Echo Cancellation - History

History

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 few hundred milliseconds), it is considered annoying. If the delay is very small (10's of milliseconds or less), the phenomenon 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 rely upon 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 are cases 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 telecommunications circuits. 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 switching attenuation 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, echo cancellers 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 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.

Voice messaging and voice response systems which accept speech for caller input use echo cancellation while speech prompts are played to prevent the systems own speech recognition from falsely recognizing the echoed prompts.