Updated 02/05/24 by KD2ZWN

C4FM, which stands for Compatible 4-level Frequency Modulation, is a digital modulation technique used to transmit digital data using 4 frequencies, which can be thought of as tones, where each "tone" represents a binary pair of either 00, 01, 10, and 11. The total bandwith these 4 tones occupy is 12.5 kHz, while they transmit at 9600 bits/second. When used for voice communications, this is both more narrowband than traditional analog FM signals, which occupy 16-18 kHz, but also allow for transmitting extra data outside of the voice signal, such as a call sign or location coordinates. Though C4FM is an open modulation technique that anyone can use, most Hams will come in contact with it in the context of System Fusion, which is an implementation of the C4FM technique to send digital voice and data packets over ham radio frequencies. System Fusion enabled sites are commonly networked together using Wires-X, which is a type of worldwide linked amateur radio system that uses CF4M as the primary digital modulation technique for the local connection.

Wires X, which stands for Wide-Coverage Internet Repeater Enhancement System, allows anyone with a radio that supports C4FM or analog FM modulation that is in proximity to a Wires-X "node" to connect to the Wires-X network. A Wires-X node is simply any transciever that has been registered and linked to the Wires-X network using a local internet connection. The Wires-X network is composed of "rooms, which are individual talkgroups owned and administered by Hams, which allow for those connected to these rooms to both talk and exchange data.This system allows for repeaters and local stations to be interlinked using the unlimited range of the internet.