modified frequency modulation (Disk) (MFM)
Modified Frequency Modulation (MFM) is an older recording method for hard disks, floppy disks and magnetic cards that evolved from the FM (Frequency Modulation) method.
In this bit-serial method, only one polarity inversion per bit is written in front of successive zeros. A data bit is only stored if it represents a "1", whereas the clock bit is written if two consecutive bit cells are "0".
The MFM method results in a doubling of the recording density compared to the FM method, where each bit requires two reversals. The improved successor to Modified Frequency Modulation (MFM) is Modified Modified Frequency Modulation( MMFM). The MFM method has been superseded by Run Length Limited( RLL) in hard disks, but is still used in floppy disks.