B.S.T.J. Briefs: A Self-Adaptive Echo Canceller

A Self-Adaptive Echo Canceller By M. M. SONDHI and A. J. PRESTI (Manuscript received October 14, 1966) Conventional echo suppressors combat echoes generated at hybrid junctions in long distance telephone connections by interrupting the return path according to some decision scheme based upon the relative levels of t h e outgoing and return signals. In this brief, a new device is described for cancelling the echo without interrupting the return path. We call this device an echo canceller to distinguish it from conventional echo suppressors. It generates a replica of the echo (which is then subtracted from the return signal) by synthesizing a linear approximation to the echo transmission path. It is self-adapting in that it automatically tracks variations in the echo path which may arise during a telephone conversation (e.g., connection or disconnection of extension phones, etc.). A schematic of such a self-adapting echo canceller is shown in Fig. 1. It is based upon an idea originally proposed by J. L. Kelly, Jr. and B. F. Logan, and incorporates modifications which simplify and improve the implementation and performance. With reference to Fig. 1, let x(t) be the input speech signal and y(t) the return signal. T h e return signal consists of an echo z(t) (which is the result of convolving x(t) with the impulse response h(t) of the echo path) and a noise n(t) (which may include a second speech signal). An estimate %{t) of z{t) is substracted from the return signal and the error signal = y(i) ~ 2U) is continuously used to improve this estimate.