Echo suppressor
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An echo suppressor (sometimes "echo suppresser") is a telecommunications device used to reduce the echo heard on long telephone circuits, particularly circuits that traverse satellite links. Echo suppressors were first developed in the 1950s in response to the first use of satellites for telecommunications, but they have since been supplanted by better performing echo cancellers.
Echo suppressors work by detecting if there is a voice signal going in one direction on a circuit, and then inserting a great deal of loss in the other direction. Usually the echo suppressor at the far-end of the circuit adds this loss when it detects voice coming from the near-end of the circuit. This added loss prevents the speaker from hearing his own voice.
While effective, this approach leads to several problems:
- Double-talk: It is fairly normal in conversation for both parties to speak at the same time, at least briefly. Because each echo suppressor will then detect voice energy coming from the far-end of the circuit, the effect would ordinarily be for loss to be inserted in both directions at once, effectively blocking both parties. To prevent this, echo suppressors can be set to detect voice activity from the near-end speaker and to fail to insert loss (or insert a smaller loss) when both the near-end speaker and far-end speaker are talking. This, of course, temporarily defeats the primary effect of having an echo suppressor at all.
- Clipping: Since the echo suppressor is alternately inserting and removing loss, there is frequently a small delay when a new speaker begins talking that results in clipping the first syllable from that speaker's speech.
- Dead-set: If the far-end party on a call is in a noisy environment, the near-end speaker will hear that background noise while the far-end speaker is talking, but the echo suppressor will suppress this background noise when the near-end speaker starts talking. The sudden absence of the background noise gives the near-end user the impression that the line has gone dead.
Echo suppressors cause effects that are frustrating for both parties to a call, although they do effectively deal with the echo. In response to this, AT&T Bell Labs developed echo canceller theory in the early 1960s, which then resulted in laboratory echo cancellers in the late 1960s and commercial echo cancellers in the 1970s.
