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DMR - How it works and how to get set up

And why you shouldn’t import too many repeaters to your codeplug like I did!

I have made an accompanying video on YouTube at https://youtu.be/dNkdYzQbISY

Setting up DMR takes some patience but once you access the repeaters and talk groups you will be rewarded with many QSOs from all over the world in glorious digital quality.

DMR (Digital Mobile Radio) is a great transmission/reception mode, it is clear audio in a resilient wrapper that can withstand a lot of QRM - interference and fading. It is pretty much either working, or not with no real distortion. It has been developed due to the availability of cheap micro processors which can process audio into digital encoding and manage complex transmission systems. The DMR amateur radio system allows people to talk one to one or through a repeater in either simplex mode (same TX and RX frequencies) or duplex mode. You can probably access many DMR repeaters in your surrounding area and they provide access to hundreds of talk groups. Now modern DMR Transceivers have thousands of memories for channels, talk groups and contacts, so it is very tempting to try and import everything just in case. BUT YOU SHOULDN’T! I will be explaining why, but first let’s have a look at how DMR manages to do many things, all within the ‘limited’ bandwidth of a repeater.

Connected to local DMR repeater GB7NF in Nottingham and lisstening to Worldwide Talk Group



DMR Mode

DMR sits on a frequency like any other transmission mode, but it uses a different modulation system and it takes a lot more control over how the signal is transmitted. This is necessary to allow the operator's radio to synchronise with the receiving transceiver or repeater. To understand this we need to look at several factors that are pretty ‘out there’ compared to FM or SSB transmissions. There is more technical information at https://www.electronics-notes.com/articles/connectivity/private-land-mobile-radio-pmr-lmr/how-does-dmr-mobile-radio-work.php


Time Slots

When you transmit, the signal will first sync with the recipient (repeater or other operators radio) It will transmit your unique RadioID which is linked to your callsign, and APRS data if that is setup. It will also receive whilst transmitting so that it can sync the time slots. The system is called TDMA or time division multiple access.

Time slots are about dividing the available Rx and Tx time into 30 millisecond slots, Slot1, and Slot2. This doubles the number of communications that can be handled by the system. It can also be used on some repeaters to segregate some Talk Groups to have a priority Time Slot. So at any moment in time two people can be transmitting through a repeater. At the same time many people could be receiving that signal just as with an FM signal. But they won’t all hear it as there are other factors involved.

Now you might imagine that this would cause problems with your audio, but each second of your audio (and other data) is processed numerically and sliced into packets that can be transmitted somewhat independently of real-time. In the same way that if you listen to BBC Radio 2 on FM and DAB there is a delay on DAB of fractions of a second, or if you monitor an Amateur Radio transmission on an FM transceiver and a SDR, there is a processing delay. This delay is short enough to be ignored, though we tend to allow a longer break at the end of an ‘over’ to allow the repeater to resync to other users.



Color Codes

The Color Codes (0-15) are a rather badly named concept as they have nothing to do with color, this is probably to do with professional radio systems which would color code their radios to exist on a named sub-domain to prevent interference between multiple systems working on the same repeater. Model aircraft used to color code Crystals and the Antenna end flag so you could tell if two people were operating on the same frequency, thereby preventing a possible crash. The color system was easier for non-technical users to manage than long frequencies which could cause mistakes. The same is true for Amateur Radio, it is just a coding system to segregate operators and the majority of current repeaters all use color code 1, but do check! This will also allow other services to use the repeater network or allow for other expansion in the future.

Talk Groups

Talk groups are probably the most exciting feature or DMR. This allows users to contact specific groups of people to talk to, there are groups themed for geographical location, regional and non-linked groups that do not use internet linking for who want contacts specific to their local area.

