Density is a measure of the quantity of information bits that can be stored on a given physical space of a computer storage medium.
[5] Optical discs store data in small pits in a plastic surface that is then covered with a thin layer of reflective metal.
A newer IBM technology, racetrack memory, uses an array of many small nanoscopic wires arranged in 3D, each holding numerous bits to improve density.
[9] Although exact numbers have not been mentioned, IBM news articles talk of "100 times" increases.
By superimposing images of different wavelengths into the same hologram, in 2009 a Stanford research team achieved a bit density of 35 bit/electron (approximately 3 exabytes/in2) using electron microscopes and a copper medium.
This is most obvious when considering various disk-based media, where the storage elements are spread over the surface of the disk and must be physically rotated under the "head" in order to be read or written.
For example, we can calculate the effective transfer speed for a floppy disc by determining how fast the bits move under the head.
Now consider an improvement to the design that doubles the density of the bits by reducing sample length and keeping the same track spacing.
The vast majority of PCs included interfaces designed for high density drives that ran at 500 kbit/s instead.
In DRAM, in particular, the amount of charge that needs to be stored in a cell's capacitor also directly affects this time.
Storage density also has a strong effect on the price of memory, although in this case, the reasons are not so obvious.
This sets a fixed lower limit, which is why the average selling price for both of the major HDD manufacturers has been US$45–75 since 2007.
In this case the cost is determined by the yield, the number of viable chips produced in a unit time.
Chips are produced in batches printed on the surface of a single large silicon wafer, which is cut up and non-working samples are discarded.
[16] The relationship between information density and cost per bit can be illustrated as follows: a memory chip that is half the physical size means that twice as many units can be produced on the same wafer, thus halving the price of each one.
As a comparison, DRAM was first introduced commercially in 1971, a 1 kbit part that cost about $50 in large batches, or about 5 cents per bit.