An Adaptive Echo Canceller

In telephone connections that involve both 4-wire and 2-wire links, an echo is generated at the hybrid that connects a 4- to a 2-wire link. The situation at one such hybrid is illustrated schematically in Fig. 1. Here S^ and S2 arc the two speech signals and E is the echo of Sx which is returned along with S2. In practice, E is an the average about 15 dB lower than »S, , but in extreme cases may be only 6 dB lower. This echo has a disturbing influence oil the conversation, which appears to increase with increasing round-trip delay.1 If no steps were taken to reduce this echo, conversation would be seriously impaired 497 498 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1067 over satellite communication links with round-trip delays of hundreds of milliseconds. The devices used at the present time to combat echo are called echo suppressors. A number of different types of echo suppressors have been designed. They are all, basically, voice-operated switches (albeit ingenious and complex ones) which disconnect the return path or introduce a large attenuation in it whenever a decision mechanism indicates that the level of Si is large compared to that of S2 + E. However, since E and S2 both share the return path, the use of such echo suppressors introduces "chopping" or interruptions of S2 during periods of double talking.* It has been shown that the S,