On Jan 16, 2006, at 9:15 PM, Bill Turner wrote:
> Just had a long chat with W7AY on 7085 using very narrow RTTY. We
> started out with 170 Hz shift and then went to 23 Hz. I transmitted a
> canned macro (the word TEST repeated 36 times) at 100 watts, then ten
> watts then one watt, using both shifts at each power level.
In thinking of narrow shifts, you can theoretically get down to
extremely small values. If you think of FSK as two OOK symbols spaced
a short distance apart, there comes a point when the sidebands of the
one symbol start to encroach in the spectrum of the opposite symbol.
300 baud packet is an example. With a 200 Hz shift, the primary
spectrum of the symbols overlap by at least 50 Hz.
I can't find the reference right now, but I seem to recall something
from 20 years ago discussing the parameters for FSK being related to
the modulation index -- this was the shift offset divided by the base-
band symbol rate. (the offset is 1/2 the shift -- offset from the
center frequency) [Hence Index = offset / 2B]. Choosing a modulation
index of 1 avoided this crossover. Values smaller than 1 tended to
have more inter-symbol interaction. Values a lot greater than 1
tended to have a "dead" space in the center of the channel where no
information is passed.
If you recall several years ago, there were a number of packet
forwarding stations operating on HF under an STA that were using a
600 Hz shift. 300 baud, 600 shift is a modulation index of 1.
So, for 45.45 baud RTTY, 90 Hz is about right. Call it an even 100
Hz. In fact, if we're going to change things, why don't we go with 50
and 25 baud? (25 baud is for (sometimes) 80 and (mostly) 160m where
the intersymbol distortion due to multipath is much greater)
50 baud, 100 Hz shift ought to fit nicely in a 200 Hz operating
bandwidth.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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