Digital Mobile Radio (DMR)
Digital Mobile Radio (DMR) is becoming increasingly popular among amateur radio operators. This page and associated pages detail some of the basics.
Digital Mobile Radio (DMR) was originally a commercial digital radio standard
Digital Mobile Radio (DMR) was originally a commercial digital radio standard designed as a commercial upgrade option to existing FM systems. Therefore, commercial DMR was designed to remain within the existing commercial FM channel spacing of 12.5kHz. However, DMR is a Time Division Multiple Access (TDMA) system with two time slots. Consequently, two channels are available within a single 12.5kHz channel. This offers advantages over the introduction of channel spacing of 6.25kHz because existing antennas and duplexers can be used.
A commercial service provider can erect one local repeater and deliver two independent systems. This can enable a retailer to have separate radio systems for warehouse and for checkouts from one mast. Another example might be a sports stadium where catering and security could be provided with independent radio communications through the deployment of DMR.
There is an important thing to say about DMR. As you might expect from a commercial system, it provides excellent sound quality. This is one of the many attractions of DMR to amateur radio enthusiasts.
Digital Mobile Radio (DMR) has become popular with amateur radio operators
With new and second-hand commercial equipment regularly for sale, DMR has become popular with amateur radio operators. And now there many models of DMR transceivers designed specifically for the amateur radio market. With handset prices are falling, prices now stand at levels most can afford, whether new or second hand. Give it a try? Once you are competent with DMR and upgrade, you will be able to pass on some of your knowledge to another amateur radio enthusiast wanting to give DMR a try. Even if you lose interest or decide to upgrade, you can sell on your radio.
Time Division Multiple Access offers two simultaneous conversations per repeater, making it very attractive for amateur radio. Consider how one time slot can be dedicated to international, national, regional calls. Therefore, the other time slot can be dedicated to local calls only.
Understanding Digital Mobile Radio (DMR)
Undoubtedly, you will want to get the most enjoyment out of DMR. To do this, you will need to have some understanding of the following topics:
- 1. Digital Mobile Radio (DMR) Standards
- 1.1. Digital Mobile Radio (DMR) Tier I
- 1.2. Digital Mobile Radio (DMR) Tier II
- 1.3. Digital Mobile Radio (DMR) Tier III
- 1.4. Amateur Radio DMR is DMR Tier II
- 2. Time Division Multiple Access TDMA
- 2.1 DMR Signals
- 2.2. DMR Uses a vocoder and not A to D / D to A
- 2.3. TDMA Data rates and dual-user capability
- 2.4. DMR provides better sound quality
- 2.5. Time Division Multiple Access TDMA – summary
- 3. DMR Code Plugs
- 3.1. Design Your Own Code Plug
- 3.2. Northern DMR Cluster hosted on Phoenix
- 3.3. User ID
- 3.4. Zones
- 3.5. Talk Groups and Reflectors
- 3.6. Repeater colour code
- 3.7. Repeater output frequency and repeater input frequency
- 3.8. Contacts
- 4. Customer Programming Software
- 4.1. Tytera MD-380 – Code Plug
- 4.2. Hytera PD-785 – Code Plug
- 4.3. Anytone AT-868UV
- 5. Net Etiquette
- 5.1. Consider other users of the system
- 5.2. QSY without changing frequency
- 5.3. Some general points
1. Digital Mobile Radio (DMR) Standards
Digital Mobile Radio (DMR) is defined by international digital radio standard developed by the European Telecommunications Standards Institute (ETSI), and first ratified in 2005.
DMR is complaint with the European Telecommunications Standards Institute Standard TS 102 361 parts 1–4. The standard is a three-tier specification. This applies whether implemented in the commercial sector or by amateur radio operators.
1.1. Digital Mobile Radio (DMR) Tier I
DMR Tier I is the radio standard for licence free PMR446 personal mobile radio use. These radios operate on specific frequencies allocated for PMR446 DMR operation. These frequencies are not in the amateur radio bands. In most counties in the EU, it is not necessary to have a licence to use these radios, whether intended for business or personal use. Ofcom has published a document for further information:
1.2. Digital Mobile Radio (DMR) Tier II
The DMR Tier II standard targets users who need spectral efficiency, advanced voice features and integrated IP data services. This tier implements Time Division Multiple Access TDMA on two time slots: a 12.5 kHz wide peer-to-peer and repeater mode specification.
1.3. Digital Mobile Radio (DMR) Tier III
DMR Tier III supports voice and short messaging handling with status messaging and short messaging and supports packet data.
