C4FM
C4FM is a 4-carrier modulation format where the carrier is shifted in frequency at a particular rate (time) to a particular location around a center frequency. This allows for each of the 4 "states" to represent a binary number. Each state is a "DiBit" or "Symbol" which contains two bits of information.

Each of the carriers depicts a particular symbol. In reality, the carriers would not be seen as individual, stand -alone carriers shifted off center from the fundamental frequency. Although each carrier has a fixed offset frequency, the carrier never returns to center frequency. Each transition of the carrier is encoded to start from where it was last positioned.