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I am wondering if someone can shed some light on how to meaningfully interpret I/Q data that is produced from an RTl-SDR dongle (RTL2832U) using the Osmocom RTL-SDR library.

I have used I/Q data before to calculate power levels of signals, but after taking a computer networking course, and having worked in an internship that revolved around (basic) digital signal processing, I am curious to capture I/Q data from unencrypted 433 Mhz transmissions (or others) and make sense of the data, given that I can correctly deduce the modulation technique. I am aware of tutorials that reverse engineer 433 Mhz weather stations (as one example of something I'd like to do), but those tutorials often gloss over the I/Q data aspect.

From what I can tell, the I/Q data that comes from the RTL-SDR library is represented in hexadecimal (unsigned), little-endian format.

So to formally state my question: what are some common algorithms, techniques or other resources for processing this I/Q data into meaningful data (i.e. binary strings)?

I am a computer science student, so I apologize ahead of time for naivety concerning these electrical engineering concepts.

Thanks!

Mayo123
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  • The I/Q data is basically the *actual radio waves*, shifted in frequency (so they fit within the ADC's capabilities) and then converted directly to digital form. So what you need to know is: what algorithm is used for decoding radio waves, and how do you write that algorithm on a computer? But for a start, you could just graph the signals (if you actually have a good signal, it should look like a wave). – user253751 Jul 03 '18 at 01:53
  • @immibis thank you for your reply. Yes, I have sought additional help, and it is becoming more clear for me. – Mayo123 Oct 12 '18 at 19:09

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I ended up answering my own question essentially...for other newbs out there, I suggest watching this GNU Radio video with Michael Ossmann: https://www.youtube.com/watch?v=rQkBDMeODHc

Thinking strictly about IQ data is too low level (for me, anyway), and GNU Radio is a great tool (along with Inspectrum https://github.com/miek/inspectrum) for picking apart signals and getting the bits.

Mayo123
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