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Tue Sep 5 14:32:44 MST 2006

(believe or not). If the no-impedence-matching is true (or even if the
technical words are slightly/somewhat incorrect), then its beginning
to appear the Adtran FXO interface is not presenting a "balanced" 
interface to the tip & ring pstn line. In other words, one side of 
the line must have some internal electronics hanging on it that 
disturbs the balance needed for pstn lines, and that imbalance is 
causing induced AC power (which is extremely common on most pstn 
lines) to be heard.

This is going to be rather difficult to explain without a drawing, but
I'll give it a try. The pstn line (all the way from the CO or fiber
mux cabinet) is nothing more then twisted copper pairs, that have a
very specific number of twists per unit of length. The twists are
actually built into the cables to ensure that whatever outside electrical
influence exists (such as AC Power), that outside source influences 
both tip and ring in "exactly" the same amount. At the end of that cable
(whether its in your house or business) if you attached a perfectly
balanced piece of equipment, it doesn't make any difference whether
that outside influence (in this case, AC power) is ten volts or fifty
volts, that influence is cancelled out and not heard. But, its because
the attached device (usually an analog phone) presents an equal load
to both the tip and ring. (That should be fairly obvious since the
typical analog phone doesn't have any real way to create an imbalance
since it doesn't have access to ground or AC power. For the real
technical types, its the differential voltage between tip and ring
that creates the sound.)

If one would connect an analog phone to the pstn line that you're 
having the hum on, and then attach a resister from one side of the
line to ground (say, from the tip to ground), you are artifically
creating the imbalance that I'm talking about. The analog phone will
now have the hum that you're hearing via the Adtran & asterisk because
of the imbalanced line. The size of the resister (whether 100 ohms or 
1,000,000 ohms) will impact the loudness of the hum; the smaller the 
value the louder the hum.

In the olden days of telephony, we use to install "repeat coils" to
isolate the imbalanced equipment (usually customer owned stuff).
(Here comes the harder part to describe in words. Really need a visual
schematic for this.)

Repeat coils were absolutely nothing more then a basic audio transformer
with two primary windings and two secondary windings. A couple of 2 ufd
capacitors and the repeat coil was all that was needed to isolate the
imbalanced piece of equipment from the pstn line, pass the DC component
needed for supervision, and elimate the hum. In the '60s and '70's, 
those repeat coils were almost the size of a beer can, and one use to
buy them from Western Electric (and others) mounted on rails that
fit a 19" or 23" rack, along with the capacitors on another set of rails.

In very unusual cases of imbalance, we use to purchase the same set of
repeat coils and capacitors implemented in a box with switches on the
front panel. The switches would allow the technician to add resistive
values onto one side of the line or the other, in an attempt to "match"
the imperfections created by water logged cables, etc. If a cable pair
had a problem that indicated 10,000 ohms of resistence to ground on 
the tip side of the line, this box would attempt to add 10,000 ohms
to the ring side of the line to "balance" out the problem and reduce
or eliminate the hum.

Since a number of the cheap analog phones today have the same problem 
with the electronics needed to sense ringing, DC power, etc, its a fair 
bet the old repeat coils exist in even the cheap phones in one form 
or another. Removing them and testing the theory shouldn't be all
that difficult.

If anyone with this type of problem has actually read this far, send
me an email off list and I'll put together a schematic of what is 
needed to try this to reduce/eliminate the hum. Keep in mind we're all
talking about the problem via email and I could be somewhat off base.

Several months ago bkw was rather strongly stating that reversing the
tip and ring on an x100p card fixed his echo problem. Assuming that
was truly the fact, then the x100p card is also very likely to be
presenting an imbalanced interface to the attached pstn line and the
repeat coil approach is likely to address some of the x100p problems
as well. (off to find out...)

radamson at

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