[GreenKeys] Might be a bad weekend...

[jhhaynes at earthlink.net]

On Sat, 1 Dec 2007, Bob Camp wrote:
>
> One way to look at a very standard 88 mhy toroid based "discriminator" is as 
> a pair of AM detectors. The output is summed to make the decision about which 
> one is "on" easy.
>
There is some interesting ancient history of FSK.  A recent article by
Mischa Schwartz of Columbia U. pointed to a paper by Armstrong published
in the January 1928 Proceedings of the I.R.E.  He was proposing two-tone
transmission for noise suppression and showed ink recorder tapes of CW
reception with and without two tones.  Presumably he was frequency 
shifting the transmitter, as it would be hard to generate two-tone
signals otherwise at that time in history.  He was not using a limiter
at the time (which was the point of the Schwartz article - he wasn't
getting noise suppression the way we think of FM as suppressing noise).

Then a paper by Carson of AT&T in the July issue purports to show why
Armstrong's scheme won't work.  I'm still trying to understand Carson's
argument.

Then Lawrence Schmitt of Morkrum (Teletype) was issued a patent
1,705,211 in 1929 for a scheme substantially similar to what Armstrong
proposed, and for the same reason.  This patent was filed for in 1924,
so it appears the two men were working on about the same thing at about
the same time, but presumably were unaware of each other's work.
Schmitt's inspiration seems to have come from polar wire-line telegraphy.
Later he received another patent 2,012,407 in which he added a bias to
permit reception of signals sent by make-and-break keying, but keeping
the mark and space channels for noise suppression.

In both cases the intuitive idea is that the telegraph signal is known to
occupy only one channel at a time, while a static crash is assumed to
affect both channels more or less equally and thus should cancel out
when the output from the two channel detectors are subtracted.

Teletype conducted some RTTY experiments in the 1920s.  Presumably this
all came to an end when the company was acquired by Western Electric.
Thenceforth radio transmission was on the turf of engineering groups at
W.E. and AT&T, which later were combined into Bell Laboratories; and
Teletype was merely a supplier of the machines it manufactured.

Commercial and military RTTY later, and amateur RTTY still later, used
FSK and limiters in the detectors, putting the technology into the realm
of FM transmission with its noise-suppression properties.  In the early
1960s some hams got interested in the possibilities of limiterless
detection.  Leaders in this were Keith Petersen and Victor Poor, with
Irv Hoff doing a lot of the experimental work.  This resulted in several
demodulator designs in which the limiters could be switched off, giving
operation along the lines Armstrong and Schmitt had proposed so much
earlier.  (I don't believe any of us at that time were aware of the
prior Armstrong and Schmitt work.)  One motivation was the same intuitive
idea that had motified Armstrong and Schmitt, and another was that in
amateur work often one of the two frequencies is clobbered by QRM while
the other remains relatively clear; thus it ought to be possible to get
copy from the single good tone.

Results, as I recall, were rather mixed.  Limiterless detection 
occasionally gave better copy, but usually when copy was so bad as
to be useless anyway.