This article is not intended to be a complete tutorial about DMR, but instead trying to convert what you may already know about repeaters to a fundamental understanding of the concepts of DMR to "flatten" the learning curve.
I was listening on a local DMR repeater the other day and heard an old-timer talking. He was on a statewide talk group on time slot 1 while there was a QSO already occurring on the same time slot on Local 1. All the hams had Digital Monitor on and could hear each other, but were talking on different talk groups. All the repeaters in the local area that were using the Local 1 talk group could listen to everything occurring on Local 1. Still, other repeaters outside of that cluster who were monitoring the statewide talk group could only hear the statewide part of the conversation. There was an attempt to get the old-timer on the Local 1 talk group, but he did not have local talk group alias names in his radios, just the talk group ID numbers. He also did not understand the terminology, and there was a lot of confusion. Once they got that sorted out, there were conversations about hot spots and trying to access a talk group on the other side of the country, which you could not do from the repeater he was currently using. He did not know that.
If you are new to DMR, the description of the conversation probably made little sense to you, but they happen all the time. DMR is a little less intuitive when compared with standard analog repeaters, but can still be understood with the learning of a few concepts. I am going to try to borrow some terms from the analog world since most hams seem to know how that works.
In terms of the most basic analog repeater, we know that the repeater listens on one frequency and transmits on another at the same time. The repeater listens on the frequency that your radio transmits on when you press the PTT. It transmits on the frequency that your radio listens to when you release the PTT. We used to program our ham radios with the front panel keypad and just needed to enter the receive frequency and the PL/CTCSS tone if it was required. Most modern amateur radios automatically compute the offset direction (positive or negative) and the shift (the MHz difference between the input and output frequencies), making it very simple and fast to program the radio. You might be able to get away with just programming the VFO with a single frequency and key up the repeater since the radio might be able to calculate the rest. Not so with DMR.
In the commercial radio world, radios are not programmable via the front panel. This is actually by FCC rule! Instead, the radios are programmed by adding the repeater parameters into a computer program. The computer program is often generically referred to as CPS (Customer Programming Software). All of the settings of the repeater are entered into a line on the CPS, including the separate transmit and receive frequencies, transmit and receive PL tones, and the name of the channel. The programmer repeats the process over and over until all of the repeaters are entered into the software. Then, the computer is connected to the radio, and the data is transferred. Amateur radio started using CPS programming in the '90s, and it made it much quicker to program the radios. Almost every amateur radio vendor provides a CPS and programming cable for their radios now. Some other third-party vendors offer this capability, such as CHIRP and RTSystems.
DMR programming still uses CPS but is exponentially more challenging to complete. Let's talk about the architecture of DMR to understand why.
An analog repeater is, well, analog. DMR repeaters are digital (Digital Mobile Radio). Digital radio is not new to any of us. Cellular phones went to digital all around the turn of the century, and you may have noticed that the call quality drastically improved. Static completely disappeared. Calls are clearer. With cell phone calls, you either hear the person on the other side of the line, or you don't. That is also true with DMR. Knowing that the digital nature of DMR is the way some of the magic happens and just being OK with that makes it a little easier to swallow. We could get an in-depth discussion about how all of the magic works, but I think that is where we lose a lot of people. Keeping this simple, let's just parallel an analog system to a digital system so we can better understand how to program the radio via the CPS.
DMR repeaters, just like the analog repeaters, have an input frequency and an output frequency. Analog repeaters use analog PL/DPL tones as a gate to the repeater. These tones are like a "key" to open the repeater to repeat what it's receiving. If it doesn't receive the PL/ DPL, it ignores the signal. There are two practical use for PL/DPL in the amateur radio world. 1) Reduce unintentional "key ups" of the repeater due to RF noise. 2) To "hide" the repeater from unauthorized users. The owner of the repeater places a PL/DPL on the repeater and gives it only to a small group then works to conceal it from others.
There are two practical uses for PL/DPL in the commercial world. 1) It is the same use as above, to reduce interference. But the second most people may not be aware of. Some commercial radio services may have several customers on the same frequency with just different PL tones. They place the tones on both the transmit and receive channels so that each customer only hears themselves and not the other user. This helps the business reuse the license investment across multiple customers. This only works if both customers aren't particularly big talkers because even with PL if two customers talk at the same time, you will get the doubling. Though commercial services do this, amateur radio rarely places to repeaters on the same frequency and just uses a different PL (though there is an exception or two in the US).
DMR does not use PL/DPL, but rather a Color Code. When you see this term, think of it precisely as PL. It's just an access gate, just like PL. Without the correct color code programmed in, the repeater will not retransmit your signal.
In the analog world, you are done. Your radio is programmed, and you are ready to talk through the repeater. On DMR, you have another couple of steps. If you don't complete them, you will not be able to use the repeater. They are talk groups and time slots.
A talk group, simply in parallel to an analog repeater, is like having multiple PLs on the same repeater as the commercial world does with analog. It's like another tone on top of the color code you are already transmitting. It's a way to quiet the radio except for other people who are monitoring the same talk group. You could have talk groups named "blue," "red," and "green." You monitor the one(s) you want to listen to. If you don't like the conversations on green, just don't monitor green, and your radio will remain quiet. However, if someone talks on green and blue at the same time, a collision on the repeater will occur.
