This is another illustration of the amazing utility of the RTL-SDR and SDR# software. Here proct0r links the output from SDR# to a program known as DSD. DSD is an open source command line program originally coded in C by anonymous for Linux for the purpose of decoding digital protocols via a PC soundcard. The program accepts audio taken from the discriminator output from a scanner or amateur radio receiver fed to the soundcard input. DSD is then run (using the appropriate arguments, depending on the mode and parameters involved) with the resulting demodulated speech directed to the soundcard output jack. Additionally, various protocol decodes (P25 among them) provide a running display of certain packet data in the terminal window. DSD relies on the libraries contained in the required MBELIB package.
It is highly recommended that all RF hackers, whether using an SDR or conventional receiver, become acquainted with these programs and examine the Linux program source code. For more information and downloads of DSD and MBELIB, visit the RadioReference Digital Speech Decoder page.