[GreenKeys] Military RATT mark high/low change

[Nick England]

I have read how ham RTTY evolved based on the simple LSMFT technique for
FSK, tones for VHF AM, and then LSB operation.

But my question is focused on military/commercial practice, initially FSK
shifted up/down from a center freq. Post-WW2, the military converted
existing CW (and on-off RATT) transmitters to true FSK with add-on
converters. New transmitters also used the same scheme, a master oscillator
followed by a 200kc section that shifted up-down from that center.
Operation was evidently mark-high.

So why the later change to mark-low?? What possible benefits offset the
disruption to existing networks?

Yes military gear is often USB only. But that’s no reason to change to
mark-low. It is just a simple matter of choosing the right tone pair for
the AFSK keyer. What’s the big deal? Surely it wasn’t because the military
just copied TU plans from RTTY Journal?

Nick, still mystified.

p.s. at some point as the military moved from FSK to USB AFSK they shifted
from 2500Hz tone center to 2000Hz center. I’m guessing probably because
2925 was getting knocked down by the voice passband filtering?

On Sun, Nov 20, 2022 at 8:52 PM Harold Hallikainen via GreenKeys <
greenkeys at mailman.qth.net> wrote:

>
>
> On Sun, November 20, 2022 3:52 pm, Nick England wrote:
> > Evidently, earlier military HF radioteletype (RATT) transmitters were
> > designed to produce mark high. Later military gear produces mark low.
> When
> > and why did this change occur? It is a puzzlement to me that anyone would
> > have created such confusion on purpose. Nets would have been a nightmare.
> > Note- I am talking about the RF frequency being low or high.
> > Cheers
> > Nick
> > --
>
> Interesting question. Back in the 1960s, I learned LSMFT (low space makes
> fine teletype). Back then, we would FSK the transmitter directly shifting
> the frequency low for space. We received with an LSB receiver so space was
> the high tone. I think a lot of commerical and military equipment only
> runs USB, so with audio space high, space ended up high on RF. Note, for
> example, the SEA 245 ( https://w6iwi.org/SEA245/SEA_245.pdf ) that I use.
> In normal maritime mode, it only supports USB. It does have an amateur
> mode that adds LSB.
>
> That is my GUESS, but I would be interested in hearing the actual history.
>
> Harold
> https://w6iwi.org
>
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-- 
Nick England K4NYW
www.navy-radio.com
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