The frequencies employed by law enforcement and first responders can be a source of vital information in emergency situations but you can't tune in using any old radio. But rather than spend big bucks on a conventional police scanner, you can easily create your own for less than $20 with this (pretty technical!) guide from Instructables member Harrison Sand.
In this guide I will explain how to build an inexpensive SDR (software defined radio) capable of following trunked, digital voice radio transmissions with a single receiver that can be used to follow police, fire, taxi, and countless other civil frequencies.
Please use the information in this guide responsibly.
The only hardware required besides a computer is going to be a SDR receiver. The bulk of these radios are just USB TV tuners, re-branded and sold as SDR receivers. In this guide, I will be using a Newsky TV28T v2 with a RTL2832U & R820T tuner. I recommend purchasing from somebody who knows about SDR, a popular dealer is NooElec.com which also has stores on eBay and Amazon.
Please use the normal download link if possible to receive the latest software packages. Secondary links are provided for redundancy.
SDRSharp: Software defined radio application. Allows for tuning and receiving of audio from receiver. A modified version will be needed that is capable of connecting to UniTrunker.
rtlsdr.dll: Updated file for SDRSharp
Zadig: Allows for receiver driver installation
UniTrunker: Provides decoding support for trunking control channels and issuance of tuning commands.
Virtual Audio Cable: Provides audio routing between components. Trial available, purchase for $25.
Digital Speech Decoder (DSD): Provides digital voice decoding support.
Cygwin: Linux support for Windows. Required by DSD.
Microsoft Visual C++ 2010 Runtime: Required by remote.dll
Download and extract Zadig (you will need a program like WinRAR or 7-Zip). Take note if using Windows XP, you will need to download the XP version. Connect the receiver to your computer and ignore/cancel any attempts Windows tries to make to automatically install drivers. In Zadig select “Bulk-In, Interface (Interface 0)”. You may need to go under options and list all devices. Select WinUSB as the driver to install and press Install/Replace Driver.
Next to Driver it should now say WinUSB and the version number. I don’t believe it is required, but if you like you can do the same for Interface 1.
Download the modified SDRSharp package and extract it. Inside the folder is a file called rtlsdr.dll that needs to be replaced. We will get the updated version from inside RelWithDebInfo.zip. Use the file in the /x32 folder. Also note RelWithDebInfo.zip contains executables that can be helpful in troubleshooting should you encounter any driver issues.
If you like you can test to make sure your radio is working properly. Open SDRSharp, set the front end to “RTL-SDR / USB” select WFM and type in a known FM radio station to the center/frequency boxes and press play. For example, if you would like to tune to 97.9 type in 97900000. You can drag the red line to align with the stream in the waterfall.
Install Virtual Audio Cable as it is required to route the audio out from SDRSharp into DSD. Using the trial version will work reasonably well however since it periodically adds a voice over the stream you may or may not have some decoding errors. A new Virtual Audio Cable device should now appear under your playback and recording devices.
DSD uses the Windows default recording device as an input. Ensure Virtual Audio Cable Line 1 is set as the default recording audio device in the windows sound properties.
Install Cygwin using all of the default settings. Once installed browse to C:/cygwin/bin and copy the cygwin1.dll file to the same folder the DSD program will be in.
Download the Windows Binary and place it in the same folder as the cygwin1.dll file.
A trunked radio system is simply a system of radios in which the frequencies used are dynamically managed by a central controller. Most police and fire radios use a trunked radio system. Unitrunker decodes the control channel, and tells the receiver which frequency to tune to.
After downloading and installing, run UniTrunker. Choose “First Time Installation” and Continue.
Two receivers must be configured in UniTrunker, a ‘Signal’ receiver used to receive the control channel signal, and a ‘Control’ receiver used to receive the voice channel transmissions. This can be a little confusing as the signal that the ‘Signal’ receiver is receiving is typically called a ‘Control Channel’.
To configure the signal receiver, press the plus button on the top and select “Signal”.
For the Audio Port setting, select the Virtual Audio Cable created earlier. Set the Sample Rate to 48000, and check all of the decoding protocols.
To configure the Control receiver, press the plus again and select “Control”.
Set the Model to “Debug”, and select all three control protocols: P25, ProVoice, and VSELP.
Finally, in the main UniTrunker options screen, click the Enable checkbox in the Listen section:
There should now be two receivers listed under the Receivers tab. One control and one signal. Select each and press the play button.
Before starting SDRSharp, open the ‘sdrsharp.exe.config’ file and ensure this line is set as follows:
add key="minOutputSampleRate" value="48000" /> (just be sure to close the brackets)
Open SDRSharp and configure with the following settings:
- Set the radio to NFM
- Press the “Front end” button, disable RTL AGC, and enable Tuner AGC
- Set the Filter bandwidth to 12500
- Ensure Squelch, Snap to grid, Correct IQ, and Swap I & Q are all disabled
- Under the Audio dropdown, ensure Filter Audio is unchecked and the audio output is the Virtual Audio Cable.
