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 subjective quality, this process increases the capacity achieved through silence suppression by preventing echo from traveling across a network.
Two sources of echo have primaryrelevance in telephony:
acoustic echo and
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 canalso be implemented in software. Echo cancellation is done using either echo suppressors or echo cancellers, or in some cases both.
In telephony, "Echo" is very much like what one would experience yelling in a canyon. Echo is the reflected copy of the voice heard some time later and a delayed version of the original. On a telephone, if the delay is fairly significant (more than a fewhundred milliseconds), it is considered annoying. If the delay is very small (10's of milliseconds or less), the phenomena is called sidetone and while not objectionable to humans, can interfere with the communication between data modems.
In the earlier days of telecommunications, echo suppression was used to reduce the objectionable nature of echos to human users. In essence these devices relyupon the fact that most telephone conversations are half-duplex. That is one person speaks while the other listens. An echo suppressor attempts to determine which is the primary direction and allows that channel to go forward. In the reverse channel, it places attenuation to block or "suppress" any signal on the assumption that the signal is echo. Naturally, such a device is not perfect. There arecases where both ends are active, and other cases where one end replies faster than an echo suppressor can switch directions to keep the echo attenuated but allow the remote talker to reply without attenuation.
Echo cancellers are the replacement for earlier echo suppressors that were initially developed in the 1950s to control echo caused by the long delay on satellite telecommunicationscircuits. Initial echo canceller theory was developed at AT&T Bell Labs in the 1960s, but the first commercial echo cancellers were not deployed until the late 1970s owing to the limited capability of the electronics of the era. The concept of an echo canceller is to synthesize an estimate of the echo from the talker's signal, and subtract that synthesis from the return path instead of switchingattenuation into/out of the path. 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.
Rapid advances in the implementation of digital signal processing allowed echo cancellers to be made smaller and more cost-effective. In the 1990s, echocancellers were implemented within voice switches for the first time (in the Northern Telecom DMS-250) rather than as standalone devices. The integration of echo cancellation directly into the switch meant that echo cancellers could be reliably turned on or off on a call-by-call basis, removing the need for separate trunk groups for voice and data calls. Today's telephony technology often employs echocancellers 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.
Voice messaging and voice response systems which accept speech for caller input use echo cancellation while speech...