Echo canceller having fir and iir filters for cancelling long tail echoes

Abstract

An echo canceller comprises a finite impulse response filter (6) connected to the incoming port of a hybrid (1), and a tail canceller (7) which includes an infinite impulse response filter (24; 40) having a multiplier (14; 33) for recursively multiplying an output digital symbol from the tapped-delay line (8) of the FIR filter with an attenuation coefficient R. A digital symbol from the IIR filter or one from the tapped-delay line (8) is multiplied with a tap-weight coefficient C.sub.1 and summed with the outputs of tap-weight multipliers of the FIR filter to produce a replica of an echo symbol, which is subtracted from a digital symbol y.sub.n from the outgoing port of the hybrid (1), producing a residual echo symbol e.sub.n. The coefficient C.sub.1 is derived from correlation between residual echo e.sub.n and a symbol from the tapped-delay line. In one embodiment, symbol from the tapped-delay line is further delayed (17) for a unit interval on the one hand, and multiplied (18) with coefficient R on the other, the difference between them being detected (19) and multiplied (20) with a coefficient K, which is either sgn(C.sub.1) or 1/C.sub.1, to generate a multiplied difference. Correlation is detected (22) between the difference and residual echo e.sub.n to derive the coefficient R. In a second embodiment, symbol from the tapped-delay line is further delayed (36) for a unit interval and multiplied (37) with K to produce a delayed-multiplied symbol. Correlation is detected (35) between it and residual echo e.sub.n to derive the coefficient R.