The Physics
An encyclopedia of scientific essays

Linear & Angular Speed of a CD

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Cutnell, John D. & Kenneth W. Johnson. Physics 3rd Edition.New York: Wiley, 1995: 239, A-18. "A CD has a radius of about 0.060 m and rotates at 3.5 rev/s for music at the outer edge. Find (a) the constant tangential speed at which the music is detected and (b) the angular speed (in rev/s) for music at a distance of 0.025 m from the center of a CD." 1.3 m/s
8.4 rev/s
"Compact Disc." Encyclopedia Americana. Connecticut: Grolier, 1999: 151. "CDs spin at about 500 rpm when read near the center, decreasing to approximately 200 rpm when read near the circumference, producing a constant linear velocity." 3–8 rev/s
Davidson, Homer L. Troubleshooting and Repairing Compact Disc Players. New York: McGraw Hill, 1996: 47. "The CD starts out at an inside diameter of around 500 rpm and slows down to approximately 200 rpm, while the 45 rpm speed is constant." 3–8 rev/s
Erickson, Grant. A Fundamental Introduction to the Compact Disc Player. University of Minnesota, 1994. "Rotational speed: 1.2–1.4 m/sec. (constant linear velocity)" 1.2–1.4 m/s
Anderson, Dave. CD-ROM. PC Technology Guide. January 1999. "As the disc rotates at between 200 and 500 rpm, the light bounces off the pits and the frequency [sic] of the light changes." 3–8 rev/s
"Compact Disk." McGraw Hill Encyclopedia of Science and Technology, 8th ed. New York: McGraw Hill, 1997: 238. "The track is optically scanned at a constant linear velocity of 1.25 m/s." 1.25 m/s

In the spring of 1980, Sony teamed up with Philips Electronics and Polygram to produce a compact disc (CD) standard. A year earlier, Philips had already produced a prototype for compact disc player in Europe. Since then, the technology for compact discs and their players have greatly evolved. Compact disc players have become more lightweight, more portable, and many personal CD players now even come with AM/FM radios.

CD technology has greatly revolutionized the idea of a spinning disc. In the mid-20th Century, people were using turntables to spin records at 45 revolutions per minute. CDs spin at an angular speed of 500 rpm when read from the center and 200 rpm when read near the circumference. Besides having an angular velocity, the CD also has a constant linear velocity (CLV). The CLV of a CD has been standardized by Philips at 1.2 to 1.4 m/s.

CDs are much more efficient than black records. Records contain grooves that have been coded with amplitudes that correspond to a specific sound. CDs have "pits"rather than grooves. The spaces between the pits are a digital representation of the recorded sound. The information on these pits is read by lasers that do not physically touch the disc as in a record. By using a digital representation of the sound, the physical wear of recording and playback is greatly reduced. The sound quality is far much better than analog systems.

CDs are not only used in the music industry, but are also utilized in the computer industry. The information of a huge library can be stored on a single CD. CDs for the computer are known as "read-only memory", meaning that the information cannot be altered. CD-ROM drives are classified by speed -- 1X being the speed of a standard audio disc (200 rpm), 2X is twice as fast, 4X is four times as fast, and so on.

Lawrence Fung -- 2000

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