A Single-Chip VLSI Echo Canceler

Abstract

A single-chip VLSI (very large-scale integration) echo canceler has been fabricated in NMOS(N-channel metal-oxide semiconductor). The canceler has a 128-tap (16-ms) delay line and a white-noise convergence rate of 70 dB/s. The chip measures 313 by 356 mils and contains 35,000 devices. I. I N T R O D U C T I O NOn 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 T h e 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 V N L , whereas the desired signal is only attenuated by 1 X V N L . Thus, the signal-to-echo ratio improves by 1 x V N L .For circuits over about 2000 miles, the amount of V N L 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

A high performance digital voice echo canceller on a single TMS32020

“…proposed in [2] consists of comparing the transmitted signal to the recent history of the reference signal and 8.12. 2 whereA= 2.…”

“…Computer simulations were undertaken to compare the (2) performance of the block update algorithm with M = 16 to that of the stochastic gradient algorithm (M = 1), taking into account the finite precision of the TMS32020. Fig.…”

Study of echo cancelling algorithms for full duplex telephone networks with vocoders

“…Researchers at Bell Telephone Lab [6] and Comsat Lab [7] have used the Widrow LHS algorithm with success. A reasonable echo reduction and speed of convergence were achieved.…”