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

[David I. Emery]

On Tue, Feb 11, 2014 at 09:46:30AM -0500, Lester Veenstra wrote:
> 4 frequency M-aryFSK      
> 
> The NSA units I had decoded four audio frequencies each separated (from the
> next lower) by a standard shift (determined by a plug-in filter group
> encased in a metal can about the size of a TV B+ capacitor with an octal
> plug on one end of the assembly), and when both channels were running the
> scope presented a square pattern. (As I recall, I only saw that once.)  It
> had a plug-board (associated with the filter plug-in) to allow for
> dynamically assigning M-M, M-S, S-M and S-S to different frequencies.  

	I had access to some of the same NSA demods (or similar, not
completely sure without photos) in the mid 60s surplus from the NSG
Winter Harbor Maine intercept station... and did find some signals back
then that had traffic on DFSK (and a square pattern).   This would be
1965-67.

	Vaguely remember some were synchronous ARQ type and some regular
start stop tty... and perhaps at least one was combined with ARQ on one
channel and 67 wpm tty on the other.   There was some ITC traffic on at
least one of the the signals I found.

	I also had some keyed (ASK) tone to TTY loop converters from
some sort of system that used these - had massive power transformers and
keyed the loop with high power audio output tubes... I was able to make
them work with the DSFK boxes..  they also had some tube type digital
logic in them to supply start and stop signals for some sort of
synchronous tty when fed a suitable clock... never did get this to work
back then.

	But the plug in filter selection was definitely in the ones I
had, as I remember it there was an input bandpass filter (sealed potted
network) and four LC tone filters and one of the plug in configuration 
plugs had tuning components (resistors and caps) for the tone filters.

	The things I had were two 12U rack chassis... tied together with
a power cable with Cannon connectors IIRC, one chassis was the power
supply and VERY heavy, and the other was the demod with a 3 inch CRT in
the center of it.   I am pretty sure mine used standard miniature 7 and
9 pin tubes of that era and not octal base large tubes... 

	Compared to more state of the art demods these things didn't
work very well... but were kind of fun to play with.

	Some time later in the 60s the AF began to use a HF RTTY mode
(TE-204) that was similar - 4 tones with one pair of tones active for
half the baud time and the other pair for the other half.   This was
most often used with 75 baud 100 WPM TTY signals... so the switching
back and forth was done at 75 HZ synchronous with the TTY 75 baud bit
rate.  Only one tone was transmitted at any time, of course, just as for
the Soviet DFSK system.   The 75 Hz switching between the tone pairs was
always present, even when the data was steady mark or space so rather
than a steady mark or space tone the signal sounded like a continuous
buzz or purr... with the RTTY data mark space sound quite audible in it.

	The 75 HZ supplied a clock that allowed slow PLL based tracking
of the signal timing and optimum integrate and dump detection of the
tones and the use of the 4 tones constituted a kind of in-band frequency
diversity... and of course the continuous 75 HZ clock signal also kept
the crypto in sync...

	That TE-204 4 tone FSK modulation continues to be occasionally
used right up to the present (it is available as a mode on some still
used gear) and from the mid 60s through the early nineties was used for
the KW-7 encrypted secure "India Oscar" HF RTTY circuit to Air Force One
and other VIP aircraft.  Sometimes they would come up in the clear on
this to send unclass traffic or chat on the keyboard and with the
correct understanding of the signal it was quite possible to copy
traffic from it. Of course anything important was KW-7 encrypted and not
copyable.

	The TE-204 modems were AFSK devices used with SSB radios and/or
other audio channels... somewhere I still have the tone frequencies they
used.   I think there was always a standard set used on a SSB radio...
unlike the DFS stuff which sometimes involved multiple frequency
division multiplexed DFS signals radiated from one transmitter on a ISB
sideband.

-- 
  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."