| 摘要 |
Acoustic echos are canceled by employing a first echo canceler having a comparatively long first impulse response synthesis capability which is conn ected between a transmit path and receive path for generating a first error signa l and for canceling echo signals in the transmit path, and at least a second echo canc eler having a comparatively short second impulse response synthesis capability connected i n 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 cancele r and is adaptively operating simultaneously with but independent of the first echo c anceler to further cancel echos in the transmit path. Specifically, the first echo can celer 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 t he more time varying, i.e., dynamically varying, component of the echo path impulse respo nse. 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 instanc e, in the canceling of acoustic echos, the most time variation in the echo path impuls e response results from the movement of objects and/or persons near either the micropho ne or the loudspeaker. This movement results in significant changes in the early por tion of the echo path impulse response. Aligning the tap coefficients of the second ech o canceler with this early portion of the echo path impulse response provides significa ntly 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 t he echo path impulse response still exists in a relatively short time interval, but the t ime varying component relative to the overall echo path impulse response may change wit h time or may not be known a priori. Under these circumstances, it is desirable to dy namically align the coefficients of the second echo canceler over the echo path impuls e 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 dyna mically generated in response to the tap coefficient values of the first echo cancel er.
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