Learn / Guide 9

DMR and digital voice

Time slots, color code, talkgroups, repeaters and hotspots: how DMR works and how to listen to the network before getting a radio.

intermediate 6 min read

Where it came from

DMR stands for Digital Mobile Radio. It is an open standard, published by the European Institute ETSI in 2005 for professional radios: security, transport, industry. Radio amateurs adopted the system when portable radios became cheap, and built worldwide networks on top of it.

Instead of modulating voice directly onto the carrier, as in FM, the digital radio digitizes the voice, compresses it with an encoder called AMBE+2 and transmits the bits in four frequency levels (4FSK), in a 12.5 kHz channel.

Two time slots on the same channel

DMR divides time into 30 millisecond slices that alternate: slot 1, slot 2, slot 1, slot 2. Each conversation uses only one of the two slices. The technical name is TDMA.

Two consequences follow from this:

• A repeater carries two independent conversations on the same frequency, one in each slot.
• The portable radio transmits only half the time, and the battery lasts longer.

This is not apparent to the ear. On a common FM receiver, DMR sounds like a low-pitched, continuous ratchet. It is worth memorizing this sound because it appears frequently on 70 centimeters.

Color code

The color code is a number from 0 to 15 programmed into the repeater. The radio needs to send the same number, otherwise it is ignored. It plays the role that subtones play in FM repeaters: it prevents a signal from another repeater on the same frequency from opening the wrong one. It has nothing to do with security or secrecy.

Talkgroups

In FM, whoever is on the frequency hears everything. In DMR there is an extra layer: the talkgroup (TG). Each transmission is addressed to a group number, and only radios configured for that group open the audio.

TGs work like rooms. Some from the BrandMeister network:

The number 724 is the country code for Brazil in mobile telephony tables, and Brazilian regional and state groups start with it. The list of Brazilian groups is on the BrandMeister wiki.

There are also private calls, from one radio to another. Each amateur radio operator has a DMR ID, a seven-digit number linked to the callsign and registered at RadioID. Brazilian IDs start with 724. It is through this number that the network knows who is speaking.

Repeaters and networks

A DMR repeater by itself covers a city. Connected to the internet, it becomes a gateway to a network. What you say in São Paulo on a national TG comes out on all repeaters in the country that have that group active.

The largest amateur radio network is BrandMeister, maintained by volunteers, with servers in various countries. There are others, such as DMR+, TGIF and FreeDMR, with their own groups and rules.

A TG can be on a repeater in two ways:

• Static: stays on all the time. Everything that passes through it goes out on the air.
• Dynamic: is activated when someone local transmits to it, and drops after a few minutes without use.

Hotspot

The hotspot is a pocket repeater for personal use. It is a small board with a minimal power transceiver, mounted on a small computer and connected to home internet. The handheld radio talks to the hotspot a few meters away, and the hotspot talks to the network.

For those who live far from a digital repeater, a hotspot is the solution. Its software (Pi-Star and WPSD are the best known) shows on a dashboard who is speaking in each group.

Hotspot transmits. The power is low, but it is still an amateur radio station and requires license and regularized equipment.

The codeplug

In FM you type the frequency and start talking. In DMR you need to program the radio beforehand, and the programming file is called codeplug. Each channel combines frequency, color code, time slot and talkgroup. In addition, there is the contact list, reception groups and zones, which organize the channels.

This is the most annoying part for beginners. Almost every "my radio doesn't work" in DMR is a wrong slot, color code or TG in the codeplug.

You can listen without a radio

• Through the network itself. BrandMeister has a browser listening service, Hoseline (hose.brandmeister.network), where you choose the talkgroup and listen live. The last heard page shows who transmitted, in which group and through where they entered.
• Via an OpenWebRX. Several receivers covering 70 centimeters have a DMR decoder. You click on the signal and the audio comes out, with the slot and group number.
• With a RTL-SDR. Programs like SDRangel decode DMR off the air.

Listening to TG 724 for a few days shows how people identify themselves, how long they wait between overs and what kind of conversation fits in a national group.

Net customs

• Leave a pause before answering. The audio travels across the internet and arrives with a delay. Two or three seconds between one transmission and another give time for the network and for someone to join the conversation.
• Wide group is for calling. National and worldwide TGs go out to hundreds of repeaters. Once contact is made, agree on a smaller talkgroup and move there.
• Identify with the callsign, as on any band. The DMR ID does not replace it.
• Digital signal does not degrade gradually. It either comes in clean, breaks up, or drops out. If people say you are breaking up, it is a weak signal to the repeater or a bad internet connection at the hotspot.

The other systems

DMR is not the only digital voice mode among radio amateurs. You will hear about D-STAR, created in Japan, and C4FM or System Fusion. There is also M17, an amateur radio project with an open voice codec. They do not talk to each other over the air, but hotspots and networks bridge several of them together.