Editing Modified frequency modulation (section)

{{Short description|Line code used in early magnetic data storage}}
{{More citations needed|date=June 2023}}
'''Modified frequency modulation''' ('''MFM''') is a [[run-length limited]] (RLL) [[line code]]<ref>{{Cite journal 
|journal=Proceedings of the IEEE
|volume=78 
|issue=11 
|date=December 1990 |title=Runlength-Limited Sequences
|author=Kees Schouhamer Immink
|authorlink=Kees Schouhamer Immink 
|url=https://www.researchgate.net/publication/2984369
|pages=1745–1759
|quote=A detailed description is furnished of the limiting properties of runlength limited sequences.
|doi=10.1109/5.63306 
}}</ref> used to encode data on most [[floppy disk]]s and some [[hard disk drive]]s. It was first introduced on hard disks in 1970 with the [[IBM 3330]] and then in floppy disk drives beginning with the [[List_of_floppy_disk_formats#IBM 53FD|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&nbsp;[[bits per second|kbit/s]] (500–1000&nbsp;kbit/s encoded) on industry-standard {{frac|5|1|4}}-inch and {{frac|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 Outside of niche applications, MFM encoding is obsolete in magnetic recording.