Modified Frequency Modulation

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Modified Frequency Modulation (MFM) is a recording method and line coding that is used, among other things, for magnetic data carriers such as magnetic tape or diskette . MFM represents a (1,3) - RLL code with subsequent NRZI coding and is an improved variant of digital frequency modulation (FM).

FM and MFM coding in comparison

With this method, the individual bits are not represented by the magnetization of the data carrier, but rather by changing their polarity. A change in magnetization (flank) represents a 1-bit, whereas a magnetization that remains the same within a certain time interval represents a 0-bit. To prevent desynchronization of the reading device, clock bits are stored in addition to the data bits. With FM coding a clock bit is written before each data bit, with the MFM method, on the other hand, only if 2 zero data bits follow one another. This both doubles the storage space and greatly increases the read / write speed, since only one change in magnetization occurs per bit duration and the bit duration can therefore be selected to be shorter. Disregarding the coding rule of the MFM process can generate “invalid” signal curves (e.g. by leaving out the clock bit between two zero bits) that can be used for synchronization on byte boundaries.

The MFM process is used in floppy disk drives . MFM was also used in the first hard drives , so they were called MFM hard drives .

literature

  • John G. Proakis, Masoud Salehi: Communication Systems Engineering . 2nd Edition. Prentice Hall, 2002, ISBN 0-13-095007-6 .