This is a fork of multimon-ng with integrated RTL-SDR support, allowing direct reception from RTL-SDR dongles without needing to pipe from rtl_fm.

multimon-ng is the successor of multimon. It decodes the following digital transmission modes:

• POCSAG512 POCSAG1200 POCSAG2400
• FLEX FLEX_NEXT
• EAS
• UFSK1200 CLIPFSK FMSFSK AFSK1200 AFSK2400 AFSK2400_2 AFSK2400_3
• HAPN4800
• FSK9600
• DTMF
• ZVEI1 ZVEI2 ZVEI3 DZVEI PZVEI
• EEA EIA CCIR
• MORSE_CW
• DUMPCSV X10 SCOPE

multimon-ng can be built using either qmake or CMake:

mkdir build
cd build
qmake ../multimon-ng.pro
make
sudo make install

mkdir build
cd build
cmake ..
make
sudo make install

The installation prefix can be set by passing a 'PREFIX' parameter to qmake. e.g: qmake multimon-ng.pro PREFIX=/usr/local

RTL-SDR support is automatically enabled if librtlsdr is detected during the CMake build:

mkdir build
cd build
cmake ..
make
sudo make install

If librtlsdr is not found, multimon-ng will still build but without RTL-SDR support. To install librtlsdr:

Debian/Ubuntu:

sudo apt-get install librtlsdr-dev

Arch Linux:

sudo pacman -S rtl-sdr

From source:

git clone https://github.com/osmocom/rtl-sdr.git
cd rtl-sdr
mkdir build && cd build
cmake ..
make
sudo make install

Install MSYS2, then from the MinGW64 or MinGW32 shell:

mkdir build
cd build
cmake ..
make

Install MinGW cross-compiler, then use the provided toolchain files:

# For 64-bit Windows
mkdir build-mingw64
cd build-mingw64
cmake .. -DCMAKE_TOOLCHAIN_FILE=../cmake/toolchain-mingw64.cmake
make

# For 32-bit Windows
mkdir build-mingw32
cd build-mingw32
cmake .. -DCMAKE_TOOLCHAIN_FILE=../cmake/toolchain-mingw32.cmake
make

So far multimon-ng has been successfully built on:

• Arch Linux
• Debian
• Gentoo
• Kali Linux
• Ubuntu
• OS X
• Windows (Qt-MinGW build environment, Cygwin, and VisualStudio/MSVC)
• FreeBSD

Files can be easily converted into multimon-ng's native raw format using sox. e.g:

sox -R -t wav pocsag_short.wav -esigned-integer -b16 -r 22050 -t raw pocsag_short.raw

GNURadio can also generate the format using the file sink in input mode short.

You can also "pipe" raw samples into multimon-ng using something like:

sox -R -t wav pocsag_short.wav -esigned-integer -b16 -r 22050 -t raw - | ./multimon-ng -

As a last example, here is how you can use it in combination with RTL-SDR:

rtl_fm -f 403600000 -s 22050 | multimon-ng -t raw -a FMSFSK -a AFSK1200 /dev/stdin

This fork includes integrated RTL-SDR support, eliminating the need to pipe from rtl_fm:

APRS on 144.390 MHz:

multimon-ng -a AFSK1200 --rtl-freq 144390000 --rtl-gain 42

POCSAG pager on 403.6 MHz:

multimon-ng -a POCSAG512 -a POCSAG1200 -a POCSAG2400 --rtl-freq 403600000 --rtl-gain 40

With custom sample rate:

multimon-ng -a AFSK1200 --rtl-freq 144390000 --rtl-samp 24000 --rtl-gain 42

• --rtl-dev <n> : RTL-SDR device index or serial (default: 0)
• --rtl-freq <freq> : Frequency to tune to in Hz (required for RTL-SDR mode)
• --rtl-gain <gain> : Tuner gain in dB (default: auto gain)
• --rtl-samp <rate> : Sample rate in Hz (default: 22050)
• --rtl-ppm <ppm> : Frequency correction in ppm (default: 0)

The RTL-SDR integration automatically handles:

• FM demodulation with proper oversampling (~1 MHz capture rate)
• Frequency offset tuning to avoid DC spike
• Downsampling to the target audio rate (default 22050 Hz)
• DC blocking filter

A more advanced sample that combines rtl_fm, flac, and tee to split the output from rtl_rm into separate streams. One stream to be passed to flac to record the audio and another stream to for example an application that does text parsing of mulimon-ng output

rtl_fm -s 22050 -f 123.456M -g -9.9 | tee >(flac -8 --endian=little --channels=1 --bps=16 --sample-rate=22050 --sign=signed - -o ~/recordings/rtlfm.$EPOCHSECONDS.flac -f) | multimon-ng -v 0 -a FLEX -a FLEX_NEXT -t raw /dev/stdin

1. You can pass -l to rtl_fm for the squelch level, this will cut the noise floor so less data gets encoded by flac and will significantly reduce the file size but could result in loss of signal data. This value must be tuned!
2. Flac uses -8 here, if you run an a resource constraint device you may want to lower this value
3. The Flac -o argument value contains $EPOCHSECONDS to make unique files when this gets restarted

To replay the recorded flac file to multimon-ng (requires sox):

flac -d --stdout ~/recordings/rtlf/rtlfm.1725033204.flac | multimon-ng -r -v 0 -a FLEX_NEXT -t flac -

qmake multimon-ng.pro PREFIX=/usr/local
make
make install INSTALL_ROOT=/

After building, run the test suite:

./test/run_tests.sh