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

Adding additional receivers

This step is optional.  Get the first voice receiver setup and working first before adding others.

This will setup an additional RTL-SDR dongle to act as an additional voice call receiver. I don’t know how many total can be added but I’ve heard of setups with 8. Probably as many dongles as available USB ports. For additional voice receiver dongles, wash, rinse, and repeat.


Close DSDPlus.

Close Unitrunker.

Click the Start orb.

Click All Programs.

Click Virtual Audio Cable.

Right-click Control panel.  Click Run as administrator, you will need the admin password.

Under Driver parameters, for Cables, enter 2.

Click Set.


Select cable 2 that was just added.

SR (sample rate) should default to 22050 – 48000.  BPS (bits per second) should default to 8 to 16.  NC (number of channels) change to 1 to 1.

Click Set.

Close Exit.


Click the Start orb.

Click Control Panel.

With Category view enabled, click Hardware and Sound.

Click Sound.

Playback tab should be selected.

If the Line 2 device is set as default (as shown earlier, with the green check mark), you do NOT want this.  Right-click your actual playback device (usually speakers) and click Set Default.

Click Line 2.

Click Properties.


Click the Levels tab.

Set the levels to 50.


Click the Advanced tab.

Under Default Format, select 1 channel, 16 bit, 48000 Hz.

Click OK.


Click the Recording tab.

Select Line 2.

Click Properties.


Click the Levels tab.

Set the level to 50.


Click the Advanced tab.

Under Default Format, select 1 channel, 16 bit, 48000 Hz.

Click OK.

Click OK on the Sound box.

Close the Control Panel.


Go to the DSDPlus directory.

Double-click DSDPlus.


A total of 4 windows will open.  In this window will be display the input and output device numbers.  The new Virtual Audio Cable device is input device #2.  The audio out (speakers) is output device #1.

Note these devices.

Close DSDPlus.


DSDPlus will not run more than once instance from the same directory, it’s probably a shared library issue.

Rename the DSDPlus-1.51 directory to DSDPlus-1.51 Line 1.

Right-click DSDPlus-1.51 Line 1, click Copy.

Right-click anywhere in the window and click Paste.

Rename DSDPlus-1.51 P25 Line 1 – Copy to DSDPlus-1.51 Line 2.


Go to the DSDPlus-1.51 Line 2 directory.

Rename P25 Line 1.bat to P25 Line 2.bat.

Right-click P25 Line 2.bat.

Click Edit.


Right-click the file just created, and click Edit.

Change the -i# option to the new device number.  In this case 2.

Click the link below. Copy and paste the lines from the text file


Change the input and output device numbers.

Click the X to close Notepad.

Click Yes to save the changes.

Alternatively, to route output to another soundcard, change -o# to that device number.


Start Unitrunker.

The other receivers should say running.

Click +.


Click RTL2832.


These will be very similar settings as the first voice receiver.

Under Receiver, for Role, select Voice.

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

Under Control, for Correction, enter the PPM correction.  In this case 57.

Check Auto Gain.

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

For Digital Output, select Line 2.  This will route the digital audio output 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.

A P25 simulcast site will look like the above with with many peaks and valleys shown above that will move in a somewhat uniform manner (CQPSK).  A P25 single site system will look more wavy (C4FM).

Close this voice receiver’s properties.


On the site information box, you will notice where there is more than one call taking place, two frequencies will be highlighted in green.  This indicates two voice receivers are being utilized.


Go to the DSDPlus-1.51 P25 Line 1 directory.

Start P25 Line 1.bat.


Go to the DSDPlus-1.51 P25 Line 2 directory.

Start P25 Line 2.bat.


You will now have Unitrunker working with two voice decoders!!!