[Asterisk-Users] RFD: With echo and other distortion, can ulaw/alaw line quality ever be good enough for faxing?

[Nicolas Bougues]

On Wed, May 05, 2004 at 03:35:25PM -0400, Darren Nickerson wrote:
> Folks,
> 
> The silence was deafening ... I had a few private replies but overall I'd
> have to conclude that most people on this list aren't interested in faxing
> thru Asterisk. You're all probably jazzed about VoIP and fax is forgotten
> for now ;-)
> 

A few things to note :
- faxing doesn't care about echo. As I explained in an earlier call,
  virtually any call on the PSTN has echo. However, the delay is so
  short that human performs auto echo cancelling fairly well. But echo
  is really there, and a modem will notice it and perform its own echo
  cancellation if required
- faxing is, AFAIK, half duplex, so basically there is no real echo
  issue (ie you don't listen, so don't care, about what's incoming on
  the line while you're talking).
- *BUT* echo cancellation algorithms can (and will) significantly alter
  the faxing carrier, to the point to make it useless. So echo
  cancellation has to be disabled when doing fax. That's why any EC
  processor has to detect the 2100 Hz guard tone and disable itself
  when hearing it.

As far as ATAs are concerned, I can say that I'm pretty happy with
Sipuras. They disable their EC during faxing, and the results are
fairly good (other leg is PRI).

-- 
Nicolas Bougues
Axialys Interactive