Echo cancellation in IP networks

Abstract

Voice transmission over IP networks imposes new DSP challenges. Package loss and latency caused by packet buffering have the fundamental influence on the speech quality. These features have also an impact on the line echo canceller (EC) performance. The constant latency greater than 60 milliseconds usually causes that an echo is very well audible even for a short echo path delay. Therefore, the EC must be deployed on each 2-wire/4-wire connection while on a conventional transmission only toll connections require EC. Such a large total delay requires short convergence time and sufficient Echo Return Loss Enhancement (ERLE). Further, package loss that is related to non-stationary intervals of speech imposes stringent requirements on EC performance in tracking statistical variations of the signal. Overally, IP telephony requires more robust and less expensive EC than conventional networks. In this paper we address the basic design issues in the EC for IP telephony. We show that classical Least Mean Square (LMS) algorithms are rather inappropriate and offer an alternative solution.