Echo

CAUSES:

Echo is a wave which has been reflected at one or more points in the transmission medium, with sufficient magnitude and time difference to be perceived in some way as a wave distinct from that of the main or primary transmission

Echo is especially annoying when the delay and intensity of the audio wave is greater. That becomes a problem in VoIP since the delay is usually greater than in the public switched telephone network (PSTN).

RECOMMENDED VALUES:

The human ear is able to detect echo waves when the delay from the original one is equal or up to 10 ms. But another important factor is the intensity of the echo wave because frequently the return wave has less power that the original one. It should not be over 65 ms and over 25 or 30 dB of attenuation.

SOLUTIONS:

There are two possibilities to avoid or solve this annoying problem:

- Echo suppression - It tries to avoid the wave comeback, turning the full-duplex line in a half-duplex line. If someone is speaking in one way the other way is closed so the wave can only be transmit in one way at the same time. The commutation time of echo suppressors is very small. They unable a total full-duplex communication.

- Echo cancellation - The emitter save the original waveform in its memory and it is able to detect in the return wave the same information (maybe with noise and attenuation). The device filters that information and cancels that voice components. It requires more processing time than the echo suppression technique.