Re: Demodulating slow phase-modulated data

[Daniel Estévez]

El 3/11/21 a las 17:28, Philip Pemberton escribió:
> Hi all,
>
> I'm working on a project to reverse-engineer the data format used by an 
> old navigation system called Datatrak. I've got a project page on my 
> website about it: https://www.philpem.me.uk/datatrak/start 
> <https://www.philpem.me.uk/datatrak/start>
>
> The data is sent by phase-modulating a ~133kHz or ~146kHz (longwave) 
> carrier at a very slow rate -- the modulating wave is sinusoidal, either 
> 37.5Hz or 50Hz depending on the part of the signal being sent. The 
> signal bandwidth is about 200Hz.
>
> I've been given some single-channel WAV format recordings from LW SDR 
> receivers, 44100Hz sample rate, with a LO frequency of 128kHz. Sadly 
> they're not I-Q recordings. I'd like to try and demodulate the signals 
> in these recordings to recover the 64 bits of "Goldcode" synchronisation 
> code (from the Trigger slot) and the 128 bits of Clock signal (from the 
> Clock slot). I already know what the Goldcode value should be.
>
> Can anyone suggest a way I could do this with Gnuradio/GRC or some other 
> tool?
>
> I figure I need to convert the WAV file into a complex (I-Q) signal, mix 
> the signal of interest down to baseband, then filter it with a 200Hz 
> bandpass filter. I'm not sure what I need to do after that to recover 
> the modulating phase signal.

Hi Philip,

Downconverting to baseband and low-pass filtering seems a good start. 
What to do next depends on the specifics of the modulation. I didn't 
understand what you mean by

"
the modulating wave is sinusoidal, either
37.5Hz or 50Hz depending on the part of the signal being sent
"

Do you perhaps mean that the carrier is amplitude or phase modulated 
with an FSK signal that uses tones of 37.5 Hz and 50 Hz to encoded the bits?

I'm not at all sure if that's what you mean, and it doesn't seem a very 
conventional modulation scheme, so perhaps it's something else. I don't 
know anything about Datatrak.

Best,
Daniel.

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