Areal Density Definition

Areal density, also sometimes called bit density, refers to the amount of data in terms of bits that can be stored in a given area on a platter in a hard disk drive (HDD).

A bit (abbreviated with a lower case b) is the most basic unit of information in computing and communications. Every bit has a value of either zero or one. Each bit in a HDD (as well as on a floppy disk or on magnetic tape) is represented by the orientation of the magnetization of a microscopic area in the magnetic media.

A platter is a thin, high-precision aluminum or glass disk that is coated on both sides with a high-precision magnetic material. Modern HDDs typically contain multiple platters, all of which are mounted on a single shaft, in order to maximize the data storage surface in a given volume of space. The magnetic coating is generally a ferric oxide or a cobalt alloy, which is, in turn, covered by a layer of a very hard material and, finally, by a thin layer of lubricant.

The magnetic media on each side of each platter is divided into a series of tracks. A track is any of the concentric circles over which one magnetic head passes while it is stationary but the platter is rotating at high speed. A magnetic head, also commonly referred to as just a head, is a small, high-sensitivity electromagnet that used for reading and writing data on the magnetic media. Most platters require two heads, one for each side.

Areal density is usually expressed in bits per square inch (BPSI). It can be calculated as the product of track density and linear density. The former is a measure of how closely the concentric tracks are packed together on a platter. The latter is a measure of how tightly the bits are packed together on a length of track.

The most outstanding trend with regard to HDDs has been the continuing increase in areal density in combination with the development of magnetic heads and other components that can take full advantage of it. This increase in density has been the result of successive improvements in the magnetic materials and the development of higher precision techniques for depositing them on the platters. There is still much room for further increases in areal density.

Created March 30, 2006.
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