[Asterisk-Users] Why echo occurs

[Eric Bishop]

OK I understand that the $5 handset may indeed have an echo but that
it occurs so fast that it is not preceived as an echo. I pose the
following questions:

1. Is the echo (regardless of it's speed) a side effect of long
distance communications or is it there by design for some technical
purpose?

2. Is only a problem in 2-wire technologies (ie analog and BRI ISDN lines)?

3. Where exactly is the slowdown occuring? For example take my Supira
3000 as a case in point. It takes no longer for the PSTN signal to
reach the Sipura's FXO port than it does my $5 handset. Going from the
other end it takes no longer for the SIP signal to reach to the
Sipura's ethernet port than it does any other IP phone. So logically
the slowdown is happening as Sipura converts the PSTN signal to SIP
and so forth. Is it just that the Sipura/TDM400 etc. have a too slow
conversion CPU. Would a faster digital to analogue audio converter
"fix" the the problem?


On Thu, 10 Feb 2005 21:10:16 -0500, timebandit001 at gmail.com
<timebandit001 at gmail.com> wrote:
> > 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
>