Comment on [FoAR] Foundations of Amateur Radio - The Art of decoding a signal. #podcast
vk6flab@lemmy.radio 5 days agoHi Kristoff,
Thanks for the heads-up on the empty data file. I think that the GitHub web interface was “helping” when I initially created the repo - since I still cannot create those from my cli. Now fixed.
I’ve added your interpretation with the sync bits to the documentation for the file. It’s a really interesting observation. I don’t know if there are more than one different types of packets, since my earlier attempt to record the data using a WebSDR failed for some unknown reason. I do know that others have also heard this signal on-air, so perhaps it’s still happening and others might share their recording.
I’d love to learn how to usse the differential signal to remove noise to see if they’re actually all the same packet, or if they are actually different. I don’t particularly want to start manually flipping bits, but then there’s only 1,461 of them, so it’s doable in a pinch.
The timing of the signal is also interesting. According to inspectrum, the baud rate is 91.81, which isn’t any standard rate, which also makes me wonder if there is any actual information being transmitted here, other than a fixed timing signal.
73 de Onno VK6FLAB
kristoff@infosec.pub 4 days ago
Well, let’s look at this high-level. Let’s look at how (say) FSK decoding is done. Let’s concider a audio signal of 1200 and 2200 Hz.
From what I’ve seen of oldl RTTY machines, how it was done in the beginning was simply by filtering: you take the audio, create two audio filters, one at the low frequency and one at the high frequency. Then, on both outputs you do signal detection (AM demodulation) and you get two voltages. Compair them (or subtract one from the other) and you get your data.
Another option, if the two frequencies are sufficient close together and you can filter the signal that you only have that signal and no other stuff, if simply do FM demodulation. After all, FM modulation is that you take in a voltage and output a sinewave of a certain frequency. The higher the input voltage, the higher the frequency. So, invert that, if you input a signal of either 1200 or 2200 Hz into a FM demodulator, you will a voltage that is (say) 1 volt for the 1200 Hz tone and 1.5 volt for the 2200 Hz. Then set a threshhold at 1.25 volt (which you can determine by taking the average of all the voltages of -say- the last 10 seconds), and you can determine if you have received a “0” or a “1” tone.
Another option, if you work with an SDR and you have IQ data, if to first downconvert the audio. The middle between 1200 and 2200 Hz is 1700 Hz. Create a sinewave of -1700 Hz (yes, minus 1700 Hz, negative frequencies!) and mix it with the received signal. As a result, you will get a signal of either -500 Hz or +500 Hz. FM demodulate that, and you get a negative or a positive voltage.
Now, in this case, the signal is quite far apart (7 KHz) and there are kind of other signals inbetween these two carriers. If you have a SDR, you can just take in the complete signal (say, sample at 20 Ksps), filtering out all the unwanted signals and then process it like any other FSK signal (as described above). Another option I guess would be to use two receivers (two sessions to a websdr), set one at 7.061 (500 Hz below the lowest) and the other at 7.067 (1500 Hz below the highest tone). Apply filtering that you remove all other signals. Your computer should then play out two tones: a 500 Hz for the lowest carrier and a 1500 Hz tone for the highest carrier. Mix and record the audio, and then then process it like a normal FSK signal.
Yes, … signal-processing can be fun.
Ah. Concerning the signal itself and the meaning of the bits. I heared your latest podcast episode after I wrote my previous post. I agree. Perhaps it simply do not mean anything. We live in strange times, so it can be anything, … or nothing :-)