Telecommunications Tech Blog May 2013
Fixing Hum on an Analog Phone Line or Analog Station Port
By Mike Sandman •
Hum on an analog phone line, analog VoIP line or analog
station port can be due to a number of reasons. The bottom line is that the
reason you're hearing the hum has to be fixed, there is no filter for
"hum" on a phone line once the hum has been created.
The three reasons for hum are:
- Induced AC caused by a pair with a telephone line
traveling parallel to a power line for some distance. That's usually caused
outside when the power lines are on the top of a pole, and telephone lines
run under them for miles. It's the real reason you never want to tie-wrap a
cable to a power cable of any kind.
- An "imbalance," which is one of the two wires in a
cable pair being longer than the other (everything having to do with
telephones must be balanced - or both wires are the same exact length). An
imbalance can be created by one of the two wires being shorted to a wire on
another phone line, a wire on a power line, or ground. That includes
problems caused by water that's leaked through broken insulation shorting
out conductors in a multi-pair cable.
- RFI (Radio Frequency Interference) or EMI (Electro
Magnetic Interference) from some kind of radio transmitter, or electronic
equipment that puts out interference.
It's harder to fix an intermittent problem that might only
happen in bad weather.
Fixing Induced AC:
Verify that induced AC is the reason for your hum by
measuring the AC from tip to ground and ring to ground, with an AC voltmeter. If
there's more than .5VAC (half a volt) you can by a device that will "drain" the
AC form the pair before it gets to your phone equipment.
AC on a phone line can cause a lot of other problems, and
you may not hear the hum. For detailed information on troubleshooting and fixing
Induced AC problems see our
Longitudinal Imbalance Tech Bulletin.
Fixing an Imbalance:
A line is imbalanced when one side of the line is longer
than the other (tip longer than ring, or vice versa). This is a legacy problem
that you will hear on a regular old 2500 set or butt-set.
The usual cause of an imbalance is a nick in the jacket of
a buried or aerial cable, where water can get to one side of the line causing a
foreign ground or foreign voltage (from other pairs) on the line. That makes
that wire longer than the other side of the line. There is no filter for
You could try to find the problem in the cable and repair
it. That's not always easy, especially on a buried or aerial cable. Or even a
cable running through a big factory with dangerous machines all over. The only
way to "find" the fault is to use a TDR (Time Domain Reflectometer) which will
tell you how many feet it is to each splice or anomaly on the cable. You then
walk that far out and take a look to see if that's your problem.
You normally choose a different pair or run a new cable if
the problem is on the inside wiring. The phone
company has to fix the fault or give you a new pair if the hum can be heard at
the demarc with the inside wiring disconnected.
Fixing Hum Caused by RFI/EMI:
RFI or EMI isn't caused by cable or wire. The wire acts
like an antenna bringing the junk to your phone or phone equipment. It could
effect a station or trunk port, ATA, gateway or an analog or digital phone.
Even if the phone equipment is digital or VoIP at some
point is has to be analog so it will work with our analog ears and mouth. Once
the hum is created by the RFI or EMI it's simply audio and there's no way to
filter audio out of a phone conversation.
Some component in the phone equipment (a station or trunk
card?) or phone is sensitive to the particular RFI or EMI, and creates the noise
interacting with other components in the analog part of the equipment. The
interfering equipment might not affect another brand of phone or phone system
the same, or the phone may not be affected by a different brand of
industrial equipment or even the same equipment in a slightly different
It could be caused by industrial or X-RAY equipment that's
not grounded or shielded properly, bad grounds in the building or bad power.
One of the most frequent causes of hum on telephones is
the transformer that powers neon signs. Bad components or badly designed
circuits in stuff like CRT computer monitors or even light dimmers can cause
The easiest way to troubleshoot these kinds of problems is
to turn off everything to see if the problem goes away. If it does, turn things
back on one at a time to see what's causing the problem. If it doesn't try
running the phone equipment off a battery backup that's not plugged into the AC,
or try removing or replacing the ground.
Some people call data noise, often picked up inductively
from nearby computer equipment, hum. There's a difference in the hum caused by
an imbalance or RFI/EMI and data noise (which is more of a consistent hiss or
You could consider data noise the same as inductive AC,
caused by the telephone wiring running parallel to data cables.
You can see more information on troubleshooting RF
problems in our RF
Troubleshooting Tech Bulletin.