[GreenKeys] Selective Fading Experiment

[David I. Emery]

On Sun, Jan 12, 2025 at 06:25:27PM -0500, Duncan Brown wrote:
> Eugene,
> 
> The Russians had a system (1960-70s ?) known as DFSK that used 4 tones.?? 
> I'm not sure if it was two channels or one channel with redundancy.?? 
> AN/FRA-86 was used to demodulate it.

	I did encounter some retired US intercept equipment for Soviet
DFS(K) system - when I was in HS in the mid 60s - that was pretty
antique vacuum tube technology stuff that I think must certainly have
dated to not much later than the mid 50s at the very latest ...

	Two independent streams... with different traffic on each. I
certainly remember actually seeing that on HF (and copying some traffic
sent that way) ...

	Basically 4FSK - one carrier on one of 4 frequencies at any one
point in time.

	I vaguely remember there might have been variants as to the
truth table of the logic mapping the two bits (one from each channel) to
the one out of 4 tones.   And different frequency spacings (shift) of
the tones.

	There also was a US four tone system used for RTTY with VIP and
flying command post aircraft (including Air Force 1)  - IIRC the first
military modem implementing it was designated as the TE-204.  	

	This was a synchronous 4FSK system used mostly for 75 baud (100
WPM) baudot in both encrypted and occasionally clear format.

	It worked with two frequency interleaved pairs of FSK tones -
one set present for the first half of a traffic bit transmitted and the
other present for the second half.  Thus the actual signaling rate was
150 baud With the odd/even pairs of tones alternating.   This provides a
timing clock allowing optimum sampling of the tone level at the 4 tone
frequencies... and continuous timing (important for cryptographic sync
in that era).

	It also provided some reduction of multipath intersymbol
interference. And thus (perhaps) better multipath rejection.

	This system was still in use by US entities - very occasionally
- well into the 21st century... in the old days it was called INDIA
OSCAR...

	The signal format this system used dates to at least the early
60s I believe.   

-- 
  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."