[GreenKeys] Military RATT mark high/low change

[John, W9DDD]

Well to muddy the waters more, my not so great memory from 1960-ish,  it 
was mark low for both AFSK for VHF AM/FM and HF.  My reasoning for that 
is that "we" started out switching in a capacitor to lower the frequency 
-- closed keyboard contacts -- cap switched in -- lower the frequency.  
That's pretty much how I remember modifying a surplus HF transceiver for 
Army Mars participation.  (man that was a heavy beast and I put casters 
on the separate PS so I could move it).

---
----
You said "tomorrow" yesterday.

The above comments or recommendations are SWAG. Use at your own risk.
John, W9DDD

On 11/21/2022 6:47 AM, Nick England wrote:
> 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 <mailto: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 <http://www.navy-radio.com>
>
>
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