Line Echo Cancellation

Introduction

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.

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.