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The term echo cancellation is used in telephony to describe the process of removing echo from a voice communication in order to improve voice quality on a telephone call. In addition to improving quality, this process improves bandwidth savings achieved through silence suppression by preventing echo from traveling across a network.
There are two types of echo of relevance in telephony: acoustic echo and hybrid echo. Speech compression techniques and digital processing delay often contribute to echo generation in telephone networks.
Echo cancellation involves first recognizing the originally transmitted signal that re-appears, with some delay, in the transmitted or received signal. Once the echo is recognized, it can be removed by 'subtracting' it from the transmitted or received signal. This technique is generally implemented using a digital signal processor (DSP), but can also be implemented in software. Echo cancellation is done using either echo suppressors or echo cancellers.
Drawbacks
Echo suppression may have the unwanted side-effect of removing valid voice signals from the transmission. This can cause audible signal loss that is called "clipping." For that reason, echo supression has been obsoleted by echo cancellation.
Echo control on voice-frequency data calls that use dial-up modems may cause data corruption. Some telephony devices disable echo supression or echo cancellation when they detect the 2100 or 2225 Hz "answer" tones associated with such calls, in accordance with ITU-T Recommendation G.164 or G.165.
Acoustic echo
Acoustic echo is generated through a telephone set when sounds directly picked up by the microphone also travel within the location of the microphone, reflect on objects at that location, and are then indirectly picked up again, with a certain amount of delay, by the microphone. The sounds may initiate from either party as well as from any other ambient noise in the location, including the telephone set speaker (ear piece).
Hybrid echo
Hybrid echo is generated by the public-switched telephone network (PSTN) through the reflection of electrical energy by a device called a hybrid (hence the term hybrid echo).
See also
"Echo cancellation"