A Single-Chip VLSI Echo Canceler

On long-distance telephone calls, echo is an impairment that is often as annoying subjectively as the more obvious impairments of low volume and noise. Since the subjective annoyance of echo increases with delay as well as level, the measures used for its control depend on circuit length. For circuits under about 2000 miles, via net loss (VNL) is used.1 The via-net-loss plan inserts increasing amounts of loss with increasing circuit length. Since the loss is inserted in the transmission paths for both directions, echo is attenuated by 2 X VNL, whereas the desired signal is only attenuated by 1 X VNL. Thus, the signal-to-echo ratio improves by 1 X VNL. For circuits over about 2000 miles, the amount of VNL needed to give satisfactory echo performance would result in unacceptable received levels.2 On terrestrial circuits over this length, echo suppressors are used to control echo. An echo suppressor is basically a voiceoperated switch that attempts to open the transmission path from the listening customer to the talking customer. Two problems associated with echo suppressors are their inability to control echo when both customers speak simultaneously (double-talk) and the chopping and 149