This command-line tool converts an FM broadcast signal into stereo sound with de-emphasis applied.
It expects 16-bit signed-integer MPX (FM demodulated mono PCM) via stdin and outputs stereo PCM to stdout.
make
This program reads and writes S16_LE samples via standard streams: single-channel input, stereo output.
./demux -r <samplerate> [-R samplerate_out] [-d time_constant_μs] [-g gain_db]
-r Input sample rate (Hz).
-R Output sample rate (Hz). Must be less than or equal to the input sample rate.
The default is to use the input rate.
-g Additional output gain (dB). Beware of clipping. 6 dB means doubled amplitude;
or you could give an amplitude ratio instead by adding an 'x'.
-d Time constant of the de-emphasis filter (microseconds).
Meaningful values are 75 for the Americas and South Korea and
50 elsewhere; the default is 50.
Listen to a stereo broadcast on 90.0 MHz with rtl_fm and sox:
rtl_fm -M fm -l 0 -A std -p 0 -s 192k -g 40 -F 9 -f 90.0M | \
./demux -r 192k | \
play -q -t .s16 -r 192k -c 2 -
Listen with aplay, resampling inside demux:
rtl_fm -M fm -l 0 -A std -p 0 -s 192k -g 40 -F 9 -f 90.0M | \
./demux -r 192k -R 44.1k | \
aplay -f S16_LE -c 2 -r 44100
Convert a 192 kHz MPX WAV into a 44.1 kHz stereo WAV:
sox mpx.wav -t .s16 - | \
./demux -r 192k -R 44.1k | \
sox -t .s16 -r 44.1k -c 2 - stereo.wav