Modified Frequency Modulation, commonly MFM, is the magnetic data recording scheme used by most floppy disk formats, notably by most CP/M machines, as well as PCss running MS-DOS.

MFM is a modification to the original FM (frequency modulation) scheme for encoding data on single-density floppy disks. MFM allows more than 1 symbol per flux transition — up to 3 — giving greater density of data. It is used with a data rate of between 250-500 kbit/s on industry standard 3½" and 5¼" low and high density diskettes.

MFM was also used in early hard disk designs, before the advent of Run Length Limited (RLL) encoding. However, except for 1.44 MB floppy disks, this encoding is now obsolete.

See also: Group Code Recording