Research On Adaptive Multi-Channel Acoustic Echo Cancellation Theory

Echo in communication system is generally undesirable and inevitable. With the wide application of teleconference and hand-free systems, the acoustic echo cancellation becomes an important research theme. However the research results of multi-channel acoustic echo cancellation are much less compared to those of mono-channel. There are many theories, approaches and implementation techniques to be studied. Adaptive echo canceller is one of the most prospective approaches in these fields presently, especially it is a main technique in the field of acoustic echo canceller. In this thesis we achieve the following for multi-channel acoustic echo cancellation.Firstly, the structure and principle of adaptive filter and adaptive algorithms are studied in this paper.Secondly, the principle and problems of the adaptive multi-channel echo cancellation are analyzed. Pre-processing methods for reducing the correlation of multi-channel input signals are introduced. Half-wave rectifier function, which is one of nonlinear pre-processing methods, has a simple and low-complexity structure. The pre-processing method decreases the channel correlation significantly and the distortion is hardly audible in an office environment.Thirdly, adaptive multi-channel echo cancellation algorithms and their characters are discussed systemically. In the teleconference systems, single-channel monophonic sound certainly is not sufficient for a listener to spatially separate one remote talker from another. Moreover, it is very difficult to understand two (or more) remote speakers talking at the same time. Synthesized stereophonic sound will help considerably in solving these problems. The methods which can be used to synthesize stereo sound are explained and are combined efficiently with stereo acoustic echo cancellation to solve these problems. Because interference can seriously deteriorate adaptive filter performance in stereophonic acoustic echo cancellation, the performance of adaptive filter under interference environment was improved by detecting interference and controlling the optimal filter.Lastly, simulations have been conducted under several different situations. Result of one simulation proves it effective to use the half-wave rectifier function in the previous section. The convergence properties of stereophonic acoustic echo cancellation with different adaptive algorithms have been showed. Another simulation result shows that interference-free stereophonic acoustic echo cancellation has better performance.