Combating Echo in Speech Circuits with Long Delay

In speech-communication systems, echo is universally present to a greater or lesser extent. It goes unnoticed, however, on short connections where the round-trip transit time is too small to give an audible separation of direct and reflected components. For circuits of moderate delay, echo suppressors have long served to alleviate the problem of echo return. Present equipment, however, does not provide satisfactory performance when the delay becomes relatively large. In view of the current interest in satellite communication, a number of studies of echo-suppressor performance for longer delays have been undertaken. The present paper is a report of one such study in which an AB-type screening test was used to evaluate the circuit innovations employed. These include diode-type variolossers for introducing and removing echo attenuation on a “soft-switch” basis and “time-logic” circuitry for reducing such attenuation during periods of double talking. Results indicate that, while the circuit innovations employed add perceptibly to suppressor performance, only a partial solution of the problems involved has been achieved. This is particularly true for circuits having line-loss and talking-level asymmetries.