In the world of hard drive manufacturing, the term “areal density” comes up a lot. With announcements about new magnetic recording technologies and less platters, the term has confused most. Users can see the benefits from all the benchmarks and various performance tests, but what is it exactly?
Areal density
Areal density is defined as the product of the linear bits per inch (BPI), measured along the length of the tracks around the platter, multiplied by the number of tracks per inch (TPI), measured radially on the platter. In short - it’s the amount of data that can be squeezed onto a given area of a platter.
Given that higher density, or larger amounts of data can be stored in the same amount of space, is better, areal density has recently become a technology growth-rate indicator for the hard disk drive industry.
How HDDs record data
Hard disk drives, or HDDs, consist of one or more rapidly rotating discs, or platters, coated with magnetic material and with magnetic “heads” arranged to write data to the surface and read it from them. Within the surface coating, the data is stored in the magnetic polarization of small patches called “tracks”, which are circular bands of data. These tracks are divided into “sectors”.
Manufacturers achieve higher areal density by moving the tracks closer together, which is called “track density”, or by making each linear bit shorter so that more of them can fit on each track (linear density).
Increasing track density means that the heads must also be able to achieve greater precision in their placements over the tracks. In turn, this considers the prospect of lessening the gap between the head and the disk itself (which as of now is as close as 10 nanometers in some drives).
Increasing areal density is a difficult task because it requires so many various technological advances to numerous hard disk drive components. A higher linear density results with interference between bits. A common solution idea is to reduce the magnetic signal strength stored on the platter. However, this creates more problems such as making sure the heads are sensitive enough and close enough to pick them up.
Future advancements in the hard disk drive industry include “patterned media”, where a disk is preformatted with magnetic domains that can be more tightly packed without interference. Another idea is “holographic storage”, where a laser writes data three-dimensionally in a crystal plate or cube.
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