The situation with digital amateur radio

From the 1950s to well into the 1990s, voice communication on VHF and UHF was mostly narrowband FM. Amateur radio was no exception. Even the early mobile telephone networks were FM and hence they could be overheard with an ordinary radio scanner. Aviation was the only exception to the FM rule. They use AM instead. AM was chosen for two reasons:

• The VHF air band was established in the early 1950s and it was truly worldwide. At that time, AM was the most widely known voice communication mode.
• When multiple planes transmit on the same frequency, the other signals can at least be noticed. For a safety critical application such as air traffic control, this offers a crucial advantage. With FM, you often hear just the strongest signal without even noticing there is another one trying to get heard. Not to mention digital modes.

As it is a truly worldwide band, it is extremely hard to change the way it is used, so it will remain AM for the foreseeable future.

Narrowband FM was the mode of choice for VHF and UHF mobile and portable radio, for police, fire brigades, ambulances, taxis and other enterprises. And so it was for radio amateurs on the 2 meter VHF band and the 70 cm UHF band.

Digital Voice Communication

Digital voice communication has several advantages over analogue FM.

• It uses less bandwidth than narrowband FM.
• Speech can be relayed through a large network without loss of quality at each link.
• Metadata (like transmitter ID) can easily be added. So a dispatcher can see immediately which vehicle is talking right now. Other data (like text messages) can easily be added too.
• Encryption can easily be added. You don’t want your communications to be overheard by scanners? Encryption get it done.

Some disadvantages:

• It’s all or nothing: you get heard with perfect clarity or not at all. No warning that you get out of range by getting a noisy signal. No hint that somebody is calling for help, even if you can’t copy the signal.
• This is not a disadvantage of digital communications per se, but of trunking networks that are often used with it: congestion when there is a distress situation. This congestion gets worse if users of the network are not well trained to cope with it.
• Cost of equipment. For mass produced items like mobile phones. cost will come down though. But any digital amateur sets are still more than three times as expensive as analogue FM sets.

Mobile phones were the first to go digital during the 1990s. Many police departments went digital in the early 2000s.

Digital Amateur Communications

One of the purposes of amateur radio is to experiment with new technologies. So even if there is no direct advantage of digital communications in day to day use, the challenge of setting up a digital network is reason enough to try it.

Currently three digital voice modes are in use for amateur radio:

• D-star was developed in the early 2000s. It was specifically developed for amateur radio.
• System Fusion (C4FM) was developed by Yaesu around 2013.
• DMR (Digital Mobile Radio) was developed in the early 2000s in Europe for professional users, but it can also be used for amateur radio.

All these systems allow you to relay voice signals over large networks, all allow you transmit data files and all allow you to fall back to analogue FM if you need to communicate with users that do not use “your” digital system.

At least when D-star was introduced, the audio codec AMBE was protected by trade secrets. So you were fundamentally not allowed to know how the PCM audio signal was converted to a lower rate bitstream and back again. This aspect is very much against the spirit of amateur radio. Everything else of the D-star protocol was open though. There existed “black box” chips containing the AMBE codec, that you could use in your own equipment. At present we do have good open-source audio codecs and even AMBE itself has been reverse engineered, so you can make your own implementation. C4FM and DMR also use AMBE as their voice codec.

From an experimentation standpoint it might be a good thing that there are three different digital voice standards for amateur radio. but it’s a bad thing in every other respect.

• Two of the digital systems are tied to a particular brand in practice. With Yaesu you get C4FM, with ICOM (or Kenwood) you get D-star. DMR is mostly used by Chinese brands that are often not designed specifically for amateur use. The brand of equipment you choose and the digital system you prefer are now linked. There are no multi-standard digital amateur sets and even if they existed, they might be too hard to configure.
• Any digital amateur repeater excludes the users of the other digital systems. It’s often paid for by all members of a national, regional or local radio amateur association. The number of repeaters is often constrained, so we won’t have repeaters for all three digital systems in the same location. Currently there are amateur repeaters that support two or even three of these standards, so there is hope for the future. But users of different digital modes cannot hear each other when using the repeater.
• Amateur radio is not exactly a growing market. Anything that constrains the number of people you can talk to, is a bad thing. You can still communicate with all other radio amateurs using analogue FM, but why buy a digital set? As soon as you use it in digital mode, the ones in the other camps can’t hear you any more.

One problem with digital amateur equipment is the complexity of programming. With analogue FM you just need to dial in the frequency and hit the PTT on your mike. For a repeater, you need to do a bit more work, such as selecting the correct repeater shift and CTCSS tone. But this can still be done on the set itself, without the need to use a computer. Enter the world of DMR and you have to set a lot more parameters. Plus you need to register with the operators of the networks, so your set can be linked to your call sign. For all practical purposes you need a computer to program your set to use with the repeaters available in your region. The required data files are available for many sets and these are called code plugs.

The situation with different digital systems for amateur radio is worse than that of the different VCR systems in the early 1980s, because a radio transceiver is useless without another compatible transceiver in the hands of a different radio amateur. At least with a VCR you could record your own TV programmes on your own tapes, without caring what systems your friends used. You couldn’t share tapes, but that was illegal anyway. Your video rental store might not have a good selection of tapes for your system and that sucked.

The future?

How will this develop for amateur radio? In some cases, one system gets to dominate the market with the others dying off slowly but steadily. We saw this with VCRs, where VHS got the dominating position. Sometimes, two standards are adopted by all equipment. We saw this with vinyl records. In the late 1940s, RCA had a 7” 45 RPM record and Columbia had a 12” $33 {1 \over 3}$ RPM record. Both systems continued to exist and before long, all new turntables had both speeds.

Multi-mode digital repeaters are already a thing, so maybe we will see multi-mode sets too? Multi-mode reception could be a good idea to add to an amateur transceiver. A transceiver supports one mode for transmission: either C4FM or DMR or D-star, but it can receive and decode all three modes (that it auto-detects). A multi-mode repeater auto-detects it in exactly the same way. At a considerable cost in equipment complexity, we would finally achieve interoperability for digital amateur radio transceivers. But this would only be effective if all sets have multi-mode reception. So it may be better to have multi-mode for both transmit and receive. As long as at least one party has a multi-mode transceiver, communication is possible with the user of any digital transceiver. These multi-mode transceivers would then require programming for all three systems, which just requires bigger “code plugs”. When essential patents expire, we could develop such sets as open source projects. And that’s fully aligned with the spirit of ham radio.

Analogue FM will be used by amateurs for a long time to come, for the following reasons:

• The fragmentation of the digital voice systems for amateur radio
• Simplicity of operation and programming the equipment
• Availability of cheap FM-only transceivers.
• Getting some signal through under marginal conditions.