rtlmic is a multichannel FM microphone receiver/demodulator for RTL-SDR cards. It outputs realtime audio to JACK.
Dependencies:
- libjack
- libvolk
- librtlsdr
To build and install, just use make && sudo make install.
Basic usage is simply:
$ rtlmic [channel 1 frequency] [channel 2 frequency]...You can capture as many channels as your CPU can handle, as long as they all fit within the capture bandwidth of the RTL-SDR.
Use rtlmic --help to see all available options.
In order to get back correct audio, you should know certain parameters about
your microphones: the companding ratio (-e), the companding tau (-E), the
deemphasis tau (-M), and the FM deviation (-w).
Most microphones use 2:1 companding (the default ratio). The companding tau is usually determined by an R-C filter in the microphone, connected to the compressor chip. In my case, the component values are 10kΩ and 1µF, which gives 10kΩ * 1µF = 10ms tau. The deemphasis tau just affects the frequency response: the higher the tau, the less high-end the microphone will have. If you do not know the exact values for your microphone, you can just try and see what sounds best.
By default, the RTL-SDR will be tuned to the frequency in between the highest
and lowest channel specified. However, if you have a channel near that point
(e.g. if you are only capturing one channel, or an odd number of evenly spaced
channels) then you may experience additional noise, as the RTL-SDR does not
perform well near DC. You can work around this by choosing a different center
frequency with -f.
You may want to play around with other parameters, e.g. adjusting the tuner gain
(-g) and squelch threshold (-s), as well as the audio gain (-a). If your
channels are spaced closely together, lower the transition width (-t) and use
a tight deviation (-w). Note that lowering the transition width increases CPU
usage significantly.
If you have persistent buffer over/underrun problems, you should try changing
the buffering settings -b and -B, as well as the JACK period size in the
JACK server. A few under/overruns on startup are normal, as it takes some time
for rtlmic to lock onto the exact ratio between the true SDR and JACK
frequencies.