Abstract

An echo canceller with an adaptive convolution processor for computing and subtracting an anticipated echo has a nonlinear processor coupled to the error signal and to the send output. The nonlinear processor is responsive to average values and attenuates the error signal over a continuous range as a function of presence of send and receive signals, rather than coupling and decoupling the error signal to the output in the manner of a center clipper. Attenuation is altered rapidly when one of the send and receive ends is active, which may be detected by their averages or by loss across a summing junction providing the error signal. Attenuation is decreased gradually neither of the send and receive ends are active. The error signal attenuation level is also used to control injection of artificial noise. An average floor noise level in the send signal is determined and a noise injection apparatus coupled to the send output variably injects an artificial noise signal in inverse proportion to attenuation of the error signal, up to a maximum of the floor noise level. The gradual adjustment of attenuation of the error signal (which includes the send-in signal), and maintenance of the output noise level at the average background level of the input make operation of the echo canceller unnoticeable to the far end listener.