Combating Echo in Speech Circuits with Long Delay

In speech communication systems, echo arises from impedance discontinuities at the various circuit junctions. In telephonic communication, echo is universally present to a greater or lesser extent. However, it usually goes unnoticed on short connections where the round-trip signal transit time of a few milliseconds or less is too short to give an audible separation of direct and reflected components. Echo suppressors have have long served to alleviate the problems of echo return on circuits with moderate delays. One of the design problems is to remove echo without interfering with the two-way flow of speech during periods of double talking. In view of recent interest in satellite communication systems, where delay times of the order of hundreds of milliseconds may be involved, a review of echo-suppressor performance under more severe operating conditions has been undertaken. This paper discusses the special problems involved in suppressing echo on circuits with long delay. It describes an experimental solid-state device for detecting and combating echo in such circuits. In particular, the questions of differential break-in and voice-controlled loss in the return path are considered. A demonstration is given of the operation of the experimental device under different conditions of circuit echo.