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.