Complete Communications Engineering

Acoustic echo cancellation (AEC) is a signal processing technique that is used to achieve echo-free full-duplex communication in a telecommunications system that has acoustic coupling between the loudspeaker and microphone. The difficulty with AEC over line echo cancellation (LEC) is the variability not only in the echo path, but also in the implementation. For LEC systems, the coupling resulting from the hybrid is relatively steady between implementations. Whereas for AEC systems, the coupling between the loudspeaker and microphone can vary significantly depending on the design of the loudspeaker and microphone enclosure as well as the acoustics of the room in which the device is deployed. Therefore, in order to achieve an ubiquitous solution for an acoustic system, intelligent control of the adaptive filter is required for the echo canceller as well as the post-filter.

In Variable Stepsize and Regularization Parameters for NLMS, it was shown that the performance of the echo canceller can be improved with variable step-size control. Optimum control of the step-size parameter is based on the convergence state of the canceller. In Post Filtering for Residual Echo Control it was concluded that a post-filter can be designed to reduce the residual echo from the linear adaptive filter. Optimum control of the post-filter is based on the estimate of this residual, which in turned is based on the convergence state of the canceller. In addition, systems which employ the two-path method require an estimate of the convergence of the foreground and background filters to decide which filter set is in the most beneficial state. From the three examples above, it is clear that the ability to obtain a quick and accurate estimate of convergence of the acoustic echo canceller is crucial to the performance of the entire  system.

To obtain an estimate of convergence or the Echo Return Loss Enhancement (ERLE), one must first estimate the coupling factor or the Echo Return Loss (ERL) of the loudspeaker-microphone enclosure. An estimate of the ERL is required to determine how much attenuation can be attributed to the echo path and how much can be attributed to the echo canceller. The coupling factor determines the attenuation or possible gain in the path.

There are two main approaches to estimating the coupling factor of an echo canceller. The first method is amplitude based while the second is cross-spectrum based. The amplitude based method to estimate ERL is the average spectral energy of the near-end signal over the average spectral energy of the far-end signal. This approach should only be updated during periods of known far-end signal energy and should not be updated during periods of double-talk. In the cross-spectrum based method, the far-end and near-end spectrum signals are multiplied and summed over a long period of frames. Then it is normalized by the far-end signal energy. This method is unaffected by double-talk of the near-end speaker and far-end speaker as long as they are uncorrelated. The downside to this method is the echo path changes are not followed accurately due to the long averaging period. However using a combination of the two methods will allow for quick and accurate estimation of the ERL, and hence, proper control of the entire echo cancellation system.

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