DMR
| DMR (Digital Mobile Radio) | |
|---|---|
| Mode type | Digital voice and data |
| Standard | ETSI (European Telecommunications Standards Institute), Tier II for amateur use |
| Access method | Two time slots (TDMA) in a 12.5 kHz channel |
| Modulation and codec | 4FSK; AMBE+2 voice codec |
| On the air | |
| Bands | Mostly 70 cm; also 2 m |
| Networks | Brandmeister, TGIF, FreeDMR, DMR-MARC and others |
| Needs | A DMR radio, a codeplug and a DMR ID |
| DMR ID from | RadioID.net (free) |
DMR, short for Digital Mobile Radio, is a digital voice standard created for commercial two-way radio and since adopted by hams in large numbers. It was developed by the European Telecommunications Standards Institute (ETSI) in the mid-2000s, and business radios built for it, from makers such as Motorola and Hytera, soon found their way onto amateur repeaters. Inexpensive amateur-market radios followed, many of which also work on FM, and DMR is now one of the most widely used digital voice modes on the 70-centimeter and 2-meter bands. Its learning curve is steeper than FM's, because a DMR radio has to be told not just a frequency but a time slot, a color code and a talkgroup.
How it works
DMR divides each 12.5 kHz channel into two time slots using time-division multiple access (TDMA). A transmitting radio sends its signal in short bursts, each about 30 milliseconds long, and a second radio can fill the gaps in the other slot. A single DMR repeater can therefore carry two separate conversations at once on one frequency pair, and a handheld that transmits only half the time gets longer battery life. Amateur DMR uses the standard's Tier II, the version for conventional (non-trunked) radios and repeaters.
Speech is compressed by the AMBE+2 vocoder, a proprietary codec, and sent as four-level frequency shift keying (4FSK), along with error correction, the caller's ID number and the destination. Like other digital modes, DMR sounds clean right up to the edge of coverage and then breaks into garbled, robotic audio rather than fading gradually into noise.
Talkgroups, color codes and codeplugs
Three ideas cover most of what is new about DMR:
- A talkgroup is a virtual channel, something like a chat room. Keying up on a talkgroup sends your voice to every repeater and hotspot listening to that talkgroup, whether across town or around the world. Talkgroups are identified by number; on the Brandmeister network, for example, 91 is worldwide and 3100 is the United States. A private call goes to a single station's ID instead.
- A color code, a number from 0 to 15, does the job that a CTCSS tone does on an analog repeater: the repeater ignores any transmission that doesn't carry its color code. If the radio's color code doesn't match exactly, the repeater stays silent.
- A codeplug is the configuration file for a DMR radio. It holds the channels, each a combination of frequency, color code, time slot and talkgroup, grouped into zones, along with a contact list of talkgroups and private-call IDs. Codeplugs are built with the manufacturer's programming software on a computer, and the format differs from one radio model to another.
Repeater owners decide which talkgroups are carried on which time slot. Some talkgroups are static, always connected to the repeater; others are dynamic, linked when a user keys up on them and dropped after a period of inactivity. The repeater's listing or the club's website explains the local arrangement.
Networks and DMR IDs
Most DMR repeaters connect over the internet to a network run by volunteers. Brandmeister is the largest; others include TGIF, FreeDMR and the older DMR-MARC network. Each network has its own talkgroup numbers, and a repeater usually belongs to only one.
Every DMR user needs a DMR ID, a seven-digit number that identifies the radio on the air and on the networks. IDs are issued free by RadioID.net after it checks the applicant's amateur license. The ID is entered in the radio's settings, and many radios can load a database that matches IDs to call signs and names, so the display shows who is talking. The ID doesn't replace identification: operators still give their call signs by voice, as on any other mode.
Hotspots
A hotspot is a small, very low-power transceiver, often a single circuit board attached to a Raspberry Pi computer, that links a DMR radio to a network through home Wi-Fi or a phone's data connection. The radio talks to the hotspot across the room at a few milliwatts, and the hotspot passes the traffic to the internet. Hotspots let hams with no DMR repeater in range use the networks, and most multi-mode hotspots also handle D-STAR, System Fusion and other digital voice modes. Free software such as Pi-Star sets them up through a web page.
A hotspot is still a transmitter, so it should be set to a frequency that fits the local band plan and kept out of the 435–438 MHz satellite segment of 70 centimeters.