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

[Andrew Kohlsmith]

On Tuesday 05 July 2005 08:40, Rich Adamson wrote:
> 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

Isn't getting your transmit and receive levels to 0dBm considered THE #1 STEP 
in combating echo?  The echo cancellers all assume that the audio will be at 
a given level and thus it's crucial to make sure this adjustment is fine.

> 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

Perhaps I have just been lucky but after adjusting the levels such that 
they're at 0dBm my echo problems are largely gone.

> 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.

If I crank up the gain to compensate for a 7dB loss would the far end not get 
audio at correct levels?  The 15kft isn't a moving target so that loss should 
be constant.  Now if the hybrid on the TDM card is causing such jacked-up 
audio to be crossed over to the rx side you would *still* only see it as a 
sidetone since it's occurring right on the card and your only delays would be 
the digitization and PCI transfer delays.

> 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.

This is *very* true.  The echo canceller is built assuming ideal conditions.

> 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.

I agree with the second part of this, but as I said I've had very good luck 
with MEC2, MMX "optimizations" and adjusting CFLAGS and KFLAGS for your true 
processor for the zaptel driver.  Once ztmonitor's saying you're tx and rx 
levels are at 0dBm everything just seems to work.  No echotraining even.  
Just an echocancel setting (usually 64, but 128 and 32 have been used too).

-A.