ECHO CANCELLER FOR TELEPHONIC TRANSMISSION BY DIGITAL CHANNELS USING A PSEUDO-LOGARITHMIC CODING LAW

摘要

A digital echo canceller is arranged in a digital telephone circuit for PCM-signals in accordance with a pseudo-logarithmic encoding law, for example the A-law of the CCITT, in the proximity of a subscriber line interface circuit having a codec in order to cancel in the near-end speech signals S supplied by the codec to the send path the echo effect Y of the far-end speech signals X conveyed to the codec by the receive path. The echo canceller has circulating shift registers for the signals X and the coefficients C, digital correlation calculation means for obtaining the coefficients C, digital convolution calculation means for obtaining an echo cancellation signal &upbar& y and digital subtraction calculation means for obtaining near-end speech signals R in which the echo effect Y has been substantially cancelled. Each of the digital correlation, convolution and subtraction calculation means incorporates only one pseudo-logarithmic-to-linear code converter, and furthermore the digital subtraction calculation means incorporates only one linear-to-pseudo-logarithmic code converter.