Monografia echo cancelation telefonia

ABSTRACT
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 primary relevance in telephony: acoustic echo and hybrid echo.
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 can also be implemented in software. Echo cancellation is done using either echo suppressors or echo cancellers, or in some cases both.

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 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 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