1.4. Amateur Radio DMR is DMR Tier II
Amateur radio DMR implements DMR Tier II. This is on the amateur radio 2m and 70cm bands, either through repeaters or in simplex mode.
At the time of writing there was thought to be:
- 11 DMR repeaters licenced for the 2m amateur band using a repeater input frequency 0.6MHz below the repeater output frequency
- 129 DMR repeaters licenced for the 70cm band using a repeater input frequency 9.0MHz below the repeater output frequency
- 14 DMR repeaters licenced for the 70cm band using a repeater input frequency 7.6MHz above the repeater output frequency.
As you can see, DMR is most popular on the 70cm band. There is a technical reason for the popularity of DMR repeaters on the 70cm band. The 70cm band covers a wide frequency range. This makes it possible to have a large frequency difference shift between repeater input and output. This makes it easier to design a duplexer to keep the input and output signals apart.
2. Time Division Multiple Access TDMA
Time Division Multiple Access certainly brings commercial benefits, but also make DMR attractive to amateur radio enthusiasts.
The intention was to offer commercial DMR as a hardware upgrade option to existing commercial FM systems running with 12.5kHz channel spacing. DMR was originally designed to offer two channels within an existing 12.5kHz channel spacing. This has advantages over single 6.25kHz channels. The existing antennas and duplexers can be utilised. This is achieved through multiplexing. Utilising Time Division Multiple Access (TDMA), DMR provides two time slots and therefore two channels.
Time Division Multiple Access offers two conversations per repeater making it very attractive for amateur radio. One time slot for international, national, regional calls and the other for local calls in and out of the local repeater.
For example, when an analogue FM repeater is hosting a scheduled net, whether worldwide or a local club net, the repeater is unavailable to others because the repeater is fully utilised.
However, when a DMR repeater is hosting a local scheduled net, other amateur radio users can still put out a call out through that repeater and all connected repeaters. Alternatively, when a DMR repeater is hosting a scheduled net linked to other repeaters, local amateur radio users can still put out a local call from that repeater.
2.1 DMR Signals
Signals are sent using 4-level Frequency Shift Keying. DMR uses 4FSK modulation at 4800 symbols/second with the dibits mapped to +1944 Hz, +648 Hz, -648 Hz and -1944 Hz in relation to the centre frequency.
2.2. DMR Uses a vocoder and not A to D or D to A
DMR does not use analogue to digital (A/D) and digital to analogue (D/A) conversion. Instead, DMR works using an AMBE+2™ vocoder (voice encoder).
In the encoder, the input is passed through a multiband filter. Control signals are generated that represent the amplitude of the frequency associated with each of the multiband channels. Only the control signals are transmitted. At the receiving end, the control signals manage the amplitude of tones synthesised at each of the multiband frequencies. It is important to understand that only the control signals are transmitted, and the control signals change slowly. Therefore, very little bandwidth is used.
Just imagine how a pianola functions. It uses punched cards to generate music. The amount of information represented in the punched cards is very small but generates the same musical effect that a piano would achieve over a wide bandwidth channel. DMR utilises this efficient use of bandwidth to good effect.
2.3. TDMA Data rates and dual-user capability
The vocoder compresses 60ms of audio with Forward Error Correction (FEC) into 216 bits of data for transmission in a 27.5ms frame. Each 27.5ms frame consists of a total of 264 bits; 108-bit payload, 48-bit SYNC or embedded signalling, and a second 108-bit payload for a total of 216 bits per frame – there is a 2.5ms break between slots. On the uplink into the repeater, your radio uses alternate 30ms timeslots and you only transmit for 27.5ms every 60ms, giving a 40% battery saving. On the downlink, the repeater transmits sequentially, TS1, TS2, TS1 and so on.
2.4. DMR provides better sound quality
There are enough data bits for Forward Error Correction. But, if the incoming signal cannot be recovered with error correction the connection is dropped, so you don’t tend to get an attack of the ‘Dalek’ sounds.
Let us look at this in more detail. Signal-to-noise and distortion ratio (SINAD) is a measure of the quality of a signal from a communications device. A typical amateur radio held VHF or UHF transceiver might have a receiver sensitivity of 0.19 μV at 12 dB SINAD. This is stating that the receiver will produce intelligible speech with a signal at its input as low as 0.19 μV.
Radio receiver designers will test the product in a laboratory using a procedure, which is typically as follows:
- With no signal present on the input, the noise and distortion of the receiver are measured at a convenient level.
- A signal is applied to the input such that the output increases by 12 dB.
The level of the signal needed to produce this is noted. In this case, it was found to be 0.19 microvolts.
