Dave Weis wrote:
On Tue, 4 Nov 2003, 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.
I've had to do similar things to lower loop current. Be sure you get at
least 1 watt resistors and put the same size on the tip and ring. Putting
resistance on only one wire will throw other things off.
dave
The 1 watt recommendation is a good one. The split resistor will
probably not make any difference on a voice only line, as the
transformed impedance of the phone line on the other side of the hybrid
won't really care. On the other hand, a big R imbalance will affect
transmission line characteristics on the line, which will have a
negative impact on data transmission. The bottom line is that resistors
are cheap and using two is probably worth the extra pennies. Just be
sure to divide the resistor value that minimizes echo in half so that
the total resistance insterted is the measured value.
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