[GreenKeys] 170 hz - cast in stone ?

[jhhaynes at earthlink.net]

I was recently reading a bunch of old RTTY literature to try to find
out how the 170 Hz shift was adopted.  It's pretty clear that the
previous standard of 850 Hz was adopted because that was what the
commercial and military people were already using.  It appears the
main reason for such a wide shift was to tolerate the drifty radios
we had in those days.  There is also a possibility of copying mark-only
or space-only when selective fading causes only one of the tones at
a time to fade out; but the early equipment didn't have much capability
for that.  850 is also ten times the standard channel spacing of wire
line carrier telegraph systems.

I didn't succeed in learning exactly how 170 came to be adopted.
People were playing with a variety of narrower shifts, and it seems
that they just gravitated toward 170.  That happens to be one fifth
of 850, which makes it convenient for tuning filters with the equipment
people had in those days.  A popular item was a 425 Hz tuning fork
controlled oscillator.  The original filter frequencies were integral 
multiples of 425 Hz.  There was some interest in 85 Hz shift, because
with that you could use a wire line carrier telegraph channel as a
modem.  For a while the MARS service dictated that shift had to be
850 or 85; they didn't allow 170.

It took a change in FCC rules to allow narrow shift.  The original
rules for FSK required 850 Hz shift.  After the change it was anything
less than 900 Hz.

Then along came the Bell System with Dial TWX and settled on a
shift of 200 Hz.  Early experimentation with packet radio used
Bell modems or equivalent, so we got 200 Hz shift for 300 baud
packet.  I'm not sure if that is how AMTOR and PACTOR came to use
200 Hz shift.

With the earlier amateur equipment you couldn't use just any old
shift because the filters were fixed-tuned.  Typically there are
three filters: one for mark and two different ones for space,
tuned 170 and 850 Hz above mark, respectively.  Later there came
to be some terminal units with tunable filters, and still later we
get into DSP filtering where the filter center frequency is rather
easy to change.  Some software will adapt to shift; with other
software you get a choice of shifts and have to pick one.

U.S. hams could never use 1300 Hz shift because of the FCC rule.
And early on if you used anything other than 850 or 170 you were
not likely to make any contacts because the receiving station
would have filters for only those two shifts.  Today you might
be able to use other shifts, depending on what the receiving software
allows.


jhhaynes at earthlink dot net