Stereo Echo Cancellation(SEC) employing Signal Decorrelation with emphasis on Affine Projection Algorithm(APA)

Abstract: Monophonic tele-conferencing systems employ acoustic echo cancellation (AEC) to reduce echoes that result from coupling between loudspeaker and microphone. Acoustic echo cancellation is simple to develope as there is a single channel. But future tele conferencing systems are expected to have multi channel communication which is necessary in hands-free multi user tele communication systems. Stereophonic echo cancellation (SEC), has been studied since the early 1990s, in hands-free tele communication applications such as tele conferencing, multi user desktop conferencing, and tele video gaming. To enhance the sound realism in order to increase the speech intelligibilty it is necessary to use two channel (stereo) audio systems. This requires SEC systems. In SEC there is a fundamental problem that the adaptive algorithm used can not identify correct echo path responses due to strong correlation between stereo signals and also the convergence is slow. In this case it is necessary to identify two echo paths for each channel thus there are four echo paths to identify which is very difficult. In this thesis, the problems with stereo echo cancellation is explained and echo cancellation with emphasis on two channel affine projection algorithm (APA) is studied. The signal de-correlation techniques are reviewed and compared. The idea behind signal decorrelation techniques is to introduce nonlinearity into each channel. This can be done by using half-wave rectifiers or time varying all-pass filters.Three methods were developed to reduce correlation between stereo signals. One, is to use two positive half-wave rectifiers on both channels considered as NLP1. Second, is to use positive and negative half-wave rectifiers on each channel (NLP2). Third, is to use time varying all-pass filters (TV-APF) on both channels with delays. Experiments were performed using MATLAB and observed echo return loss enhancement (ERLE) and misalignment(MIS) with different scenarios. The euclidean norm distance calculation has been used to find out MIS between filter coefficients and true echo path models. It is observed that NLP1 and NLP2 lack signal perception even though ERLE was good. The MIS falls down below 25dB with decorrelated stereo signals. The use of TV-APFs gives good echo cancellation and does not effect the signal perception, the ERLE in this case was 40.3231dB.