[GreenKeys] Filterless TU

[Harold Hallikainen]

Interesting discussion! Someone mentioned running the audio through a
capacitor and taking advantage of the reactance change. This is pretty
much a differentiator where the output voltage increases 6 dB when the
frequency goes up an octave. It's a simple linear FM detector. Other
linear FM detectors include the ratio detector, discriminator, quadrature
detector, pulse counting detector, etc. I use an SA639 chip in an IR FSK
receiver (FSK RF on IR). It has a quadrature detector giving a linear FM
output. I determine what is half way between the two voltages and compare
the quadrature detector output to that. The pulse counting demodulator was
very popular in FM modulation monitors used in broadcast stations. They
operated at a pretty low IF (a few hundred kHz). The incoming IF drove a
monostable multivibrator that output a fixed width pulse on every positive
zero crossing (or maybe every zero crossing). As the frequency varied, the
time between the pulses varied, but the pulse width remained the same.
This resulted in a varying duty cycle. This variable duty cycle pulse
train was run through a low pass filter to recover the modulated audio.
Clever technique that gave very good signal to noise ratio. In DSP, I
understand that a common FM demodulation technique is to multiply the
current sample times a previous sample and run the result through a low
pass filter.

But, for FSK, it APPEARS that it is better to consider the signal similar
to a diversity on/off keyed signal (on/off keyed on two different
frequencies). We previously discussed my idea of using an XR2211 for
demodulation. It's a PLL linear FM demodulator driving a comparator. But,
its susceptible to interference. So, I'm now thinking of duplicating the
vacuum tube TU I had in high school, only doing it in DSP. The vacuum tube
TU used 88 mH loading coils to form LC band pass filters (for 850 Hz
shift). Here, I'm thinking of detecting the tones with the
https://en.wikipedia.org/wiki/Goertzel_algorithm , then doing a comparison
between the two tone levels to determine if it's a mark or space.

So... stuff to play with!

Harold


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