One person speaks while the other listens, and they speak back and forth. In the earlier days of telecommunications, echo suppression was used to reduce the objectionable nature of echos to human users. If the delay is slightly longer, around 50 milliseconds, humans cannot hear the echo as a distinct sound, but instead hear a chorus effect. If the delay is very small (10s of milliseconds or less ), the phenomenon is called sidetone. If the delay is fairly significant (more than a few hundred milliseconds), it is considered annoying. In telephony, echo is the reflected copy of one's voice heard some time later. ITU standards G.168 and P.340 describe requirements and tests for echo cancellers in digital and PSTN applications, respectively. This technique is generally implemented digitally using a digital signal processor or software, although it can be implemented in analog circuits as well. Once the echo is recognized, it can be removed by subtracting it from the transmitted or received signal. Echo cancellation involves first recognizing the originally transmitted signal that re-appears, with some delay, in the transmitted or received signal.This muting prevents the speaker from hearing their own voice returning from the far end. 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. 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.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.ДЄlthough 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: 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. 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). Echo suppressors were developed in the 1950s in response to the first use of satellites for telecommunications.Д®cho suppression and cancellation methods are commonly called acoustic echo suppression ( AES) and acoustic echo cancellation ( AEC), and more rarely line echo cancellation ( LEC).

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. JSTOR ( January 2011) ( Learn how and when to remove this template message)Д®cho 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.Unsourced material may be challenged and removed.ДЇind sources: 'Echo suppression and cancellation'В вЂ“ news

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