DMR is growing in popularity within the amateur radio community. Many areas of the UK have repeaters and hotsposts / gateways that allow access to the various networks that are available.
How does DMR work?
The DMR standard was created by the ETSI (European Telecommunications Standards Institute).
One of the advantages of DMR is the quality (readability) of the QSO is maintained over a large area. However, unlike FM, this degrades rapidly outside of the footprint.
Like other digital modes, it can also operate when the signal is not optimal.
An example of this is where a QSO on FM is an S4 but with a lot of white noise. Working DMR simplex on the same frequency allows for a clean 5 and 4, rather than the previous 3 and 4 signal report.
Below are some of the features of DMR:
- TDMA Time-division multiple access
- 12.5KHz bandwidth (6.25 kHz per slot)
- Colour codes (to prevent station overlaps)
- Talk Groups
- Digital message service
- AMBE+2 vocoder
- 4-state FSK (symbol rate of 4800symbols/sec Each symbol carries 2 bits of data, so the equivalent data rate is 9600bits/sec)
Time Division Multiple Access
TDMA is basically the slot system. The idea is that two radios can transmit at the same time, one on each slot. You can think of the slots a bit like FT-8.
In FT-8 you have a situation where one person transmits on the even 15 seconds and then you transmit on the odd.
The two slots transmit in a similar way so slot 1 could be imagined as on the evens and slot 2 on the odds. This is not technically accurate, but helps you grasp the basic concept.
Another benefit of TDMA is you only use 50% duty cycle. So your radio is only transmitting 50% of the time. This helps to prolong the battery life of your HT.
Just like a standard analogue repeater, you have the Transmit and Receive frequencies. These will be published on the repeaters details page. You can also use the same frequency for both if you are working simplex.
To prevent interference between two repeaters on the same frequency, DMR uses colour codes. You can think of these a bit like CTCSS tones.
Talk groups are a bit like a digital squelch. Several groups can transmit on a time slot, but you can set the radio to only hear or transmit to one specifically. You can also set the radio to listen to many of them.
For the rest of this article I will refer to these as TG’s.
User Activated Talk Groups
You will often hear users QSY to TG81-83 after putting out a call on TG235 UK Wide Calling Channel. These channels are listed by repeaters as UA or User Activated. If you are listening to a QSO on TG235 and they do QSY you will no longer hear them.
If you wanted to listen to them, you would need to change to the channel you have set for TG81 and press the PTT quickly.
This will ask your local repeater to switch on TG81 and you should now be able to listen to their QSO.
Please bear in mind that doing this means everyone using your repeater now hears the QSO on TG81 which will probably be on Slot 1.
It is considered polite to go back to TG9 when you are done and press the PTT quickly again. This stops random QSO’s being retransmitted on the repeater and leaves Slot 1 clear for just the calling channel or local TG’s.
DMR has two slots. Literally called slot 1 and slot 2. Many of the repeaters have specific talk groups available on specific slots. It is important that you use the correct slot for the talk group you wish to use.
This system allows both slots to transmit at the same time. So you could be in a QSO on slot 1 with someone in another part of the world, while someone else is on slot 2 chatting to someone locally.
Only one person can use a slot at a time, much like analogue.
When setting up a channel on a DMR radio, you will need to choose the “Admit Criteria” (this might be called something else, depending on the radio). This governs when the radio is allowed to transmit.
This is the “impolite” option. This allows you to transmit regardless of the repeater already being used or transmitting.
This is the most “polite” option. This will not let you transmit if the frequency is in use.
Colour Code Free
This is the preferred option. The radio checks if the colour code is being transmitted and will not allow you to transmit if it is. The radio will usually make a sound to let you know you cannot currently transmit.
These seem to confuse a lot of people. I have read various guides on the internet that try to explain these. The older radios and certainly Motorola need you to set the talk group you wish to hear on a channel. You set this in the RX Group. Tytera and others no longer require this and will automatically listen on the talk group the channel has been set to transmit on.
The idea of the RX Group is to allow you to listen to more than one group.
As DMR was primarily intended for commercial use, there are situations where organisations want all staff to hear on a certain talk group, but only operate on their own. This is where setting RX Groups comes in to play.
This is basically where you set up your talk groups and add contact info (DMR id to callsign lookups).
You give the talk group a name you would like to be displayed and then make sure the id is the number of the talk group. Also ensure ‘group call’ is set.
Private callsign data is stored as ‘Private Call’ this allowing you to see who is operating by callsign and name, rather than just the DMR id.
DMR Networks (Amateur Radio)
Brandmiester and Phoenix (DMR-MARC).
It is important to note that both of the networks use different talk groups.
Individual repeater keepers also govern which talk groups are available. They also govern which UA (User Activated) talk groups exist.