[GreenKeys] Tri-tone RTTY

[Harold Hallikainen]

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.
> ______________________________________________________________
> GreenKeys mailing list
> Home: http://mailman.qth.net/mailman/listinfo/greenkeys
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:GreenKeys at mailman.qth.net
>
>>>> Jordan Spencer Cunningham's GreenKeys Search Tool:
>>>> https://teletype.net/gksearch
>>>> 2002-to-present greenkeys archive:
>>>> http://mailman.qth.net/pipermail/greenkeys/
>>>> 1998-to-2001 greenkeys archive:
>>>> http://mailman.qth.net/archive/greenkeys/greenkeys.html
>>>> Randy Guttery's 2001-to-2009 GreenKeys Search Tool:
>>>> http://comcents.com/tty/greenkeyssearch.html
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to harold at w6iwi.org


-- 
Not sent from an iPhone.