[GreenKeys] Military RATT mark high/low change

Mon Nov 21 22:12:23 EST 2022

 Deflecting this a bit to non-military use, when Voice of America was running theRTTY schedules out of Greenville to the stations in Europe and Africa, we usedAFSK on the ISB transmitters.   I don't think I have any access to any of the oldschedules to back me up on this, but I don't think we cared if the AFSK audio wason USB or LSB, simply because we demodulated the RF signal back to audio, sothe upper/lower tones were always correct to feed to the AFSK tone convertors.
One point which might have been valid for some transmitters, the upper tone beingat the end of the audio bandpass of the channel is that all of our exciters had six kHzchannels, for program audio, so there was no issue with any high frequency roll offaffecting the higher of the two tones.
We also used two tone pairs, and I have no idea what tone pairs were used, but both tone pairs were keyed simultaneously (and at the same time), and the toneconvertors on the receive side used the two sets of tones in a diversity process, whichhelped a bit in achieving good copy.
Greenville was still using Northern Radio tone keyers and tone convertors when Ileft in 1989, and I don't know if there were any changes to when we moved from the RTTYcircuits to the email based communications.   
Too far back for some of the memory cells.
73
Sheldon
    On Monday, November 21, 2022 at 11:13:27 AM EST, W2HX <w2hx at w2hx.com> wrote:  

> 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? 

That most almost certainly has to be the reason.

As for the other question it is a mystery.

From: greenkeys-bounces at mailman.qth.net <greenkeys-bounces at mailman.qth.net>On Behalf Of Nick England
Sent: Monday, November 21, 2022 7:47 AM
To: harold at w6iwi.org
Cc: greenkeys at mailman.qth.net
Subject: Re: [GreenKeys] Military RATT mark high/low change

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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