Abstract: Echoes occur in telephone systems whenever signals in one channel of a two way conversation spill over into the other channel. The present echo canceller is particulary intended for use with a hands free telephone where the spill over is in the form of an acoustic echo from the loud speaker (2) to the microphone (1). The signals are sampled at a rate 1/T and are made available as digital samples. A synthetic copy of the echo signal is generated by means of an adaptive digital non-recursive time domain transversal filter. Instead of tapping the filter at unit time intervals equal to the sampling period T in the conventional manner, the filter is tapped at unit intervals equal to twice the sampling interval. For a given amount of calculation in the filter, this has the effect of doubling the length of the filter's impulse response.