[Asterisk-Users] Why echo occurs

[Steve Underwood]

Eric Bishop wrote:

>Just out of interest, 
>
>When echo occurs (the type where I hear myself echoing as I talk) what
>is bouncing against. Is it the other caller's equipment, the central
>office or something in between?
>  
>
With 2-wire analogue line you will have echo from the hybrid at the 
local exchange.

If the far end has a 2-wire analogue line you will have echo from the 
hybrid in their phone.

Whatever kind of phone is at the other end there will be echo from the 
acoustic coupling between earpiece and mic (in cellular phones and IP 
phones this is usually eliminated by a local echo canceller).

If you try to cancel these echos at your end it will only work if the 
path has a precisely constant length. If the path length  changes (e.g. 
the far end is an IP phone), the echo canceller's training will keep 
falling apart. If the path length is constant, and the canceller well 
training it should do a very good job of eliminating the echo from the 
local analogue loop. It won't give more than about 30dB of suppression 
of the echo from beyond the A-law/u-law section of the link, due to the 
inherent distortion of the codec. It is normally necessary to suppress 
small residual signals to avoid hearing a weak echo from the distant 
phone when its user is silent.

Regards,
Steve