System Fusion is Yaesu's digital voice system. Its radio signal uses C4FM, a four-level form of frequency modulation, and it is designed to live alongside ordinary FM: Fusion radios and repeaters can switch between analog FM and digital automatically.
Four levels, two bits each
A plain FSK signal jumps between two frequencies, so each symbol carries one bit. C4FM uses four, so each symbol carries two bits: twice the data at the same symbol rate, in the same narrow channel.
Idealised. Real transmitters smooth the steps to keep the signal narrow. Four-level FSK is also written 4FSK; C4FM is a constant-envelope form of it.
The same trick, four-level FSK, appears in other modes too; see P25 and NXDN and M17. The audio chain is the usual digital voice one:
The vocoder is the heart of digital voice. Different systems use different vocoders and different ways of sending the bits.
Two flavours of digital voice
Fusion radios offer a choice of how to spend the available bits. Two of the digital settings, in Yaesu's terms:
Setting
Idea
V/D (voice and data)
Voice shares the bits with data such as GPS position.
Voice FR (full rate)
Most of the bits carry voice, which suits good audio on a strong signal, with little room for data.
Features worth knowing
Automatic mode select (AMS) lets the radio choose FM or digital to match what it hears, so you can use one memory for either.
Group monitor shows which Fusion stations are nearby and active.
WIRES-X links repeaters and hotspots into "rooms" over the internet. You pick a room from the radio, much like selecting a talkgroup. See EchoLink, IRLP and AllStar.