[GreenKeys] Tone frequency history?

[Jim Haynes]

I will immediately confess that I don't know.  What became amateur 
practice no doubt came from imitating commercial practice.

Going back to the 1930 edition of the AT&T Long Lines "green book" 
Principles of Electricity applied to telephone and telegraph work, there 
is a description of voice frequency telegraph carrier systems saying the 
carrier frequencies are multiples of 85 cycles (Hertz) and the channels 
are 170 cycles apart.  425 to 2295 are the channel frequencies involved. 
This system uses make-and-break keying rather than FSK.  The cover of the
book says it is a revised edition, but I don't have the earlier one.

Now we know that during and after WW-2 the standard shift was 850 Hz,
and this was embodied in the AN/FGC-1 demodulator designed by Bell Labs.
There is also the article by Sprague in the November 1944 issue of
Electronics magazine which describes the FSK system used by Press Wireless
and was the main printed source for hams in the early days of RTTY.

It was fairly recently that I learned that E.H. Armstrong published an
article in Proceedings of the I.R.E. in January 1928, "Methods of
Reducing the Effect of Atmospheric Disturbances" in which he proposed
using FSK for telegraphy.  This was quite some time before he got into
the FM system for which he is famous.  His telegraph proposal did not
use a limiter, so it was not true FM.  His hope was that the mark or
space signal would be detected in the appropriate filter, but that noise
bursts would affect both filters simultaneously and thus could be
subtracted from each other.  Carson of AT&T published a comment in
July of that year showing that Armstrong's proposal would not work.
Although Armstrong's ink-recorder traces showed that signals received
by FSK were clearly better than those received by make-and-break keying
of a single frequency.  Armstrong later had some bad words for people
who can show something mathematically when real tests with real hardware
show a different result.  I tend to believe that the actual difference
can be accounted for by the roughly doubling in transmitter power that
FSK gives, since the transmitter is transmitting power all the time
rather than only during the marking pulses.

At about the same time, and presumably unknown to each other, Larry 
Schmitt of Morkrum Co. (ancestor of Teletype Corp.) filed for a patent
1,705,211 in 1924, issued in 1929, proposing to use FSK for telegraph
transmission with the very same hope, that noise would cancel out as
mark and space signals were subtracted from each other.

There are various things in Bell System Technical Journal.  Such as
"Fequency Shift Telegraphy - Radio and Wire Applications" in BSTJ April
1948.  Also published in AIEE Transactions 1947, p. 479.  "Carrier
Telegraphy in the Bell System" Bell Laboratories Record, February 1948.

It has long been my belief, without any evidence, that the reason for a
shift as wide as 850 Hz was to accomodate frequency drift in the radio
equipment.  Well one bit of evidence is that in the AN/FGC-1 the mark and
space filters are rather flat in frequency response over a rather wide
range, so that the signal drifting in frequency would not produce much
change in the detector output until the drift got near the edges of
the filters.  In the Sprague paper they used a discriminator that was
linear over a very wide range of frequencies, and then low-frequency AC
coupled the discriminator output to the printer driver.  So as the average
DC level out of the discriminator varied with frequency drift the
DC component would be removed and the difference between mark and space
would be preserved.

John Williams W2BFD of the New York area was one of the very early
founders of amateur RTTY.  He wrote a history in QST, "The Story of
Amateur Radio Teletype", October 1948.  Obvously at that date the story
was only beginning.

So somewhere in all that may be the explanation of how we got our 
frequencies.

Jim W6JVE

 	---

 	"Ya can argue all ya wanna, but it's dif'rent than it was."
 	"No it ain't! No it ain't!  But ya gotta know the territory."
 		Meredith Willson, The Music Man