Modified Frequency Modulated
MFM is on of the available technologies to convert binary numbers (bits) into magnetic information. MFM technology is used by floppy disks and was used on the first hard disk drives (ST-506 standard). On hard disk drives this technology was replaced by another one called RLL.
MFM modulation works like this:
On the bit flow coming from the device head, when there is a ”1“ bit the signal is inverted. When there is a ”0“ bit after a ”1“ bit, the signal isn’t inverted. And where there is a ”0“ bit after another ”0“, the signal is inverted (see picture, where 10001111 is being modulated).
click to enlarge
Before MFM floppy disks used another modulation technique, called FM (Frequency Modified). On FM modulation, each bit corresponds directly to a signal inversion, so there is no data ”compression“. Because of that, FM modulation uses the double of the space when compared to MFM. This explains why the first floppy disks used on the PC (360 KB and 720 KB) were called ”double density“: because they used MFM modulation, which stores the double of data on the same physical space compared to FM modulation.
Since the first hard disk drives used on the PC used this kind of modulation, several people called them ”MFM hard drives“. In fact, these ”MFM hard drives“ are ST-506 hard disk drives (ST-506 is a hard disk drive standard used by the very first PC hard drives).