[GreenKeys] How were teletypes connected across long distances?

[Jim Haynes]

On Fri, 24 Nov 2023, Chuck Robertson wrote:
> 
> If they didn’t have modems or routers, and the machines send and receive
> only by 110v current, does that mean there were dedicated wires across long
> distances between cities, carrying electrical pulses between sending
> machines and receiving machines?

Let's go back to Morse telegraph days.  A typical circuit was a single
wire passing through a bunch of stations, all in series.  Such as along
a railroad line.  The number of stations depended on traffic load,
all the way down to a two-station circuit if there was a heavy traffic
load between the two stations.  Traffic was heard at all the stations,
so operation was very much like a radio net where each station had a
call sign and operators of stations not called could ignore what they
were hearing.

The reasons for series connection were several.  One is that batteries
were a lot of work to maintain, and with a series circuit the high voltage
many-cell batteries were needed only at the ends of the wire.  Small
batteries were used at stations just to operate the sounders from relays
on the main line.  Also if we ignore leakage the series circuit delivers
the same current to all stations.  One drawback is that the circuit has
to be kept closed, consuming battery current, when it is not in use.

The length of wire, wire gauge, and number of stations in series 
determines how far the circuit can send.  But there were repeaters
that could connect two circuit segments to each other and extend
the range.

Since it is not economically possible to connect directly all pairs
of stations that might wish to communicate, there were message relaying
offices where telegrams were received as hard copy and retransmitted
by hand on other circuits to get them to their destination.

Then there was duplex, which allowed transmitting separate messages in
opposite directions on one wire.  And then quadruplex which allowed
two messages in each direction.  A drawback to this is that the receiving
operator cannot interrupt the sending operator if something goes wrong.

When we get to more modern times the multi-station circuit was still
often desirable but was not implemented as a single wire pass through
all the stations.  Instead the wires ran between telephone or telegraph
offices, but there were repeaters which relayed traffic between the
offices and the customers' premises and created a virtual single series
circuit.  But the inter-city and the office-to-customer wiring could
be any of a number of things.  Time division multiplexing was popular
within Western Union, while frequency division was preferred by the
Bell System since the latter had a need fo intercity voice-grade circuits
anyway.  And time division back then using motor driven distributors
required high technology for the time to keep the transmitters and
receivers synchronized.

In the telephone biz there was also "compositing".  A "composite
set" is a set of high pass and low pass filters allowing one pair
of wires to operate as a voice grade circuit and also as two telegraph
circuits.  Since the voice grade circuits terminating in telephone
switchboards could not pass the 20 Hz ringing current there were
"composite ringers" which converted the ringing signals between
20 Hz and 135 Hz that would pass through the voice frequency circuit.
And then there was 1000 Hz ringing which could pass through voice
frequency repeaters needed on long circuits.

Frequency division multiplex for telegraphy was a popular idea going
way back.  Recall that A.G. Bell was trying to develop a "harmonic
telegraph" multiplexing system when he accidentally discovered the
principle of the telephone receiver.  His competitor Elisha Gray
succeeded with the harmonic telegraph, while the Bell System had
no interest in pursuing it further until there was electronic
means.  Western Union had a lot of single wire ground return telegraph
circuits, while the competitor Postal Telegraph built its lines with
balanced wire pairs so they could use frequency division multiplex.
When W.U. got out of the time division multiplex operation they had
a lot of trouble finding good pairs of wire on their pole lines
since they had never needed pairs before.

There is some objection to transmitting telegraphy and telephony
on circuits in the same cable because the higher voltage higher
current telegraph signals cause noise on the voice circuits.

In the 1950s-60s there was some use of electronic time division
multiplexing on radio circuits, mainly by the military.  But
ultimately this was replaced by frequency division multiplex
because of its greater flexibility.  And of course you know that
radio with Morse code uses on-off keying, whereas frequency shift
keying was found to be superior for teleprinter operation.  In
fact I learned late in life that Armstrong had proposed FSK for
Morse telegraphy but that fell by the wayside when he got into
FM radio.

The Bell System had a number of frequency division carrier telegraph
systems, and someone mentioned a Lenkurt product of the same sort.
I saw one of those installed in a Bell telephone office in my youth.
One of the Bell carrier systems was called 43A1.  Bell developed a
single-channel telegraph system using just one channel of the 43A1
system, so this could be considered an early form of telegraph modem.

I'll mention one example from Western Union.  At first every telegraph
circuit, whether intercity or office-to-customer, was an individual
thing, with the power supply and current adjustment located anywhere.
This caused a lot of difficulty with patching around circuit or equipment
failures because you couldn't just substitute one wire for another.
So they came up with a concept called Polar Centralized Circuit Handling,
or PCH.  In a PCH switchboard every circuit has a two-conductor jack
where one conductor is a sending leg and the other is a receiving leg.
To patch one thing to another you use a patch cord with a reversal, so
that it connects a send leg to a receive leg and vice-versa.  Signals
are polar and at a standardized current level so anything can be patched
to anything else without worrying about current levels.

Later on there came to be low-voltage patching bays and repeaters and
things.

References for all this: Western Union Technical Review (online)
AT&T Long Lines book "Principles of Electricity Applied To Telephone
and Telegraph Work" several editions, at least one of which is online.
Army manual TM11-680 "Teletypewriter Circuits and Equipment (Fundamentals)
TM11-655 "Fundamentals of Telegraphy (Teletypewriter)"
Other military manuals