Comments on: How To: Tune rxgain / txgain for Zaptel / DAHDI FXO interfaces on Asterisk PBX

By: Kris Boutilier

Kris Boutilier — Thu, 18 Jun 2009 21:32:28 +0000

Thanks for the credit, I’m glad the information was of use to you.

If anyone is curious why rx/tx gain and echo management are so interconnected this post might also be of interest: http://www.mail-archive.com/asterisk-users@lists.digium.com/msg83286.html

By: admin

admin — Thu, 23 Oct 2008 14:33:42 +0000

Hi Kline,

Sorry for the late response - but you got it dead on

By: Kline

Kline — Sat, 27 Sep 2008 17:16:28 +0000

OK; think I’ve figured it out.

If I want to calibrate line 1… I have the tone generated on line 1 and use line 2 to call into line 1. I use ztmonitor to watch the rx level on line 2 and adjust the tx gain on line 1 until the rx level on line 2 is at or around 14844.

Got it - right!

By: Kline

Kline — Sat, 27 Sep 2008 16:54:02 +0000

I’m not understanding the tx part. So let’s say I have two lines, 1 & 2. Do I send the tone out on the line I want to calibrate? Say I want to calibrate line 1… do I send the tone out on line 1 to line 2 and use ztmonitor to monitor line 1… looking at the rx number but adjusting the tx because it is what’s affecting the rx?

By: Brian

Brian — Thu, 05 Jun 2008 03:14:35 +0000

I tried the three Bell Canada Milliwatt numbers in your URL there. Unfortunately, all out of service now. Maybe tomorrow (as opposed to nearly midnight now) I will try calling a few *-1185 in my area and see if I can find one.

By: mwatson

mwatson — Thu, 05 Jun 2008 02:46:57 +0000

Hey there Brian,

I hear ya on feeling that 10.9 feels high. When I did my main site I was using values in the +6 - 8 range on rxgain and -1 - 2 on txgain. I however have done a site where I was up into +11.5 area. I had the same feeling as you, it just seemed ridiculously high… it did seem to do the trick though, the customer has been happy ever since we did that.

Both sites I mention though were on MTS Allstream, also in Canada. My site I know for sure was served off of an Allstream switch, the other one however might of been being served off an Bell Canada switch through Allstream’s “resale” platform as they call it.

I have read pages on voip-info.org that do suggest not going beyond +/- 10, however there main caution seems to be around causing potential problems with DTMF detection, Fax negotiation, etc.

My advice is just try it out with the values you’ve used and see how it does. Fortunately its easy and simple to revert the values if you need to.

As for the Milliwatt numbers… I can’t say I’ve had a ton of luck getting them from the telco’s either… I asked my Allstream sales rep, who sent me to his “Sales Engineer”, then told me to call Allstream technical support… I personally didn’t feel like trying to explain what I was looking for Tier-1 support, so I opted for the google method. The number I used was long-distance, but not too long… I was calling Toronto from the Niagara Region, so not too bad… certainly far from ideal though. These are the numbers I used - Milliwatt Test Number List. When I tuned my first interface, I tried calling all 3 numbers, each of them were giving me nearly identical results, so I figured I’d be ok.

By: Brian

Brian — Wed, 04 Jun 2008 17:42:01 +0000

I had to set my rxgain to 10.9! I have no scientific basis for what alarms me about that but it seems really high. I did have to use a milliwatt number that is not in my own area code, never mind exchange or the ideal case, Central Office. I am terribly skeptical of the phone company here, Bell Canada giving out milliwatt numbers just because I want one.

Thots?