB increase in SNR. There is no need to transmit additional information and a normal decoder can be used together with additional circuitry.