[GreenKeys] Polar Relays

[Jim Haynes]

A number of the early amateur RTTY receiving converters used polar relays
to combine the mark and space filter outputs, so they were sometimes
effectively part of the discriminator.  But they fell out of favor,
replaced by vacuum tubes directly keying the 60ma loops.  Maybe one
reason they went out of favor was hams ham-handedly fiddling with the
adjustments.  I remember the late Bob Weitbrecht had an insignia
depicting a polar relay with an axe chopping through it as part of his
campaign to get rid of them.

Of course in the telegraph biz polar relays were widely used, sometimes
because polar signaling was employed, other times as components of
repeaters or ways to get between the TTY signal on the wire lines
outside the machine and the 60 ma loop inside the machine.

A good reference is the AT&T Long Lines book, Principles of Electricity
applied to Telephone and Telegraph Work.  The 1953 edition is scanned and 
online https://long-lines.net/sources/att_principles_ocr.pdf

Way back the great E. H. Armstrong was proposing FSK for radiotelegraph
transmission.  Edwin H. Armstrong {\it Methods of
Reducing the Effect of Atmospheric Disturbances.} Proceedings of
the I.R.E., January 1928, p. 15. (discussion, p. 27)
This was before he developed wide band FM for high-fidelity noise free
broadcasting.  He seems to have hoped that the desired signals would
be distinguishable as mark or space, while things like static crashes
would affect both mark and space equally and could be cancelled out.
John Carson of AT&T showed mathematically that this would not in fact
work.  John R. Carson {\it The Reduction of
Atmospheric Disturbances.} Proceedings of the I.R.E., July 1928,
p. 966.  However Armstrong's ink recorder traces accompanying his
paper showed substantially cleaner signals from FSK.  Perhaps the
mathematical explanation is that FSK uses approximately twice the
transmitter power of CW because the transmitter is on all the time.

At roughly the same time Lawrence Schmitt at Teletype was experimenting
with radioteletype and proposed FSK for essentially the same reason,
that noise would cancel because it affected both mark and space channels
equally.  There is no reason to suspect that Schmitt and Armstrong were
aware of each other's work.  Nothing much came of Schmitt's work because
about that time AT&T bought Teletype.  AT&T considered matters of
telegraph transmission to be on their turf; all they wanted from
Teletype was the printing telegraph machinery.  Schmitt's patent shows
a polar relay being used to compare the mark and space signals and
deliver a decision.

We know Teletype sets were offered with built-in polar relays at
the customer's request.  The simplest sets, without polar relays,
were usable on neutral 60ma or 20ma loops from the outside world.
A polar relay could help clean up signals on neutral loops and was
essential if the incoming signals were bipolar.  A polar relay could
also be part of some auxiliary scheme such as a busy line detector
or a motor control arrangement.

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 	"Ya can argue all ya wanna, but it's dif'rent than it was."
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 		Meredith Willson, The Music Man