Echo canceller using end delay measurement

Abstract

An echo canceller is connected in the four-wire, voice-frequency path of a communication circuit and includes an end delay analyzer which is bridged between the four-wire transmit and receive paths, and a variable delay and an echo synthesizer connected in series between the received signal path and one input of a summation circuit. The other input to the summation circuit is connected to the four-wire transmit path along which the return echo signal is propagating. Any difference appears as a residual echo at the output of the summation circuit. The end delay analyzer, in response to the received signal and the echo return signal, provides an output control signal that adjusts the variable delay to compensate for the terminal circuit delay which occurs between the echo canceller and the four-wire to two-wire hybrid. This inserts delay in the received signal, which is applied to the echo synthesizer, thus simplifing the echo synthesizer structure as the variable component now must only compensate for the active part of the impulse response of the terminal circuit.