[Milsurplus] Mystery FSK/DFSK demodulator CV-2310/G

[David I. Emery]

On Tue, Nov 27, 2012 at 11:54:32PM +0000, Sheldon Daitch wrote:
> Nick,
> 
> not sure what DFSK means, but I wonder if it means diversity FSK?
> 
> Way back when we ran the HF RTTY schedules at the Greenville VOA
> operations, we used audio frequency diversity on those RTTY circuits.
> 
> I am in TDY status and have much slower internet access and can't 
> verify the models.  We used a low set of tones and a high set of tones
> on the Northern Radio tone keyers, both keyed with the same 
> TTY keying.  We used the NR Type 174 (I think) which matched
> the same set of tones and there was a comparator circuit in the demodulator
> using both sets of tones to determine whether the pulse was a mark or space.
> 
> Whether or not that is how the CV-2310 is set up, don't know. 

	The things I played with as a kid were 4 tone FSK.   These days
that is usually abbreviated as 4FSK.   Four tone FSK transmits two bits
per symbol by having the FSK'd carrier on one of 4 frequencies each one
corresponding to the one or zero (mark or space) states of the two
channels or bits of information.

	To clarify, F1 might be mark A and mark B, F2 might be Mark A
and space B, F3 might be Space A and Mark B and F4 might be Space A and
Space B.

	For those who are familiar with either P25 phase 1 modulation or
Flex paging modulation this 4 tone FSK format should be familiar.

	The difference between those formats and the DFSK or DFS format
was the classic HF 1950s-1960s DFSK allowed the bit streams in the two
channels to be completely asynchronous with each other so the  pattern
of signal on the 4 tones could have shorter and longer pulses than the
bit lengths of info on either channel.

	The advantage of this technology was that on one class C final
high power FSK transmitter one could get two independent channels or
circuits of RTTY without having to go to shorter pulses with worse noise
and multipath distortion performance in the HF environment and/or the
need for some kind of complex and expensive time division multiplexing
(complex by the standards of stuff built using vacuum tube digital logic
that is).

	Why this particular RTTY format was so favored by the Soviet
empire I do not particularly know.   But it was.

	I believe the US/NATO world developed digital logic based time
division multiplex systems (lots of 4 channel ones) during about the
same era... which mostly put faster bit streams on standard 2 channel
FSK signals.

	Then of course the transition to SSB/ISB systems lead to
extremely wide use of VFT tone packs on HF ... with up to 16 or 20
channels of side by side narrow shift FSK tones (usually 85 Hz shift on
170 hz tone spacing).

	As for diversity, the antique NSA demods did have provisions for
diversity IIRC (it is LONG ago)... I believe they would switch back and
forth to the stronger input of two inputs.

	As to the actual bit assignments to the 4 frequencies I forget
the details - I think one channel was usually upside down from the other
but I frankly forget whether one channel selected a wider or narrower
shift and the other selected mark or space (eg above or below the center
of the signal) or whether one had a bit weight of two and the other a
bit weight of one and the selected tone was the sum...


-- 
  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."