Intelligible speech can be detected 12 dB above the receiver’s noise floor (noise and distortion). In the real world, lower SINAD values (more noise) can still result in intelligible speech, but it is tiresome work to listen to a voice in that much noise.
2.5. Time Division Multiple Access TDMA – summary
Digital Mobile Radio (DMA) Tier II utilising Time Division Multiple Access TDMA delivers better access to repeaters for amateur radio users.
In addition, the use of forward error correct provides a potential improvement in sound quality with no penalty from having two DMR conversations occupying the space of a single FM channel.
3. DMR Code Plugs
DMR Code Plugs are the frequencies and settings that must be uploaded as a file into a DMR radio. This will enable access to local repeaters and the UK simplex frequencies.
Most amateur radio operators will be familiar with the CHIRP software. This can be used to load frequencies and settings for analogue operation into Baofeng, Yaesu and other radios.
To load frequencies and settings into a DMR radio, you will use software similar to CHIRP which is usually referred to as Customer Programming Software (CPS). This software is typically specific to each radio manufacturer. In the case of manufacturers Tytera, Hytera, and Anytone, the CPS is available as a free download. Motorola CPS has to be purchased.
Once you have collected together all the frequencies and settings you intend to use and entered them into your code plug, you use the CPS to upload that information into your radio. Alternatively, another amateur might have a suitable code plug and upload this into your radio for you.
3.1. Design Your Own Code Plug
Most amateur radio operators like to design their own personal code plug. To do this yourself you will need the following information:
- User ID
- Talk Groups and Reflectors
- Repeater colour code
- Repeater output frequency and repeater input frequency
3.2. Northern DMR Cluster hosted on Phoenix
The Northern DMR Cluster hosted on Phoenix covers a major part of the North of England.
- GB7MR – Rochdale
- GB7HS – Batley
- GB7HX – Huddersfield
- GB7LE – Leeds
- GB7TD – Wakefield
- GB7EL – Nelson
- GB7RV – Ribble Valley
Therefore, I am going to focus on the Northern DMR Cluster. There is an alternative system called Brandmeister. However, the new GB7MN repeater planned for Brandmeister is now to go on to Phoenix. Therefore, the following sections cover the information you will need to add Northern DMR Cluster / Phoenix repeaters to your own code plug.
3.3. User ID
Your User ID enables both DMR and D-Star systems to recognise your callsign. Digital transceivers and digital repeaters function by transferring blocks of digital information representing voice communications between operators. To make this possible, the DMR system needs to be able to uniquely identify all amateur radio operators.
To achieve this, each amateur operator must have their own unique User ID. Each User ID is stored on the DMR/CCS7 database. Both DMR and D-Star systems coordinators have agreed on a single registration system. You apply for your User ID at the following link: http://register.ham-digital.net/ .
Once you have a User ID, both the DMR and D-Star systems will be able to identify you.
Each User ID consists of seven digits. You can identify the country of origin of callsigns on DMR from the User ID. UK amateur radio operators in the UK have numbers in the range 234XXXX. Similarly, Italian amateur radio operators have numbers in the range 222XXXX.
The most popular DMR radio at one time was the Tytera MD-380 / Retevis RT3. These radios let you select a ‘Zone’ and have the capacity for selecting from 16 Talk Groups within the Zone using the selector knob. Once a Zone, typically a single DMR repeater, has been selected on the radio, the user can switch through each of up to 16 Talk Groups. It is therefore general practice to design a code plug with a Zone for each DMR repeater, where each Zone has up to 16 Talk Groups.
Each of the 16 Talk Groups in a Zone is individually defined as a Channel. The link between the Talk Group name in text and the Talk Group number is defined for each Talk Group as a Contact.
Another popular DMR radio is the Hytera PD785 which also follows the same convention for Contacts, Channels, Talk Groups, and Zones.
The Customer Programming Software links Zones to Talk Groups to Channels to Contacts.
3.5. Talk Groups and Reflectors
Talk Groups and Reflectors enable DMR users to specify where their calls are directed.
Calls between repeaters are directed over the internet. The internet sees Talk Groups and Reflectors as the same thing. Talk Groups are accessed via repeaters. Reflectors are accessed via repeaters and via hotspots.
DMR Plus is the original network that developed tools to interconnect ETSI Tier 2 DMR repeaters. DMR Plus also supports the traditional DMR-MARC Talk Groups and DMR Plus Reflectors. A number of evolutions mark the adaption of commercial DMR to meet the needs of amateur radio users. In the UK, the DMR-Marc network became OpenDMR which is better known as Phoenix. Northern DMR Cluster is hosted on Phoenix.
