This thread describes how to setup your PC to decode both D-Star and DMR Digital Voice using a low cost RTL SDR dongle and some software which I have attached. This method also works with the more expensive Funcube Dongle (FCD) which I have, or with the I/Q IF output available from the USB of an Icom IC-7100 transceiver which I don't have!
The process starts with downloading SDR Sharp and setting it up with the RTL Dongle. These are available from eBay for approximately £10 from China, and as they usually have an oddball MCX Connector, an adapter may be useful if you want to connect this to your outdoor antenna.
To install SDR Sharp, copy the files from the ZIP into a folder i.e. C:\Amateur Radio\ and then click install.exe and it will create a folder called sdrsharp for you and download all the files it needs.
Select the appropriate source on the left hand menu and then click the play button - you should then be able to use your RTL SDR dongle to listen to FM broadcasts, air band and of course amateur radio transmissions by typing in the frequency.
If you tune to a D-Star (or DMR) Signal then all you will hear is the same digital noise that you would hear if you used a conventional FM receiver, as this is yet to be decoded.
Virtual Audio Cable
The next stage is to get the received digital signal into a decoding software. This is done by means of a shareware program that allows the audio output of the receiver to be connected into the audio input of the decoding software - I currently use a paid version of Virtual Audio Cable to connect the output of SDRSharp into DL-FLDigi for monitoring high altitude balloons so I'm not covering this!
Digital Speech Decoder
The Digital Speech Decoder software performs the task of converting the digital signal received on the Windows default audio input and decodes this into speech which is directed to the Windows default audio output - these need to be configured before starting the software from the CMD window.
John G8TPX has put together a YouTube video which walks you through the setup process from here and shows the various software running - linked from this are his instructions for using an IC-7100. For those who use Linux George M1GEO has written a comprehensive set of instructions for building DSD for Linux.
Some food for thought!