Modified Frequency Modulation, commonly MFM, is a line code used by most floppy disk formats, notably by most CP/M operating system machines, as well as IBM PC compatibles running DOS.

MFM is a modification to the original FM (frequency modulation) scheme for encoding data on single-density floppy disks. MFM allows more than one symbol per flux transition — up to three — giving greater density of data than FM. It is used with a data rate of 250-500 bits per second (500-1000kbit/s raw MFM) on industry standard 5¼ and 3½ ordinary and high density diskettes. MFM was also used in early hard disk designs, before the advent of more efficient types of Run Length Limited coding. Now, at the turn of the millennium, however, except for the steadily disappearing 1.44 MB floppy disk drives, MFM encoding is largely obsolete.

= Coding =

Notice that two ones can t appear together, and the maximum number of zeros in a row is three. This bit stream is the NRZI encoded to be written to disk, a 1 bit representing a magnetic transition, and a 0 bit no transition.

Example:

1 0 1 0 0 0 0 1 ($A1) 0100010010101001 ($44A9) 0100010010001001 ($4489 - sync mark) ^ missing clock

Hexadecimal 4489 is typically used as a unique synchronization mark.

= See also =

*Group Code Recording