Several years ago I stumbled into it. And the only thing that was digital at the time in the area was St. Vincent's hosipital, then later UWGB public safety.
You take the discriminator audio from your analog radio (available on the data port of many mobile ham rigs) and feed it into the sound card input.
It allows you to listen to popular formats like; P25/APCO25, DMR/Mototrbo, D-Star, IDAS/NXDN.
Here is what I did to install it on Ubuntu 10.04 :
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apt-get install libsndfile-dev cmake apt-get install build-essential libssl-dev curl git-core git clone https://github.com/szechyjs/mbelib.git git clone https://github.com/szechyjs/dsd.git cd mbelib mkdir build cd build cmake .. make make install cd cd dsd mkdir build cd build cmake .. make make install ldconfig
The audio recreation part (mbelib) isn't very smooth on a pi, as its a bit underpowered. So use a normal computerroot@tlb:/dev# dsd -i /dev/dsp -o /dev/dsp
Digital Speech Decoder 1.7.0-dev (build:v1.6.0-26-ge40e89a)
mbelib version 1.2.5
Audio In/Out Device: /dev/dsp
There are hourly time P25 annoucements on the output of Matt's repeater 147.435.. if you need a signal to try and decode.