[GreenKeys] Tri-tone RTTY

[William Wuttke]

You also have to watch your power output.
Don't forget you've got another DSP TU available for live QSOs. It would be
an interesting experiment.
When I was in the Air Force, we used the Collins TE-204, 4 tone modem which
also had time diversity between the two sets of tones.

Bill
KE3BK

On Thu, Nov 16, 2023 at 20:38 Harold Hallikainen via GreenKeys <
greenkeys at mailman.qth.net> wrote:

> I've been giving this some more thought today. I wonder about doing RTTY
> with these tones:
>
> Mark: 2125, 1785, 1445
>
> Space: 2295, 1955, 1615
>
> The tones are 340 Hz apart with a 170 Hz difference between mark and
> space. Ideally, at least one of the mark and space tones would get through
> on a selective fade. Existing TUs would be able to tune any of the tone
> pairs. On the demodulator side, there would be 3 mark filters and 3 space
> filters. I'd probably full wave rectify the outputs of all the filters
> (absolute value in DSP), add the mark filters to drive a data low pass
> filter, and add the space filters to drive another low pass filter.
> Dynamic threshold control would still be used. No limiter could be used
> since there would be simultaneous tones. Terminal units with limiters
> would need to have a bandpass filter before the limiter to get rid of the
> extra mark and space tones.
>
> On the transmit side, in DSP, I'd just add the results of three DDS tone
> generators. The sum would have to be divided by 3 to bring the peaks down
> to the single tone level.
>
> Multitone FSK is interesting. Most use one tone at a time (like Olivia).
> But FreeDV uses OFDM with 29 QPSK carriers, so "multitone" is being done.
>
> I might implement it in my DSP TU using the frequencies listed at the top
> of this email. I'll do my typical test of sending a test tape from here in
> Tucson AZ and recording it at the KFS SDR. I can then try demodulating the
> recording with one tone for each mark and space and with three tones for
> each. It will be interesting to see the error rates.
>
> Harold
> https://w6iwi.org
>
>
>
>
> On Thu, November 16, 2023 10:09 am, Nick England wrote:
> > I just came across an interesting tidbit - in WW2 the German Lorenz SZ42
> > encrypted 50 baud RTTY used three simultaneous tones each for mark and
> > space - providing diversity to overcome fading
> > *Mark* 900Hz, 1620Hz, 2340Hz
> > *Space* 540Hz, 1260Hz, 1980Hz
> >
> > At the bottom of this page are some audio files to hear tri-tone and
> > regular RTTY,
> > https://lorenz.virtualcolossus.co.uk/technical.html
> >
> > Nick K4NYW
> > Contributor, FWIW Dept.
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