Modified frequency modulation
encoding in communication technology
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 at which 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 Outside of niche applications, MFM encoding is obsolete in magnetic recording.
Magnetic storage
Magnetic storage devices, like hard drives and magnetic tape, store data not as absolute values, but in the changes in polarity. This is because a changing magnetic field will induce an electrical current in a nearby wire, and vice versa.
Begin with the source’s own compact description: “Modified frequency modulation” is encoding in communication technology. The dossier treats that line as a proposition to test through Modified, frequency and modulation, not as a finished interpretation.
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This entry incorporates text from “Modified frequency modulation” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.