The purpose of
minidemod-wfm.c is to have a few lines of code that can actually demodulate WFM from an I/Q input.
The whole DSP chain is built by OS pipes, just like this:
rtl_sdr -s 240000 -f 89500000 -g 20 - | tcc -lm -run minidemod-wfm.c \ | sox -t raw -r 240000 -e unsigned -b 8 -c 1 - -t raw - rate 48000 \ | mplayer -quiet -rawaudio samplesize=1:channels=1:rate=48000 -demuxer rawaudio -
Read like this:
rtl_sdr (acquires samples) | minidemod-wfm (demodulates) \ | sox (low pass filter + decimation) | mplayer (audio output) `
[For systems with aplay, you can use this instead of mplayer: aplay -r 48000 -B1000000 ]
To run it, you will need a Linux box with
rtl_sdr tcc sox mplayer installed.
I've also added
minidemod-wfm-atan.c with a detailed explanation of the demodulation process. It does sound better, but uses more CPU (still not more than 10% on my box).
At SDRA-2018 I gave a talk on writing a simple AM/FM/SSB receiver in C:
The code (which actually fits on 2 sheets of A4 paper) can be found here:
If you need a fully fledged command-line DSP tool for SDR, see my CSDR project here:
How this tool can be used as a demo on SDR
I used this tool to introduce some fellow students to SDR, after a short explanation on SDR and modulations in general.
I showed them the code, but previously deleted the formula that does the actual calculation, turning this into a fun exercise: they had to figure out that one line themselves.
If they entered the correct formula, the success was immediate: audio was playing through the speakers.
András Retzler email@example.com
Also tnx dnet for the small fixes.