magnetic disk

storage A flat rotating disc covered on one or both sides

with some magnetisable material. The two main types are the

hard disk and the floppy disk.

Small areas or "zones" on a magnetic disk are magnetised. The

magnetisation is aligned in one of two opposing orientations

with respect to the recording head. The head writes data

by altering the alignment and reads data by detecting current

pulses induced in a coil as zones with different magnetic

alignment pass underneath it.

Data is stored on either or both surfaces of discs in

concentric rings called "tracks". Each track is divided

into a whole number of "sectors". Where multiple (rigid)

discs are mounted on the same axle the set of tracks at the

same radius on all their surfaces is known as a "cylinder".

Data is read and written by a disk drive which rotates the

discs and positions the read/write "heads" over the desired

track(s). The latter radial movement is known as "seeking".

There is usually one head for each surface that stores data

though, for increased performance (from reduced rotationallatency), it is possible, though expensive, to position

multiple heads at equally spaced angles around the discs.

Therefore there are two states that can be detected for each

zone - a change in alignment, or no change.

Ideally a data bit of one or zero can be recorded in each zone

of magnetisation, however, if a zero represents an absence of

magnetic change, the detection of several consecutive zeros

would imply accurately measuring the length of time of the

absence, i.e., measuring the time between two magnetic

changes. However, the variability of motor speed limits the

number of consecutive zeros which can be read reliably.

The best recording methods accurately follow the

characteristics of the magnetic and rotational aspects in

recording the disk, to be as dense as possible in recording

bits.

Compare magnetic drum, compact disc, optical disk,

magneto-optical disk.

(2003-03-10)