Tuesday, June 12, 2012


Bob Witte, K0NR wrote an interesting column for CQ-VHF's Spring 2012 magazine issue.

It's titled "TRBO Hits the Amateur Bands."

Its about Digital Mobile Radio (DMR) sometimes called MOTOTRBO.

He points out that its a new digital standard that is gaining traction on the VHF and UHF amateur radio bands. He reports there are over 90 DMR repeaters up and running in the U.S with more planned.

DMR originated as a European Telecommunications Standards Institute (ETSI) standard.

The DMR Association is the industry body promoting adoption of the standard and includes these companies as members: Harris, Hytcra, Icom, JVC, Kenwood, Motorola, Tait Communications, Vertex Standard, and Zetron.

DMR takes a novel approach to spectral efficiency. The bandwidth of the radio signal is nominally 12.5 kHz with two signals sharing the channel via Time Division Multiple Access (TDMA). Simply put, two mobiles working through a DMR repeater share the channel by cycling their transmitters on and off in a synchronized manner. This is similar to how some cellular-phone systems handle multiple phones operating on the same channel. The cellular-phone base station controls the synchronization of the various phones so they do not interfere with each other. Similarly, with DMR the repeater has to synchronize the two mobiles using the same 12.5-kHz wide channel.

"A DMR installation looks a lot like a normal repeater system but with the benefit of two channels built in."

For those totally unfamiliar with TDMA, I recommending checking out my older blog titled "Understanding Digital," where I reference some Hak5 videos on Pulse Code Modulation. And the following week, Time Division Multiplexing (TDMA).

These sort videos give a good tutorial on how this works.

But if you really need the nitty-gritty, the TDMA time slots are 30 msec in duration.
In the 30 msec slot, the transmitter is required to ramp up to full power in 1.5 msec, send data for 27 msec, then power down in 1.5 msec.

For further good reading and information, I suggest reading K0NR's article. I believe this is the first printed ham article on DMR.