DSD+ stands for Digital Speech Decoder Plus and is a software program that can allow you to decode digital voice signals such as P25 and MotoTRBO/DMR. DSD+ is under continual development, and in their last public update they began offering early access to the latest DSD+ features in development through their fast lane subscription. The fast lane subscription costs $10 USD for one year and $25 for unlimited early access. Information about joining the fast lane service can be found in the readme file of the latest DSD+ 1.074 public release.
Over on YouTube user John Miller has been testing the latest early access version DSD+ 1.08t. This new version adds trunking support which allows you to follow conversations. Previously other software like Unitrunker was required to follow the trunking signal. On YouTube John has uploaded a video first showing trunking in action, and a second video showing how to set up DSD+ 1.08t for trunking.
Usually to listen to trunked radio systems, two RTL-SDR dongles are required. One for decoding the trunking control channel and another for listening to the audio channel. However if the audio channels are within the same chunk of received bandwidth as the control channel it is possible to use just one dongle to follow trunked conservations.
NOTE: There is now a plugin available for SDR# that will decode TETRA fairly easily. It is still in beta and misses a few features found in telive. Check it out in this post.
TETRA is a trunked radio communications system that stands for "Terrestrial Trunked Radio". It is used heavily in many parts of the world, except for the USA. Recently, a software program called Tetra Live Monitor (telive) was released on GitHub. This software can be used along with the (patched) Osmo-TETRA software to monitor and listen to unencrypted TETRA communications.
Below we show a tutorial on how to listen to TETRA communications using a RTL-SDR RTL2832U software defined radio. This tutorial is based heavily on the telive_doc.pdf file that is written by the author of telive and included in the telive git download. Please refer to that pdf file for further details on how the software works. We have modified their tutorial slightly to make it a little easier to understand. As this code is still under heavy development if you have trouble please check their PDF file for modifications to the procedures.
Most of this tutorial is performed in Linux and we assume that you have some decent Linux experience. We also assume you have some experience with the RTL-SDR dongle and have a decent antenna capable of picking up TETRA signals in your area. If you don't have a RTL-SDR dongle yet see our Buy RTL-SDR dongles page.
Note: As of October 2016 there is now a Windows port of the Telive decoding software available. This may be an option for you if you prefer to run in Windows. More information here.
First, we will need to find some TETRA signals. The easiest way to do this is to open SDR# or another program like GQRX and look for them. TETRA signals are continuously broadcasting with a bandwidth of around 25 kHz. In most European countries they can be found at 390 - 470 MHz. In some countries they may be found around 850 MHz or 915 - 933 MHz. There may be several TETRA signals grouped in close proximity to one another. See the example images below.
A Zoomed in TETRA Signal
A Grouping of TETRA Signals Zoomed Out
An example audio clip of a TETRA signal recorded in NFM mode is shown below.
Once you have found some TETRA signals, record their frequencies. Now close SDR#, or whatever software you were using and boot into Linux. In this tutorial we use a 32-bit Ubuntu 14.04 virtual machine running on VMWare Player as our Linux system. Some of the commands may vary if you are using a different system.
The popular trunking decoding software Unitrunker now supports the RTL2832U R820T RTL-SDR directly in its new version. This means that extra SDR receiver software like SDR# is no longer required to use Unitrunker.
In a normal radio system, one company (or talkgroup) might use a single frequency for radio communications. However, this is very inefficient as the frequency may not be in use for the majority of the time. In a trunked radio system, a small set number of frequencies are shared between a large number of talkgroups. Each radio receives a special computer controlled control channel. The control channel determines a vacant frequency that a particular talkgroup should use. This helps to make radio frequency allocations more efficient.
Because a talkgroup might switch between various frequencies often, it can make listening to a conversation difficult for radio scanners. Unitrunker can be used to decode the control channel and follow a voice conversation as it hops across various frequencies. With two RTL-SDR dongles you can set up a trunking receiver station with just Unitrunker. What follows below is a tutorial on how to set this up.
Reddit user rtlsdr_is_fun has updated the SDRSharp Unitrunker trunking plugin so that it works with the latest versions of SDRSharp. See the thread here for more information and a download link. Use this plugin to allow Unitrunker to control SDRSharp, so you can listen to trunked radio conversations. A new tutorial on doing this can be found here.
