What’s digital radio is all about
Welcome South Coast Reflector
Digital Radio
First of all, what digital radio isn’t, is the “sit at a computer and type” mode that some, ill-informed, people have called it. It’s proper radio, with RF which follows the same laws of physics as any other mode. It has aerials which, if you don’t install them correctly, still don’t work – just like with FM. It has mobile and handheld radios (which are almost all dual-mode so they can be programmed with normal FM channels as well). It also has repeaters which can be networked together to increase their coverage area. Next, we are talking about a telephony mode – that is, you talk to people. With a microphone. So, it’s definitely not a data mode and there’s no typing! If you want that, go elsewhere. Lastly, it’s not elitist (or similar) – no more so than someone you know using a band you don’t have (yet). Some people want to use it and some don’t. If you want to join in, sort yourself out with some gear, you’ll be very welcome.
So, to continue….
As pressure on radio spectrum increases, the professional communications world is coming up with ways of squeezing more into less. That is more conversations in less spectrum. Inevitably, this technology is finding its way into the amateur world, often helped along by those same professionals.
There are two fundamental ways of achieving this improved efficiency and every technique really boils down to one of these solutions. The first is to make each channel narrower – in the past we went from 25kHz channels to 12.5kHz and now there’s pressure for 6.25kHz channels. This is the method the likes of Yaesu’s fusion uses – in Fusion’s “voice and data” mode, a voice channel is fitted into 6.25kHz along with a data channel in a further 6.25kHz – still a total of 12.5kHz. In Fusion’s “voice only” mode, the single voice channel offered is still the same 12.5kHz wide that traditional analogue modes use. This is called FDMA or “frequency division multiple access”. FDMA is the spectrum allocation method radio amateurs are most familiar with – a given band is divided into fixed bandwidth channels (for example the FM portion of 2m or 70cm). “Going digital” using this method doesn’t offer us very much. We’d still get one digital channel on a given RF channel and we already have the ability to connect to other repeaters around the world – on the Isle of Man, our existing AllStar FM network is hard to beat in that respect.
The second answer is to squeeze more conversations into the same space. This is called TDMA or “time division multiple access” and is the approach TETRA and DMR both take. In the case of DMR, the system rapidly switches between two channels on the same frequency. The total spectrum bandwidth needed is still 12.5kHz but now there are now two speech channels in the same space instead of one. This is where the term “timeslot” first appears when talking about digital radio – the first time period is “timeslot 1” and the second is “timeslot 2”. The picture below shows six timeslots but, theoretically, there can be any number. Remember, DMR is two – this means that can be twice as many DMR conversations fitted into a given band when compared with traditional analogue systems.
Finally, all digital systems offer potential advantages is improved coverage – some are better at it than others. This should not be confused with “bigger coverage area” though - it's still RF and it follows the same laws of physics of any other radio system - for a start, it still gets blocked by big bits of rock. By the nature of digital encoding and modulation, signals are very much either there or not – think of DAB and digital television broadcast systems. What that means in practical terms is that, firstly, there’s no mobile flutter and secondly, for areas where FM is noisy, digital systems will deliver clear speech. Experience has shown that DMR will work flawlessly down to a signal strength of around -120dBm, which is a horrible ropy and noisy, S1 signal in an FM system.
Why DMR and not Fusion?
The obvious question that comes up after reading the glossy adverts in the amateur radio magazines is “why doesn’t everyone use Fusion?” – after all, Yaesu are virtually giving the repeaters away. Apart from the fact that we wanted to keep what we had and add to it, rather than swapping it for something else, here are some thoughts…
The first answer is that their system is entirely proprietary – you can only use Yaesu radios with a Yaesu repeater. Even if you think Yaesu will still be making Fusion gear in five or ten years time, that’s only OK if you like Yaesu radios and they make one that suits you – there’s not much choice of gear.
The second answer is that DMR is an open standard – the opposite of things like Fusion. This means that there are several manufacturers that make equipment that inter-operates. That’s the way we’ve always worked – imagine having different types of FM so that only people with Icom radios could talk to other Icom owners but people with Kenwood gear couldn’t join in. There is one thing to say regarding the open-ness of DMR and that is that it is only the core that is open – there are various things that don’t work between manufacturers. This is limited to the likes of text messaging and security which are hardly a problem for amateur radio. Whatever, as a user, you still get the biggest choice of equipment with a DMR system.
