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LineNoise.txt
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1999-01-28
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SOME NOTES ON LINE NOISE by The Mad Phone-man
===========================================================================
Well since that's my field let me elucidate abit....
Most of what you refer to as line noise isn't really noise per-se on the
line, but uneven response to tones (and that's what we use in our modems).
This can be caused by any number of factors. If you call up good old NONEX
and tell them that you have "line noise" your asking for a service bill and
no remedy. You will find that the average lineman they send out, has the IQ
of his shoe size.
Case in point- the old number I used to run my bbs at. I had phantom rings
on the line, so did my neighbor across the street. When it was very damp or
had rained for a while (several days) the "line noise" would appear, and
cause me grief. Well,with the similar problem and a moisture related one
at that across the street, it seems like the problem is out around the pole
in front of my place.
Well,after three visits, and three phantom excuses for repairing it
needless to say I was pissed.(The CO will often blast the phone line with
what it calls cable test voltage. Its around 600 volts and will vaporize
the MOV surge protector in your modem or answering machine) MOV= Metal
Oxide Varistor for you non-techies. But it will clear many of their "bad
lines". Now one fine day, they finally send out a guy from the "Cable
department" and I talk to him, I explain the past problems and suggest he
check past my place and for some reason during wet times, we have
inter-cable leakage. He found a spot down the street that had been zapped
by lightning some time before, and all the wire ends were charred and
black. He showed me a ball of burnt wire that could very well been the
cause. Well it was, as the problem is lessened about 95%. I say 95% because
I still have a bit of the same problem, possibly due to another charred
spot or a more recent lightning strike. But this problem seems to respond
to my calling my number and letting it ring for say a half hour, this drys
out the line enough that it stops acting screwy.
Well on to the noises that can be heard over the line that really do have
influence over transmission quality. The most prevalent is a crackling or
a dial tone drawn intermittently. (Listen to the line with a hi-z butset)
Ringing voltage is usually about 90 volts and20-30 hz, so there's usually
no problem in ringing the phone, but when you pick up, the phone voltage
drops to 30-50 volts and the smallest problem will surface. The phone
company has this habit of checking the line to ground (it should never be
connected in a regular home phone line) and they will clear off water,
insects, and other crap with a 600 volt blast as mentioned earlier. This
applied across the line to ground sure does away with the water, but in
cases where spike protection or impulse protection from lightning is
present, WATCH OUT! the MOV's are typically manufactured to
short at around 130-150 volts, and are connected to the third pin (Ground)
of a three wire plug on your answering machine, your cordless phone, and
some modems. The MOV will explode as it shorts itself to ground to do its
job of surge protection, leaving a short on the line which is readily
detectable by the CO. IF THIS HAPPENS, inform them immediately that you
have a problem, and they will make restitution for repairs in CASH! I got
$75 for repairing a $1.98 MOV (available at Radio Slack)
Now more on the noises....
==========================
After the infamous popping, the next most bothersome to the modemer is a
low level whine, muchlike some of the shitty MCI ports,sounding like a jet
engine varying in rpm, itis particularly evident behind
dialtone, and if you drop the dialtone, its still evident. This is digital
whine caused by some circuit in the path not exactly in phase. This is
particularly annoying because the digital equipment tries to compensate by
stuffing or removing bits from the digital path to attempt phase lock, and
thus, data is occasionally lost or garbled. I have two lines at home that
are heavily used and have this problem, its not apparent to modem traffic
until you hit 2400 baud, then the time frame of the modem kinda gets
jitters from the digital whine.
Lastly, of importance, is signal level. In analog, as the signal goes down,
the signal tends to corrupt on the lowest
levels while some will pass fine during peaks. If this is the problem, you
see multiple errors on x-modem, then good blocks....this problem can have
numerous causes, fading of a microwave path, followed by a switching of
diversity receivers, bringing the signal backup to par temporarily.
Also in this category are bad amps at either the CO or in your own
equipment, that won't limit gain till noise actually takes the place of the
signal, This is a "slow attack" increase of noise followed by a quick
quieting as the redundant amp takes over (In your CO).
Crosstalk:
----------
If you have ever picked up the phone and swore you could hear someone else
talking or in some cases, actually been able to talk to the other party,
you have been exposed to crosstalk. This obviously will knock the shit outa
a modem conversation due to the fact the voice is mixing non-linearly with
the data or in some cases, overpowering the data signal itself.
This is a fairly common problem, common to analog FDM multiplex equipment.
This equipment tries to keep the conversations apart using SSB channels
arranged throughout a radio's baseband (IF) frequency. Because the channels
are without carrier which would enable the equipment to check signal level,
frequencies called "pilots" are inserted into the clusters of channels
allowing their levels to be monitored and controlled.If there is a "Hot"
pilot or a "Hot" channel in the group, it will bleed over into the other
channel's passband and crosstalk will be heard. In the cases of two-way
communications via crosstalk,the group pilot is usually so hot, everybody
in both directions is splashing over and both directions are being heard.
Basically the solution to line noise problems is to know about
them, and being able to pinpoint the exact type of problem will make it
easier for the Telco to service it quickly.
Another article in the continuing series by The Mad Phone-man
"Helping everyone get all from what he pays for".
(C) 1988 Mad Phone-man Enterprises
Permission granted for reprinting this article in its entirety.
From Lunatic Labs UnLimited: 213-655-0691