Compact Disc - Physical Details

Physical Details

A CD is made from 1.2 millimetres (0.047 in) thick, polycarbonate plastic and weighs 15–20 grams. From the center outward, components are: the center spindle hole (15 mm), the first-transition area (clamping ring), the clamping area (stacking ring), the second-transition area (mirror band), the program (data) area, and the rim. The inner program area occupies a radius from 25 to 58 mm.

A thin layer of aluminium or, more rarely, gold is applied to the surface making it reflective. The metal is protected by a film of lacquer normally spin coated directly on the reflective layer. The label is printed on the lacquer layer, usually by screen printing or offset printing.

CD data is represented as tiny indentations known as "pits", encoded in a spiral track moulded into the top of the polycarbonate layer. The areas between pits are known as "lands". Each pit is approximately 100 nm deep by 500 nm wide, and varies from 850 nm to 3.5 µm in length. The distance between the tracks, the pitch, is 1.6 µm.

Scanning velocity is 1.2–1.4 m/s (constant linear velocity) – equivalent to approximately 500 rpm at the inside of the disc, and approximately 200 rpm at the outside edge. (A disc played from beginning to end slows down during playback.)

The program area is 86.05 cm2 and the length of the recordable spiral is (86.05 cm2 / 1.6 µm) = 5.38 km. With a scanning speed of 1.2 m/s, the playing time is 74 minutes, or 650 MB of data on a CD-ROM. A disc with data packed slightly more densely is tolerated by most players (though some old ones fail). Using a linear velocity of 1.2 m/s and a track pitch of 1.5 µm yields a playing time of 80 minutes, or a data capacity of 700 MB. Even higher capacities on non-standard discs (up to 99 minutes) are available at least as recordables, but generally the tighter the tracks are squeezed, the worse the compatibility.

A CD is read by focusing a 780 nm wavelength (near infrared) semiconductor laser through the bottom of the polycarbonate layer. The change in height between pits and lands results in a difference in the way the light is reflected. By measuring the intensity change with a photodiode, the data can be read from the disc.

The pits and lands themselves do not directly represent the zeros and ones of binary data. Instead, non-return-to-zero, inverted encoding is used: a change from pit to land or land to pit indicates a one, while no change indicates a series of zeros. There must be at least two and no more than ten zeros between each one, which is defined by the length of the pit. This in turn is decoded by reversing the eight-to-fourteen modulation used in mastering the disc, and then reversing the Cross-Interleaved Reed-Solomon Coding, finally revealing the raw data stored on the disc.

CDs are susceptible to damage from both normal use and environmental exposure. Pits are much closer to the label side of a disc, enabling defects and contaminants on the clear side to be out of focus during playback. Consequently, CDs are more likely to suffer damage on the label side of the disk. Scratches on the clear side can be repaired by refilling them with similar refractive plastic, or by careful polishing.