Acoustic Echo Cancellation

This article explains the problem of acoustical echoes and their cancellation. It focuses on the hands-free telephone as one of the major applications for echo cancelling devices. Beginning with a short discussion of a system formed by a loudspeaker and a microphone located within the same enclosure, the properties of speech signals and noise are described. The major part of the echo can be cancelled by an adaptive filter connected in parallel to the loudspeaker and the microphone. Residual echoes are suppressed by an additional filter within the outgoing signal path. Algorithms for the adaptation of the echo cancelling filter are described. Because of its robustness and its low computational complexity the NLMS algorithm is primarily applied. Measures to improve the speed of convergence and to avoid divergence in case of double talk or strong local noise are discussed. Echo cancellation for stereophonic systems, cancellation in subbands, and the applications of block processing techniques conclude the article. Keywords: acoustic echoes; loudspeaker enclosure microphone system; adaptive algorithms; step size control; stereophonic systems; subband echo cancellation; block processing algorithms

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