DMR, D-STAR, C4FM (System Fusion) and NXDN: which digital standard to choose

Digital voice in amateur radio long ago outgrew any single standard. Today four main protocols coexist on the VHF/UHF bands: DMR, D-STAR, C4FM (System Fusion) and NXDN. All of them carry voice digitally, but they are built differently — and the choice of standard affects the price of the radio, the audio quality, network availability and your ability to talk to other operators. Let's look at each one on the merits.

Overall architecture: TDMA versus FDMA

The first fundamental difference is how the radio channel is used. DMR uses TDMA (Time Division Multiple Access): one channel of 12,5 кГц is split into two time slots. This lets two independent conversations run simultaneously on the same frequency — very valuable for repeaters and hotspots. The other three standards use FDMA (Frequency Division Multiple Access): each conversation occupies a whole separate channel.

• D-STAR works in a 6,25 кГц bandwidth — the narrowest of the four standards. This means less spectrum "occupancy," but also one conversation per channel.
• C4FM (System Fusion) uses 12,5 кГц FDMA. One channel — one conversation, but with a higher codec bitrate.
• NXDN is the most flexible in bandwidth: there are 6,25 кГц and 12,5 кГц variants, both FDMA.

The practical takeaway: a DMR repeater or hotspot can theoretically handle twice as many conversations on the same frequency. For understanding the basics of DMR this is one of the standard's key advantages.

Codecs: who speaks what

Voice in digital radio is compressed by a vocoder. Three of the four standards use AMBE-family designs from DVSI — AMBE (Advanced Multi-Band Excitation):

• DMR — AMBE+2 at 3,6 кбит/с (voice) + 0,85 кбит/с FEC per time slot. In total each slot takes 4,8 кбит/с out of the overall 9,6 кбит/с stream.
• C4FM (System Fusion) — the same AMBE+2, but in DN (Digital Narrow) mode at 4,4 кбит/с; in VW (Voice Wide) mode the bitrate rises to 9,6 кбит/с with higher audio quality.
• NXDN — also AMBE+2, developed jointly by Kenwood (the NEXEDGE brand) and Icom (the IDAS brand). Both manufacturers use the same over-the-air protocol, but their internet linking has historically differed — a converter-reflector is needed for them to work together.
• D-STAR — an older variant of AMBE (not AMBE+2) at 3,6 кбит/с. The protocol was developed by JARL (the Japan Amateur Radio League) and is promoted by Icom. The audio quality is subjectively a bit lower than AMBE+2, especially under interference.

D-STAR: the pioneer

D-STAR (Digital Smart Technologies for Amateur Radio) appeared back in the early 2000s — it is the oldest of the standards considered here. Its distinctive features:

• Data transmission (callsign, GPS coordinates) was built into the standard from the start — before it became fashionable.
• The global network of REF reflectors and cross-platform XLX nodes lets you connect with operators worldwide.
• A strong dependence on Icom: most D-STAR radios are the Icom IC-9700, IC-705, IC-7100 and the like; entry-level prices are noticeably higher than for DMR equivalents.
• Registering your callsign in the system is mandatory — without it, internet access is impossible.

D-STAR is still active, especially in Japan and Western Europe, but in Russia the repeater infrastructure is quite limited. For more on the repeater situation, see the article DMR repeaters in Russia.

C4FM (System Fusion): Yaesu in its element

Yaesu System Fusion appeared in 2013 as Yaesu's answer to Icom's D-STAR. Key traits:

• Automatic Mode Select (AMS): the radio detects whether an incoming signal is digital or analog and switches on its own — convenient for mixed networks.
• VW (Voice Wide) mode delivers the best audio quality of all four standards under good propagation conditions.
• The YSF (Yaesu System Fusion), FCS and Wires-X (Yaesu's proprietary internet linking) networks cover the whole world.
• Yaesu radios are traditionally noted for good audio ergonomics, but the ecosystem is Yaesu-only. You can't use a radio from another manufacturer in full C4FM mode.
• Through an MMDVM hotspot you can connect to YSF/FCS reflectors with any supported radio — which opens up extra possibilities.

