P25 Trunked Tracking and Decoding with RTL-SDR, Unitrunker, and DSDPlus


Unitrunker will decode the packets from the control channel and direct the voice receivers to the frequency of the voice all for DSDPlus to decode.


Go to http://www.unitrunker.com/.

Click download (diskette).


The stable releases will appear in the download directory.  Beta releases are in the preview directory.

Click UniTrunker-  Save it in your Downloads folder.


Double-click UniTrunker-

Hate these things.  Uncheck Always ask before opening this file.

Click Run.


Check I accept.

Click Install.  If you’re running as a standard Windows user, you will need the admin password.


Installation will begin.


Click Finish.


Click the Start orb.

Click All Programs.

Click Unitrunker.

Click Unitrunker.


If you see this error, you will need to create a directory first.


Click the Start orb.

Click Computer.

In the location bar, type %APPDATA% and press Enter.


You will be brought to the Roaming directory.  Click New folder.

Type Unitrunker.  Press Enter.

Close Computer.

Start Unitrunker.


Select First Time Installation.

Click Continue.


Click the Options tab.

Under Listen, check Enabled.

This will allow Unitrunker to follow the voice calls.

Signal (control channel) receiver


Click the Receivers tab.

Click +.


Click RTL2832.


Under Signal, for RTL Device, select one of the RTL-SDR dongles.

Under Control, for Correction, enter the PPM correction.  In this case 55.

Check Auto Gain.

The Chase option is a useful if the system has a control channel that rotates through the primary and alternative control channels… or fails over often 🙂

Check Mute.  This will mute any output from this receiver.

For Audio Output, select Unspecified.  This will not route any audio from an analog signal.

For Digital Output, select Unspecified.  This will not route any audio from a digital signal.

For Park, enter the control channel frequency.  In this case 774.78125.

Check P25.  This will tell Unitrunker the system type so it can follow calls with the voice receivers designated the same system type.


Click Play.

Under Decode, the Protocol will fill in.  In this case APCO P25.

Health will give a decode indicator number between 0 and 100 with 100 being the cleanest decode.  This IS an indicator of how well the signal is being decoded.


Click the Scope tab.

P25 will be a signal with many peaks and valleys shown above that will move in a somewhat uniform manner.

Close this signal receiver’s properties.


If the control channel signal is being successfully decoded, a box will appear with site information.

Click Download this site (gear).


Enter your Radio Reference username and password.

Check Remember Password.  This will allow for quicker updating of the system in the future.

Click Download.


This will lookup and download the site and system information including talk-group labels.


You will see this sites’ known frequencies and talkgroup labels.  Yellow are the alternative control channels and red is the primary.

Voice receiver


Click +.


Click RTL2832.


Under Receiver, for Role, select Voice.

Under Signal, for RTL Device, select the other RTL-SDR dongle.

Under Control, for Correction, enter the PPM Correction.  In this case 75.

Check Auto Gain.

For Audio Output, select Unspecified.  This will not route any audio from an analog signal.

For Digital Output, select Line 1.  This will route the digital signal audio to Virtual Audio Cable.

For Park, enter the park frequency.  In this case 771.40000.

Check P25.  This will tell Unitrunker this receiver is setup to decode P25.

Click Play.


Click the Scope tab.

When receiving a voice call, P25 will be a signal with many peaks and valleys shown above that will move in a somewhat uniform manner.

Close this voice receiver’s properties.


Go to the DSDPlus directory.

Double-click the P25 Line 1 batch file.


If you see things appearing on screen and hear audio through your speakers, it’s working!


Setup shown with Unitrunker and DSDPlus.

Now that a voice receiver is programmed, the site window will hightight the frequency in green that the voice receiver is tuned to.  In this case the Willoughby Hills / Waite Hill PD  talkgroup is being decoded by DSDPlus.