Echo cancellation

Echo suppression and echo cancellation are methods in telephony to improve voice quality by preventing echo from being created or removing it after it is already present. In addition to improving subjective quality, this process increases the capacity achieved through silence suppression by preventing echo from traveling across a network.

These methods are commonly called acoustic echo suppression (AES) and acoustic echo cancellation (AEC), and more rarely line echo cancellation (LEC). In some cases, these terms are more precise, as there are various types and causes of echo with unique characteristics, including acoustic echo (sounds from a loudspeaker being reflected and recorded by a microphone, which can vary substantially over time) and line echo (electrical impulses caused by, e.g., coupling between the sending and receiving wires, impedance mismatches, electrical reflections, etc., which varies much less than acoustic echo). In practice, however, the same techniques are used to treat all types of echo, so an acoustic echo canceller can cancel line echo as well as acoustic echo. "AEC" in particular is commonly used to refer to echo cancelers in general, regardless of whether they were intended for acoustic echo, line echo, or both.

Echo suppressors were developed in the 1950s in response to the first use of satellites for telecommunications, but they have since been largely supplanted by better performing echo cancellers.

Although echo suppressors and echo cancellers have similar goals—preventing a speaking individual from hearing an echo of their own voice—the methods they use are different:

ITU standards G.168 and P.340 describe requirements and tests for echo cancellers in digital and PSTN applications, respectively.

In telephony, "echo" is very much like what one would experience yelling in a canyon. Echo is the reflected copy of one's voice heard some time later and a delayed version of the original. On a telephone, if the delay is fairly significant (more than a few hundred milliseconds), it is considered annoying. If the delay is very small (10s of milliseconds or less), the phenomenon is called sidetone, and while not objectionable to humans, can interfere with the communication between data modems. If the delay is slightly longer, around 50 milliseconds, humans cannot hear the echo as a distinct sound, but instead hear a chorus effect which sounds like talking in a tunnel or cave.

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