It’s not always obvious when you get into DMR exactly what makes DMR so ‘special’ and why everyone is jumping on the bandwagon, compared to most other types of digital voice modes, one of the coolest things about DMR over most other forms of digital voice modes is TDMA. that’s the secret sauce.
Conventional FM repeaters, along with older FDMA types work very well indeed, but more can be done, especially nowadays. On an FM transmitter, as soon as you transmit, your transmitters PA (RF power Amp) shoots right over to solidly pumping out 100% constant carrier, this is simple to understand, as indeed is a FM repeater’s basic operation, and we’re all used to and happy with that.
However, one conversation on a FM (or FDMA) repeater generally means that you have to wait until the repeater is free before you can use it of course, so if your message is important, you’re stuffed, unless you have another repeater to make the call on.
Imagine though, a repeater which can carry 2 separate conversations at the same time using the same two frequencies an FM repeater would use ? How ?
The answer, is TDMA, Time Division Multiple Access, it has been around years, longer than you might think, but it’s basically a channel access method – a way of data streams sharing the same channel, (because *everything* is data nowadays) and there are different types of channel access too, but as we are discussing DMR really, we’ll stick to that for now. Most of us, back in the 1990s will likely be familiar of the annoying and somewhat unpleasant ‘buzz’ or interference when your old cell phone was near an amplifier or stereo (remember those?) and the rat-a-tat buzz from the 2G Nokia cell phone you had.. that was a TDMA transmission.
Very basically, and because it’s a little abstract, i’ll attempt to give a ‘duffers’ way of imagining it. I know very little, but there seems to be very few write-ups anywhere on the web about it, particularly for hobbyists or folks who aren’t computer whizz kids, there are many DMR websites and discussions on forums and Facebook, but many others, though thorough and correct, require the imagination the size of a planet and a leap of faith to match.
Here goes, if you were to chop your FM transmitter on and off very quickly indeed, you could also imagine that in between your brief TX bursts, and the snapping of your PTT, that someone else, if just as quick, could send their quick TX bursts out too, on the same frequency in-between yours, no-one would be harmed, but you couldn’t speak fast enough to get your voice over could you ? and neither could the station that you’re trying to talk to.
Now, digital radio takes all your TX audio, throws away what it thinks is unneeded, and compresses the rest.
Now your voice is digital and in squeezed into a tiny IC in your radio, we can do fancy things with it, we can, for instance, break it up into blocks and transmit these blocks in, er, blocks, or packets, – the quick pulses we mentioned earlier, but faster and electronicaly timed and controlled.
The receiver collects these ‘blocks’ and re-constitutes them, back into a whole lump, like adding water to a 1970s Vesta Chow Mein and you’ve got your audio back at the receiver, albeit after a lot of processing, and of course, you can do this through a repeater or simplex. You can even have two concurrent QSOs on simplex, on the same frequency!
A basic one way QSO through a DMR repeater is pretty simple to envisage,
here’s the first of my (sad and tragic) attempts at basic animation in The Gimp, a open source, free software graphics program, (For Windows, Mac & Linux) which i only found out did animation a couple of hours ago, so be kind to my first attempts !
TDMA – the Secret sauce
As you can see above, transmitting one DMR transmission over a DMR repeater all seems to make some kind of sense, you can see the packets arriving at the repeater, and the repeater transmitting them on.
The interesting bit about DMR is whilst they’re nattering on there, we can start up another QSO on the same repeater and the other QSO will be totally unaware, so, if there’s just one QSO on the DMR repeater, it will be sat idle a good proportion of the time, yet still continue to relay a full QSO on one of the slots, but there’s another slot we can play with, lets look at that.
Slots o’ fun
Above, on grubby animation number two, is what the repeater is seeing when someone else calls through the repeater, hugely slowed down, as for a short time it listens in the (lets call it a ‘gap’) and if it hears any valid DMR on the input at that time, it calls that slot 1, then automatically and without waiting, listens again on the same frequency in the other ‘gap’ or slot, and if a valid chunk of tasty DMR comes in at the expected slot 2 time, well, we will call that slot 2 then, and so it goes, back and forth, many times a second, constantly, until both transmissions stop and it can rest.
So, to recap, there are now two simultaneous QSOs going on in the repeater, and as we can see, as the repeater is easily hearing both transmissions, the DMR repeater is also sending back control information to your radio, stuff like telling both transmitters when to TX their blocks of data back, all DMR receivers expect to hear packets or these blocks of data, so everybody is happy. Win, Win.
It’s a busy, busy busy job …
Above, in crap animation number three, (I’ll stop soon, i promise) – is a very poor attempt to envisage a DMR repeater in the midst of, er, repeating. Two QSOs are taking place on the one repeater at the same time, neither QSO knows or needs to know about the other QSO,
QSO one (slot 1) is blue, and QSO two (slot 2) is green, neither know or can hear, (or join in with) the other QSO, they’re both completely independent, try doing that without TDMA.
In the crude graphic, massivley slowed down, the repeater is flashing the colour of the QSO it is currently TXing, it really onl haas one transmitter and one receiver really, but as you can see, it switches between both ‘slots’ automatically and very fast indeed, with the effect that the end user can’t tell. congratulations, because instead of having to buy another repeater, you’ve just got a free one by using DMR.
There are more DMR benefits too, stuff that the FM and FDMA stuff can only dream about,Myself, i like the old P25 phase 1 digital, a mostly American emergency service digital system, but it’s old and FDMA, not TDMA and and has only half the capacity of a these two slot TDMA system, bit TDMA is so important in comms, that P25 ‘phase 2’ is now TDMA, because it makes NO SENSE to invest in a poor propriety digital amateur only mode, or really any FDMA nowadays, unless it’s foe low end or retro fun, it’s a technological dead end street now, technology has moved on, some companies thought they’d lock users into substandard and propriety modes and dropped the ball completely or refused to update, as their cash cow was still bringing in the $$$ whilst other, superior standard, industry wide modes, well supported and still in active development continue to offer future development, rather than just at the whim of some proprietary bean counting exec.. but i digress..
One feature we enjoyed when we started playing with DMR, was to de-key each other .. the locals here are an often cruel and cheeky bunch who used to delight in, lets say, if someone was transmitting for a really long time, and listeners were starting to doze off or get bored, we send a DMR command from our rigs to the Guy who is still in waffle mode – still in TX -and instantly setting the (surprised) waffling station’s rig back into receive mode – whilst they still had their finger on their PTT ! for a while it was a challenge to not be boring or waffle too much, lest you would be remotely de-keyed in mid sentance !
Such is the fun we had, but alas the tumbleweeds have taken over the hobby and i have to get my kicks by writing articles on MBARS 🙂
“Talkgroups what the bloody hell are they then”