We should also talk about the availability of the talk groups. They must be "activated" to be heard. For example, the blue talk group may be a listen always talk group, referred to as "full-time." That means the repeater is always listening for and repeating transmissions for that talk group. But red may be PTT-activated, which means it is not active until someone first transmits to the repeater on the red talk group channel. This only matters on repeaters that are interconnected. If repeater A on the network that is utilizing the red talk group and is connected to repeater B where the red talk group has not been activated, someone listening to repeater B on the red talk group will not hear the conversation unless they transmit once to activate the red talk group. Then, they will hear the traffic on red. Repeater B will only listen to the red channel for so many minutes (typically 5 to 15) before deactivating the talk group from the last time someone keyed on the talk group from repeater B. This is called a "hold-off timer." This means that if you are listening only, after a few minutes, the connection will close. There are usually no announcements from the repeater that it has opened or closed a talk group. It just happens. Limiting talk groups like this helps to keep a repeater quiet where there may be no one interested in participating in the conversation on that talk group, or allow the repeater to be used with another talk group altogether. When using an analog repeater, all repeaters connected to the network are keyed. With DMR, only the ones with that talk group activated retransmit the traffic.
Then there is the time slot. DMR repeaters have two time slots called time slot 1 and time slot 2. The best way to think of this is just like a separate repeater. A DMR repeater can support two time slots allowing it to support a conversation on time slot 1 and time slot 2 at the same time without interference. So, if green was on time slot 1 and blue was on time slot 2, both talk groups could be active at the same time without interference. This is where the "digital magic" occurs, and you just have to "believe."
Talk Group Naming
There are standards for naming talk groups. Each talk group has a number and an alias (name). The number could be something like "1234". The alias could be "green." These names and numbers are determined by the groups that created the standard. There are a few standards that loosely follow each other, but not entirely. One standard is DMR-MARC, and another is BrandMeister, among others. With the DMR-MRC standard, the talk group availability and which time slot it is assigned to is determined by the repeater, and the computer systems it is connected to, called a c-bridge. That's why it is crucial to check Repeaterbook and the repeater's web page for available talk groups. You need to know the talk group number and time slot for each one. You also need the name of the talk group so that it is more meaningful to you when displayed on your radio's screen. BrandMeister repeaters are a bit more open, and typically you can access all BrandMeister talk groups (hundreds of them) on your whim. Sometimes repeater owners only want you to do that on one of the time slots and leave the other one for local traffic. Again, research is critical. BrandMeister repeaters do not use a c-bridge.
This is like using EchoLink in an analog repeater, which is a way to use the Internet (VOIP) to connect multiple repeaters. Repeaters may be connected locally to each other or across the globe, all determined by the limitations of the repeater owner, his equipment, and the network. BrandMeister and DMR-MARC repeaters cannot talk to each other except for common bridged talk groups. That is important to note when trying to make a cross-continent connection. The rules about the activation of the talk group mentioned also apply.
It can be challenging to establish these connections.
Many new people to DMR are interested in trying out the technology because of the interconnectivity. They may not be initially attracted to the digital quality of the communication for rag chewing because all of that is already available on analog repeaters without the complication. They want to be in Arizona and talk to their friend in Maine, for example. However, this is the more complex and frustrating part of the technology.
The repeater you are using must be on the same talk group standard as the repeater you want to context (i.e., DMR-MARC or BrandMeister). With the DMR-MARC standard, repeaters are limited to the number of talk groups they can support, so they typically choose some local talk groups along with their statewide talk groups, some bordering states, and maybe a regional talk group. Then they might throw in a national talk group or two. Rarely will a repeater on one side of the country carry the statewide talk group of a repeater in another distant state. So, your only option may be to try to contact your friend on a national talk group supported by both repeaters.
An example is talk group 310, known at Tac 310. This talk group is usually pretty busy, so repeater owners who do carry it make it PTT-activated only. You activate the talk group on your repeater, and your friend activates the repeater on his end. Then you wade through the log jam that comes with a nationwide talk group, and your conversation becomes less than intimate and less fun.
BrandMeister is a little more flexible as it is easier to dial-up a lesser-used talk group since BrandMeister repeaters don't have the talk group limitations that come with c-bridges. You both still need to activate the talk group on either end.
There is a whole other option in hotspots, which are like mini DMR nodes, typically used on the BrandMeister network. These micro nodes give you all the flexibility you want to call anyone else you want on the BrandMeister standard, and you can even choose which talk groups to monitor full time. You just can't use them very far away from your home. Some c-bridge operators on the DMR-MARC standard also provide gateways into their networks.
The final word here is about programming the DMR radio in the CPS. Where analog is easy and you program one line for each repeater, DMR typically requires programming multiple lines for each repeater. Each additional talk group you desire to use requires it's own line. You program into the line the repeater's input frequency, output frequency, and color code. On the same line, you select the talk group to use and the time slot to use it on. A lot of tedious research is required to get it right. Typically, when you select the talk group, you are selecting an alias talk group. In another section of the CPS, usually called "Contacts," you enter the talk group ID and the alias name. The radio needs to know the talk group ID (the numeric code) to transmit to tell the repeater's computer which talk group you are trying to access. You should program in the contacts list before entering the repeater or, the talk group won't be available in the drop-down list when creating the repeater entry.
Good luck with your endeavors. There are plenty of articles out there, some better than others. Hopefully. this one has helped to "flatten the curve" and introduce you to some of the basics of DMR.