Echo cancellation is the process of eliminating echo from voice communication to improve the quality of the call. It is necessary because speech compression techniques and packet processing delays generate echo, of which there are 2 types, acoustic echo and hybrid echo. Echo cancellation improves voice quality in VoIP calls and also reduces the required bandwidth due to silence suppression techniques.
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Echo suppression and echo cancellation are methods used 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 audio quality, echo suppression increases the capacity achieved through silence suppression by preventing echo from traveling across a telecommunications network. 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.
Echo suppression and cancellation 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.
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:
- Echo suppressors work by detecting a voice signal going in one direction on a circuit, and then muting or attenuating the signal in other direction. Usually, the echo suppressor at the far end of the circuit does this muting when it detects voice coming from the near-end of the circuit. This muting prevents the speaker from hearing their own voice returning from the far end.
- 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 digitally using a digital signal processor or software, although it can be implemented in analog circuits as well.
ITU standards G.168 and P.340 describe requirements and tests for echo cancellers in digital and PSTN applications, respectively.