[Asterisk-Users] Calibrating both RX and TX gain?

[tmassey at obscorp.com]

asterisk-users-bounces at lists.digium.com wrote on 10/17/2005 12:45:13 PM:

> > > Here's a couple of ways to determine levels...
> > >
> > > 1. using the model 4 transmission test set, attach the tone 
generator
> > > to one analog pstn line and the transmission level test jacks to a
> > > second pstn line. Dial from one line to other and measure the tone.
> > > Divide by two, and the result is the loss associated with a single
> > > analog pstn line from your location to your central office.
> > 
> > Remember, I'm not working with simple POTS lines.  I've got an 
> Adtran TA 612 providing CO lines
> > from a T1.  There is nothing that says that the RX and TX settings
> on the Adtran are the same...
> >  Therefore, just dividing by 2 won't work.
> 
> Obviously I _assumed_ you were working with analog pots lines. Sorry.
> Since I don't have access to your previous/original postings, now I'm
> somewhat confused as to exactly how the T1 and 612 are interconnected
> wtih asterisk. Is the T1 terminated on asterisk or the CO? Are the ports
> on the 612 FXS (for phones) or FXO (for CO lines)?

It's a "Smart T1":  Internet and CO lines on the same T1, which are broken 
out by the Adtran.  We have 6 CO lines:

PSTN T1 -> Adtran 612 FXS -> TDM400 with FXO Modules -> FXS modules 
            Ethernet                                         or
               |                                        local snom 190's
               V
            Firewall
              (to rest of network)

My original e-mail, with a lot more detail regarding my problem (way low 
sound and much echo) is included at the end.

An additional point:  When I call on a cell phone, there is no echo. Their 
echo cancellers kill it.  Their cancellers are so good, though, that when 
I use the echo test, all I hear is a very small amount of quiet garbled 
noise at the beginning of each word.  Very impressive!

When will Asterisk's echo cancellers get that good?  :)

Unfortunately, I did not realize that when I installed the system, and I 
used calls to my cell phone to determine connection quality.  Did I 
mention that the system is about 800 miles away from me now?  :(

> > Also, couldn't there be an issue on standard POTS lines where the 
> effect upon a singnal between
> > TX and RX is different?
> 
> I think I need a better understanding of how your assets are 
interconnected
> before I utter more inaccurate statements. From a telco perspective, 
> a customer line (whether an analog pstn copper pair, or T1-extended)
> should never have a different tx vs rx gain/loss at the rj11 point. 
Should 
> be exactly the same in both directions.

I guess that's kind of the definition of a hybrid?  :)

> > So how important or valuable will getting a milliwatt test number be?
> 
> Fairly important if you want to identify audio quality/level issues.
> Not so important if you were just trying to adjust rxgain/txgain on
> a digium TDM analog card.

Well, I've got +15db rxgain and -3db txgain.  This gives me barely 
acceptable levels both ways, yet I still have lots of echo.  Yet when I 
put an analog handset on the line, both RX and TX levels are fine.

In other words, even if you leave out the large echo I'm getting, why 
don't my TDM interfaces give me audio levels anywhere *near* what a $10 
analog handset gives me?  Line loss isn't an issue:  there's 12 feet of 
Cat5 between the channel bank and the TDM card!  :)  It sure feels like 
something more than simple levels and delay:  something like badly matched 
impedance.  I can't figure out why a handset would sound fine in both 
directions, when my rx and tx gains have to be *so* out of whack.

> In any case, you can still use a distant milliwatt generator to obtain
> realistic measurements, regardless of how you use those measurements.

OK, then, with that said:  Anyone want to give me a milliwatt test number? 
 The closer to Camden, South Carolina or Detroit, Michigan, the better? :)

Thank you *everyone* for all of your help and suggestions.  I greatly 
appreciate any information you can add.

Tim Massey


Original E-mail:


Hello! 

I'm having an echo problem with a TDM card.  The TDM card is being fed by 
a channel bank just 12 or so feet away.  When you put an analog handset on 
the line, both the RX and TX volume seem to be just fine.  However, when I 
use the TDM card, I have to have an rxgain of 13.5, and even then, the 
audio is relatively quiet.  I'm also getting echo on these lines, so I 
have turned the txgain down as low as I can and still be heard.  Right 
now, it's at -6, but it will have to come up some because that is too 
quiet.  But I still have echo. 

I am in the middle of trying to get a milliwatt test line to calibrate the 
rxgain properly.  However, this won't help me with the txgain, will it? 
How can I properly calibrate the txgain?  By ear?  Or is there a more 
scientific method? 

For example, once I have the rxgain calibrated for all of the lines, could 
I then call into, say, Zap/3 from Zap/4 and run Milliwatt() on Zap/3 and 
use ztmonitor on Zap/4 to calibrate it?  I'm sure it's not perfect, but 
would it be close enough? 

A second question:  doesn't it seem wrong that my rxgain and txgain are so 
far off when I'm just talking to a channel bank 12 feet away?  I sure 
don't have cable loss.  It sure seems like the impedance is way off or 
something.  Is there a way to test this further, rather than just cranking 
up the gain?  My guess is that using the milliwatt line will just tell me 
to make the rxgain higher, which will probably just make the echo issues 
worse... 

Tim Massey
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.digium.com/pipermail/asterisk-users/attachments/20051017/5de7b131/attachment.htm