Two-Path Method for Acoustic Echo Cancellation

In a two-path echo canceller system two sets of filters are used.
The foreground filter is the filter used for the output of the AEC system, while the background filter is generally continuously adapting its coefficients.
Based on several tests, when the background filter is perceived(认为) to have achieve better cancellation than the foreground filter, the coefficients of the background filter are downloaded to the foreground filter.
Since the background filter is allowed to adapt freely, it is able to better model the echo paths changes.
Meanwhile during doubletalk situations in which the background filter to diverges, the output of the system is unaffected as the coefficients of the foreground have not been updated.
The use of the two-path method provides a security blanket of an AEC system and does not rely heavily on the tuning of thresholds as compared to doubletalk detectors in single-path systems.

在双路径回声消除系统中，用到了两个滤波器。
前景滤波器是当前AEC输出正在使用的滤波器。
背景滤波器通常一直在持续的调整系数。
经过多轮测试，如果认为背景滤波器的回声消除效果好于前景滤波器，此时可将背景滤波器的系统更新到前景滤波器。
因为背景滤波器可以自由的调整，所以可以很好的来模拟回声路径变化的情况。
同时在双讲时，当背景滤波器是发散的，系统的输出不会受到任何影响，因为前景滤波器的系数还没有更新。
双路径方法为AEC系统提供了更强的保障。而且与单路径中的双讲检测方法相比，双路径方法不再严重依赖阈值。

References:

https://www.vocal.com/echo-cancellation/two-path-method/