Analysis of bursting in telephony loops with adaptive hybrids

Abstract

Adaptive filtering techniques have been successfully applied to attenuate echoes in long-distance telephone communications. Recently, with the increased ability to process signals cheaply, adaptive echo cancellation is being utilized on shorter telephone circuits. To avoid the possibility of entrapment in an echo-canceller initialization that causes singing, doubletalk detectors cannot be used aggressively on such highly variable short connections. While echo cancelers tend to be effective on the shorter circuits, in the absence of doubletalk detectors a novel (and undesirable) phenomenon called bursting has been observed in real-time laboratory tests of such applications. Bursting is characterized by long periods of successful echo attenuation alternating with short periods of wildly oscillating signals. By analyzing a simplified abstraction of this problem, a theoretical explanation is presented of the kernel of the bursting problem: an imbalanced excitation combination. The structural source of the problem is the use of an adaptive filter in a feedback loop. >