Analysis and Control of the Signal Dependent Performance of Adaptive Echo Cancellers in 4-Wire Loop Telephony

The central 4-wire loop telephone system, with an adaptive echo canceller at either end of the loop, typically achieves good echo suppression. However, poorly cancelled echoes and/or signal bursting have been observed. Such undesirable behavior has been linked to the presence of cross correlation between and autocorrelation of the signals transmitted by the subscribers. The primary aim of this paper is to quantify the relationship between the correlation levels of the subscriber signals and echo canceller performance in the double echo canceller (DEC) 4-wire loop system. This is achieved by carrying out dynamical analyses of the DEC system. The analyses lead to the determination of conditions on the subscriber signals’ correlation levels, satisfaction of which result in guaranteed stable behavior. Furthermore, the analyses indicate that good echo cancellation requires the subscriber signals to have minimal cross correlation and narrow autocorrelation functions. These results lead to the investigation of the whitening and orthogonalizing capabilities of scramblers. A line coding scheme, based on these devices, is identified, which, for essentially all subscriber signals, leads to good echo cancellation