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Acoustic echo cancellation is a signal processing technique that is used in a telecommunications system that has acoustic coupling between the loudspeaker and microphone, in order to achieve echo-free full-duplex communication. The difficulty in acoustic echo cancellation over line echo cancellation is the variability not only in the echo path, but also in the implementation. In line echo cancellation, the coupling resulting from the hybrid is relatively steady between implementations. In acoustic echo cancellation, the coupling between the loudspeaker and microphone can vary significantly depending on the design of the loudspeaker and microphone enclosure and acoustics of the room, in which the device is deployed. Therefore, in order to achieve an ubiquitous solution for an acoustic echo cancellation system, an intelligent control system for the adaptive filter for the echo canceller and the post-filter is required.
It has been discussed previously in Variable Stepsize and Regularization Parameters for NLMS, that 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 discussed that a post-filter can be designed to reduce the residual echo from the linear adaptive filter of the echo canceller. Optimum control of the post-filter is based on the estimate of the residual echo, which in turned is based on the convergence state of the canceller. In addition, acoustic echo cancellation systems which employ the two-path method, also require an estimate of the convergence of the foreground and background filters, in order 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 echo canceller is crucial to the performance of the entire acoustic echo cancellation system.
In order to obtain an estimate of convergence or Echo Return Loss Enhancement (ERLE), one must first estimate the coupling factor or Echo Return Loss (ERL) of the loudspeaker-microphone enclosure. An estimate of the ERL is required to determine how much attenuation of the echo 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 echo path.
There are two main approaches to estimating the coupling factor of an echo canceller. The first method is amplitude based. The second method is cross-spectrum based. The amplitude based method 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 update during periods of double-talk. In the cross-spectrum based method, the far-end and near-end spectrum signals are multiple and summed over 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 downfall to this method is the echo path changes are not followed accurately due to the long averaging period. 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.