[GreenKeys] Tri-tone RTTY

[Harold Hallikainen]

Interesting! I did find the manual on the Northern Radio 153 (
https://www.tmchistory.org/NorthernRadio/Manuals/northern_153_instr.pdf ).
It appears to be a two channel AFSK generator and not specifically
intended for frequency diversity, though I imagine it could be used for
that.

I could not find the manual for the 154. Looking at providing frequency
diversity, I think I'd combine the tones right after rectification after
the tone BPF (or, in the case of DSP, absolute value of the samples). The
mark discriminator level would drop to 1/3 the level that was present when
all three tones were present should two of them be lost, but, it's still
there. The interleaving of the tones would LIKELY cause both mark and
space levels to drop together. Then, dynamic threshold control would deal
with any remaining bias distortion.

Time interleaving seems more difficult unless the data is in blocks or
packets. With only two blocks, you can't determine which one is right
unless you add a checksum, CRC, or FEC. While easy enough to do in
software, this seems to be moving away from teletype.

Note that on the DSP TU, I now have a command you can type during transmit
(!ER) that sends an error report on the last reception. By counting good
start bits and bad stop bits, it determines an error rate (not perfect,
but an error rate!).

Part 97 is a bit difficult to read as far as permitted emissions. But, at
this point, especially once the new 2.8 kHz bandwidth rule becomes
effective (30 days after publications in the Federal Register), it APPEARS
multitone FSK would be permitted. I'll give it a try when I get caught up
on some other stuff.

Harold
https://w6iwi.org




On Fri, November 17, 2023 11:03 am, Sheldon Daitch via GreenKeys wrote:
>  On the subject of keying multiple sets of tones with the same data, in
> the long gone days of the Voice of America RTTY schedules, thoe AFSK
> circuits on HF did indeed use two sets of tones keyed with the same data.
> I can't tell you what equipment was used at VOA locations other than
> Greenville, but at Greenville, we used Northern Radio Type 153 tone keyers
> and the Type 174 tone convertors.  There were two sets of tones, a low
> pair and a high pair, 85 Hz shift each.  I wish that I had kept notes on
> the two pairs used, but I don't know the tones frequencies involved.
> Another form of RTTY diversity was time diversity.   There may be other
> systems involved, but Barry Research marketed their 6028 system for very
> critical TTY communications over HF links.   I found this link for the
> BR system:Â 
>
> https://www.sigidwiki.com/wiki/BR-6028
>
> Enjoy.
> 73Sheldon
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