An Adaptive Echo Canceller in a Nonideal Environment (Nonlinear or Time Variant)

In a previous paper 1 we have described the performance of an adaptive echo canceller in a linear time invariant environment. Qualitatively speaking, the environment in which an echo canceller will operate appears to be mainly linear and time invariant; however, some echo paths will be nonlinear and/or time variable. Some typical causes of nonlinearity are the volume dependent gain in compandored circuits 1 and harmonic distortion in amplifiers and repeaters. Time variability, on the other hand, may be caused by spurious modulation of the carrier of long-haul single-sideband suppressed carrier systems. This is commonly referred to as incidental FM or phase jitter. In certain cases these anomalies are of sufficient magnitude to degrade the performance of an adaptive echo canceller. In this paper we examine the operation of an adaptive echo canceller studied previously in an ideal environment 1 in a nonideal environment (nonlinear, time variant). We restrict ourselves to nonlinearities which do not possess infinite memory and to time variability caused by phase jitter. In both cases we derive a lower bound on the suppression provided by the echo canceller and empirically verify t This problem occurs when the compressor portion of the compandor is not perfectly compensated for by the expandor. 2779