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Command-line FM stereo decoder with deemphasis

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stereodemux

This command-line tool converts an FM broadcast signal into stereo sound with de-emphasis applied. It is tested against Stereo Tool.

It expects 16-bit signed-integer MPX (FM demodulated mono PCM) via stdin and outputs stereo PCM to stdout.

Requires

Compiling

We have a Makefile for reasons of tradition (you may have to configure include paths yourself):

make

We also have a more modern and automated route, using meson:

meson setup build && cd build && meson compile

You can choose either or; none is better than the other.

Usage

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.

Examples

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