For binary information to be stored magnetically, it has to go through some sort of a conversion. This conversion is know as encoding. Each bit which is stored on the hard drive is saved in a pattern of magnetic fluxes. A flux is whenever the polarity is switched, like from north to south, and each flux requires 2 areas. For storage, the most rational storage would be north orientation for 1's and south orientation for 0's. A little problem arises in that when there are groups of similarly polar fields, they strengthen each other to become more powerful and potentially destructive to nearby areas. This would occur mostly at the end of the drive when the drive is not full of information. The similar polarity of all of the zeros would disrupt the information around it. To prevent this, information is stored on the hard drive as a series of fluxes, using specific formats that govern the method in which this is accomplished.

Frequency Modulation
The very first hard drives and floppy disk drives used frequency modulation. Frequency modulation was accomplished by using a flux followed by another data bit. The data bit would be a flux for a 1, and no flux for a 0. This scheme would neutralize the additive nature of the magnetic fields. This required that 4 magnetic areas were used, and this proved to be a real waste of space.

Modified Frequency Modulation
MFM improved on the frequency modulation in that it added an extra flux only between 2 consecutive zeros, where frequency modulation would add one before ever piece of data.

DATA	- Frequency Modulation	- Modified FM
1111	FFFFFFFF	FFFF
0000	FNFNFNFN	NFNFNFN
1010	FFFNFFFN	FNFN
	F=flux, N=no flux

MFM almost cut the average usage down by half, but there were still problems in that the amount of data that a disk could hold would change depending on the binary patterns. MFM is still used with floppy disks because changing them to another format would make them incompatible with older versions.

Run Length Limited
RLL is what is used on most hard drives. There are many different variations to this format, which are chosen to best optimize the hard drive usage. This format only specifies the use of two parameters, run length and run limit. Run length is the minimum space that is allowed between two fluxes. Run limit is the maximum space allowed between two fluxes. Data is analysized in chunks, rather than bit by bit. RLL follows set of patterns for certain data sequences which are defined by the type of RLL being used. This optimizes the storage of data to use the least amount of space possible, increasing hard drive capacity. The types of RLL are expressed as "X,Y RLL". The most commonly used pattern is "1,7 RLL".

Partial Response Maximum Likelihood
PRML was designed by the Quantum Corporation, and is used solely on their hard drives. This format was designed to allow a higher areal density without corrupting the data. Instead of translating fluxes directly to bits, the flux pattern is analysized, an the most likely data pattern is extracted. This allows the magnetic storage to be 30-40% more compact, but still remain readable.

Floppy disk drives operate in almost the exact same way as hard disk drives do. The only difference is in their construction. Floppy disks use a flexible medium, it is basically the same magnetic coating which is used in hard drives, but only it is affixed to a pliable plastic instead of a metal platter.

Floppy disks do not rotate as fast, and only have one storage layer. This layer is double sided to increase storage capacity. Floppy drives use the same technology as the older hard drives did, and were never upgraded because it wasn't necessary. Floppy disks were never made to store great volumes of information, that is what a CD-R is for.

Floppy disk actuator arms are controlled by a stepper motor. This is a lot less precise than a voice coil, but more inexpensive to manufacture. A stepper motor turns rotates, spinning the actuator arms. The motor is capable at stopping in predefined spots, and each time the stepper makes one complete revolution, the actuator changes one track position. A 1.44MB floppy disk has a track density of only 135 tracks per inch, while a modern hard drive has an upwards of 7500 tracks per inch.

Construction | Storage & Operation | Geometry & Partitions | Data Encoding & Floppy Drives