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Radio Systems – Digital vs Analogue

Digital communication systems need to carry a number of different signals. Voice being the obvious one, but alongside will be data in the form of perhaps text, plus other information that identifies the radio, and it’s settings. As a result, the various systems have been designed to maximise the limited bandwidth available in the radio systems allocated by each country.

Two main types of digital protocols were developed.

Time Division Multiple Access (TDMA) and

Frequency Division Multiple Access (FDMA)

Both systems offer different ways to carry more information in a standard communications channel. In this way, frequency allocations can remain the same, with users simply making the switch to digital fairly simply.

In TDMA systems, the data is compressed in time, and two separate channels can operate sequentially. Usually this is two time slots. A radio can operate on slot one, and it will transmit in short bursts, leaving slot two empty. Another radio can operate in this time slot, making multiple access possible.

In FDMA systems, the two data channels are split in frequency – each channel separated from the nominal centre frequency. These both operate continuously.

The two systems broadly offer similar data capacity, but each has some technical hurdles that must be considered.

FDMA is continuous streams of data on different frequencies, TDMA is data in packets on the same frequency.

In contemporary radio communications systems, both FDMA and TDMA is in use, and usually are described as DMR – Digital Mobile Radio, which is the TDMA system, and dPMR – Digital Private Mobile Radio, which is the FDMA system.

If any research is carried out all the above information is easily found, but much of the available comment really only relates to large multi-user systems. TDMA is often recommended as the ideal digital system, and for these large systems, and amateur radio users with thousands of users around the world, perhaps it is? For smaller users the situation is much less clear.

Scenario 1 – the Sports Shop.

Consider a shop selling many goods that need fetching from a stock room. Assume there are two sales assistants dealing with customers and two working in an upstairs stock room. In the shop, the assistant calls the stock room to request a certain type of shoe, in a particular size. Upstairs they find it, and either bring it down, or call the assistant back to say they are out of stock.

Off-the-shelf PMR446 radios would work perfectly well – apart from that tricky set of shelving near the water tank, where reception is poor. Messages from downstairs in this location may be hissy, garbled or incomplete. The worker in the store room may have to walk just a few metres away to get proper reception again. The system copes. Perhaps business increases, and it becomes annoying or distracting for the assistant in the men’s area to hear the messages between the shop floor ladies area and the ladies shoe area in the stock room? A simple solution (CTCSS or DCS) exists, even on cheap PMR446 systems where each user has a different tone setting, even if they are on the same working frequency. Tone 1 is used by the men’s department and tone 2 by the women’s department.

Imagine what happens when a move to digital is made. The trouble spot near the water tank now becomes a bigger problem. Digital systems tend to either work, or not work. Noisy and weak signals are not heard at all. In the problem area near the water tank the best that can happen is a signal that chops in and out, or worse, simply not heard at all. Digital is not always better.

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