Crosstalk Elimination Technical Bulletin
Crosstalk is being able to
hear a conversation on
telephone line from another line,
or a station on a phone system from another station.
In this Tech Bulletin we're talking about crosstalk on real wired analog phone lines,
analog lines from a cable or VoIP provider (from a box), or analog station ports
on phone systems or ATAs.
If the crosstalk is coming in on a line from the phone
company (two conversations heard), there's nothing you can do about it
except report it accurately to the phone company.
If crosstalk is on real phone lines, even if it's not heard by the
telco repairman, their automated tests should show a problem with the
line you're reporting it on. It will have a short to ground or a short
to foreign voltage, probably caused by a defect in the jacket of the
cable. It could be a cross with one of the other lines in the premise,
or one or more lines going to another premise.
While you may have crosstalk on the RF section of a
cordless phone or on a digital T1/PRI, that would be very unusual and
would be harder to fix - way beyond the scope of this tech bulletin.
The first thing to do is figure out where the
crosstalk is coming from:
On the inside wiring
telephone equipment (phone system KSU or KSU-less phone) either
because it's broken / poorly designed or you're using the wrong type
of modular line cord.
- Between two outside lines
- Between two analog station ports
- Between a POTS line and an analog station port
- On the phone line (POTS or digital) that's coming
in from the phone company, well before it ever gets to the premise.
Figuring out where the crosstalk is coming from is
often the hardest part of the service call. It takes time. An
accurate log kept by the customer noting the lines/stations it was heard
on and the times really helps. Especially since crosstalk is almost
You may not be able to fix a crosstalk problem if you
can't hear it when you're on the service call, but there are some easy
tests to perform that will let you check pretty accurately.
There's one of two reasons you're going to hear crosstalk:
1. A physical short between
conductors in telephone cable... caused by:
Bare wires physically touching each other, either as a
direct short (all the time) or intermittently. An intermittent short on
a phone line is referred to as a "swinger" - as in when the wires strung
between two telephone poles swing into each other when it's windy,
causing an intermittent short.
Telephone lines originally were bare
conductors strung between insulators on telephone poles, spaced out
horizontally so they couldn't touch (there was no insulation on the
When you have an intermittent short on a phone
line, you'll also hear static.
If you short a speaker
wire from your stereo, you'll hear the volume drop either a little or a
lot. There is only AC audio on the pair. No DC at all.
The DC on a telephone pair makes crackling static a
If you short a wire on a phone line the volume will drop either a
little or a lot, but you'll also hear a click when the short is placed
or removed from the conductor, or more commonly a crackling type static as an
intermittent short changes resistance between the conductors - or
conductor and a "foreign voltage" or ground.
A foreign voltage could be from a conductor from
another phone line, some other source of AC or DC voltage, or even from
a wire from a phone line or station port touching an AC power cable.
The short can be from tip to ring (on the same line -
but then you wouldn't hear crosstalk), from tip or ring to ground, or
any combination caused by water invading the cable (water conducts
electricity) through a defect in the insulation.
Not all cable is made
to be waterproof, but most will resist water for at least a short time.
Animals biting into the cable definitely causes water to get into it.
It takes a while for wet cable to dry out by itself. Some cable
can be dried using air, which is how the phone company protected many of
their cables for years. Modern cable for use outside (aerial or direct
burial) has a waterproofing gel inside and a heavy jacket to try to
In the old days before electronic telephone equipment (when there
were just switches and relays), a repairman would use a battery box
(burn box) to
put a very high current down a pair that was wet. The idea is that the
conductors would get hot and dry out the water in the cable.
That worked until subscribers started putting in electronic phone
systems, when the high current would burn up the trunk cards on these
new systems. The phone company didn't want to stop drying cables that
way but they were getting sued by the subscribers whose expensive new
phone system was destroyed. It's seldom done today.
The phone company's last choice, and yours, should be to replace a
wet buried or aerial cable. The first thing you try is finding a spare
pair that isn't wet/bad. If you can't find a good pair you may be able to
detect where the fault is (where the water is getting into the cable)
with a TDR (Time Domain Reflectometer) which will tell you how many feet
away you are from the problem in the cable. You walk that far out and
dig the cable up, or otherwise look for the cable and try to repair the
fault in the cable.
If it's a really easy cable run replacing the cable is probably
better and faster than fixing it.
