Apparently you did not read my entire message.
I specifically stated it would be non-optimal and a bare-bones solution.
While 600 ohms may be the characteristic impedance of the wire run, a mismatch at either end will change the impedance of the entire path to some value that is the related to the severity of the mismatch and *the distance from it*. The CPE "sees" an impedance of X because it is at Y distance from the mismatch. You could have a horrible mismatch somewhere on the line and still see 600 ohms because you happened to be at the right distance.
And yes, the longer the run, the higher its DC resistance even though it's impedance is still 600 ohms. However, in the AC world, there is a measure of impedance and loss per fixed distance. So the loop might be 20 miles long, 600 ohms impedance, and 1.5 dB loss per mile (as an example). Since the resistance increases with distance, something must be happening within the wire to help overcome the resistance in order to stay at a fixed 600 ohms: capacitance and inductance. However, the loss increases with distance, so you may have a 600 ohm match at the far end but no signal to pick up.
If you wanted to fix an impedance mismatch the "right" way, you'd use a matching network. In it's simplest form, you could use a transformer to convert the 150 ohms impedance at the jack to 600 ohms for the equipment. You could also use a dynamic matching network with variable capacitors and inductors to create just about any impedance you want.
On Oct 12, 2004, at 2:58 PM, Rich Adamson wrote:
Adding resistance to one side of the line only begs for problems as it creates a tip-ring imbalance that will "cause" echo, etc, when other imperfections exist.
If that approach works at all for anyone, its addressing a symptom and not the root cause.
Try this one: Each customer loop is made up of copper and the longer the copper, the more resistance. Yet the impedance (in the US) is consistently 600 ohms. A short loop might be a 100 ohms while a long loop might be well over 1500 ohms; still both are 600 ohm impedance.
------------------------
Impedance is the measure of total opposition (resistance, capacitance, and inductance) to alternating current flow. Adding resistance will raise the impedance of the line.
On Oct 12, 2004, at 12:58 PM, Rich Adamson wrote:
Impedance does not equal resistance. Apples and oranges.
------------------------
If you're certain it is an impedance problem and the impedance of your
line is lower than that of the CO, you can increase the impedance of
your line by putting a potentiometer in-line and adjusting it until
the
sidetone disappears. This is a bare-bones solution and decreases the
efficiency of the line because you're putting in pure resistance.
If your impedance is higher than the CO, or if you want to be more
efficient, you'll need a more complicated impedance matching network.
On Oct 12, 2004, at 10:26 AM, Kris Boutilier wrote:
I have Asterisk connected to a channel bank via a t100p card. There
excessive sidetone generated on the analog side due to an impedance
mismatch
- I am very close to my serving CO which brings the line down to
about
150ohms and the channel bank is expecting 600ohms. However, the very
loud
sidetone is being fairly effectively supressed by the zap echo
canceller and
I have quite usable lines as a result.
From time to time calls are placed to other PSTN numbers (even
terminating
off of the same CO) that are introducing their own far end echo
component on
the line and it isn't being supressed at all - thus my outbound
callers
begin to hear thei rownvoiceandgetveryfrustrated.Ihaveto
assume the
second reflection couldn't be supressed by design - the zap echo can
has
already locked on to the sidetone echo after all.
So, I have a fix for the sidetone/impedance problem (PRI on order) however is it possible to insert another can into the system somehow? For example, if I were to run TDMoE to a second box and access to the t100p/channel bank through there?
Any suggestions welcome (apart from curing the sidetone) :-)
Kris Boutilier Information Systems Coordinator Sunshine Coast Regional District
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