<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
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 rotational latency), 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
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
Compare magnetic drum, compact disc, optical disk, magneto-optical disk.
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