Modified Frequency Modulation

Modified Frequency Modulation (MFM) is a recording method and a line-coding, which applies inter alia, magnetic media such as magnetic tape or disk. MFM is a (1,3) RLL code is followed by a NRZI encoding, and is an improved version of the digital frequency modulation (FM).

In these methods, the individual bits are represented by the magnetization of the data carrier, but the same by switching. A magnetization change ( edge) is a 1-bit is a uniform magnetization in a given time interval, however, a 0 -bit. To prevent desynchronisation of the reading device, can be stored in addition to the data bits or timing bits. In FM encoding a clock bit is written before each data bit, only when two zero bits follow in the MFM method, however, each other. Thus both the space is doubled as the read / write speed also greatly increased as per bit duration to no more than a magnetization change occurs and thus the bit duration can be made shorter. Failure to comply with the provision of the MFM encoding process " invalid " waveforms can be generated (eg, by omitting the clock bits between two zero bits ) that can be used for synchronization on byte boundaries.

The MFM method is used in floppy drives. MFM has also been used at the first hard drives, they were therefore called MFM hard drives.