[Asterisk-Users] Why echo occurs

[Scott Laird]

On Feb 10, 2005, at 5:57 PM, Eric Bishop wrote:
> Hi all,
>
> Can someone give me a simple rational explanation why a $5 analog
> handset  gives me no echo whatsoever on an analog PSTN line, but
> PSTN-VoIP devices such as the TDM400 and Sipuras do and thus require
> software-based echo cancellation. Surely a $5 analog handset does not
> have an "echo canceller".

Conventional wisdom says that it takes two things to get audible echo:

1.  A 2-wire leg to the connection, such as a POTS line.
2.  A substantial delay.

If you don't have a 2-wire leg, then you won't generally get an echo.  
If you have a minor echo, but the delay is short, then you don't 
perceive it as an echo--your voice in the earpiece just sounds a little 
bit louder then normal.  If you have both an echo and a delay, though, 
then you get a noticable echo, because the sound of your voice echos 
off the far end of the call and comes back late enough for your ears to 
hear it.

So, you don't hear an echo with a POTS line because it's too fast to 
hear.  You don't hear it with a pure VoIP call because the two ends of 
the call never get mixed together.


Scott