[GreenKeys] Military RATT mark high/low change

[Harold Hallikainen]

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