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