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My aim with this short series of posts is to give an overview of DMR features, starting with those most important to Radio Amateurs. There is plenty of further information around the Internet for those who wish to dig deeper.

As mentioned in my previous post, DMR is complex because it was originally designed to cover the many and diverse requirements of businesses. Business radio users are not, in general, interested in playing with their radios but just in their effective use. Managers typically want to have to provide minimal training and to delegate the setting-up of the radios to specialists within the business or, in the case of small businesses, to their radio suppliers. Radio Amateurs are somewhat different. We have a much more hands-on approach to radio matters and look at the technology, and its use, from a different viewpoint. Furthermore, the business model built into DMR does not work well with us unless the whole world-wide community of Radio Amateurs is treated as a single “business” for DMR purposes. More on that in Part 2. Meanwhile, in Part 1, I’m covering just the basics of Channels:-

Channels: Frequencies

The division of the available frequency range into discrete frequency channels is already familiar to those using FM on the VHF and UHF bands. DMR follows this model of use. Communication may take place in simplex mode, normally on channels designated in our band plans, or through repeaters using different Rx and Tx frequencies. Nothing new to learn so far. What follows assumes repeater use because this is actually (in my view) easier to understand, when it comes to DMR, than the vagaries and nuances of simplex.

Channels: Time Slots

An analogue voice repeater provides a single channel with different Rx and Tx frequencies and a bandwidth of 12.5 kHz. A DMR repeater is able to split such a channel into two 6.25kHz sub-channels – thereby doubling repeater capacity – using time-division multiplexing. It switches, every 30ms, beween two “time slots” (TS1 and TS2) to do this. When setting up a radio, therefore, it is necessary to set up two* channels for the same frequencies, using the slot number to distinguish them, in order to make full use of the repeater. For practical purposes, these channels must be given different names. I just append the slot number to the repeater’s call sign, e.g.: GB7HD1 and GB7HD2.

Channels: Colour Codes

The information you use to set up an analogue repeater channel in your radio includes a CTCSS tone frequency, such as 77Hz, which you must transmit continuously to the repeater, as part of your signal, in order to communicate through it. DMR uses a so-called “colour code” instead, for the same purpose. This has nothing at all to do with colour; it is simply a 4-bit code passed betwen radios (including repeaters, of course) as part of the DMR protocol. All values (0..15) are valid, though 15 should not be used by us mere mortals because it is reserved for use, internally within the protocol, for synchronising timings between radios. If intending to use a particular DMR repeater, just put the supplied colour code number in your channel specifications for that repeater. The value 1 is, by convention, normally used for simplex channels but values 0..14 are available if there is some special need for them.

*Note

Each DMR channel specification also requires the inclusion of a default “contact” (aka “talk group”). Because of this, it is possible (and occasionally convenient) but never actually necessary to set up an arbitrary number of channels using the same frequencies but distinguished by slot and by talk group. Early DMR advice, on the Internet, often included setting up a separate channel specifcation for every combination of frequency, slot and talk group you were ever likely to use! That way madness lies, in my opinion, but there’ll be more about talk groups and related topics in Part 2.