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Modified frequency modulation (MFM) is a run-length limited (RLL) line code [1] 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.
A modified frequency modulation, or (1,3) RLL encoding, stores each data bit as two bits on tape, but since there is guaranteed to be one 0 (no flux reversal) bit between any 1 (flux reversal) bits, then it is possible to store 6400 encoded bits per inch on the tape, or 3200 data bits per inch.
Frequency modulation encoding, or simply FM, is a method of storing data that saw widespread use in early floppy disk drives and hard disk drives. The data is modified using differential Manchester encoding when written to allow clock recovery to address timing effects known as "jitter" seen on disk media.
English: Graph illustrating encoding of byte 0xA1 (10100001 in binary) using Frequency modulation (above) and Modified frequency modulation (below). Individual bits being encoded are displayed at the top; blue dashed line (short dashes) indicates synchronization pulse; red one (long dashes), the pulse carrying the actual data (change of phase corresponds to "1" and lack thereof to "0").
Eight-to-fourteen modulation (EFM), used in compact discs; Hamming code; Hybrid ternary code; Manchester code and differential Manchester; Mark and space; MLT-3 encoding; Modified AMI codes: B8ZS, B6ZS, B3ZS, HDB3; Modified frequency modulation, Miller encoding and delay encoding; Non-return-to-zero (NRZ) Non-return-to-zero, inverted (NRZI)
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Frequency modulation and phase modulation are the two complementary principal methods of angle modulation; phase modulation is often used as an intermediate step to achieve frequency modulation. These methods contrast with amplitude modulation , in which the amplitude of the carrier wave varies, while the frequency and phase remain constant.
MFM 97.1, former name of Heart Wirral, ... Modified frequency modulation, a data encoding method used on floppy disks and older hard disks; Other