An echo canceller which cancels an echo by generating a replica of echo from the transmit data when the signal from the transmit line appears as an echo signal on the receive line, and subtracting the replica of echo from the echo signal in a circuit for connecting a transmit line and receive line to...http://www.google.com/patents/US4868874?utm_source=gb-gplus-sharePatent US4868874 - Echo canceller