There is a one to one link between Talk Groups and Reflectors. Not all Talk Groups are matched with a Reflector, but this should not affect repeater users. The following is a list of Talk Groups and Reflectors on the Northern DMR Cluster.
|Talk Group||Time Slot||Description||Reflector||Repeater|
|TG1||1||Worldwide Calling Channel||none||GB7HS (Batley), GB7HX (Huddersfield), GB7LE (Leeds), GB7MR (Manchester), GB7TD (Wakefield), GB7EL (Nelson), GB7RV (Ribble Valley), GB7HM (Caergwrle)|
|TG2||1||European Calling Channel||none||GB7HS (Batley), GB7HX (Huddersfield), GB7LE (Leeds), GB7MR (Manchester), GB7TD (Wakefield), GB7EL (Nelson), GB7RV (Ribble Valley), GB7HM (Caergwrle)|
|TG13||1||Worldwide English Channel||none||GB7HS (Batley), GB7HX (Huddersfield), GB7LE (Leeds), GB7MR (Manchester), GB7TD (Wakefield), GB7EL (Nelson), GB7RV (Ribble Valley), GB7HM (Caergwrle)|
|TG113||1||WW English User Activated||none|
|TG123||1||WW English User Activated||none|
|TG119||1||WW User Activated||none|
|TG129||1||WW User Activated||none|
|TG235||1||UK Calling Channel||DMR+4400|
|TG80||1||User Activated Talk Group 80||DMR+4401|
|TG81||1||User Activated Talk Group 81||DMR+4402|
|TG82||1||User Activated Talk Group 82||DMR+4403|
|TG83||1||User Activated Talk Group 83||DMR+4404|
|TG84||1||User Activated Talk Group 84||DMR+4405|
|TG810||2||South West TG810||DMR+4410|
|TG820||2||North West TG820||DMR+4420||GB7MR, GB7EL, GB7RV|
|TG860||2||North East TG860||DMR+4460||GB7HS, GB7HX, GB7LE, GB7TD|
|TG862||2||M62 Corridor Roaming||none||GB7HS (Batley), GB7HX (Huddersfield), GB7LE (Leeds), GB7MR (Manchester), GB7TD (Wakefield), GB7EL (Nelson), GB7RV (Ribble Valley)|
3.6. Repeater colour code
Potentially, two repeaters in different geographical regions on the same frequencies can cause interference. Operators might inadvertently work through a different repeater to the one intended. For example, extreme radio propagation conditions can make this more likely to happen.
In the case of FM repeaters, the Continuous Tone-Coded Squelch System (CTCSS) prevents inadvertent access of geographically remote repeaters. This is because CTCSS frequencies are allocated on a regional basis. Therefore, the CTCSS tone sent to access a local repeater will not be recognised by a remote repeater.
In the case of DMR, the repeater colour code prevents signals intended for one repeater being accepted by another. The repeater colour code is simply a number between 0 and 15. Technically, the repeater colour code is a binary number between 0000 and 1111 occupying just four data bits in the digital signal transmitted and has nothing to do with colour. But you do not need to worry about this. You just look up the repeater colour code number for the repeater you want to access and enter it in the DMR code plug.
To find the repeater colour code on the ukrepeater.net website, just ‘click’ on the repeater name and look for the digit (for example, in the case of GB7MR you are looking for cc:2). Or alternatively, you can find repeater frequencies and repeater colour codes for your DMR code plug for the Northern DMR Cluster repeaters at http://www.northerndmrcluster.com/talkgroups.html
3.7. Repeater output frequency and repeater input frequency
You can look up the repeater output frequency and repeater input frequency on the Radio Society of Great Britain repeater website: https://www.ukrepeater.net/ . When you enter the repeater frequencies into your DMR code plug, bear in mind that the ukrepeater.net website details the repeater output frequency which in your DMR code plug editing software will be your radio receive frequency. Similarly, the repeater input frequency will be the frequency you transmit on. This applies for the software for Tytera and for Hytera radios, and probably most other radios.
Frequencies for all UK digital repeaters are listed within the website at https://www.ukrepeater.net/repeaterlist4.htm .
The Contact section within the Code Plug fulfills two functions.
Firstly, the Talk Groups title or name are entered in the Code Plug as text. However, these need to be converted to numbers. In the Contacts table, the Contact Name (which is the Talk Group Name) is highlighted as a Group Call linked to a Call ID that is the Talk Group number we are already familiar with.
Secondly, just as we know our own User ID and callsign, you can enter other peoples’ User ID and callsign so that their details appear on your radio when you receive a call.
