Analog, D-Star, P25, Fusion, DMR. Why do we need another digital mode for UHF and VHF communications?
When the PAPA System started talking about DMR I was very much in the mode if being happy with Analog and D-Star in my stable. Why would I want to spend a bunch of money on another digital mode?
My initial reaction was to compare and contrast DMR with D-Star. You can do this, but it’s kind of like comparing D-Star with analog…it’s similar, but it’s different.
There are things I like about all three modes. DMR does offer a few stand-out features.
First, and I cannot stress this enough, DMR is an open standard that has been adopted by lots of manufacturers. Unlike D-Star, you can buy DMR radios from at least ten different manufacturers, starting at about $1o0.
As good as D-Star audio sounds, it does not degrade well. When the signal gets weak, the audio gets garbled. There is no error correction, when the signal is marginal you end up with random garbage going into the decoder and garbage coming back out. This is referred to as “R2-D2” as it sounds much like the Star Wars robot.
DMR has forward error correction. This means you can drop about 15% of the data before you start having problems decoding. As soon as you move beyond the algorithm’s capabilities audio is lost. So you go from great audio to nothing.
From experiments we’ve done in the club, we can carry a simplex signal further on DMR than we can on Analog. This has to do with how the signal is modulated and the error correction.
DMR is a TDMA, or Time Division Multiple Access. Two full conversations can be carried on a channel/repeater at the same time. They will not interfere with each other in any way. This doubles the capacity of each channel. You will here people talk about Time Slot 1 and 2. You select a time slot indirectly by selecting a talk group. In the amateur world talk groups are defined primarily in regions. In the corporate world they might be along department lines, something like security, maintenance, etc.
Here in California we have a statewide talk group, Southern California, Central California, Northern California, Southwest, and many others. There are also nationwide talk groups. Here is the talkgroup matrix we use for PAPA codeplugs: http://www.papasys.com//dmr/TG-TS/
I like that I have a conversation local to a given repeater, or expand to a wider region. I select talk groups based on who I’m trying to reach and the distance between us.
There is also a way to go onto a talk group that covers a large area without keying up all the repeaters in that region. This is called a Dynamic talk group. It will only be on the two repeaters being used for the conversation.
For people that travel in a region DMR offers a feature that is unique…roaming. Roaming allows me to select a talk group, say SoCal. The radio will listen for beacons from each of the repeaters that offer the SoCal talk group, the radio will tune itself to the repeater that offers the best signal. When I drive from Orange County to San Diego this is really convenient. I don’t have to remember to change repeaters as I drive. Roaming is only offered on Motorola and Hytera radios at this time. This one feature makes the more expensive radios important to many DMR users.
Most of the DMR radios also support analog FM, so you can use them with your analog repeaters.
Now, as with anything, there are some challenges in DMR.
Most of the radios do not allow front panel programming. This means you are required to use software, and sometimes an expensive cable, to program the radio.
You no longer just put in a channel. Each repeater will carry between four and fifteen talk groups. To add a repeater, you setup a memory channel for each talk group. You also setup scanning and roaming. Adding a repeater can take about 20 minutes in front of the software.
If a repeater frequency is changed, you have to make that change for each talk group on that repeater.
You really have to plan ahead. If you are going into a new area you have to program your radio before you go. You can’t just flip into VFO and get it working.
All of the radios are single band. You buy them as a VHF or UHF radio. Here in Southern California that isn’t a problem as we only have UHF repeaters.
What are your thoughts on DMR? More articles coming soon.