[GreenKeys] 75 baud RTTY

[Jim Haynes]

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