> [TELECOM Digest Editor's Note: A couple things I do not understand
> about voice communication over electrical power lines: Some say it
> will not work; others say it is okay. My own experience has been that
> (a) Chicago Transit Authority for many years (has?) used the third-
> rail for telephone conversations between control towers/trains/station
> agents. (b) I personally have tried so-called 'wireless intercoms'
> between different locations nearby; sometimes they worked (although in
> a rather piss-poor way; other times not at all. I have no personal
> experience with (a) but have been told the connections are very
> 'noisy' many times, and (b) when they worked, they seemed to have a
> lot of 'hum' in the background. When they did not work (all I got
> was hum with no audible voice at all) I am told this was because the
> two intercom stations involved were on opposite 'legs' of the
> transfomer. Can anyone explain this better to me?
I am not an expert on the technology, but it is not hard to "piggyback"
voice communication over an electric power line in various ways:
a) Some college and high school radio stations use facility power lines
as their transmission media. A regular radio tuned to the proper
frequency will pick up the signal, but the radio has to be in a school
building.
b) As mentioned, there are home intercoms that transmit over house
wires.
c) Some subway-el systems transmit trainphone over the 3rd rail. I
don't think the quality is that good and there is a lot of "hum" in the
power background. (Even regular telephone circuits pick up the hum).
I believe some systems started off with 3rd rail carrier but switched
to true radio, which requires the stringing of antenna through all the
tunnels and stations, but gives a better signal and allows hand held
walkie-talkies.
An electric wire can carry currents of different frequencies, so AC
power current, DC power current as well audio frequency and radio
frequency can be all carried on the same line. For example, a RR line
powered by AC has both the AC power (25 or 60 Hz) as well as the
control signal (100 Hz and others) sharing the medium. Some lines
even have multiple power sources from a separate wire, such as DC via
third rail. There are "filters" (IIRC, "impedence bonds") that
separate out the stuff.
So, from a purely theorectical point of view, voice and data
communications can be carried on power lines. How well and expensive
that would be in practice remains to be seen.
> I know that the third-rail seems like an awful way to transmit voice
> communications. On the one occassion I had to see the CTA system in
> action, I called into the CTA main headquarters phone number (MOHawk
> 4-7200) and the operator switched me to a supervisor in one of the
> control towers several miles away for whom I had a question. The
> connection, frankly, was not all that good. Once I also called Grand
> Central Station in downtown Chicago to the Lost and Found; she
> switched me to the Lost and Found in Baltimore, OH, also via the
> trackside phone lines. That connection sounded pretty bad also. PAT]
For the calls above I doubt they used the trainphones, which are meant
more for trains. Before SEPTA switched over to Bell, it's internal
communication system between towers and the like was pretty bad. That
was the result of just physical decay of the private network -- old
wiring, open pole wire, old telephone sets and switchgear,
interference from power sources, etc. SEPTA dumped it's own system
and switched over the Bell Centrex which was a huge improvement.
PATCO's original internal telephone system (a used SxS switch) had a
terrible hum in the background since the switch was atop a power
substation and telephone lines near traction power lines. It has been
replaced and it's much nicer now; presumably the new system has better
filters to keep out the power noise.
Well into the 1980s a lot of carriers had handcrank local battery
telephones still in use as wayside telephones; many of those phones
used antique components.
Keep in mind that railroads and transit carriers almost always built
their own voice communications network instead of relying on Bell; it
was more cheaper given the distances involved. I suspect the
widespread deployment of ESS allowed Bell to offer reasonable rates on
a regional Centrex system that wasn't possible before. For example,
the Upper Darby Terminal of the Mkt-Fkd line is out of the city limits
and it's a 3 message unit between one terminal and the other. Given
the numerous times the dispatchers, motormen, towermen, cashiers,
supervisors, and shopmen need to communicate with each other 24 hours
a day the message unit charges would be horrendous; it's free with a
private system. Further, carriers already have trained staff who
maintain the traffic control signals and they maintain the telephone
network as well.
In the last 30 years, both railroad and transit have upgraded much
(but not all) of their systems. Railroads have gone heavy into fibre
optic and even satellites. Transit has gone into advanced systems
"applique" phones. NJ Transit gives train crews Nextel cellular
phones for emergency use and to double as walkie-talkies.
Would anyone know the history of telephone ownership on the New York
City subways and IRT, BMT, and IND predecessors? I think whatever
private networks they might have had were dumped long before other
subways.
hancock4@bbs.cpcn.com