Multistage echo canceller including time variation compensation

Acoustic echos are canceled by employing a first echo canceler having a comparatively long first impulse response synthesis capability which is connected between a transmit path and receive path for generating a first error signal and for canceling echo signals in the transmit path, and at least a second echo canceler having a comparatively short second impulse response synthesis capability connected in parallel with the first echo canceler between the transmit and the receive path. The second echo canceler is supplied with the first error signal from the first echo canceler and is adaptively operating simultaneously with but independent of the first echo canceler to further cancel echos in the transmit path. Specifically, the first echo canceler is intended to capture the substantially stationary and any slowly varying components of the echo path impulse response, and the second echo canceler is intended to capture the more time varying, i.e., dynamically varying, component of the echo path impulse response. I have recognized that in certain applications, the more time varying component of the echo path impulse response may exist in a relatively short time interval. For instance, in the canceling of acoustic echos, the most time variation in the echo path impulse response results from the movement of objects and/or persons near either the microphone or the loudspeaker. This movement results in significant changes in the early portion of the echo path impulse response. Aligning the tap coefficients of the second echo canceler with this early portion of the echo path impulse response provides significantly enhanced echo cancelation performance over that achievable with only the first echo canceler. This is realized by placing a "short" fixed delay in series with the receive path signal to the second echo canceler. In other applications, the time varying component of the echo path impulse response still exists in a relatively short time interval, but the time varying component relative to the overall echo path impulse response may change with time or may not be known a priori. Under these circumstances, it is desirable to dynamically align the coefficients of the second echo canceler over the echo path impulse response. This is realized by employing a dynamically adjustable delay in series with the receive path to the second echo canceler. In one embodiment, the delay value is dynamically generated in response to the tap coefficient values of the first echo