Modified frequency modulation

Modified frequency modulation (MFM) is a recording method for magnetic data media such as magnetic tape or disk. It is an improved variant frequency modulation (FM) - procedure.

With these procedures the individual bits are represented not by the magnetization of the data medium, but by the changethe same. A magnetization change represents a 1-Bit, a continuous magnetization within a certain time interval against it a 0-Bit. In order to prevent a Desynchronisation of the read device, additionally to the data bits still clock bits are stored. During the FM coding before each data bit a clock bit is written, with the MFM methodhowever only, if 2 zero-bits follow one another. Thus both the storage location is doubled and writes/strongly increased reading rate.

The MFM procedure is used today still in floppy disk drives, in non removable disks became it only by the more modern RLL - procedure and today by PRML and EPRML replaces.

See also: Group Coded Recording (GCR), run length Limited (RLL), and Partial Response/maximum Likelihood (PRML)


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