After the comment I got and posted about yesterday in my last post I seriously contemplated not even bothering writing any new posts in this blog. But after quite a bit of thought on the subject I realised why should I let the views of one gutless person dictate whether I continue with this blog or not. So here I am with the next post on DMR and the Radioddity GD-77.
The next sections are not meant as a comprehensive definition of each of the parts that make up the DMR system. Instead they are meant more as an overview of the terms to help take some of the mystery out of them.
Firstly what is DMR radio
DMR as you might have already guessed means Digital Mobile Radio.
It was first developed in about 2005 as a commercial system of communications, but as with many modes we use, we have borrowed the concept from commercial systems and adapted them for amateur radio use.
What are Time slots
Lets first think about a conventional analogue repeater. This repeater uses 12.5KHz bandwidth for both reception and transmission meaning that only one user can be transmitting to the repeater at a time. However as a DMR signal is only 6.25KHz wide, that means that two people can be using the same repeater at the same time, one using time slot1 and the other using time slot2. As both users are only using 6.25KHz each they can both occupy the same total 12.5KHz bandwidth that a conventional analogue repeater uses also allowing us to use the same 25KHz channel spacing. Now obviously there is a lot more to it than that but I don't really think for the purposes of this guide we really need to go into the details of bitrates, compression or the fact that you are only transmitting for 30ms out of every 60ms.
What are colour codes
This is probably the simplest of them all but probably not the most obvious.
Lets go back to the analogue repeater system for a moment again. To open that repeater for use you used to use a 1750Hz tone, later we moved to CTCSS tones which are sub-audible tones added to the carrier that the repeater recognises and will open if the right CTCSS tone is sent. It also transmits that same tone on the output of the repeater to open the squelch on your radio.
A colour code is really exactly the same as a CTCSS tone but in the DMR world it's called a colour code. I have only ever seen 1 and 3 used so I have no idea how many their are, but the details for the repeater you intend to use will tell you which number it is set to.
I find it very weird that when talking about a colour code you are actually referring to a number instead of an actual colour, but hey, this is the weird and wonderful world of confusing DMR terms.
What are Talk groups
Talk groups are you can probably imagine different areas of the DMR network where different types of conversations go on. These can world wide groups such as TG1 where anyone from anywhere will congregate, but bear in mind that you are expected to use english as the language for all conversations on this talk group. There are many different talk groups covering many different languages and regions of the world so you should easily be able to find one to suit where and what you want to talk about.
There are in theory 99999999 available talk groups although that is only a theoretical number and far far fewer are used. You are encouraged not to just ragchew with your buddy on one of the major talk groups like TG1 and to move to another free TG# agreed upon between yourselves.
I am sure there will be a list of talk groups and their designated usage on the internet but I have not yet had a look for one.
DMR registration
In order to use the DMR network you are required to register your callsign whereby you will be given your own unique ID number. Without this number you will not be able to use the full DMR network and in almost all cases your transmissions will be rejected by the system.
Your DMR ID number is a unique 7 digit number that personally identifies you and your callsign. This serves a few purposes, the main two being that nobody can impersonate you unless they know your ID number. The second and most important one is to prevent abuse of the system by non hams or sadly that very small minority of hams and non hams that deliberately try to spoil things for the rest of us. I don't know about you, but many a time I have had a keyer appear on an analogue repeater and there is very little you can do to stop it.
To register for a DMR ID# you will need to go to https://www.dmr-marc.net/cgi-bin/trbo-database/register.cgi and follow the simple instructions. They say that you should get your ID# within 72hrs but in my case I got my ID# back in about 7-8 hours. I guess the key here is to just be patient as most services like this are staffed by volunteers and can only process your application in their spare time.
DMR radios and linear amplifiers
The simple answer is don't even bother trying to connect your radio to an amplifier.
The reason why is that because of the way that a DMR radio transmits (30ms on 30ms off) there are currently at the time of writing no amplifiers that can switch on an off that fast. So all you will end up doing is making your transmissions worse, unintelligible and also cause interference to other hams. The best thing you can do if you cannot reach your local repeater reliably is to get a better antenna for your handheld or to put up a bigger internal or external antenna sited in a place that will enable you to reach that repeater.
There are other options, but I will explain at least one of those in a later installment.
Well I think that about covers it for the first section on this series, but if I have missed something important then I will come back and add it later.
Until next time
73's Paul M0CNL
