The crew over at RTL-SDR.com has posted a simple tutorial on using the RTL-SDR dongle and free SDR# and Digital Speech Decoder software (dsd) to monitor P25 digital radio voice comms. They describe, “Digital radio voice communications are becoming more commonly used in the radio spectrum. This is due to the various improvements offered over traditional analogue voice radio systems. Unfortunately for radio scanner hobbyists, digital radio is difficult to receive, as special radio scanners which can be expensive are required to decode the digital audio. Additionally, digital radio systems can be encrypted making it impossible for communications to be decoded by a hobbyist. However, most users of digital radio do not bother to encrypt their systems as it can introduce lag and extra battery drain in portable radios.
The most common digital speech codec is APCO P25, which dsd is able to decode. Dsd is also capable of decoding other common digital codecs such as DMR/MOTOTRBO and ProVoice which no hardware scanner is currently capable of.”
One part of this project that isn’t open source is their use of Virtual Audio Cable software to route the SDR# output to the DSD.