[GreenKeys] 75 baud RTTY

[Nick England]

FWIW Dept.

I have several models of USN FSK keyers that were used late in the war and
just after WW2 to convert existing CW transmitters for RTTY use. I also
have several models of the Navy’s first post-war generation of
transmitters. All of these use oven-stabilized 200kc crystal oscillators in
the FSK section. Transmitter drift was not an issue.
Later AFSK systems transmitted reduced carrier as a pilot for receiver
system AFC, and drift was REALLY not an issue.
850 shift was in widespread military use at least through the 1990s and is
still covered in the current (2017) MIL-STD-188-110D.

And, of course, we have an 850 shift net on 40m on Saturday mornings with
most folks running military gear.

Ham RTTY has evolved with different constraints, of course.

Cheers
Nick

On Sun, May 21, 2023 at 10:28 PM Jim Haynes <jhhaynes at earthlink.net> wrote:

> On Sun, 21 May 2023, Harold Hallikainen via GreenKeys wrote:
>
> >
> >
> > On Sun, May 21, 2023 8:58 am, Russ Miller wrote:
> >> Jim,
> >>
> >> One thing that changed from the 1950s?
> >>
> >> 850 shift was hampered with selective fading.  170 shift improved copy
> >> during those situations.
> >>
> >> Russ
> >>
> >
> > But, does 850 Hz work better with dynamic threshold control or "slideback
> > voltage"? With 170 Hz shift, it is more likely that both tones will fade
> > into the noise than with 850 Hz shift where MAYBE one tone is still there
> > during a fade.
> >
> > Harold
> > https://w6iwi.org/rtty
> >
> It has long been my belief, with no supporting documentation, that 850
> shift was chosen to get some tolerance for frequency drift in the radio
> system.  Hams didn't mind too much carrying on a contact with one hand
> on the receiver tuning knob, but for commercial/military work they would
> want hands-off copy for hours at a time.  And the radio equipment just
> wasn't all that good.
>
> The standard method of generating FSK was to frequency modulate a 200 KHz
> oscillator, the output of which was beat with a frequency 200 KHz removed
> from the operating frequency.  So the 200 KHz oscillator was prone to
> drift even if the transmit frequency was derived from a crystal, and of
> course in amateur use it was a VFO, with frequency multiplication to get
> to the higher frequency bands.  And then the receivers were - well -
> SX-28, SP-400, and others of that ilk.
>
> It was also noted when we went to 170 Hz shift that there was a reduction
> in QRM.  With 850 the CW ops did not take kindly to such a wide band
> signal in their portion of the bands.  But when we got to two-tone
> demodulation rather than limiter-discriminator demodulation there was
> some interest in the frequency diversity of the two tones, with selective
> fading affecting only one at a time.  In the era of DSP demodulation
> K6STI wrote a piece in QST advocating a return to wide shift.
>
> Getting back to the baud rate matter, I suspect the preference for slower
> speeds is more pronounced on the lower frequency bands, e.g. 80M.
>
> Jim W6JVE
>
> ______________________________________________________________
> 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 navy.radio at gmail.com
>
-- 
Nick England K4NYW
www.navy-radio.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/greenkeys/attachments/20230522/1bf4286a/attachment.html>