[Asterisk-Users] How to read dbm or voltage via ztmonitor ?

[Rich Adamson]

> Strange... I have read something about this, but probably misunderstood :
> http://lists.digium.com/pipermail/asterisk-users/2004-November/071301.html
> "3. Start 'ztmonitor' on the target trunk in 'quantitative' mode.
>  4. Dial the CO Milliwatt test line from inside Asterisk via the target
> trunk. Note the Rx level - it should be near 14844 
>  5. If level is < optimal make the the rxgain for the channel more +ve. If >
> than optimal, more -ve."
> 
> Have you already read that ?
> Is somebody has done adjustements with this procedure ?
> If not possible, what are your recommandations to help to correctly adjust
> analog lines interface levels and reduce echo to be optimal (not by hearing
> it) ?

Yes, I've read that. Ztmonitor is simply a very _basic_ tool that provides
you with a little bit of feedback to adjust the rxgain and txgain
settings to something relatively close to what the human ear considers
reasonable audio levels.

The tool cannot detect or determine what settings are reasonable for
echo. Therefore, the end result is you need to listen to a conversation
on the pstn line and adjust the same rxgain & txgain to balance audio
levels _and_ echo at the same time. (Must stop and start asterisk after
each change in rxgain or txgain for those changes to become effective.
A reload will not cut it.)

The TDM card (and associated software-based echo canceller) is difficult
to set as the canceller operates over a rather narrow range. If your
system falls outside that range, you'll end up having to adjust the
gain settings to rather low audio levels to minimize the echo. That
has basically been an issue since the TDM card was released, as well
as with its x100p predecessor.

Historically, those * systems that are further away from the Central
Office have more issues then those that are located close by. The reason
for that is the greater the pstn cable loss (between asterisk and the
central office), the more difficult it is to adjust the audio levels
to compensate for that loss without generating echo. In other words,
if your asterisk system is 15,000 feet from the CO, you will typically
see about a 7 db cable loss. That loss is very noticable to the human
ear. If you crank up the audio levels to help compensate for that loss,
echo will occur and the software based echo canceller is not able to
handle it.

In addition to the above, if the analog pstn line has any noise, 60hz hum,
impedence mismatches, or other imperfections, the echo canceller tends
to not handle the true audio in a reasonable manner. You may also find
the canceller is more/less effective with different human voices; high
pitched voices will be different then low pitch voices in terms of echo.

So, the bottom line is that you can use ztmonitor to help find initial
settings as a starting point, but then you have to listen to real
conversations to further adjust the gain settings to minimize echo
while maximizing your ability to hear any conversation. Using some other
asterisk implementor's rxgain & txgain settings is 100% useless as
those settings are dependent upon the distance between your asterisk
system and your central office.

There are lots of different commercial test sets that can be used to 
evaluate an analog pstn line, but those test sets do not measure the 
audio through the TDM card, drivers, echo canceller, etc. Therefore,
the human ear becomes the test set of choice for asterisk.