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Main physical parameters
The main parameters of the CD (taken from the September 1983 issue of the compact disc specification) are as follows:
* Scanning velocity: 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.)
* Track pitch: 1.6 μm.
* Disc diameter 120 mm.
* Disc thickness: 1.2 mm.
* Inner radius program area: 25 mm.
* Outer radius program area: 58 mm.
* Center spindle hole: 15 mm
The program area is 86.05 cm², so that the length of the recordable spiral is 86.05/1.6 = 5.38 km. With a scanning speed of 1.2 m/s, the playing time is 74 minutes, or around 650 MB of data on a CD-ROM. If the disc diameter were 115 mm, the maximum playing time would have been 68 minutes, i.e., six minutes less. A disc with data appearing 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 micrometre leads to a playing time of 80 minutes, or a 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 will be.
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