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

[Steve Underwood]

Stephen R. Besch wrote:

>    5) Attempt to balance the hybrid at the 2-line to 4 line interface.
>            Why:  99% of the time, this is where the echo originates 
> and this is where is should be fixed.  Unfortunately, this is not for 
> the faint of heart, but if your line card has a hybrid balance 
> adjustment (many don't), use it.  Also, with multiple simultaneous 
> calls, this may be the only real solution.  Part of the problem arises 
> from the use of lower impedance telephone wiring nowdays. The typical 
> characteristic impedance of Cat5 twisted pair is about 100 ohms and 
> many line cards are optimized for a 600 ohm line. This is made worse 
> if the DC resistance of the wiring to the CO switch is relatively 
> low.  I haven't tried this myself, but you might try something as 
> simple as a 500 ohm variable resistor in series with the ring line and 
> adjust for minimum echo.  If it gets worse, you haven't lost anything, 
> just take the resistor out of the line. If it works, measure the value 
> of the resistor when set for minimum echo and replace it with a fixed 
> value resistor.

Tweaking the hybrid is really a waste of time. Most don't permit 
tweaking for this reason. Any change to the circuit, like changing to 
another phone (perhaps even of the same model) generally defeats the 
effect of any tweaking on the short lines of most PBXs. A well designed 
hybrid is fairly relaxed about termination, though the return loss can 
vary a lot across the audio band. Most approvals specs only call for 
about 12dB of return loss, and you will seldom see more than 20dB - even 
with hand tweaking. Whatever you do with the hybrid, only proper echo 
cancellation will clean things up well enough for good VoIP (or 
cellphone calls, which suffer similar high latency).

Actually twisted pair are generally below 150ohms. No line can have an 
impedance higher than 377ohms. 600ohm termiation is a fudge. On long 
lines the fudge doesn't work well, and loading coils are needed to 
refudge things. ADSL can't work with the loading coils in place, so they 
are being stripped out in many places. Such is life - messy!

Regards,
Steve