[GreenKeys] FRF FSK conv. (was - Two eBay Novelties)

[David Burns]

Duncan..
     The NSA units (I pulled out of a sand pit!) had had their tags 
removed.  I later learned that those monster units (which looked like a 
rush-job from an engineering perspective using octal-based tubes) were 
later redesigned (using miniature tubes) into a two-unit job that took 
up about half the rack-space.  I saw those auctioned off at the Navy 
Yard in D.C. (around 1970)  but never owned one. Was told they were 
functionally identical to the older big units.

     It would seem that a four-tone system (where each tone frequency 
indicates the current mark/space state of -both- channels) gains 
immunity to noise that would not be present in a "two-tones-at-once" 
system, since you could design the decoder logic to be relatively 
insensitive to the other three frequencies (for a short period of time 
following a transition).  I don't know whether that was done in those 
units.  I did have a circuit diagram (acquired from "a source"), but it 
was wildly complicated for me to follow as a high-school person.  I got 
the drift, but not the details.

     Any thoughts?

-Dave in Boston