MFM encoding
MFM encoding (Modified Frequency Modulation) is, as its name implies, a modified version of FM encoding. It was originally developed for use in early IBM hard disks (where it was used starting in 1970), but became most notable for its widespread use in floppy disks, including the standard PC/MS-DOS formats and most CP/M formats as well as some others.
Similarly to FM, MFM encoding depends on changes of signal rather than absolute magnitude, but it uses a more compact scheme for representing data bits. FM encoding required two transitions for some bits, while MFM uses only one transition per bit at most, zero for some bits, though extra "clock bits" and "sync marks" are also added. Elaborate rules for the placement of all these signals are needed to prevent multiple zero bits encoded with no transitions appearing in a row, since it is rather hard to count the number of consecutive occurrences of the absence of a signal transition.
