Echo & conversation crosstalk

Hi all, hope I am in the right place. I’ve recently installed an asterisk based system at my church, and it is 99% great. The 1% is quite annoying. Hardware is a pretty fast (less than 4 years old) computer, and Digium TDM400B, with 2 FXO cards for our outside PSTN copper; inside is all IP, using Polycom 430 phones. This replaced an old Meridian key system that we could no longer get phones for.

My issues.

  1. Echo. Used to be really bad, using standard MG1. After much reading, found OSLEC, and have had really good success - until today, when the principal called me (tech issues), and over the course of the conversation, the echo on her side came and went several times. I haven’t gone the route of the HPEC yet, since it needs tied to hardware, and I want to replace the computer. Any thoughts?

  2. Almost as frustrating… If both PSTN lines are in use, with active conversations, when a person is quiet on one line, they can actually hear the conversation taking place on the other line. I don’t know if it is both sides of the conversation, or just one side (i’ve experienced it, it was our side that time). Obviously, in a church or school, being able to hear another conversation is… not good.

Your thoughts? Somewhere else I should be posting?

Thanks very much!

for the echo canceller to work well, the FXOs must be set up to match the characteristics of the phone lines.
This is done with ‘fxotune’ (which is left in the zaptel source directory after zaptel is built - I’d copy it to /usr/sbin so it’s available as a command).

With zaptel running but asterisk stopped, run
fxotune -i
This will scan through the available zap channels and calibrate the board to the lines. It takes a minute or so per channel.
(The result is saved in /etc/fxotune.conf)

When the machine starts, it needs the calibration reloading by running
fxotune -s
after zaptel has started, but before asterisk.

As long as the line characteristics are constant, you don’t need to re-run it very often.

(fxotune dials a single digit to get a quiet line, I think the default is 4. If it does not get a quiet line, the calibration will not be accurate. You can put a single or multi-digit number after the -i, try using a normal phone to see what works. e.g. for the UK, a single ‘1’ gives 20 seconds silence - ‘fxotune -i 1’. The first few digits of a normal phone number may work.)

The crosstalk and varying echo sound like a fault on the external phone line, such as a bad connection, corroded terminals, water in a junction box, cracked cable etc…
Try connecting normal phones direct to the lines and see if the problem is still there. If it is, call the phone company - if not, report back on here and see if anyone has more ideas.

Thanks, yes, I have run fxotune. I’ve also recently discovered comments about running fxotune daily, and so am running fxotune every night “whether it needs it or not”. (out of curiousity, since I’m running the “advanced” one - how exactly does that data scale? I’d love to graph it the results.)

The IP phone system I put in replaced an old key system, and there was never any mention of crosstalk or echo with it, so I’m making the first assumption that the fault lies “on my side” of the PSTN termination.


You would not notice echo on a basic analog phone system.

If the line matching is poor, all it does is change how much you can hear yourself in the earpiece. As there is no processing delay there is no echo, and as you can always hear yourself talk you don’t notice it.

With a digital system there are conversion and processing delays both ways, so by time your speech has got from the mike to the line & back to the earpiece you can hear it as a distinct echo.

I’ve run fxotune a couple of times, weeks apart, on my system & the resulting files are identical. If the figures vary with time it indicates something changing with the line characteristics.

This could tie in with the crosstalk, if there is a short or bad connection somewhere the line this could have odd effects.

I’d double-check all the cables and connections, it could be a corroded terminal or poor crimp plug somewhere.

On a slightly wilder idea, what’s the earthing like on the Asterisk PC? Hypothetically, if there is no earth or a very poor earth, the signal from one line could be modulating the ground voltage on the machine & affecting the other line. Try an earth from the case metalwork to a earthed pipe or similar?

Possibly a poor connection on one leg of a line could have a similar effect.

I’d still also try a basic phone on each line to see if one has problems, such a weak audio or clicks/crackles/hiss which could indicate a connection fault somewhere.