A few examples from Brandmeister are below, though the TG numbers are pretty universal across many systems, the local groups will be specific to the repeater area -

TG 1        Local    
TG 2        Cluster    
TG 8        Regional    
TG 9        Local    
TG 91        World-wide    
TG 92        Europe    
TG 93        North America    
TG 94        Asia, Middle East
TG 95        Australia, New Zealand
TG 98        Radio Test    
TG 202    Διεθνές Ελλάδα    
TG 204    Nederland    
TG 206    Belgium    
TG 208    France    
TG 213    Andorra    
TG 214    Spain    
TG 216    Hungary
TG 218    Bosnia and Herzegovina
---- and further down, UK specific groups as also exist for many nations --
TG 2350        UK & Phoenix - QSY to 2353    
TG 2351        Chat 1    
TG 2352        Chat 2    
TG 2353        Chat 3    
TG 2354        Ireland    
TG 2355        Scotland
TG 2356        Wales    
TG 2357        Isle of Man
TG 2358        London    
REF 4000 Disconnect ***
TG 9990 Parrot


Promiscuous Mode or Digital Monitoring

This allows the Talk group setting to be overridden, this means that you can hear any digital talk group or color code to be heard. Some radios allow ‘single time slot’ and ‘dual time slot’ monitoring. Just remember that if you transmit, you may well be transmitting to a different call group or time slot than the one you are listening to. This is handy for checking that you can receive the signals, but best ignored for day to day use.

Lots of activity on the Brandmeister dashboard
 

DMR Etiquette

Due to the operation of DMR repeaters being able to support multiple modes in and out, many of which are via internet links you initially will need to ‘Login’ via a quick PTT press to gain access. You will also see that your radio sends and receives a couple of times in quick succession. At the end of using a talk group it is polite to send a REF 4000 Disconnect. You will be timed out eventually, but you may be restricting the number of available slots, so it is best to disconnect properly. Also, after each over it is best practice to wait 4 seconds or so (groups will advise) to allow the repeater to reset it’s configuration as it may need to switch from Echolink to D-Star or DMR or other connectivity. If you yourself are changing Tx modes, then you should wait 30 seconds after sending a disconnect before reconnecting in a new Tx mode.
 

Most DMR radios will show who you are talking to with name and location

Setting up your DMR Radio

Registering with RadioID.net

To gain access to repeaters you will need to register your callsign with RadioID https://www.radioid.net/ and send a scan of your licence.


Channels

There is a specific DMR simplex calling channel at 145.2000MHz also called DV16. Further info is at https://www.dmrguide.uk/index.php/dmr-simplex/.

Now just because many radios can store 4,000 channels, 10,000 talk groups and 500,000 digital contacts doesn’t mean that it is a good idea to do so. I strongly recommend storing the usual VHF and UHF simplex channels, though you probably only need to store calling channels and VFO the rest.


Repeaters

Again it is tempting to import every repeater within a 50 mile radius, and while you might travel around and access many of these on FM, it is a much better idea to only store one of two Digital ones, for starters you will need to contact most repeaters controllers to gain access, and secondly you will need to program one channel for each frequently accessed Talk Group too. You can however manually enter additional talk groups for occasional use.

A sample of DMR repeaters at repeaterbook.com


Talk Groups

The same applies here, not all repeaters have all talk groups and some local ones have different numbers. So best to economise, have a listen around and work out what groups are a) in use and b) of interest. There is a lot of crossover especially with other English speaking nations so you will often hear American Australian, Kiwi and ex-pats on local groups and Worldwide TG 9 is a great one to have as well as UK Chat TG 2350. Only add more when you need to.

A sample of Brandmeister talk groups at PI-Star



Scanning

Scanning is of little use with DMR repeaters due to the long connection times and repetition of channels with different talk groups. This will not trigger squelch so don’t bother to add them to scan lists. Just preset them to your favourites list.


Useful Links

https://www.dmrguide.uk
https://www.dmrfordummies.com
https://ukrepeater.net
https://www.repeaterbook.com
https://www.pistar.uk/dmr_bm_talkgroups.php Brandmeister, but relevant for most digital links
https://brandmeister.network
https://secure.echolink.org Useful for monitoring activity to check your settings
 

Also Facebook groups and Twitter





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