[GreenKeys] 10 Hz shift RTTY?

[David I. Emery]

On Tue, Jul 25, 2017 at 04:55:36PM -0400, Nick England wrote:
> So, who used 10 cps shift?
> I just noticed that the AN/URA-8B (CV-89A) manual has a change notice (T-4)
> to improve operation when using 10-cycle shift deviation. My, that isn't
> very shifty!

	The only use I ever remember knowing about was the 60s program
to FSK certain torch level 50 KW clear channel AM stations to provide a
survivable comms system - sort of a grandfather of GWEN...

	There may possibly have been similar experiments with
international SW transmitters as well...

	From a theoretical standpoint there is no BW advantage to really
narrow FSK shift below MSK... and worse BER performance. FSKing an AM
transmitter would generally be less audible in various kinds of AM
reception of the signal the narrower the shift and also less audibly
obvious from beat notes in hetrodynes... so really narrow shift keying
of such would have advantages as respects non-interfering with the
primary program material - which was paramount for use with major
broadcast signals.

	I would also love to hear some war stories about other uses and
users however... 

	Needless to say during that time period and slightly before the
narrow shift multitone TTY VFT FSK systems were at their apex and there
were lots of narrow shift FSK tone complexes on various kinds of  comms
circuits (hardly just HF SSB/ISB by any means)...

	But these used more or less the narrowest possible shift to
reduce occupied bandwidth per tone (and thus get the most info in a
standard 3.2 KHz voice channel), but not narrower.   And demods for them
always involved serious matched audio bandpass filters...

	Today, of course, (and starting with the R-1051 era) frequency
stability suitable for such is easy and relatively normal in ordinary
gear, in the era I am talking about it was not and took a lot of
expensive and power hungry stuff to implement (and even that a bit
marginal)... thus there was certain advantage against unsophisticated
opponents to such - a very mild speed bump of covertness...


-- 
  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."