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<DIV><FONT face=Arial size=2>There are a large number of potential sources of
hum and each situation will narrow them. The first thing would be to
quantify the observation. I am assuming it is power line frequency,
although that may not be the case. It is also useful to notice whether it
is fairly pure or rich in harmonics, or even if it is predominately the second
harmonic, which would be produced by full wave rectification in a power
supply.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Classically in telephony, hum is caused by
imbalance or things like split pairs that disturb the balancing capability of
the twisted pair. However, as has been pointed out, hum can only enter a
system at an analog point, and the OP indicates that the only analog point in
the system would be inside the SIP phone, unless there is something analog
beyond the T1, but that is probably outside of his control.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Since Asterisk is connecting a T1 to a SIP phone,
there is no way anything inside the box (PC) can be the cause of the hum.
If it had FXO or FXS cards in it, magnetic coupling or even electrostatic
coupling inside the PC would definitely be a consideration, and would most
likely manifest itself as some sort of buzz that was NOT related to the power
line frequency.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>The comment about the wall transformer on the SIP
phone is very germane. If it has a bad or inadequate filter capacitor that
could allow hum into the SIP phone through the power voltage. It would
most likely be 120 HZ (US, 100 Hz in many other countries) and would have
additional harmonics, because it isn't sinusoidal. One test would be to
power the phone from a known good laboratory type power supply as a
test.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>The other main culprit in this case would be
coupling of magnetic fields into the phone itself, either the electronics inside
the housing, or the handset components, or even the handset cord.
Proximity to any electrically powered device with a large transformer would be a
potential source. Even a nearby CRT terminal or monitor. An easy
test for this would be to move the phone and/or handset. Generally just
rotating it 90 degrees will make the loudness of the hum change
noticeably. If that is the case, then identify the offending source and
move or replace it.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>This situation is actually simplified because the
SIP phone is the only thing that could produce the symptom. Its just a
matter of determining how it is entering the phone, through the power supply or
directly into the electronics from a nearby source.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Wilton</FONT></DIV>
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