What is DMR?

What is DMR?

Digital Mobile Radio (DMR)
Digital Mobile Radio, commonly known as DMR, is a commercial digital radio standard that has been successfully adapted by amateur radio enthusiasts.

DMR -v- ANALOGUE Graph

TDMA double capacity. Compatible with Analogue system.Amateur radio DMR
Tytera MD380 – typical amateur radio DMR radio
The first thing to say about DMR is that it is a commercial system that provides excellent sound quality. With new and second-hand commercial equipment regularly for sale, DMR is becoming increasingly popular to amateur radio operators. In addition, transceivers built to the DMR standards specifically for the amateur radio market are increasingly available. Therefore, handset prices are falling. Prices now stand at levels where most can afford to buy a handset to ‘give it a try’ and sell on if they lose interest or want to upgrade.

DMR Standards
DMR is complaint with the European Telecommunications Standards Institute Standard TS 102 361 parts 1–4. The standard is a three-tier specification, whether implemented in the commercial sector or by amateur radio operators utilising the amateur radio 2m and 70cm bands.

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 without licensing for business and personal use in most countries of the EU. These frequencies are not in the amateur radio bands. There are specific restrictions on the types of radio that can be used. Ofcom has published a document for further information:

https://www.ofcom.org.uk/__data/assets/pdf_file/0025/85156/ir_2009_analogue_and_digita1.pdf

DMR Tier II
DMR Tier II is a standard targeted at those 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.

DMR Tier III
DMR Tier III supports voice and short messaging handling with status messaging and short messaging and supports packet data.

Amateur Radio DMR is DMR Tier II on the 2m and 70cm bands
Amateur radio DMR implements DMR Tier II 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. There were about 129 DMR repeaters licenced for the 70cm band using a repeater input frequency 9.0MHz below the repeater output frequency. In addition, there were 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. One of the factors behind the popularity of the 70cm band for DMR repeaters is that the band covers a wide frequency range. This makes it possible to have a large frequency difference shift between repeater input and output, which in turn makes it easier to design a duplexer to keep the input and output signal apart.

When you enter the repeater frequencies in to 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.

There are some things to understand about DMR. Just as you need to program your FM transceiver with: –

The repeater output frequency
the repeater input frequency, and the frequency used for Continuous Tone-Coded Squelch System (CTCSS) or Tone Calling 1750Hz for some European repeaters;
with DMR you will need to program your DMR with: –

Your unique User ID,
repeater output frequency and repeater input frequency,
repeater colour code, time slot,  and DMR talk group.
Accessing DMR repeaters has similarities to accessing FM repeaters, but there are some new aspects to understand. DMR repeaters deliver Time Division Multiple Access TDMA on two time slots making it possible to have two independent QSOs or conversations taking place at the same time. Therefore, users must program their radios to select a time slot and DMR talk group. The information you need to program in to your radio is quite easy to understand. To try and keep things simple and not bamboozle people with too many new terms, the file containing the information to be programmed in to a DMR radio is called a ‘Code Plug’. We shall now look at these new aspects and how they are added to a Code Plug.

DMR code plug – the basics
Some basic information needs to be programmed in to a DMR radio to make it function. These settings are collectively referred to as a DMR code plug. To better understand these settings, we can compare them to the essential settings used to access an FM repeater.

Just as you need to program your FM transceiver with: –

The repeater output frequency
the repeater input frequency, and the frequency used for Continuous Tone-Coded Squelch System (CTCSS) or Tone Calling 1750Hz for some European repeaters;
with DMR you will need to program your DMR with: –

Your unique User ID
repeater output frequency, repeater input frequency, and repeater colour code.  DMR repeaters deliver Time Division Multiple Access TDMA on two time slots making it possible to have two independent QSOs or conversations taking place at the same time. Therefore, users also must program their radios to select: –

Time slot and DMR talk group.
The information you need to create a DMR code plug for your DMR radio is quite easy to understand. We shall now go through each of the DMR details to be entered in to your code plug.

