Complete Communications Engineering

The challenge within echo cancellation lies in the ability to rapidly track changes in the echo path while remaining robust to large bursts of disturbances, such as double-talk. These two objectives are contradictory because in order to rapidly train to the changes in the echo path, a system requires an adaptive algorithm with a fast convergence speed, which in turn means it will easily diverge in the presence of double-talk.

In most systems, a double-talk detector is implemented to freeze adaptation and prevent divergence. The decision threshold for a double-talk detector generally requires some tuning to the signal processing environment. This is an undesirable characteristic, thence, most systems are tuned to be overly conservative. In other words, since a double-talk decision miss is more important to prevent than a double-talk false alarm, a system is tuned to insure no missed decisions occur. This can result in a system that can produce false alarms at a high frequency, preventing a system from adapting when it should.

The application of robust adaptive algorithms to a echo cancellation system allows the double-talk detector to be less conservative. As the majority of the double-talk misses occur at the onset and offset of speech segments, robust adaptive algorithms attempt to make the adaptation of the filter coefficients impervious to disturbances until a double-talk decision can be made. A scaling factor that is regulated by the level of the past error signals, controls the rate for the adaptive algorithm. Thus, a system that has not converged for example, will result in a high scaling factor. In a condition of double-talk, it’s onset will be delayed as the past error signals values are used.

The Two-Path Method for Acoustic Echo Cancellation (AEC) paper, discussed this topic further. The complications of double-talk detectors and the purpose of using two sets of filter coefficients as an approach to mitigating the problem of fast convergence and double-talk become clearer. The problem with the two-path method is the requirement of two FIRs operations, which imposes a necessary increase in memory and computational complexity to the system. The application of a robust adaptive algorithms can help alleviate the problem of overly conservative double-talk detectors without the added burden of the two-path method.

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