I received this from, Floyd, K8AC, re: 850 vs 170 Hz shift.
It seems routed in things like why it's USB on some bands and LSB on
others and how 50 ohms became the antenna standard. It was based on
what was available or practical in the last century.
No reason not to use software to reduce the shift and create a new
standard and increase available bandwidth and accommodate more signals
in crowded condx. Is there?
73 - Steve WB6RSE
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Exactly how 850 was chosen I don't know, but I do know that terminal
units of the day used low Q tuned filters for the two tones comprised
of 88 mH coils wound on toroids. You could get the coils surplus and
just had to add the necessary parallel capacitors to tune to the right
frequencies. The resulting filters were very broad and it's likely
that using a smaller shift just wasn't possible as there would be too
much of mark leaking into the space filter and vice versa. With the
advent of more sophisticated filtering, it became possible to build
fairly selective filters that allowed the shift to be reduced. How
170 was chosen as the value I don't know. Since HAL Co. was leading
the way with a family of terminal units at the time, they may have
influenced the choice of 170. Their TUs were capable of running 170,
400, and 850 cycle shift (never did see 400 cycle used). You may know
that HAL still makes high end TUs today (ST-8000 family). The other
thing that never caught on was higher speed RTTY, running 100 WPM
instead of 60 WPM. Apparently the error rate on noisy bands was
higher than desired, but I used it successfully on 20 meters for a
while.
73, Floyd - K8AC
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