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Modified Frequency Modulation (MFM) is a method of recording digital data on magnetic media. In the olden days of sub-100 Megabyte hard drives, most used MFM to record the data onto the disks. MFM used a set of rules that specified the way data was represented as magnetic flux changes. These rules were used to pack as many bits of data in the smallest amount of media area. MFM was self-clocking, and the controller was able to sync automatically to the data stream. Other methods of recording data was RLL and, to a lesser extent, NRZ.

MFM or modified frequency modulation is a method of encoding data on magnetic media which is more efficient than the FM encoding. As in FM, a zero is represented by a bit without a flux variation in the middle, and a one is represented by a bit with a flux variation in the middle. However, the clock ticks (the flux variations which serve solely to mark the beginning of a new bit) are omitted where possible.

A one has the flux variation in the middle, and doesn't need a flux variation at the start as well. (The drive can keep track of roughly how far the heads have travelled between flux variations, although it would lose track if it had to go too long with no variations at all.) A zero preceded by a one also doesn't need a flux variation at the start, because there was one in the middle of the previous one. A zero preceded by a zero does need a flux variation at the start - otherwise there would be long strings of zeroes with no flux variations.

MFM encoding is twice as efficient as FM, since it only uses 0.75 flux variations per bit on average. It is still used for encoding data on floppy disks, although hard disks tend to use newer schemes such as RLL.

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