Digital Data on a Compact DiscBinary data (0's and 1's) are encoded on a compact disc in the form of pits in the plastic substrate which are then coated with an aluminum film to make them reflective. The data is detected by a laser beam which tracks the concentric circular lines of pits. The pits are 0.8 to 3 micrometers long and the rows are separated by 1.6 micrometers.
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Index CD concepts Sound reproduction concepts Reference Rossing Physics Teacher, Dec. 87 | ||
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Compact Disc Drive DetailsIn a compact disc player, a laser beam must track a spiral row of pits which are 0.5 micrometers wide with track spacing 1.6 micrometers. Tracking is aided by a three-beam laser arrangement. In addition to staying on the track, which is much narrower than the 100 micrometer groove separation on a vinyl record, the rotation speed must be adjusted as the beam tracks inward or outward. A linear speed of 1.25 m/s is maintained by increasing the rotation speed from 3.5 to 8 revolutions per second as the beam tracks inward toward the center of the disc. |
Index CD concepts Sound reproduction concepts Reference Rossing Physics Teacher, Dec. 87 | ||
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Laser Tracking on the CD
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Index CD concepts Sound reproduction concepts Reference Rossing Physics Teacher, Dec. 87 | ||
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Scaled Views of a Compact Disc
A reflective layer of aluminium is applied to reflect the laser beam. A protective coating is then applied to the top. The laser system reads the data from below.
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Index CD concepts Sound reproduction concepts Reference Rossing Physics Teacher, Dec. 87 | |||
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Detection of CD Pits
The reflected light from the pit is then 180° out of phase with the reflection from the flat area, so the reflected light intensity drops as the beam moves over a pit. The threshold of the photodiode detector can be adjusted to switch on this light level change. |
Index CD concepts Sound reproduction concepts Reference Rossing Physics Teacher, Dec. 87 | ||
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