[RTTY] ARRL attack on current RTTY users

[Kok Chen]

On Nov 21, 2013, at 10:05 AM, Robert Chudek - K0RC wrote:

> This discussion / education thread would be a great opportunity to apply the "Fair Use" clause of the US Copyright Statutes. In that context, excerpts from copyrighted materials could be posted here to further educate the reflector participants. In other words, few in the audience have access to old issues of IEEE Proceedings, 73, CQ, QST, or other technical journals that may contain a technical article relevant to this topic. Citing your source is, of course, prudent.

For reference, let me directly quote Frank Gaudé, K6IBE/4 from his article "Two-Tone, Shifts and Filters, Part II" in the February 1964 issue of the RTTY Bulletin:

"I have continued the literature survey concerning selective fade vs. mark-space frequency separation.  If one analyses all the data taken over the past ten years, the conclusion can be drawn that selective fade is independent of frequency separation down to at least 170 cps shift.  Some years were a little better for wide shifts but most years showed no difference. There were many times when there was more selective fade at the narrow shifts than the wide, which is indeed, surprising."

The RTTY Bulletin is not an academic refereed journal (it is the precursor of the "RTTY Journal") but neither are QST or the ARRL Handbook :-).

BTW, in modern times, we can actually "see" selective fading right on our waterfall displays.  What you can see is a narrow band of fading sweeps through the waterfall.  If you look at a moderately wide band digital signal like 500 Hz MFSK16 or Olivia, you will see just a few tones get "notched" out selectively.  And the notch moves.  And to confirm Frank Gaudé's observations, the notches are often narrower than 170 Hz.

PSK31 has no defense against selective fading, since the "notch" will take the PSK31 signal out completely.  But one of the 170 Hz shift RTTY tones will survive since the "notch" is narrow enough to let at least one of the two complementary tones through. This, by the way is why it is not that easy to quantify whether PSK31 is better or worse than RTTY on the HF bands.  PSK31 is better than RTTY when conditions are good, but we also know that RTTY can get through when PSK31 folds up if there is any significant multipath, as long as the RTTY demodulator has a decent ATC circuit/algorithm.

73
Chen, W7AY