I’ve search around quite a bit, and have yet to find anyone who mentioned getting results like this from fxotune. I’m running Trixbox 1.1.1 with Asterisk 220.127.116.11 and Zaptel 18.104.22.168 with 2 x TDM04B quad-FXO analog Digium cards, txgain is set to 0.0, and rxgain is set to 8.0 (can’t hear people on the other end if it’s below this, and it’s a little quiet even at 8.0).
Prior to any tuning, if I run “fxotune -d -b x” for all 8 channels, it reports an echo ratio ranging from 0.0039 to 0.0443. From what I’ve read, those values are low enough that the software echo cancellers in Zaptel should be able to take care of the rest. I’ve only made a couple of test calls so far, but there doesn’t appear to be any echo.
Now, if I tune my channels using “fxotune -i 1 -m 3”, load the settings with “fxotune -s”, and then check the echo ratios again with “fxotune -d -b x”, my ratios now range from 0.1033 to 0.1125 across the 8 channels.
Previously (yesterday) I had another box connected to the same 8 POTS lines, using the same exact wires. That box was Trixbox 1.1.1 with Asterisk 1.2.11 and Zaptel 1.2.8, and also had 2 TDM04B cards (but not the same exact cards). Prior to tuning, on that box my echo ratio ranged from 0.163 to 0.2071, and after tuning ranged from 0.0414 to 0.0534 across the 8 channels. Even after the tuning, I was still getting some pretty bad echo on some calls, while other calls were OK.
I suppose it’s possible that Zaptel 22.214.171.124 has some vast improvements in echo cancellation, but I don’t see how the echo can be made worse after using fxotune. Hopefully someone can shed some light on this, or at least reproduce the scenario to confirm my findings.