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Modified frequency modulation (MFM) is a run-length limited (RLL) line code used to encode data on most floppy disks and some hard disk drives. It was first introduced on hard disks in 1970 with the IBM 3330 and then in floppy disk drives beginning with the IBM 53FD in 1976. 
MFM is a modification to the original frequency modulation encoding (FM) code specifically for use with magnetic storage. MFM allowed devices to double the speed data was written to the media as the code guaranteed only one polarity change per encoded data bit. For this reason, MFM disks are typically known as "double density" while the earlier FM became known as "single density".
MFM is used with a data rate of 250–500 kbit/s (500–1000 kbit/s encoded) on industry-standard 5+1⁄4-inch and 3+1⁄2-inch ordinary and high-density floppy diskettes. MFM was also used in early hard disk designs, before the advent of more efficient types of RLL codes. Outside of niche applications, MFM encoding is obsolete in magnetic recording.
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