ANALYSIS AND CONTROL OF THE SIGNAL DEPENDENT PERFORMANCE OF ADAPTIVE ECHO CANCELERS IN 4-WIRE LOOP TELEPHONY

The central 4-wire loop telephone system, with an adaptive echo cancel ler at either end of the loop, typically achieves good echo suppressio n. However, poorly cancelled echoes and/or signal bursting have been o bserved. Such undesirable behavior has been linked to the presence of cross correlation between and autocorrelation of the signals transmitt ed by the subscribers. The primary aim of this paper is to quantify th e relationship between the correlation levels of the subscriber signal s 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 condition s on the subscriber signals' correlation levels, satisfaction of which result in guaranteed stable behavior, Furthermore, the analyses indic ate that good echo cancellation requires the subscriber signals to hav e minimal cross correlation and narrow autocorrelation functions, Thes e results lead to the investigation of the whitening and orthogonalizi ng capabilities of scramblers. A line coding scheme, based on these de vices, is identified, which, for essentially all subscriber signals, l eads to good echo cancellation.