[asterisk-users] b410p + fax (echo cancellation)

[Steve Underwood]

Zoilo Gomez wrote:
> I really don't get it ....
>
> From several emails in this list archive, I had clearly understood 
> that it is important to switch Echo Cancellation off for fax-channels, 
> or faxing would not work properly.
>
> However, faxing (B410P ISDN bridged to TE410P PSTN) seems to work fine 
> with EC at 256 taps on the B410P.
>
> I am confused; can anyone enlighten me?
With EC on, modems might work, or they might not.

If you have little or no echo, the EC actually does very little, so it 
won't significantly affect the one way at a time modems used for faxing.

If you have a lot of echo, then having the EC switched on will degrade 
the modem signal. Things might still work, as a modem adapts itself to 
line conditions. You probably get more bit errors, but the fax will 
still get through. However, it doesn't take too much echo before the 
modems are pushed beyond the limits of what they can compensate for, and 
the receive side cannot extract the bit stream at all.

So, the bottom line is it is not black and white, but EC off is more 
reliable.

Regards,
Steve