[GreenKeys] Filterless TU

[Keelan Lightfoot]

Harold,

A while back I came across this project:

https://sourceforge.net/projects/openbaudot/

It uses the mechanism you speak of; there are separate bandpass filters for the mark and space frequencies, the output of which is fed into a lowpass filter, and compared with the total noise level to determine if the signal is in a marking or spacing condition.

I wanted to make it work on a real time audio stream on a Raspberry Pi, and rather than faff about with cross-compiling cross-platform audio code for a pi, I ported the openbaudot library (in spirit) to a golang implementation that will happily and easily compile for any platform:

http://code.beefchicken.com/keelan/gortty

I used this to create a "reverse modem" that is supposed to act as a AFSK<->Internet gateway using AT commands:

http://code.beefchicken.com/keelan/rttymodem

While waiting for better audio hardware for my Raspberry Pi to come in the mail, I got distracted and haven't touched the project since, but as I left it it worked as advertised. YMMV.

- Keelan

September 16, 2019 3:14 PM, "Harold Hallikainen" <harold at w6iwi.org> wrote:

> 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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