VoIP Mailing List Archives

[creslin at digium.com]

Hey all,
First of all, some background:

Fxotune is a utility that is used to tune the hybrid on FXO modules
For all of you with FXO modules out there, fxotune can help you adjust
the analog and digital hybrid that is on the FXO interface and tune it
so that it maximizes echo return loss. This means that it will reduce
your default echo which is received, and will help any echo cancellers
on the line to do a better job. If using one of the open source
software echo cancellers, using fxotune can be the difference between
having echo problems and not having echo problems.

This is the update:

I just committed a new version of fxotune which uses a better technique
for measuring echo return loss. Before, there was a simple power
calculation which was done on the samples that would indiscriminately
check the power of all samples received. This works well when the line
is silent, but if there are any sort of tones in the background or noise
due to noisy line conditions, this calculation can yield results which
may improve things, but are not the best results.

The new method involves using fourier analysis of the tones used in the
test reference which is sent out. Using fourier analysis instead of the
power calculation, we can cut through any background noise which is not
related to our test sequence's set of tones, producing a much more
accurate and noise immune calculation.

If you have run fxotune before on your lines, I recommend you re-run it
with the updated version of the utility. As of this moment, it is not
yet in a released version of zaptel, but if you check out either latest
1.2 or 1.4 branches, it will be there. If you run fxotune with the -v
option, it will tell you what the return loss it calculates for each AC
impedance and set of coefficient parameters in dB.

In order to use the new analysis calculations, you do not need to pass
any sort of special parameters to fxotune, it does the new analysis
technique by default.

Please let me know if you have any issues as well. Thanks!

--
Matthew Fredrickson
Software/Firmware Engineer
Digium, Inc.