Constant bit-density

Areal density is a measure of the quantity of information bits that can be stored on a given length of track, area of surface, or in a given volume of a computer storage medium. Generally, higher density is more desirable, for it allows greater volumes of data to be stored in the same physical space. Density therefore has a direct relationship to storage capacity of a given medium. Density also generally has a fairly direct effect on the performance within a particular medium, as well as price.

Hard disk drives store data in the magnetic polarization of small patches of the surface coating on a disk. The maximum areal density is defined by the size of the magnetic particles in the surface, as well as the size of the "head" used to read and write the data. The areal density of disk storage devices has increased dramatically since IBM introduced the IBM 350 disk storage, the first hard disk drive in 1956 at an areal density of 2,000 bit/in². In 2015, Seagate introduced a hard drive at a density of 1.34 Tbit/in², about 600 million times that of the first disk drive. It is thought that current recording technology can continue to scale beyond 1 Tbit/in². New technologies are under development and are expected to continue magnetic areal density progress.

Compact discs (CDs) store data in small pits in a plastic surface that is then covered with a thin layer of reflective metal. The standard defines pits that are 0.83 micrometers long and 0.5 micrometers wide, arranged in tracks spaced 1.6 micrometers apart, offering a density of about 0.90 Gbit/in².

DVD disks are essentially a higher-density CD, using more of the disk surface, smaller pits (0.64 micrometers), and tighter tracks (0.74 micrometers), offering a density of about 2.2 Gbit/in². Single-layer HD DVD and Blu-ray disks offer densities around 7.5 Gbit/in² and 12.5 Gbit/in², respectively.

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