The new DMR System...
The Radioddity GD-77 Dual Band Handheld The Hytera MD785 Mobile The Retevis RT3/TYT MD380
DMR (Digital mobile Radio) is actually nothing new and has been around for some time. It is however new to the amateur radio world. Originally sold as a commercial system by Motorola under the brand MotoTRBO it is a highly effective system with a long pedigree and good track record not just for voice but also data.
Interest in all things digital in the radio world has been spurred on by the interest and growth of Icom's D-Star and Yaesu's C4FM Fusion systems. However for many the extremely high cost, (£400+ for a handheld, £500+ for a mobile) even when imported direct, cannot be justified. This is where the DMR sets come into their own for around 1/3 of the price.
However they do work in rather different ways to the other main amateur radio systems, and this is where some confusion can set in... like anything new it can be complicated until you get the hang of it. Icom & Yaesu system is mainly a case of programming in your amateur callsign and the mode FM or a digital one (Auto C4FM, Narrow Digital [for voice and data], Wide digital [for voice only], D-Star etc. You also have the ability to input a frequency either in memory or to use a direct entry VFO. DMR is different - it has no VFO, all information for each frequency/mode must be entered into a channel memory - then crossed link with a common setup Zone. Each callsign must have a Radio ID which must be obtained from a central international registry. Like D-Star & Fusion, you can direct your call through a repeater network that is linked around the world to a specific location, or call set to set. The way this is done however is different with DMR in that it must be contained within the memory for each frequency, repeater or link. Again like other digital systems, voice transmissions are broken up into data packets of a fixed time duration and interleaved with header information to route the call to the appropriate destination. With DMR however, this is done by using TDMA.
Time-division multiple access (TDMA) is a channel access method for shared-medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots.[DMR uses 2 slots] The users transmit in rapid succession, one after the other, each using its own time slot. This allows multiple stations to share the same frequency channel while using only a part of the channel capacity. C4FM uses FDMA [Frequency-Division Multiple access]. see below. Each have their own advantages and drawbacks.
So which is best?
Thats a bit like asking which is better red wine or white wine. The answer is simply, its up to you.
Some people prefer red wine, some prefer white, some don't like either. In the case of Digital Radio a lot will depend on the depth of your pocket, the tolerance of your other half, your personal preference or what others are doing in your area. But for many it may well come down to cost. Can you justify spending ~£1000 for a mobile and handheld from the big 3 manufacturers (I, K & Y) or £300 - £400 for a DMR setup? (remember the days of VHS vs Betamax? Betamax was probably the better system but could not compete with the lower prices of VHS)