[GreenKeys] FSK shifts

[Harold Hallikainen]

On Tue, August 24, 2021 10:49 pm, Ralph Mowery wrote:

>
> 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.
>

This is a very good point!

> 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.
>

I could see a simple high pass filter having an issue with slow data. For
a cinema closed captioning system I designed (ask for closed captioning
the next time you are in a movie theater!), I send 10kbps FSK data at 1.8
MHz with a total shift of 200 kHz (+/- 100 kHz). The receiver uses a
quadrature detector to linearly FM demodulate the signal. The threshold
for the slicer is set using a diode RC network that charges one capacitor
to the highest voltage out of the demodulator and the other to the lowest.
A voltage divider gets the mid-point of these two voltages, and the
demodulated signal is compared with that. There is the possibility that a
very long mark signal would result in both capacitors being at the same
voltage setting the threshold at the mark level (which would not be good).
In the closed captioning system, however, the data is sent continuously at
10 kbps.

Harold





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