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

[Stephen R. Besch]

Brian,

I think some of the confusion comes from what end of the line we are 
looking at and the nature of the imbalance.  While the resistor may fix 
the near end, it will probably cause some termination problems at the 
far end. Reflections mostly, which on a short run analog line shouldn't 
be much of a problem. I haven't looked into this in detail.  Also, if 
you look at the structure of a balanced transmission line, it should be 
really important to not have any imbalance out on the distributed part 
of the line, such as caused by having one wire of the pair having a 
different resistance, or by having a resistance anywhere but at the line 
termination - say 2000 feet out.  If I interpret things correctly, this 
would give a line which has two termination resistances at which there 
is a peak of power transfer to the load, and neither of them would 
appear purely resistive, giving phase shift errors which make balancing 
the hybrid difficult or impossible, and degrading data transmission 
capability.  Re the gain specs, I don't have a reference to them, but I 
suspect that there is stuff to be found on Google.

Brian D Heaton wrote:

>Stephen,
>
>	Very interesting.  I know I've seen all manner of messiness in the past
>when folks have monkeyed with balanced pairs.  I'll take your word on
>the modeling data.  I've not gone that far in depth with it.
>
>	You don't have the specs on gain adjustment handy do you?  I've
>probably got it buried in an old pub somewhere, but I don't have
>anything in soft-copy.  
>
>			THX/BDH
>
>
>	
>
>On Tue, 2003-11-04 at 18:34, Stephen R. Besch wrote:
>  
>
>>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.
>>
>>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.
>>    
>>
>
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