Now we need to locate a control channel. This part can be a bit tricky, but after some practice it gets easier. You can start by looking for channels in the 850-900MHz range, but another good place to start is in the radioreference.com frequency database.
Browse to your area and find the section labeled “Trunked Radio Systems”. Browse through the systems and find one that has a good amount of information. See the example below.
We are interested in the control channels, shown in red and blue.
Test each control channel until you find one you can receive. For example, if I want to test 856.21250a then I need to enter 856212500 into the Frequency and Center boxes in SDRSharp. (Note the extra 0, and no decimal point)
Control channels will be a continuous stream in the waterfall window, unlike voice transmissions which are intermittent.
If you cannot find a signal, try other trunked systems listed in your area, listed in surrounding areas, or by manually browsing the spectrum.
If you find that a control stream is to the near right or near left of the red line after typing in a known frequency, manually move the line to be in the center of the stream. It may be helpful to zoom in. This problem will be dealt with under the calibration section.
After about 10-15 seconds of being tuned to a control channel, a new window should pop up in UniTrunker.
After a few seconds to a few minutes several frequencies should populate and channel activity will start scrolling by. If you see a red “Frequency Needed” and 0.00000, wait a few more minutes to see if it goes away. If not, click on the small calculator icon at the top and then standard.
Because this tuner will be tuned by the program, it is essential that it's correctly calibrated.
Enter the known control frequency (either from radio reference, or the red frequency listed in UniTrunker) in the center and frequency boxes.
You may see that red line not aligned with the channel. Open the “Front End” dialog box and adjust the Frequency correction value until the red line is centered onto the channel.
After you have found a control channel and calibrated the tuner you will need to lock UniTrunker down to only receive that control channel. Everything would technically work without this step, but it helps UniTrunker find the control channel faster as it switches between the control and signal.
Double click the Signal line under the Receivers tab and change the “Lock Mode” to Dedicated.
Double click the Control line and change the Park frequency to the control channel. In my case it’s 856.21250.
Copy Remote.dll from the SDRSharp directory into the UniTrunker installation directory. Remote.dll requires Microsoft Visual C++ 2010 Runtime, so you will need to download and install that as well.
Close and then reopen UniTrunker. Ensure SDRSharp is tuned to the control channel and UniTrunker is receiving the control signal.
Under the Trunking tab in SDRSharp set the “UniTrunker Install Directory” to the directory containing the ‘sdrsharptrunking.log’ file. This is the file that UniTrunker makes to tell SDRSharp which channels to tune to.
NOTE: The code currently makes the poor assumption that you’re running UniTrunker directly from the install directory. If you run it from the shortcut, you will find the ‘sdrsharptrunking.log’ file in your %AppData% directory, typically C:\Users\\AppData\Roaming\UniTrunker. This is the folder you need to select, and because it does not see UniForm.exe or Remote.dll in the same directory, it may complain. Ignore that.
Check the “Delay Re-Tune Until Call Completes” box. This setting controls the level at which SDRSharp will re-tune back to the control frequency after an audio transmission ends. This value will need some adjusting as you go along, but to roughly set it look at the peak signal strength of the control channel, in my previous screenshots you can see it’s around -15db. Subtract another 15db or 20db and set it to that number.
Set the “Tune to Control Channel” to the known control channel frequency.
At this point we can launch the Digital Speech Decoder executable and check the Enable box under Trunking in SDRSharp.
You will more than likely need to adjust the AF Gain value under the Audio dropdown. As a general rule you want to try and get the inlvl value in DSD to around 30% for the best quality audio decoding. Note that your computer may work better at another value, so experiment a little.
In my case, setting AF Gain to around 60% worked well. Also experiment with adjusting the AGC settings in the “Front end” dialog box. Try enabling/disabling the RTL AGC, Tuner AGC, and RF Gain values to get the best signal. Note if you change the AGC values you will likely need to change the “Delay Re-Tune Until Call Completes” value as well.
Keep in mind this setup is not ideal; it’s a basic setup using minimal components. You will not receive all transmissions and there can be a delay when re-tuning back to the control channel. For an optimal setup you will want to have two SDR receivers—one dedicated to the control channel and one dedicated to the signal. Connecting the USB receiver to your computer through a USB extension cable can help reduce interference as well.
Most of the information in this tutorial is stuff pulled together from the web. Here are some great places to get more information:
- SDRSharp Quick Start Guide
- Digital Speech Decoder (DSD) Install Tutorial
- Digital Speech Decoder (DSD) Guide
- Unitrunker Configuration Guide
- Massive Radio Frequency Database
Republished with author's permission from Instructables