4. Customer Programming Software
Customer Programming Software enables code plugs to be generated and uploaded to DMR radios. Customer Programming Software is specific to each radio manufacturer. When you buy a radio you should read the code plug from the radio and save it, just in case. In the case of Tytera / Retevis radios, you can download a code plug from someone, add your details, and write it to your radio. In the case of Hytera radios, the code plug supplied with the radio contains a unique code that matches that code plug to that radio. You cannot directly write a code plug obtained from someone else to your radio. You must enter data (or cut and paste from a code plug you want to copy) into your software and then write to your radio. This can take time.
4.1. Tytera MD-380 – Code Plug
Here are the screens you will use to enter your User ID, Zones, Channels, Contacts.
4.1.1. User ID
You need to enter your User ID in to your code plug.
Typically, a Zone will be associated with a single DMR repeater. In this example, the Zone for GB7MR calls up Channels that describe each Talk Group for GB7MR.
4.1.3. Talk Groups for specific repeaters are stored in Channels
Talk Groups for specific repeaters are stored in Channels. Also stored in the Channel are the repeater output and input frequencies, Colour Code, and Time Slot.
You will need to add Contacts that define the Talk Groups.
You will also need to add Contacts that define other users so that the name and callsign shows on your screen during receive.
4.1.5. Write Code Plug to your radio
All that remains is to write the Code Plug to your radio.
4.2. Hytera PD-785 – Code Plug
4.2.1. User ID
Setting the User ID is set on a similar screen to the one used for the Tytera.
4.2.3. Talk Groups for specific repeaters are stored in Channels
If you didn’t get all this, M6CEB has a helpful YouTube video: https://www.youtube.com/watch?v=LqKXb6Nt6cw
4.3. Anytone AT-868UV
The Anytone AT-868UV has become hugely popular with amateur radio enthusiasts, probably for two reasons. Firstly it covers both 2 metres and 70 centimetres, bands. Secondly – a full Code Plug for the UK including the full User ID register is available free of charge from Moonraker:
Express Zip File Compression can be downloaded free to open the RAR file. This will give you the code plug – AT-D868UV – CPS1.33 – V10.00 (REL 2019 03 10).rdt.
Another attraction is that the Anytone AT-868UV can accommodate multiple Radio IDs. This would be useful for an amateur operator who would also like to use a club callsign.
And that is all you need to know to programme the Anytone. No wonder the Anytone AT-868UV is so popular!
5. Net Etiquette
5.1. Consider other users of the system
- Don’t use network Talk Groups when a local call would suffice – it wastes internet or 4G data which has to be paid for.
- Don’t stay on a Calling Channel depriving other system users from being able to put a call out.
- QSY to User Activated Talk Groups for network calls so that only the necessary repeaters use the internet.
- Disable GPS/APRS as these are not supported by the repeater, will tie up a slot, and also use up data unnecessarily.
5.2. QSY without changing frequency
- Start off on TG9, S2, moving to TG9 S1 if S2 is busy.
- If you do not receive any replies to your calls on TG9, then try TG820, which is our regional TG.
- Should you be unsuccessful on TG820, then try TG860, which is the M60 corridor (roaming).
- If there is nobody on TG860, then try TG235, which is U.K. wide.
- Once a contact had been established on TG820, TG860, or TG235, move to one of the UK UA TGs to continue your conversation:- TG80, TG81, TG82, TG83.
- You may go straight to TG235 for a UK wide call or TG1 for a World Wide call.
- Once a contact has been established, move to a UA TG to continue your conversation. Ask the other station what UA TGs are available to them. This would normally be:- TG113, TG119, TG123, TG129.
- If there are no common UAs available when on TG1, then keep the conversation to a few overs.
5.3. Some general points
- Disable GPS/APRS as these are not supported by the repeater, will tie up a slot, and also use up data unnecessarily.
- Repeaters are generally set to 3 minutes Time Out, so make sure your radio’s TOT is set to 3 minutes or less.
- Leave a gap, around 2 seconds, after the other station has finished talking. This will allow the repeater time to reset.
- Regularly check if there are other stations on the Talk Group who want to come in.
- There is no need to call CQ, but it is helpful to mention the Talk Group and Time Slot when you do call. This is helpful when driving, or when your radio is on scan. A couple of suggested calls are as follows:- “This is G0ROC, Talk Group 9 Slot 2, standing by for any calls”“ or “G0ROC, Talk Group 9 Slot 2, listening and standing by”.
- If you are local to the repeater, then try to use TG9 S2 where possible.
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