User ID
Digital transceivers and digital repeaters function by transferring blocks of digital information from one specific amateur radio operator to other operators. To make this possible, the DMR systems need to be able to uniquely identify all amateur radio operators using the system.

To achieve this, each amateur operator must have their own unique User ID. Please note that the User ID, stored on the DMR/CCS7 database, enables amateur radio operators to be uniquely identified on both the DMR and D-Star systems. Although DMR and D-Star are essentially independent of each other, the organisers of both systems have agreed 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 systems will be able to identify you.

A User ID consists of seven digits. In the case of amateur radio operators in the UK, User ID numbers are being issued in the range 23453XX (June 2018). You can actually identify the country of origin of calls on DMR from the User ID. For example, numbers in the range 234XXXX are unique to Great Britain and in the range 222XXXX are unique to Italy.

Repeater output frequency and repeater input frequency
Amateur radio DMR
https://www.ukrepeater.net provides the repeater output frequency and repeater input frequency for your DMR code plug
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/ . There is a lot of information on the website. Frequencies for digital repeaters are listed within the website at https://www.ukrepeater.net/repeaterlist4.htm .

Repeater colour code
There is the potential problem of two repeaters in different geographical regions but on the same frequencies, interfering with each other. Operators might inadvertently work through a different repeater to the one intended. Similarly, the output of one of the repeaters in this example could be detected by the other repeater. This problem might be made more likely through extreme radio propagation conditions for example.

In the case of FM repeaters, the Continuous Tone-Coded Squelch System (CTCSS) prevents interference between repeaters. An operator accessing their local repeater will not inadvertently access a more distant repeater with the same receive frequency but a different CTCSS tone frequency.

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 cc:1).

DMR repeaters deliver Time Division Multiple Access TDMA
We now need to understand the time 2lot and talk group parameters, which arise from the way DMR facilitates Time Division Multiple Access TDMA. How does that effect how we programme and use our radios? Well, firstly we need to look at how DMR digitises voice calls.

Amateur Radio DMR
Figure demonstrating how TDMA facilitates two voice calls on a single channel (figure posted on Twitter no copyright claimed)
DMR does not use conventional analogue to digital and digital to analogue conversion. Instead, DMR uses a voice encoder (vocoder) system that passes the voice signal through an audio 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 receive 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.

If you are interested, the control signals are sent as 4-level Frequency Shift Keying. DMR uses 4FSK modulation at 4800 symbols/second with the dibits (2 bits per symbol) mapped to +1944 Hz, +648 Hz, -648 Hz and -1944 Hz in relation to the centre frequency.

Time Slot
DMR occupies a 12.5 kHz bandwidth. Due to the efficient use of bandwidth through the vocoder system, DMR is able to transmit enough information to represent two separate audio channels within the single 12.5 kHz channel. In fact, enough information is transmitted within 30 milliseconds for the receiver to synthesise a full 60 milliseconds of audio. Therefore, a DMR repeater is able to transmit one conversation for 30ms and then to transmit a second conversation for 30ms and to repeat this cycle continuously without either receiver perceiving any breaks. This enables any DMR repeater to carry two simultaneous conversations. In effect each voice call occupies just 6.25 kHz of bandwidth. Each separate 30 millisecond slot within the full 60 millisecond cycles of continuously alternating data is referred to as a time slot.

If you imagine the first 30 milliseconds of information to be Time Slot 1 and the second 30 milliseconds to be Time Slot 2, then the DMR system can allocate Time Slot 1 for one specific type of conversation and Time Slot 2 for another types of calls. That is called Time Division Multiple Access TDMA.

Talk Groups and Reflectors
At this point it might be helpful to mention that in the UK there are two DMA conventions for utilising TDMA in the North of England. One is the Northern DMR Cluster, which sits as a guest on the Phoenix DMR system, part of OpenDMR and the other is known as Brandmeister. Both conventions route QSOs through Talk Groups and Reflectors. However, there are differences in the way the two systems handle QSOs.

DMR Scotland – Digital DMR Repeaters for Radio Amateurs