NXDN: a narrowband niche standard

NXDN is a commercial standard from Kenwood and Icom for professional communications, adapted by radio amateurs. Its features:

• A very narrow bandwidth (6,25 кГц) — it uses the spectrum efficiently.
• Good audio quality and resistance to interference.
• In the amateur world it is used extremely rarely: few repeaters, a small community.
• The compatibility problem between Kenwood NEXEDGE and Icom IDAS for internet linking is solved through special converter-reflectors.
• NXDN radios do exist, but the selection is modest and the prices aren't budget-friendly.

For a beginning operator, NXDN is not the best choice: the network is small and finding a contact is hard. It is more of a professional tool that ended up in the hands of enthusiasts.

DMR: the mass-market choice with an open ecosystem

DMR (Digital Mobile Radio) is an ETSI standard developed for commercial communications, but long and thoroughly adopted by radio amateurs. Why did it become the most widespread?

• Radio prices: entry-level DMR radios (Baofeng RD-5R, TYT MD-380, some AnyTone models) cost several times less than comparable D-STAR or Fusion models. For more, see the review best DMR radios.
• A huge network: BrandMeister is the largest amateur DMR network in the world, with thousands of repeaters and thousands of talkgroups. There are also TGIF and other networks.
• Two time slots: double the efficiency of frequency use — important for repeaters and for static and dynamic talkgroups.
• DMR ID: each operator gets a unique numeric identifier (more on this: DMR ID registration), used to address calls and for private communication.
• An open ecosystem: MMDVM, Pi-Star, WPSD, DMRGateway — all of it works with DMR radios from many manufacturers.

A detailed comparison of DMR operating modes is in the article DMR: operating modes.

MMDVM: one hotspot — every standard

The MMDVM (Multi-Mode Digital Voice Modem) platform deserves a separate discussion. It is a hardware-and-software solution that lets a single hotspot work with all four standards: DMR, D-STAR, C4FM, NXDN and even P25. You can learn more about the platform in the article what is MMDVM.

Built on MMDVM are:

• The Pi-Star and WPSD software stacks for Raspberry Pi — the most common solutions.
• Hotspots based on ready-made boards (HS-HAT, Nano-HAT, ZUMspot and others).
• Branded solutions like the SharkRF openSPOT — pricier, but with a convenient web interface.

Crucially: switching between modes happens in the Pi-Star/WPSD settings, and physically the same hotspot runs DMR today and D-STAR or C4FM tomorrow. This makes MMDVM a universal tool for experimenting with different standards. The DroidStar app for Android lets you connect to most digital networks straight from your smartphone over the internet, with no radio at all.

Comparison table: four standards at a glance

• DMR: TDMA, 12,5 кГц, AMBE+2, 2 slots, BrandMeister/TGIF/your own networks, radios from budget level up — the best choice to start with.
• D-STAR: FDMA, 6,25 кГц, AMBE (old), 1 channel, REF/XLX, radios predominantly Icom — a good ecosystem, more expensive.
• C4FM (Fusion): FDMA, 12,5 кГц, AMBE+2, 1 channel, YSF/FCS/Wires-X, Yaesu only — excellent audio quality, closed ecosystem.
• NXDN: FDMA, 6,25–12,5 кГц, AMBE+2, 1 channel, niche networks — a professional standard, little present in the amateur world.

What is more popular worldwide and in Russia

On a global scale, DMR is the steady leader by number of active operators, repeaters and hotspots. C4FM holds a firm second place — especially in Japan and North America, where Yaesu is traditionally strong. D-STAR retains a loyal audience but shows no growth. NXDN remains a niche.

In Russia the picture is similar: DMR repeaters and DMR hotspots significantly outnumber the D-STAR and C4FM infrastructure. Finding a contact through BrandMeister or through your own network is substantially easier than through a D-STAR reflector. Questions of frequencies and licensing in Russia are covered in detail in the article frequencies and the law.

Bottom line: which standard to choose

If you are just starting your journey into digital radio, DMR gives the best ratio of price/capabilities/audience. Affordable radios, the largest worldwide networks, a rich open ecosystem and the ability to use an MMDVM hotspot make it the practical number-one choice.

C4FM is worth considering if you are already in the Yaesu ecosystem and value maximum audio quality. D-STAR is appealing for those who work with Icom and want data embedded in the air. NXDN is a highly specialized choice for professionals or collectors of rarities.

And if you want to try all the standards without buying several radios — one MMDVM hotspot + the DroidStar program open access to DMR, D-STAR, C4FM and NXDN at once. For more on choosing a hotspot, see the article MMDVM hotspot.