Modified Frequency Modulation

Modified frequency modulation (MFM), is a run-length limited (RLL) coding scheme used to encode the actual data-bits on most floppy disks. It was first introduced in disk drives in 1970 with the IBM 3330 hard disk drive and then in floppy disk drives beginning with the 'double density' IBM 53FD in 1976. MFM is a modification to the original digital FM (digital frequency modulation also known as delay coding) scheme for encoding data on single-density floppy disks and some early hard disk drives. Modified frequency modulation (MFM), is a run-length limited (RLL) coding scheme used to encode the actual data-bits on most floppy disks. It was first introduced in disk drives in 1970 with the IBM 3330 hard disk drive and then in floppy disk drives beginning with the 'double density' IBM 53FD in 1976. MFM is a modification to the original digital FM (digital frequency modulation also known as delay coding) scheme for encoding data on single-density floppy disks and some early hard disk drives. Due to the minimum spacing between flux transitions that is a property of the disk, head and channel design, MFM, which guarantees at most one flux transition per data bit, can be written at higher density than FM, which can require two transitions per data bit. It is used with a data rate of 250–500 kbit/s (500–1000 kbit/s encoded) on industry standard 5¼-inch and 3½-inch ordinary and high density diskettes. MFM was also used in early hard disk designs, before the advent of more efficient types of run-length limited codes. Except for the steadily disappearing 360 KiB/1.2 MiB (5.25-inch) and 720 ~ 880 KiB/1.4 ~ 1.6 MiB (3.5-inch) floppy disk formats, MFM encoding is obsolete in magnetic recording. FM encoding (delay encoding) is the encoding of binary data to form a two-level signal where (a) a '0' causes no change of signal level unless it is followed by another '0' in which case a transition to the other level takes place at the end of the first bit period; and (b) a '1' causes a transition from one level to the other in the middle of the bit period. Delay encoding is used primarily for encoding radio signals because the frequency spectrum of the encoded signal contains less low-frequency energy than a conventional non-return-to-zero (NRZ) signal and less high-frequency energy than a biphase signal. Delay encoding is an encoding using only half the bandwidth for biphase encoding but features all the advantages of biphase encoding:To be rewritten: It is guaranteed to have transitions every other bit, meaning that decoding systems can adjust their clock/DC threshold continuously.One drawback is that it lacks easy human readability (e.g. on an oscilloscope). Delay encoding is also known as Miller encoding (named after Armin Miller, its inventor). Some RFID cards, in particular EPC UHF Gen 2 RF cards, use a variant called 'Miller sub-carrier coding'.In this system, 2, 4 or 8 cycles of a subcarrier square wave are transmitted for each bit time. The Miller encoding transitions are indicated by 180° phase shifts in the subcarrier, i.e. the subcarrier pauses for 1/2 of a cycle at each transition. (The resultant binary subcarrier is itself either ASK or PSK modulated on another carrier.) As is standard when discussing hard drive encoding schemes, FM and MFM encodings produce a bit stream which is NRZI encoded when written to disk. A 1-bit represents a magnetic transition, and a 0-bit no transition. Data encoding has to balance two factors: