[Asterisk-Users] Anyone using * in a live production environment?

[Stephen R. Besch]

Brian D Heaton wrote:

>IIRC, proper functioning of the 2-wire to 4-wire hybrid depends on
>proper balance between the the individual wires of the pair.  If you
>upset the balance you're going to get all kind of problems.
>
I just finished modelling a standard 4-transformer hybrid coupled to a 
balanced RC transmission line. Cross talk was zero when the hybrid was 
balanced. Inserting a single resistor in  series with tip or ring 
imbalanced the hybrid and cross talk appeared. This could be completely 
compensated with the proper RC on the opposite side of the hybrid, as 
predicted. It made absolutely no difference to the cancellation if the 
resistor was split.  Since a balanced hybrid appears as a pure 
resistance (complex terms are 0)  to the transmission line, placing a 
simple resistor in series with the hybrid (on either side) at the 
termination point will just look like 2 resistors in series and will 
properly terminate the line.  There should be no effects at all from 
doing this other than the loss of some energy in the termination 
resistor, which can be made up for with a boost in Rx gain.

>
>Along the same lines, overdriving the hybrid is a big source of echo. 
>
That's because the cores saturate on transformer based hybrids.  This is 
not as likely to occur with active hybrids built with op-amps (which are 
found in almost all modern line cards), although it is possible if the 
gains are high enough.  However the distortion from the clipping would 
be far worse than the echo.

>The correct way to set the RX/TX gain values would be to get the number
>for the "milliwatt test signal" from a friendly telco tech.  You can
>dial that number through * and then adjust the RX gain value so that the
>signal is loud, but not overdriving the ADC on the analog interface
>card.  A good starting point on the TX gain would be whatever the RX
>gain ends up at.  The proper way to set TX gain would be to use a
>digital version of the miliwatt tone (1004Hz at 0dBm IIRC) outgoing and
>adjust it for a proper level through to another line on the same telco
>switch or across a T1 span that you can observe with a test set.
>
>I don't recall the exact specs for what percentage of the full-scale
>value milliwatt tones should be at on the digital systems.  Anyone have
>the reference?
>
>			THX/BDH
>  
>