Digital Mobile Radio

Digital Mobile Radio

Lately I’ve become more involved in the HAM radio DMR community. Digital Mobile Radio, or DMR, is a Time Division Multiple Access (TDMA) digital voice system suitable for VHF and up. Most HAM radio applications utilise the UHF bands with typically a frequency split of 7.6 MHz, very much like traditional FM repeaters. More and more DMR enabled repeaters are popping up and there have been competing networks to link the repeaters up over the internet.

Other digital voice standards

There are many other digital voice standards operated by the HAM radio community such as TETRA, D-STAR, C4FM and many more. DMR is a relatively new technology, developed as an open standard by the European Telecommunications Standards Institute (ETSI). Comparing the different technologies is beyond the scope of this blog post, but DMR is a relatively new technology built for commercial use. This means there is still plenty of room for experimentation to make it suitable for HAM radio use whereas there are not many new developments on the D-STAR network.

DMR Repeaters

DMR repeaters are mostly built on using ready-existing, commercial repeaters form various vendors like Motorola, Hytera, RadioActivity, etc. But the HAM community has started to experiment with more open alternatives, like the excellent Multi-Mode Digital Voice Modem (MMDVM), DVMEGA and DV4mini projects. The MMDVM project aims to build a full capable, open source DMR repeater system while the DVMEGA and DV4mini allow end users to run their own local DMR hot spot.

Linked repeaters

CQ DMR map

Various projects exist today that link up the various DMR repeater systems. This can be somewhat confusing to the end user, because not all systems link up to the other existing systems, so there are multiple views of what is the global DMR platform. You can get an overview of how the various networks are spread on the http://www.cqdmrmap.com/ site.

More and more sites are adapting the Brandmeister DMR technology. Brandmeister is driven by a very talented group of HAM radio operators from all over the world. What makes Brandmeister so powerful? The people driving the project.

In my humble opinion Brandmeister is a prime example of what is possible when you combine modern day technologies with a group of driven HAM radio operators. Brandmeister offers support for a very wide variety of vendors and is fully decentralized. Also, Brandmeister is promoting experimentation with home built solutions by offering Homebrew protocol support, so HAM radio operators can plug in to the Brandmeister network.

Voice synthesis

DMR uses digital speech synthesis using the Advanced Multi-Band Excitation, or AMBE codec developed by Digital Voice Systems, Inc.. Initially the AMBE codec is what scared me away from DMR, because I think it’s very important that we keep the HAM radio world free from patented technologies that carry crazy license fees. Now over the last couple of months, DVSI has made the AMBE chips more affordable and Chinese radio manufacturing companies like Tytera are implementing low-cost software versions of the AMBE codec in their radio systems.

Luckily, the DMR specification leaves plenty of room for other voice codecs to be carried over the DMR network. So hopefully, the HAM radio will start experimenting with open voice standards like Codec2 or Opus.

Adressing on the DMR network

Addressing on the DMR network uses 24 bit identifiers to uniquely identify actors on the network. There are two standard modes that carry voice information (indicated by the Full Link Control Opcode or FLCO) that indicate wether a voice transmission is destined for a group, or for a (private) user to user transmission. If a group voice transmission is sent, the destination identifier is usually referred to as a Talk Group.

Brandmeister

Since HAM radio operators don’t want to get vendor locked, a team from Russia has started the Brandmeister project to link up repeaters using different technologies. More talented developers from all over the world have joined the Brandmeister project and the development team continues to expand.

Starting off small, the Brandmeister project now has over 250 repeaters linked up from all over the world. This is still a relatively small network compared to D-STAR which has over 1000 repeaters linked, but the network growth rate is fenominal. The repeaters setup a link to a regional master, that has a full mesh with all the other regional masters around the world. This allows anyone on the Brandmeister DMR network to open up remote Talk Groups and talk with the other side of the world as if they are on the same local repeater.

Hose line

Piggy backing on the full mesh routing capabilities, I have started to develop a web based front end dubbed Hose line which allows users to listen in to the voice streams that are on the network. The engine is entirely built in Go and Javascript. This is how it all works.

Full mesh routing

All the regional masters on the Brandmeister network have a list of what other regional masters exist on the network. If a user on the Brandmeister DMR network starts a voice conversation to a Talk Group, all of the voice transmissions are relayed to all the other regional masters on the network. In turn, the regional master relays all of these packets to all of the repeaters. The repeaters keep track of what Talk Groups are local to them, or what Talk Groups are recently activated and decides if the voice frame will be relayed to the radio interface or not.

Streaming

The Hose line project uses the full mesh routing capabilities of the Brandmeister DMR network to receive voice frames from all the regional masters around the world. When a voice frame is received for a Talk Group, Hose line check if there are subscribers listening to the stream and decodes the voice frames on-demand so it can relay it as plain audio to the user’s browser. All of this is powered by a very simple publish-subscribe (or “pubsub”) bus written in the Go programming language.

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