Echo suppression and cancellation

echo cancellationecho cancellerecho suppressorAcoustic Echo Cancellationecho suppressioncancellersecho cancelerecho suppressingecho-suppression
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.wikipedia
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Voice activity detection

VADdetecting a voice signalvoice-detection
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.
VAD is an integral part of different speech communication systems such as audio conferencing, echo cancellation, speech recognition, speech encoding, speaker recognition and hands-free telephony.

Silence suppression

In addition to improving subjective audio quality, echo suppression increases the capacity achieved through silence suppression by preventing echo from traveling across a network.
For further bandwidth gains, silence suppression is normally done after echo cancellation.

Videotelephony

videoconferencingvideo conferencingvideo chat
Videoconferencing is one area where full bandwidth audio is transceived.
Acoustic echo cancellation (AEC) is a processing algorithm that uses the knowledge of audio output to monitor audio input and filter from it noises that echo back after some time delay.

Adaptive filter

adaptive filteringadaptedadaptive
This technique requires adaptive signal processing to generate a signal accurate enough to effectively cancel the echo, where the echo can differ from the original due to various kinds of degradation along the way.
Echo cancellation

Sidetone

If the delay is very small (10s of milliseconds or less ), the phenomenon is called sidetone.
Echo cancellation

G.165

Some telephone devices disable echo suppression 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.
G.165 is an ITU-T standard for echo cancellers.

Modem

modemsdial-up modem56k modem
Echo control on voice-frequency data calls that use dial-up modems may cause data corruption.
Echo cancellation was the next major advance in modem design.

Speakerphone

speaker
A standard telephone or cellphone in speakerphone or hands-free mode
These phones are much more complicated and often employ sophisticated digital signal processing algorithms to sort out the incoming sound and the sound in the room for echo cancellation.

Loudspeaker

speakerloudspeakersspeakers
Physical coupling (vibrations of the loudspeaker transfer to the microphone via the handset casing)
Echo cancellation

Signal reflection

reflectionreflectionsreflected
Signal reflection
When a sufficient degree of impedance matching is not practical, echo suppressors or echo cancellers, or both, can sometimes reduce the problems.

Audio feedback

feedbackguitar feedbackacoustic feedback
Audio feedback
Echo cancellation

Telephony

digital telephonytelephonedigital
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.

Echo

echoesechoedechoing
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.

Telecommunications network

networkcommunication networkcommunications network
In addition to improving subjective audio quality, echo suppression increases the capacity achieved through silence suppression by preventing echo from traveling across a network.

Digital signal processor

DSPDSPsdigital signal processors
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.

Software

computer softwaresoftware technologyprogram
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.

Chorus effect

choruschorus pedalchorused
If the delay is slightly longer, around 50 milliseconds, humans cannot hear the echo as a distinct sound, but instead hear a chorus effect.

Attenuation

attenuateattenuatedattenuating
In the reverse channel, it places attenuation to block or suppress any signal on the assumption that the signal is echo.

Bell Labs

Bell Telephone LaboratoriesBell LaboratoriesAT&T Bell Laboratories
In response to this, Bell Labs developed echo canceler theory in the early 1960s, which then resulted in laboratory echo cancelers in the late 1960s and commercial echo cancelers in the 1980s.

Digital signal processing

DSPsignal processingdigital signal processing (DSP)
Rapid advances in the implementation of digital signal processing allowed echo cancellers to be made smaller and more cost-effective.

Nortel

Nortel NetworksNorthern ElectricNorthern Telecom
In the 1990s, echo cancellers were implemented within voice switches for the first time (in the Northern Telecom DMS-250) rather than as standalone devices.

DMS-100

remote switching centreDMS-250
In the 1990s, echo cancellers were implemented within voice switches for the first time (in the Northern Telecom DMS-250) rather than as standalone devices.

Voice engine

Today's telephony technology often employs echo cancellers in small or handheld communications devices via a software voice engine, which provides cancellation of either acoustic echo or the residual echo introduced by a far-end PSTN gateway system; such systems typically cancel echo reflections with up to 64 milliseconds delay.

Public switched telephone network

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

Squelch

DCScarrier squelchnoise squelch'' or ''carrier squelch
This can cause audible signal loss that is called "clipping" in telephony, but the effect is more like a "squelch" than amplitude clipping.