[GreenKeys] FSK shifts

Tue Aug 24 18:49:25 EDT 2021

I am a member and it would not download for me.  I don't really need it.

I have watched the 170 hz shift tones selective fade.  When I first started
with rtty the shift was already at 170hz.  At the time (around 1980) many of
the pictures were being sent on 20 meters and lots more signals on .  So the
narrow shift worked well.  

Feeding a ssb transmitter any set of tones that pass the filter 170 hz apart
can be received with any demodulator that is set for 170 shift.  That is if
I transmit by feeding  in 1000 hz and 1170 hz you can receive it if you
demodulator is set for 2125 and 2295 you would be able to copy just fine but
your receive frequency on your display will be different that my
transmitter.  This will not work on AM and FM transmissions.  Tones much
higher than 2300 hz will usually be reduced by the ssb filter.   If doing
what I call true FSK where the carrier is shifted then the shift will not
really matter as far as the transmitted signal.

Most of my rtty receiving during those years was with a Heathkit Sb101.  I
put a capacitor on the LSB crystal to shift it so that I could use the CW
filter.  This really helped on receive. Many of the mid to high price
transceivers with the electronic filters can be set to do a similar thing
by the menus.

Normally you would not want to use a tone of 1000 Hz.  You should stay above
1500 hz mainly because if your audio tone has any harmonics the 1500 hz and
higher would be filtered out by the sideband filter.

The threshold corrector works well at machine speed.   For signals sent by
hand and the hunt and peck typers the corrector does not work very well.  

For much more info on this, look at articals by Hoff in QST from around the
years of 1965 and 1966.

Ralph ku4pt

-----Original Message-----
From: greenkeys-bounces at mailman.qth.net
[mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Harold Hallikainen
Sent: Tuesday, August 24, 2021 5:04 PM
To: greenkeys at mailman.qth.net
Subject: Re: [GreenKeys] FSK shifts

On Tue, August 24, 2021 7:33 pm, Jim Haynes wrote:
>
> You'll want to read the article by K6STI in the April 1996 issue of
> QST, pages 78-79
>
>  	---

Just read that article. Interesting. ARRL members can search for it at
http://www.arrl.org/arrl-periodicals-archive-search .

In general, he suggests that the change from 850 Hz shift to 170 Hz shift
was so the tones would fade together. Fixed threshold detectors would then
work better than if the tones faded separately. With the introduction of
"automatic threshold control," however, that advantage disappeared.

A simple automatic threshold control can be achieved by just AC copuling
(high pass filtering) the combined demodulated data (where a mark tone
drives the signal positive, and a space tone drives it negative). If the
tones are imbalanced, such as during a selective fade, the resulting DC
shift is filtered out. This is done in the TU-170 (see V24 ast
https://w6iwi.org/rtty/tu170/Flesher%20TU-170%20Terminal%20Unit.pdf ).

However, with 170 Hz shift, the tones tend to fade together (which was an
advantage with fixed threshold detectors) while with 850 Hz shift, they
fade separately. If, for example, mark gives a 1V output, space gives a 0V
output, the data going to the slicer would be 1 Vpp centereed about 0 V
(shifting between -500 mV and +500 mV). The slicer at 0V would properly
decode this. Ideally, noise is added equally to both channels, so it
cancels out.

It would be interesting to calculate out (I'd probably use LTSPICE) how
FSK behaves with selective fading. Assuming selective fading is a single
reflection with almost equal amplitude to the original signal, we could
simulate the sum of three equal signals. One is the reference, the second
is down 170 Hz, and the third is down 850 Hz. Te "direct" signal would be
combined with a delayed sum that results in a full cancellation of the
reference signal. How far down are the other two signals?

http://mai.hallikainen.org/org/FCC/FccRules/2020/97/307/section.pdf
authorizes up to 1 kHz shift at 300 baud.

Are the bands so crowded that this could not be done today?

We used to use 2.125 kHz and 2.925 kHz tones for AFSK with 850 Hz shift.
But most SSB transmitters would severely attenuate a 2.925 kHz tone. When
we changed to 170 Hz shift, we kept the mark at 2.125 kHz and put the
space up 170 Hz. Europe, however, put space at 2.126 kHz and mark down 170
Hz. I note that Navy LF LSB uses a 1 kHz center and shifts +/- 85 Hz (see
https://w6iwi.org/rtty/ ). I wonder if it would make sense to use a center
frequency of 1 kHz and shift up and down 85 Hz for 170 Hz shift and 425 Hz
for 850 Hz shift.

Harold

-- 
FCC Rules Updated Daily at http://www.hallikainen.com
Not sent from an iPhone.
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