Modified Frequency Modulation
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Modified Frequency Modulation, commonly MFM, is the magnetic data recording scheme used by most floppy disk formats, notably by most CP/M machines, as well as PCs running DOS.
MFM is a modification to the original FM (frequency modulation) scheme for encoding data on single-density floppy disks. MFM allows more than one symbol per flux transition — up to three — giving greater density of data than FM. It is used with a data rate of 250-500 kbit/s (500-1000kbit/s raw MFM) on industry standard 5¼" and 3½" ordinary and high density diskettes. MFM was also used in early hard disk designs, before the advent of Run Length Limited (RLL) encoding. Now, at the turn of the millennium, however, except for the steadily disappearing 1.44 MB floppy disk drives, MFM encoding is largely obsolete.
Coding
| Data | MFM |
|---|---|
| 1 | 01 |
| 0 | 10 if following a 0 |
| 0 | 00 if following a 1 |
Notice that two "ones" can't appear together, and the maximum number of zeros in a row is three.
Example:
1 0 1 0 0 0 0 1 ($A1)
0100010010101001 ($44A9)
0100010010001001 ($4489 - sync mark)
^ missing clock
Hexadecimal 4489 is typically used as a unique synchronization mark.
See also
- This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.