Analysis and control of the signal dependent performance ofadaptive echo cancellers in 4-wire loop telephony

摘要

The central 4-wire loop telephone system, with an adaptive echoncanceller at either end of the loop, typically achieves good echonsuppression. However, poorly cancelled echoes and/or signal burstingnhave been observed. Such undesirable behavior has been linked to thenpresence of cross correlation between and autocorrelation of the signalsntransmitted by the subscribers. The primary aim of this paper is tonquantify the relationship between the correlation levels of thensubscriber signals and echo canceller performance in the double echoncanceller (DEC) 4-wire loop system. This is achieved by carrying outndynamical analyses of the DEC system. The analyses lead to thendetermination of conditions on the subscriber signals' correlationnlevels, satisfaction of which result in guaranteed stable behavior.nFurthermore, the analyses indicate that good echo cancellation requiresnthe subscriber signals to have minimal cross correlation and narrownautocorrelation functions. These results lead to the investigation ofnthe whitening and orthogonalizing capabilities of scramblers. A linencoding scheme, based on these devices, is identified, which, fornessentially all subscriber signals, leads to good echoncancellation