Compact Disk
Device developed by Philips starting on 1972, first using an analog storage method, then migrating into the digital system used until today. Philips was facing a lot of read errors on their prototypes, so they joined in 1978 with Sony, which had an error correction algorithm. The companies announced the Compact Disk standard in 1980 and the first players reached the market in 1983. Today it is a mature standard and is used as a starting point for newer digital discs standards, like DVD.
The CD is basically a 1.2 mm (0.047") thick plastic piece (polycarbonate disc). During its manufacturing process this plastic is pressed with a spiral-shaped protuberance resembling the old vynil records. On this protuberance there are spots that reflect light directly to the CD sendor, called lands, and spots that don't reflect light directly to the CD sensor, called pits. Over the polycardbonate disc a very thin aluminium layer is applied, covering the protuberance. Over the aluminium layer an acrylic layer is applied.
The tracks are spiral-shaped because the CD was projected to store music, which should be played in a continous way.
These tracks are approximately 0.5 mm (0.0197") thick, with a distance of 1.6 mm (0.06304") between the tracks. With we could take off this spiral from the CD, it would be 3.1 miles long.
The CD uses digital audio encoded using PCM (Pulse Code Modulation) standard.
Inside a CD there is no sound recorded, only numbers that represent sounds. Thus, the CD player can use error correction methods, eliminating read errors that would cause noise.
Since it stores only numbers, CDs can be used to store digital data, like computer data.
