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

[Duncan Brown]

Photos & description of the AN/FRA-86 at 
http://www.navy-radio.com/ct-equip/4a.PDF

Duncan
K2OEQ

On 11-Feb-14 19:57, David I. Emery wrote:
> 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.
>


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