How a C4FM Repeater Works

C4FM is the digital voice format at the heart of Yaesu’s System Fusion, and a Fusion repeater has a personality all its own. It carries one conversation per channel like D-STAR, but it does something neither DMR nor D-STAR can: on a single frequency it will accept an old analog FM handheld and a modern digital radio in turn, sorting them out automatically. Add Yaesu’s own internet system on top — Wires-X rooms — plus the open community network that grew up beside it, and you have a lot of moving parts. Here is how the machine and its linking work.

It starts as an ordinary repeater

A Fusion repeater listens on an input frequency and retransmits on an output frequency, the same offset idea you know from FM. Underneath, the digital signal is C4FM — four-level frequency-shift keying, running about 9600 bit/s in a 12.5 kHz channel — and the vocoder is AMBE+2, the same codec family DMR uses. Access is FDMA: one conversation per channel at a time, with no DMR-style timeslots.

One mode selector, several personalities

What makes C4FM flexible is that the same radio can transmit in several different ways, and a Fusion radio lets you choose — or lets the repeater choose for you. The everyday choices are:

Each bar is one 12.5 kHz channel — same radio, different split. DN voice + data protected voice data VW voice wide full-rate voice — best audio, no simultaneous data FM analog ordinary FM — legacy radios still welcome

DN carries voice and data together; VW spends the whole channel on voice; analog FM is still on the menu.

AMS: the repeater that speaks both languages

Here is Fusion’s signature trick, and the reason so many clubs chose it. Automatic Mode Select (AMS) lets the repeater examine each incoming transmission, recognize whether it’s analog FM, DN, or VW, and answer back in the matching format — transmission by transmission, with nobody touching a switch.

Why AMS matters

A single AMS repeater can serve a club mid-migration: the member with a decade-old analog HT and the member with a brand-new Fusion radio share the very same machine and frequency, each heard correctly. No other common ham digital mode blends analog and digital on one channel this gracefully — it’s what let Fusion repeaters replace analog machines without stranding anyone.

Addressing: DG-ID, DP-ID, and Group Monitor

Older Fusion gear kept groups apart with a digital squelch code; current radios use the cleaner DG-ID system — a Digital Group ID from 00 to 99. Set a transmit and receive DG-ID and you’ve chosen a lane: matching IDs hear each other, and DG-ID 00 is the open setting that hears everything. On many repeaters the DG-ID also selects which Wires-X room or node you’re steering, so the same knob that groups local users also drives the internet link.

Two companion features round it out. DP-ID (Digital Personal ID) pairs individual radios to each other, and GM (Group Monitor) uses it to keep automatic track of which of your group’s members are in range, quietly exchanging their positions and status while you operate.

Linking out: Wires-X rooms — and the open network beside it

On its own, a Fusion repeater is local. To reach the world it needs an internet node, and here Fusion has two parallel worlds.

Yaesu’s native system is Wires-X. A repeater or personal node connects through an HRI-200 interface (or a software equivalent) to a PC and onto the Wires-X network, where the meeting places are called Rooms and each access point is a Node. The elegant part is control from the radio: press the Wires-X button and you can search for a room by name or number and connect to it right from the front panel — no computer needed at your end.

Beside Wires-X, the community built an open alternative: the YSF reflector network (along with FCS and YCS), typically reached through Pi-Star or MMDVM-based nodes. It plays much the same role as Wires-X rooms but is open-source and hotspot-friendly, and it’s how a great deal of Fusion traffic actually moves today.

Solid = C4FM RF over the air   ·   dashed = internet Your HT DN / VW / FM Repeater AMS · DG-ID auto-detects mode Node HRI-200 / Pi-Star Wires-X Rooms Yaesu native network YSF / FCS / YCS open community network RF One machine, two networks — the node points at Wires-X rooms, the open reflectors, or both.

An AMS repeater’s node can face Yaesu’s Wires-X rooms, the open YSF/FCS/YCS reflectors, or both.

Follow a transmission through

Say you’re on a Fusion repeater in DN mode, its node linked to a Wires-X room:

  1. Your radio — the AMBE+2 vocoder encodes your voice as C4FM in DN mode, with your callsign, DG-ID, and GPS riding in the data half of the channel.
  2. Uplink & AMS — the repeater receives on its input frequency, and AMS recognizes the signal as digital DN and answers in kind. Being FDMA, it’s handling one conversation at a time.
  3. To the node — if the DG-ID and configuration call for a network link, the repeater passes your frames to its Wires-X (or Pi-Star) node.
  4. Out to the room — the node sends your frames to the connected Wires-X room (or YSF reflector), which copies them to every other node joined there, near or far.
  5. Downlink — each destination repeater transmits your frames on its own output frequency, and every Fusion radio in range decodes them back into your voice.

The one-sentence version

A C4FM repeater is an FDMA machine that uses Automatic Mode Select to serve analog and digital users on one frequency, groups them with DG-IDs, and reaches the world through a node aimed at Yaesu’s Wires-X rooms or the open YSF network.

That covers the machine and its linking. For the mode itself — how C4FM encodes and how it compares to the others — see How C4FM (Fusion) Works; for the open reflector side, Understanding YSF, YCS, and FCS Reflectors; and for the backstory, History of C4FM and Yaesu System Fusion.

A noncommercial hobby reference compiled by N6JET, gathered from public sources and shared freely for anyone interested in amateur digital voice.