Echo cancellation on time-variant circuits

Echo cancellation is rapidly becoming a viable alternative to the conventional method of echo suppression currently in use to curtail echoes on long-distance telephone circuits. Echo suppressors introduce a switched loss into the return speech path when echo is present, and are inherently incapable of sustaining reduction of echo during periods of double-talk-while both parties are talking simultaneously. Echo cancellers use an adaptive filter to model the echo path, and compute a replica of the echo component present in the return speech signal. Subtraction of the replica from the return speech signal leaves it devoid of echo, allowing only the speech from the other party to be passed. Lack of synchronism in some classes of carrier system give rise to cyclic time variation in the echo path, termed "phase-roll," which must be tracked by the adaptive filter. Established adaptive echo cancellers are ineffective on circuits exhibiting phase-roll rates in excess of 0.5 Hz. The nature of the phase-roll problem is examined and two solutions discussed; use of compensating frequency offsets and cancellers with high-speed convergence. A prototype capable of satisfactory performance with speech on circuits exhibiting phase-roll rates up to 1.8 Hz is described. Modifications are proposed whereby the performance attained on speech signals can be made to approach the observed 3.4 Hz phase-roll limit with random noise input.