DMR – Joining the Dark Side!

As a birthday present to myself I have just ordered a Retevis RT3 DMR hand-held off eBay which should be delivered just after Christmas.

South Kesteven ARS had a talk in October by Sean Burton 2E0ENN about amateur DMR where he demonstrated some handsets and the new DV4Mini which allows gateway and internet linking for the various networks, DSTAR, DMR and System Fusion. This piqued my interest in the DMR scene.

As I posted last time I dabbled a few years ago with decoding PMR DMR using software and a sound card but they were very hit and miss at the time. I reacquainted myself with the various projects and using the FUNCube Dongle Pro+ and the latest version of DSDPlus (support forum at RadioReference.com) monitored the nearby GB7RR DMRPlus repeater managing to get some reasonably clear decodes of some amateur transmissions.

I decided to dig out my Realistic PRO-2022 scanner and using a discriminator tap fed into the sound card got slightly better quality decodes.

Doing some research and reading a couple of reviews had decided I was going to get a Tytera TYT MD-380 when funds allowed but spotted the Retevis RT3 which appears to be identical and slightly cheaper.

I know some people wince at the thought of DMR and issues with proprietary technology used in some of the systems but I think the genie is out the bottle and it isn’t going away soon. Adoption of DMR appears to be growing with talk of restructuring of talk groups needed to deal with the growth (whatever that means!) so I should at least get my feet wet and understand the technology.

I registered for an ID, now off to decode the jingo and understand all this talk of codeplugs, talk groups and time slots.

Latest antics

Here I am a month after the last post and it is has been a month of very little ‘radio antics’.

I was acutely aware that since the end of September my wife had become a radio widow so promised not to lock myself away in the shack for a while and have been doing some much needed painting and decorating around the house.

I haven’t been in much of a radio mood anyway as I have been unwell and am still not fully over my last wobble. Band and weather conditions have been rubbish with a sustained period of high wind and rain including storms Abigail and Barney. As a precaution I dropped the pole and it became apparent I had some maintenance to do on the OCFD.

The shack too had been in need of some sorting out, which I thankfully I did muster enthusiasm to tidy up.

While being largely uninspired I haven’t been completely radio silent, I did get on air for the South Kesteven ARS 2m net but found myself suffering some QRM again


It isn’t the first time I’ve seen this sort of signal, but I had thought it had gone away, it seems it is back and stronger! This was an ARISS contact I monitored back in 2013 before I was licensed with a similar noise.


After using the SDR to identify the noise I realised I have been neglecting the FUNCube Dongle for far too long. So ordered some new SMA adapters from HamGoodies and pressed it back into service. I have been using it to decode the telemetry from the FOX-1A (AO-85) satellite with the updated software and have now got myself on the leader board even if the collinear is currently horizontal about four feet off the ground!

South Kesteven ARS had a talk in October by Sean Burton 2E0ENN about amateur DMR where he demonstrated some handsets and the new DV4Mini which allows gateway and internet linking.

I remembered I dabbled a few years ago with decoding PMR DMR using the SDR and a scanner with a discriminator tap using various programs but they were very hit and miss at the time. I reacquainted myself with the various projects and had a go at decoding some amateur transmissions.

I downloaded the latest program called DSDPlus  (support at RadioReference.com) and monitoring the nearby GB7RR DMRPlus repeater managing to get some clear decodes with little effort.

Finally this week I gave a presentation at SKARS on the subject of HABs and how to plan a HAB launch. Following on from the Eggsplorer-1 and Hamfest “Pigs In Space” HAB launch I decided to try to explain everything I had learned for anyone else contemplating giving it a go!

It was a long talk (perhaps too long) as I covered everything from building the electronics, software, making the payload box, getting the right balloon, parachute, gas, obtaining permission and then the prediction, launching tracking and recovery.

It was a great turnout with a lot of interest.

The joys of RTL-SDR and Taxi MDT Decoding

I have owned and used radio scanners for many years, and loved them as my posts before December 2011 will testify.

In that month I became the proud possessor of a FUNCube Dongle Plus and discovered the joys of software defined radio, since then I purchased a FUNCube Dongle Pro+ and extended my SDR adventures in to the realms of HF and I have several of the insanely cheap RTL2832 based dongles.

As much as loved my scanners there was a major flaw with them, which has been brought into sharp focus now that I have used SDR.

No matter how fast or as sensitive as the scanner is are you are still playing a game of chance. You are limited by the frequency steps, demodulation modes and scanning rate of the receiver and you could zip through the band all day and still miss those elusive signals.

SDR and the waterfall display is a revolution, you can view a portion of the spectrum in real time and actually see the signals, they may be short lived bursts of data and voice, or continuous data transmissions.

The RTL-SDR dongles excel in this respect with their wide sampling rate you can view up to 2MHz of the spectrum at once, the following images show typical waterfalls captured this morning using one of my RTL-SDR dongles.

The first one, shows the cluster of data channels (was the old Vodaphone Paknet system) around 164.2 – 164.4 MHz, a trunking control channel and various data bursts, which are mostly Taxi Mobile Data Terminal (MDT) transmissions.

A little lower down the spectrum and another trunking control channel, a speech conversation, more data bursts and a faint digital channel.

Further up the spectrum into the UHF, a cluster of data transmissions.

Simply moving the cursor on the display and you can hear the transmission, if necessary change the demodulation type, widen or narrow the filter bandwidth, save the frequency. If you capture the IQ file you can then replay it endlessly tweaking and refining until you extract the information you want.

During the weekend I was experimenting and noticed there was lots of data bursts in the 163-168MHz range, I confess that I already knew what most of them were as I have experimented before with a scanner (with a discriminator tap) and Ian Wraith’s Java based Taxi MDT decoder. I decided to reinvestigate them using the RTL-SDR as the receiver.

While many taxi companies still use voice transmissions, many have adopted automated data terminal systems, where the dispatcher sends information about jobs to terminals in the cars, the drivers then can accept jobs, get information and send information back to the dispatcher.

Ian’s decoder which requires the Java runtime environment decodes systems that use the same physical layer as MPT1327 i.e 1200 Hz and 1800 Hz tones transmitted at 1200 bps. The two main systems used in the UK, are the Autocab and Auriga. The Taxi MDT Decoder currenly decodes the Autocab, but the coding for the Auriga system is still an unknown, so just outputs the raw data.

More information about Taxi MDT Decoder can be found here I confess to having one slight niggle with it, often I couldn’t get it to accept sound from the selected input. A work around I found was to first open the Audacity sound editor which I had installed and select the input and start a recording, then opening the decoder seems to make it work!

Ian has also written the excellent DMRDecoder which allows analysis of the DMR digital mode which is becoming more widespread. I intend to post some details soon about decoding digital modes, keep watching.

I created a video showing the Taxi MDT Decoder in action, the quality is pretty dire but you can get the idea, I identify the Auriga as being encrypted, it might be but as nobody on the team knows the protocol yet!