Writing a RF weather fax program
This is received on 8038 kHz, with a cheap Tecsun 1 kHz step SSB receiver, indoors with a small telescope antenna: wget
So this is basically 2 kHz frequency modulated... (has greyscale) This is what the 'hamfax' program makes of it (without further adjustment to be honest, it expects 1.9 kHz f0):
This is what my program makes of it (converted to jpeg from bit map sort of thing):
I left it running (not waiting for the sync field), so from top to bottom you see: the end of the previous weather map noise (as no signal) start tone (300 Hz modulation) sync pattern and image data end tones
The noise is now 'noise', you can see it breaking up in places, halfway through I switched the Tecsun receiver to wide band, it seems to help against the noise. Note that a lot if the interference bands are now not present as in hamfax...
I use a different FM demodulator in software, just the old zero crossing detector, gives me 20 data points per period, average over some 20 points, 500 ms per line, converts nicely to a 1200 pixels width from 48000 Hz (integer division). You can clearly see the UK, Spain, Netherlands, outlines.
Am adding auto save, so it will just keep running and save received charts. Has some more auto adjustments... Settings saved, keys to adjust picture. Who needs Fourier...
-rwxr-xr-x 1 root root 30552 May 18 16:56 pwfax*
30 kBBTW I use the same system to detect the 300 Hz start tone... And that seems to work too. Its faster than hamfax (a lot), ist smaller, it uses no other libraries than xlib itself...