[asterisk-users] Echo problem

[Steve Davies]

On 12/20/06, Jason Bachman <jason at bachman.cc> wrote:
> As I understand it, the echo cancelers in Asterisk only work with the
> Analog cards (FXS/FXO).

Not true, echo is caused by any number of things "in the voice
network", so Asterisk will echo cancel any Zap device. We use it to
cancel ISDN2e and ISDN30 E1 lines very successfully.

> If you are getting echo on a digital line,
> there is a problem with either a DAC, the T1 clocking, or you are
> getting bit errors.

Again, not true. The echo is (mostly) not caused in or by asterisk, it
is caused "out there". Even if a call is digital end-to-end, there is
the posibility of acoustic echo in the handsets. Of course the above
problems might also cause echo, but I expect they would also cause a
log full of errors :)

> You have a Switch in the middle - perhaps the
> switch is doing doing digital-analog conversions instead of sending the
> digital data straight through. The cause of the echo could very well be
> there, and the echo cancelers (even if they worked on a digital line)
> would not help because the cause of the echo is somewhere else, not at
> the Digium card. Check your Tadiran switch for any echo cancel
> options.  I'm not familiar with that switch so I am no help to you on
> that, but I am pretty sure that its not the Digium card or Asterisk.

I agree, that is a very good candidate. AD/DA conversions in this
device would IMHO make it responsible for cancelling any resultant
echo, and the conversions could indeed add significant delay.

> Regards,
> --Jason Bachman
>
> Scott Gifford wrote:
> > Hello,
> >
> > We're in the process of setting up an Asterisk server, and are having
> > echo problems.  We have a Digium TE110P, and have tried the MG and
> > MARK2 AGGRESSIVE echo cancellers, with a variety of gain levels and
> > training times, and with both trunk and 1.2 branch versions of
> > Zaptel, Libpre, and Asterisk.  In all cases, callers from the PSTN
> > hear their own voice echoed back after 1.5-2 seconds; none of these
> > adjustments made a difference, except adjusting gain made the echo
> > quieter.

1.5 to 2 seconds. That is a HUGE delay. echo delay is normally
measured in tens or perhaps hundreds of milliseconds, and you are
unlikely to find a software EC that can deal with a 1.5 to 2 second
delay!

This sounds as if there is something very broken in the voice network,
causing huge amounts of delay. As suggested above, check the
intermediate switch.

[snip]

> >
> > We have done loopback tests with the Digium card with a loop plug in
> > it.

What were the results?

Cheers,
Steve