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

[Stephen Davies]

On Wed, 5 May 2004, Ryan Thrash wrote:

> I'd be interested in working/testing the fax side of the equation. It 
> really *should* work in Asterisk. I find your comment below about echo 
> canceling changes particularly interesting. We have multiple daily 
> complaints of echo at the office, despite the fact that voice calls 
> come in on a T1 PRI. The theory as I've been told is that you 
> *shouln't* need echo canceling with a T1.

There's potential for echo whenever there is a 4 to 2 wire hybrid
anywhere along the path.  So if the remote caller/callee is on an
analogue 2-wire phone line then echo can be introduced.

How much echo gets introduced depends on how well the hybrid is
balanced.  Whether you hear it or not depends on the overall delay of
the link.

Acoustic echo can also be an issue - where the remote phone's
microphone picks up the sound form the earpiece.  This sort of echo is
independent of digital/analogue lines and the rest.

Steve