Line Echo Cancellation
From Zenitel Wiki
Contents
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.
Line Echo Cancellation in AlphaCom
Line Echo Cancellation (LEC) is introduced in AMC X10.23 (2007-12-11) SW. The LEC can be applied on VoIP connections. The LEC function allow ASLT based stations to have open Duplex VoIP conversations when using the handset.
The AlphaCom LEC runs on 16 kHz samplingrate, which means that the full 7.5kHz bandwidth of G.722 is still avaliable!
When LEC?
The LEC does its magic in the following situations:
- Between two ASLT stations over AlphaNet, both have handset lifted. (Both exchanges must be be upgraded to 10.23!)
- Between a ASLT station with handset lifted, and a AlphaCom IP station.
- Between a ASLT station with handset lifted, and a SIP system.
Switched duplex is still used for any conversation involving at least one ASLT station in loudspeaking mode.
Handset conversation between ASLT stations within the same exchange, goes in open duplex without LEC, as always has been. This is because of the short delay.
All statements above on "ASLT stations" also applies to telephones connected to ATLB/ATLB12.
To summarize: LEC is used instead of switched duplex when a ASLT station or a ATLB phone is in a VoIP call with the handset lifted, and the AlphaCom can assume that the other end also useEcho Cancellation.