FM encoding (which stands for Frequency Modulation, just as it does with FM radio) is a method of encoding data bits for storage or transmission, also known as Differential Manchester encoding or biphase mark code (BMC). It is used in some formats of floppy disk and magnetic tape, as well as in networking protocols.
The essential characteristic is that bits are encoded by the change in signal from one part of the medium to another, rather than the absolute value of the signal at any point. A "1" bit might be encoded as a change from "on" to "off", or from "off" to "on", depending on where the signal was left at the end of the previous bit. A "0" bit would be encoded by a different set of signal changes, perhaps two in rapid succession leaving the signal at the same state it started at. This characteristic allows the content to remain unchanged when the signal is completely inverted.
Floppy disk formats using FM encoding
- Acorn single density 5 1/4" disk
- Atari 810
- DEC RX01
- IBM Type 1 (33FD)
- IBM Type 2 (43FD)
- IBM 23FD
- IBM 3740