The third answer is that DMR is a TDMA system, so we get two channels for the price of one. Extra for free is always good! That means that, depending on who’s doing what, we can potentially get two QSO’s on a repeater at once.
The fourth answer is that Fusion is an amateur system – that’s good on the one hand because it’s designed to be easy to set up but it’s all based on amateur grade hardware which is not designed to sit on commercial radio hilltop sites, running for ever, with little attention.
Do you want another reason? The fifth answer is that it’s the way the world is going. Whether we like it or not, analogue systems are slowly being phased out in the commercial world and, at the moment, DMR seems to be the most rapidly growing system there. That means the supply of equipment suitable for us to use is unlikely to dry up. There’s gear on eBay and plenty of choice of suppliers of new equipment. As well as the established suppliers such as Motorola, the Chinese are beginning to enter the market with less expensive handheld radios and most PMR radio equipment manufacturers are making DMR equipment as well as their traditional analogue systems.
Amateur DMR Repeaters
We’ve decided to go digital with DMR for all the reasons already mentioned above.
This is being achieved with the installation of two Motorola repeaters, co-sited and sharing antennas with the existing 70cm FM repeaters at Carnane and Bride. This is being done without adversely affecting the FM repeaters.
Amateur DMR repeaters are generally inter-connected but it should be mentioned that there are two, separate DMR networks. The first is DMR-MARC and the second is DMR+. The difference is really down to the hardware used by the repeater network (as distinct to the radios individuals use to access the network). DMR-MARC is Motorola based and DMR+ is Hytera based. DMR-MARC originated in America with the Motorola Amateur Radio Club and is, by far, the biggest network in the UK. DMR+ originated in mainland Europe and is the biggest network in Holland and Germany and although our close neighbour, GB7FC in Blackpool is part of the DMR+ network, it is very much the minority network in the UK.
At the moment, the two networks are not connected to each other meaning that repeater keepers have to choose. There are, however, some interesting software developments currently underway which might mean that’ll change some time in the future.
We’ve decided that our repeaters will soon be connected to the DMR-MARC network since this includes most of the existing UK DMR repeaters.
Talkgroups, timeslots and colour-codes
Talkgroups, timeslots and colour-codes are the buzz-words that are used to describe the bits of information used to define the operation of a DMR radio system. Everyone who uses a DMR radio in the amateur radio world needs to know what they mean to avoid too many mistakes….
Timeslots have already been mentioned – they simply describe the time periods used for the two channels we can access. The first one is “timeslot 1” and the second is “timeslot 2”. It doesn’t matter why 1 is 1 and 2 is 2, just so long as every radio in the system has the same idea.
Colour codes are a bit like CTCSS tones in FM systems and are used to distinguish between repeaters on the same frequency. In the FM system on the Isle of Man, Carnane uses a CTCSS tone of 110.9Hz and Bride uses 71.9Hz even though they are both on a frequency of 430.8250MHz. The DMR system uses a colour-code of “2” for Carnane and “3” for Bride. For a user, it’s just a number you need to know when you programme a DMR radio.
Talkgroups are an extra thing we get in DMR systems. It’s a phrase that comes from commercial trunked radio systems and they are a way of distinguishing between groups of users. It can be thought of by comparing our DMR system with the government’s TETRA system where one talkgroup could be used by the Snaefell trams and another by the bus drivers. Each channel programmed into an amateur DMR radio is configured to only un-mute when signals directed to its talkgroup are received.
There’s more on talkgroups and timeslots as applied to the DMR-MARC network later.
DMR ID number
Every radio in a DMR system has an ID number, whether it’s a repeater or a user’s radio. This is a bit like a callsign which is embedded in the transmitted signal. Every transmission includes the ID number for that radio – this is normally used in the amateur radio world to display the callsign and name of a transmitting station on a user’s radio.
In the amateur world, ID numbers are generally allocated, one at a time to individuals and repeaters. The exception to this is for shared radios – in that case each radio has an ID. Generally though, the rule is that an individual number is allocated to each piece of equipment that could conceivably appear on the network at the same time. So, if you have a handheld and a mobile that only you use, you’d have one number applied to both but if you loan your handheld to other people, you’d apply for a second ID number.
The ID numbers are organised on a regional basis – for example, a number beginning with 235 is allocated to the UK (and islands) – like a “G…” callsign. From that set of numbers, anything beginning with 2356 is allocated to the Isle of Man, like a “GD… “ callsign.