[Asterisk-Users] Why echo occurs

[timebandit001 at gmail.com]

> 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".
> 
> The echo I mean is when I hear myself while talking to another party.

When you talk on the PSTN with an analog phone, in fact you have echo,
but it's coming back so fast, that you think that you just ear
yourself while you are talking.

No mix in the fact that you are talking on a VoIP phone, that takes
the voice, encode it in the proper codec, send it on the network to
your * box, * decode it, plays it on the PSTN line, takes what it
ears, encode it back in VoIP, send it on the network to your phone
that decodes it and play it back to you. Now, this adds a little
delay, that'S why you ear yourself talking just after you actually
said it.

This delays make it so that you ear it in echo. While when you are
directly on the PSTN, the echo comes back so fast that you ear it
"almost" at the same time that you say it. When you are going only
VoIP to VoIP, you don't have echo at all because there's no analog
link (that's where the echo is)

I hope I explained it well enough.

Please correct me if I'm wrong

> 1. It is not in the Asterisk box because IP to IP calls do not suffer
> this malady
Exactly

> 2. It is not from the Central Office to my premesis because my $5
> analogue handset works without echo. Also PRI ISDN works without echo.
Listen more closely, you'll see that there is echo.

With echo cancellation, you ear the echo only for the first seconds of
the call. Then the echo cancel is trained enough to suppress it

hope this help