Line Echo Cancellation

Introduction to Echo Cancellation

In voice calls you will usually hear an echo of what your self are saying, due to overhearing at the far end terminal. In line-switched networks over short distances, the delay of the echo is very small, and you feel it as a natural and pleasant feedback.

With VoIP technologies, the transmission delay will always be significant, due to transmission in packets. Echo of your own voice with 50 to 100 ms delay can be very annoying.

Echo Cancellation is a class of advanced signal processing techniques for reducing the echo from voice terminals. Echo Cancellation is needed for open duplex conversations over IP.

Echo Cancellation usually removes echo its own end of the conversation. Most networks require each end to take care of its own echo. It is best to locate the Echo Cancellation as close as possible to the terminals. Note that having good Echo Cancellation at your end, is something you need in order to be nice to your contacts. If your end has bad or none Echo Cancellation, it is not your self which suffers, directly. It is your contacts, which will find it very annoying to talk to you.

Line Echo Cancellation (LEC)

Line Echo Cancellation (LEC)is a term used in telephony for Echo Cancellation on normal telephone lines. The LEC sits in the exchange, and cancels echo due to 2-wire hybrids and acoustic overhearing in the handset. The LEC usually does not have to deal with long echo-tails.

Acoustic Echo Cancellation (AEC)

Acoustic Echo Cancellation handles acoustic echo from loudspeaking terminals. AEC has to handle the direct acoustic path from the speaker to the microphone, as well as multiple echoes from walls and ceiling in the room . AEC is usually implemented in the terminal itself, because the phone network traditionally does not take responsibility for loudspeaking terminals. AEC has to deal with long echo tails, which may change during the call, as people move around in the room. AEC is often combined with a howling suppressor, needed due to the positive feedback usually present in conversations between loudspeaking terminals.

AlphaCom IP stations has built-in AEC.

Traditional ASLT-based stations has no AEC, and the AlphaCom itself does not support AEC on VOIP calls. Loudspeaking VoIP conversations to a ASLT station has to go in switched duplex.