2. Crosstalk induced onto one pair
from another because the wires are too close together, and/or they
In the old days some telephone station wire wasn't twisted. The old
red/green/yellow/black (JAKE or JKT) inside wiring cable had basically
no or very random twists. It was CAT nothing cable and it really picked
up crosstalk if there were two lines on the four wires in the single cable. It also picked
up RFI (Radio Frequency Interference) from radio stations like crazy.
That cable was first used when essentially nobody had two lines in their
house. The third and fourth wires were used for the princess phone dial
transformer in houses, and for A-Lead control on single line phones attached to
1A key systems in businesses (to light the light on key phones when someone was
using the single line phone).
Regular twists in a cable resist crosstalk and interference. Even some
twists, like CAT1 which would be very lightly twisted, does a pretty good job of
reducing voice crosstalk and RFI. CAT3 does a great job.
Essentially all twisted pair voice cable is now CAT3. Either 4 pair or 25/32/50/100
CAT5 cable has more twists per foot, but seldom does a better job for voice
than CAT3 except in high RF situations (like at a radio station).
Speaking of radio stations, some of the engineers decide to run their own
telephone cable and they use shielded audio cable. That usually causes
crosstalk and RF. If you see that stuff or the red/green/yellow/black cable on a case
of trouble like crosstalk or RF, just abandon the cable and run twisted pair.
If someone has done something really goofy either on-purpose or mistake, and
used one wire from each of two pairs, like the white/blue wire and the
orange/white wire, that's not a pair at all and could easily get crosstalk or
Likewise if you double up pairs the wire is no longer twisted pair.
Don't do that!
You can also get crosstalk if speaker wires or something with loud audio is
on the same cable as a telephone line, or tie-wrapped to a telephone cable. The
telephone network was carefully engineered to set the maximum volume of a phone
line to prevent crosstalk. The level of audio on a pair going to a 70V paging
speaker, with the speaker wire tie wrapped to a telephone cable, is definitely
going to bleed over to the phone pairs.
Generally speaking you want to have a speaker wire cross a phone wire at a 90
degree angle, never run parallel to the phone wire.
You may have noticed that if you put a toner used to trace wires on a
phone pair it's usually heard faintly on lots of other
pairs. That's because the toner is much louder than what
the phone network was designed for - which is OK because it's just a
tool used to repair stuff and not left in-place for very long.
Checking analog lines for crosstalk
If you determine that you hear two conversations on a
single incoming line at the demarc, the rest is easy.
Well, kind of easy
because if it's intermittent it will be hard to get the phone company to
fix it if they can't hear it. On the good side if the cable was wet but
is drying out when the phoneman checks it and he can't hear crosstalk (maybe
because the other line isn't being used at the time), the phone company also
runs an automated test on the line - which will most likely show that there's a
problem on the pair.
This checking procedure will work for two lines or 100 lines. I'm going
to use three lines at the RJ-21 as an example.
You must know the phone number for a 1KC (1000 cycle) tone (milliwatt)
line. You can't use a toner for tracing wires for this procedure since
the tone it puts out is way too loud. You aren't testing for how loud the tone
is, you just need to listen for it, so you can use any 1KC tone you have the
number for. The local 1KC tone here in Roselle, IL is 630-980-9940, which
doesn't seem to time-out. That will work OK for this application.
I've used 312-856-9996 on service calls here in Chicago for years but AT&T
changed it from a 1KC tone that would stay on the line (with a slight pause
every 10 seconds) to 8 seconds of tone followed by silence (silent termination).
That's just not useful for checking crosstalk. You need a tone that will stay
there for a while, like the 630-980-9940 line. Don't hog the line because there
are others that need to use it!
If you need test numbers for your local CO we may have them in our
CO Lookup (enter the area
and exchange code to see if we have any test numbers for that CO).
You'll also need two butt-sets. The second butt-set doesn't
have to be fancy, but you need two.
It's best to do this before or after hours so you're not fighting the users
trying to use the phone!
The simplest test you can do is simply use one butt set to go on each line
and listen in monitor mode, and then go off-hook to see if you hear anything
that doesn't sound right. If there is a serious imbalance you'll hear that the
line sounds different from the others right away. You can skip that if you're
going to perform a real crosstalk test because you'll be doing it anyway.
