This is very interesting material, and thanks for digging it up. The documents indicate that the U.S. military switched to MARK-low FSK for compatibility with new STANAGs prescribed by its European NATO partners. The documents do not indicate just why the European countries chose MARK-low, so this remains a mystery, at least to me. It was either an arbitrary choice, or based on some prior European convention I'm unaware of. Both MARK-high or MARK-low shift polarity provide equal performance, and one has no advantage over the other, provided everyone agrees on a convention.

Memorandum SGM-223-61 suggests the NATO Standing Group prescribed MARK-low as the standard in 1961, but was then informed that the U.S. already had a standard which was the "conjugate" of this (that is, MARK-high). The memorandum states the U.S. would accept the new STANAG if the following sentence was added: "For universal compatibility, keying polarity reversing facilities will be provided in both receivers and transmitters, to permit operation on either the keying standard prescribed in this STANAG or on its conjugate which is the U.S. standard."

As it happened, the U.S. did not provide reversing facilities for BOTH transmit and receive in all their FSK equipment. Some FSK modems, like the MD-522, provided only reversible receive capability, and transmit was fixed at MARK-low. This met the NATO standard yet maintained compatibility with the MARK-high AN/GRC-46 (also reversible on receive) and other legacy U.S. equipment.

The other documents are primarily concerned with setting standards for a new generation of synthesized SSB equipment which would be used with AFSK modems, and has some bearing on the choice of tone center frequency. Of interest here is IMSM-0216-67, which sets requirements for SSB filter characteristics, including passband width, ripple, group delay, and stopband rejection. A BW of 2700 Hz was chosen, extending from 350 Hz to 3050 Hz, minimum, with +/- 2 dB ripple.

The upper band edge of earlier military SSB equipment was a bit wider, typically at 3250 Hz, and 2550 Hz tone center was often used (upper tone 2975 Hz). This was cutting it too close for the proposed new NATO standard SSB radios, so the tone center was reduced to place it nearer the passband center, which was 1700 Hz. However, 2000 Hz was an alternative AFSK tone center already in use with SSB equipment having narrower filters, so this was chosen instead. Again, either 1700 or 2000 Hz tone center would have worked equally well from a purely technical viewpoint, and the choice was dictated by a prior convention.

So the transition to MARK-low FSK can be traced to a European NATO standard, and the use of 2 kHz tone center for AFSK equipment was chosen for compatibility with new NATO SSB radios with an upper passband limit of 3050 Hz.

I was reading some old notices on NATOs website (Very interesting all the available orders and rule changes, Just look up RATT and things like that)

While I can’t find an exact change I wonder if it was just a way to force standardization. I read some earlier STANAG 5003 notices that specified everyone needed to use 50baud except for the USN (Probably because of the installed base) Then the USN switched to 75 Baud which was still not the NATO standard in 1968 with a caveat that everyone convert to 75 on an agreed upon date.

This combined with the fact the Mark would be at the top end of the frequency as others have stated in this thread probably forced the issue.

Either way still not a definitive answer but a lot of interesting reading that spans about 10 years time from 1957-1968 and the changes that happened in NATO communications.

This link still shows Mark High in 1961

SGM-0223-61_ENG_PDP

PDF Document · 1.1 MB

This one talks about the limits of bandwidth, not really specifying RATT in particular but they do worry about bleed over if I am readying right.

IMSM-0216-67_ENG_PDP

PDF Document · 1.4 MB

This one specifies the use of USB only

IMSM-0468-68_ENG_PDP

PDF Document · 4.6 MB

SGM-0488-63_ENG_PDP

PDF Document · 521 KB

IMSM-0429-68_ENG_PDP

PDF Document · 651 KB

Daniel Jones (K6YIC)
[email protected]
http://K6YIC.com

HH Phone #11973

On Mar 8, 2024, at 16:16, Dave Horsfall <[email protected]> wrote:

On Fri, 8 Mar 2024, John Vendely wrote:

The reason for various military branches switching from MARK high to
MARK low is surely one of the world's great mysteries, on par with the
construction of the pyramids. From an engineering perspective, the
choice is entirely arbitrary, but so far as I can see, prior to the U.S.
Army's introduction of the MD-522 in 1967, MARK high was the
convention. So why change? This must have been a bit confusing at
times for Army RATT operators, since their existing GRC-26 and GRC-46
equipment was MARK high...

Perhaps it may have something to do with higher frequencies being
attenuated or something?

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