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RTL-SDR Tutorial: Decoding Digital Voice (P25, DMR, NXDN, D-STAR) with DSD

April 11, 2013 admin

The RTL-SDR software defined radio combined with SDRSharp and a program called “digital speech decoder” (DSD) can be used as a radio scanner to easily and cheaply listen to unencrypted digital radio voice conversations.

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 signal. 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, monetary expense 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, NXDN, D-STAR and ProVoice which no hardware scanner is currently capable of.

Super cheap software defined radios such as the RTL-SDR can be used to decode these digital voice communication signals instead of expensive radio scanners. While this tutorial is aimed at the RTL-SDR, other software radios such as the Funcube dongle, Airspy, HackRF and BladeRF will also work. Hardware radios with discriminator taps connected to a PC may also work.

Examples of DSD Decoding Digital Voice with RTL-SDR as a Radio Scanner

YouTube user Geoff Wolf shows a video where he uses RTL-SDR as a police scanner to listen to public safety P25 digital radio using DSD, SDRSharp and virtual audio cable.

YouTube Video

YouTube user Superphish uses RTL-SDR as a radio scanner to decode MOTOTRBO digital voice of a taxi company, using DSD, SDRSharp and virtual audio cable.

YouTube Video

Here YouTube use Superphish uses the RTL-SDR to decode D-STAR conversations using DSD 1.7, HDSDR and virtual audio cable.

YouTube Video

Tutorial – Decoding Digital Voice using SDRSharp, DSD and RTL-SDR (For Windows)

It is assumed you have an RTL-SDR dongle set up and working with SDRSharp. If not, see the Buy RTL-SDR, and Quickstart pages first before attempting this tutorial.

There are two different version of DSD that need to be mentioned. There is DSD 1.7 which is open source software that is under active development and there is DSD+ which is closed source software. DSD+ has vastly superior decoding especially for weak signals but it cannot yet decode D-STAR. DSD 1.7 has poorer decoding, but it is capable of decoding D-STAR. To download each version of DSD see the links below. Once downloaded extract the contents zip file to a folder on your PC.

Next you will need an audio piping utility such as the paid version of Virtual Audio Cable or the free VB-Cable.  Also ensure virtual audio cable or VB-cable is set as the default recording audio device in windows sound properties as DSD will use the default sound device.

If you are unfamiliar with what a digital voice signal may look like, two waterfall examples are shown below, with audio examples recorded in NFM mode.

P25P25 Waterfall Example

DMR/MOTOTRBODMR/MOTOTRBO Signal Waterfall

D-STAR

DSTAR

 

Now to decode the digital voice signals follow these instructions.

  1. Open SDRSharp and set the audio output to Virtual Audio Cable or VB-cable.
  1. Set the receive mode to NFM, with a bandwidth of about 12.5 kHz.
  1. Tune to a digital voice signal frequency. You will need to Google for these frequencies by your location. The radioreference databases may be a good place to start looking for these frequencies. Most digital voice frequencies for many countries will be around 460 MHz or 850 – 900 MHz. D-STAR frequencies are usually at around 145.670 MHz.
  1. To use DSD 1.7 for D-STAR: Open a command prompt from Start->All Programs->Accessories->Command Prompt, and navigate to the folder where DSD 1.7 is located. For those who are not familiar with the command prompt, use the “cd” command to change directories within command prompt. For example, if your DSD 1.7 folder is located in c:\Radio\dsd-1.7, you would type in command prompt “cd c:\Radio\dsd-1.7″.
  1. Now type into the command prompt the command “dsd -i /dev/dsp -o /dev/dsp -fd” to begin decoding. This will use the default sound device set in Windows sound recording properties
  1. To use DSD+: Simply double click on the DSDPlus.exe executable and the GUI will open up.

At this point, text should be scrolling through the command prompt window when a digital signal is broadcasting. Whenever someone speaks into the radio you should see the words “voice” in the window and hear voice.

DSD Command Prompt
DSD Command Prompt
DSDPlusv1p5
DSD+ GUI Windows

DSD+ can also be used to decode LRRP signals from Motorola (MOTOTRBO/DMR) radio signals. Some Motorola radio broadcast GPS coordinates every few minutes or on request. This is useful for tracking a fleet of vehicles for instance. To show decoded LRRP coordinates on a map when using DSD+, simply open LRRP.exe. Note that many LRRP radio users use a third party GPS software system which cannot be decoded by DSD+. If you do not see any coordinates in the DSD+ event log when an LRRP event occurs this may be the case.

To get good decodes (for most sound cards), the volume settings in SDRSharp and Windows should be played with until decoding begins to improve.

To stop DSD, simply press “ctrl + c” at the command prompt while it is running.

Some Tips

  • If you don’t know what P25, MOTOTRBO, ProVoice or any digital voice signals sound like, this page has some more example audio files.
  • You can simply manually scan through various signals, and see if DSD starts scrolling text to see if a signal is supported. DSD will start attempting to decode immediately. Note however that DSD will also scroll text on trunking channels, but will not be able to decode them. See the next section for information on decoding trunking channels.
  • A strong signal is required for DSD to decode audio well. Ensure you are using a good antenna and have set the RTL-SDR gain correctly.
  • DSD will listen to the default windows sound recording device. Make sure virtual audio cable or VB-cable is set as the default device. No text will be scrolling within DSD if the wrong audio device is used.
  • Stereo mix can be used, but you will be hearing both the digital signal as well as the decoded voice at the same time. Also, the decoded voice audio will be pumped back into DSD causing a detrimental feedback loop.
  • DSD is software in development and may not perform as well as a commercial digital radio.

More Advanced Digital Radio Scanning

We can set up a digital trunking radio scanner using these tools, two RTL-SDR dongles and another piece of software called Unitrunker. See our Unitrunker Trunking tutorial for information on setting this up.


If you enjoyed this tutorial you may like our ebook available on Amazon.

The Hobbyist’s Guide to the RTL-SDR: Really Cheap Software Defined radio.

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