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

[David I. Emery]

On Thu, Feb 27, 2020 at 01:37:31PM -0600, Jim Haynes wrote:
> It's an interesting machine, but not very useful in my opinion.  You can
> choose the mark and space frequencies, but the tuning indicator doesn't 
> tell you which way to go if you are off.  Apparently intended for copying
> a single channel of a multi channel frequency division multiplex signal.

	Clearly intended for use in situations (SIGINT/COMINT and 
various monitoring of communications systems) where there are other
means in use for determining tone frequencies - such as audio spectrum
analyzers - or actual hard information about the tones in use in a
system from documentation and/or knowledge of the gear in use.

	By the 70s or early 80s when these things were designed and in 
use most systems of interest were implemented using VF audio channels
with little or no inherent frequency shift error - examples are ISB
signals from  the stable HF receivers and synthesized transmitters
typical of that era (and later) (such as WJ, Racal, Harris and even
R-1051 HF receivers) and any number of FDM-SSB carrier systems on
satellite, wireline and point to point LOS microwave radio circuits.  
Most of these VFT systems of course did pack many TTY signals into one
VF circuit -  though the 1280 also has provisions for wider bandwidth
mark and space filter operation with up to 600 (or more) baud higher
speed wider bandwidth FSK tone pairs in addition to slow narrow shift
lower speed stuff.   VFT tone pack signals with mixtures of narrow slow
and wider faster FSK tone pairs in one voicegrade audio channel were
extremely common in commercial use in the late 60s through 80s on
wireline, satellite and microwave networks.   Many of these were
eventually replaced by voice grade modems and stat mux gear... which
could stuff many more circuits into one 3.2 KHz voice channel.

	Even the remaining traditional single channel FSK signals on HF 
back then were quite likely to be generated from AFSK tones input to a
SSB modulator in a synthesized transmitter, or by mixing a LF VCO which
did the FSK with a synthesizer that generated the basic carrier
frequency.    And most receivers used (past the R-390A era) were also
synthesized and very stable and accurate.   Thus tweaking the  tuning
continuously was mostly a thing of the past back then.

	These boxes are ALSO explicitly intended for computer control in
monitoring systems... and probably were used that way more than in any
manual mode.

	However as far as available tuning indicators go, the units DO
have the ability to output the recovered mark and space tones at the
(audio) IF frequency which could of course be used with a XY scope of
some sort to display the classic RTTY crossed ellipse tuning indicator
pattern.   This is not very obvious from the documentation in the
manual, but very obvious if you study the schematics and experiment.  
The relevant mark and space tones at the audio IF ARE brought out to
terminals on the rear connectors.

> The 1280 has three board slots, one for the microprocessor and the other
> two can be a transmitter and a receiver, or two transmitters or two 
> receivers.

	The two receiver version can do either space (multiple receiver)
or frequency (same thing on different tones or another RF channel) diversity
reception and the 1280 can do AM limiter's or limiter type detection.


-- 
  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."