Modified Frequency Modulation, commonly MFM, is a run-length limited coding scheme used to encode the actual data-bits on most floppy disks. It was first introduced in disk drives with the IBM 3330 hard disk drive in 1970. Floppy disk drive hardware examples include Amiga, most CP/M machines as well as IBM PC compatibles.
MFM is a modification to the original FM (frequency modulation) scheme for encoding data on single-density floppy disks and some early hard disk drives. Due to the minimum spacing between flux transitions that is a property of the disk and head design, MFM, which guarantees at most one flux transition per data bit, can be written at higher density than FM, which can require two transitions per data bit. It is used with a data rate of 250 – 500 kbit/s (500 – 1000 kbit/s encoded) 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 (RLL) coding. Except for the steadily disappearing 880 kiB and 1.4 MiB floppy disk formats, MFM encoding is obsolete.
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