[GreenKeys] FREDERICKS (Plantronics) Model 1280 RTTY Terminal -Modem

[David I. Emery]

On Thu, Feb 27, 2020 at 06:42:41PM -0500, Ralph Irish wrote:
> Is there any chance this unit is for F6 demodulation? This is DFSK, or two simultaneous AFSK pairs 
> on the same carrier frequency. NOT a form of multiplex. I was told that it was used commonly 'at the 
> Embassy level' for safe communications. Designated as, "Dual Frequency Shift Keying", in the Mode 
> Lists of the FCC at one time. With better, faster encryption and other computer based communications, 
> F6 may be totally pass?? at this time. 

        When I was in HS in Maine (prep school) in 1965 or 1966 or so we
acquired (mostly me) some DFS intercept demods  from military surplus
lots sold to schools and colleges...
  
        I believe they were used by the NSG/NSA station at Winter Harbor
 Maine..
  
        They were enormous with 3 big rack chassis linked together with
cables ... one of which had a tuning scope on it, and another was a huge
power supply...  output from these things was on/off keyed audio tones
on two channels.
  
	I was actually able to make these tune in real USSR DFSK which
was still quite noticeably in use in that era...  and even copy some
traffic.

	DFSK was 4 tone MFSK by modern terminology... with these boxes
having plug in configuration jumpers (in an octal tube socket thing) to
assign the four tones to the two output channels.

	As I remember it, these boxes had tone filters with (again) plug
in octal tube socket configuration devices that contained capacitors
that tuned the filters for various shifts.   And a plug in input
bandpass filter providing  various center frequencies and bandpasses for
the whole MFSK signal.

	Basically the DFSK I saw combined two more or less asynchronous 
RTTY bit streams to select one of 4 tones, only one of which was
transmitted at any given moment.

	The receive demodulator determined which of the four tones was
active coming out of the input limiter and from the mapping of tones to
marks and spaces on both channels output the correct thing to each
output as a keyed audio tone.
	
	A few years later I became aware of a vaguely related 4 tone
mode in use by the US military - originally with a Vietnam era HF modem
designated the TE-204.   This mode combines a 75 baud TTY signal
(usually but not always crypto in 7.0 unit code) with a 150 baud
alternating mark and space sequence synchronous with it in a similar
fashion to the DFS system to  produce two interleaved FSK signals each
present a half bit time of the 75 baud.

	The 150 baud 101010 clock is always present even if the 75 baud
signal is sitting at mark... which allows a receive demod to lock timing
onto the clock signal so it correctly samples both time interleaved FSK
signals at the correct moment even if no transitions on the data have
occurred for a while.

	This signal format provides a kind of time/frequency diversity
plus actual synchronous bit timing low bandwidth PLL locked to the
transmit signal rather than pure edge triggered start stop timing
without an associated clock.   It also provides some capability to
detect characteristics of the radio channel for optimizing demodulation.

	HF modems capable of this format are still in occasional use to
this day by the US military... and from the 60s to the early 90s  were
actively used for secure (and occasionally in the clear) RTTY to the SAM
VIP aircraft (in particularly AF-1).   They have a characteristic sort
of buzzing RTTY sound and tests in this format show up every once in a
while even to this day on the various HF SSB global command system
channels used by the USAF.


-- 
  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."