Multiple Echo Cancelation
Original Publication Date: 1989-Mar-01
Included in the Prior Art Database: 2005-Jan-27
Any phone conversation generates one or several echoes at the 4-wire/2- wire transition point (see Hybrid in the figure). These echoes may be particularly disturbing, and they should be canceled. Digital echo cancelers use an adaptive digital filter (FIL), dynamically adjusted to generate an echo replica and subtract the replica from the echo spoiled signal. Dynamic adjustment calls for rather complex operations made to synthesize the impulse response of the echo channel and then set the filter coefficients accordingly. In theory, the filter should be as long (timewise) as the echo path. But this is not practical, since the echo path response would show quasi flat sections corresponding to zero-valued coefficients which might be dropped from the filtering operation.