[RTTY] Narrow RTTY

[Bill Coleman]

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 at arrl.net
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901