I usually do not remove the bridging clips on the RJ-21X when doing this test
since the test will catch problems looking out to the phone company and looking
in to the telephone equipment. Once I identify a suspect lien, then I do testing
with the bridging clips removed.
• For three phone lines I put one butt-set on the last line and dial the 1KC
test number (and make sure I hear it).
• I lay that butt-set down (depending on how quiet the phone room is I may
cover the receiver with my hand so I don't hear the tone coming from the
butt-set itself), and take the other butt-set and clip it on line two. I dial a
Silent Termination phone number (like 312-856-9997 which times out after a
• I listen for the tone I've got dialed up on Line 3, and for other
unusual noise. You must dial a silent termination number
(or your office to talk to someone). Just listening in monitor mode
going off-hook with the butt-set and not dialing a number is not a
valid test. An unterminated line is likely to pick up noise and
crosstalk, so you must dial a phone number to perform a valid
• If I hear noise or the tone I make a note of which line it's on. You really
need to make a chart while you're doing these tests.
• If I don't hear the tone I move the butt-set to Line 1 and dial the Silent
Termination number and listen for the tone or noise.
• Then I remove the butt-set from Line 3, put it on Line 2 and dial the 1KC
number and lay that butt-set down.
• I put the other butt-set on Line 1, dial the Silent Termination number and
listen for the one or nose.
This procedure tests all of the lines with the least amount of work.
If you have ten lines you'd start by dialing the 1KC number on Line 10, then
checking with silent termination from Line 9 through Line 1 for the tone or
noise. Then I'd dial the tone on Line 9 and check Line 8 through Line 1 for the
tone or noise, repeating the process until I finally dialed the Tone on Line 2
and checked for the tone or noise on Line 1.
If the crosstalk is between two of the lines at that premise you will find
the problem with that test. On any lines I heard the tone or noise on I then
repeat the test with the bridging clips open so I can be sure whether the
crosstalk is coming from the phone company or the telephone equipment /
If I hear the crosstalk between two or more lines with the bridging clips
open I report the lines to phone company repair. There's nothing more I can do
at that point.
If I don't hear the tone checking for crosstalk my only choice is to try to
get the customer to note which CO line they are hearing crosstalk on, and if
it's always the same line(s) I just report what the customer told me to the
phone company and hope for the best.
Just like a bad card in a phone system could cause crosstalk, a bad card in
the CO or a SLC in the neighborhood can be causing the crosstalk - not related
to cable at all.
We make a 1KC Tone
Generator and Silent
Line Terminator that you could carry with you, or even install at your
Damage from Crosstalk
So how much damage can crosstalk cause? If a customer or competitor
hears something they shouldn't it could cost a lot of money.
The biggest loss of money due to crosstalk was the divorce settlement of
former GE CEO Jack Welch.
It turns out he had GE's KSU-less multiline phones in his house. Those phones
are known to be poorly designed (as are many of the two or more line consumer
phones), and have crosstalk built-in.
Well, one day he was talking to his girlfriend on one line and his wife went
to use the other line (the phones have busy lights). Because of the built-in
crosstalk (defect from the factory) his wife was able to hear the conversation.
So how much could that possibly cost? The divorce settlement
was reportedly $180 million.
Crosstalk Caused by Modular Non-Twisted Line Cords
The GE and RCA KSU-less phones have poorly designed circuit boards and
they just have crosstalk no matter what anybody does (except throw the
2 Line, 3 Line and 4 Line KSU-less phones normally come with a 7 foot long
modular mounting cord (or sometimes two cords for a 4 Line phone). Some come
with a special twisted pair mounting cord, which helps prevent crosstalk. Even
if it's not twisted, a 7 foot flat mounting cord (not twisted) may not cause
When the 7 foot cord won't put the phone where the user needs it, they just
get a regular flat 14' or 25' modular cord. The phone works with that flat cord,
but they end up having crosstalk between the lines on the flat cord (because the
pairs aren't twisted in the flat cord - they're laying next to each other).
HERE to see our 14' and 25' 2 Pair Twisted Pair Modular Cords for those
situations. The wire is round, not flat, and contains two twisted pairs. It has
regular modular plugs on it (not the bigger RJ-45 8 pin type). This line cord is more
expensive but it will prevent crosstalk compared to a flat cord, and also reject RFI.