November 2007
Intermediate to advanced
736 pages
46h 32m
English
The information on the disc is contained in a spiral that is read from the inside to the outside. For this purpose the ‘read’ objective and other optical elements are mounted on a sledge driven by a small dc motor, and moving radially under the disc. With an average track pitch of 1.6 μm and the disc rotating at 1800 r.p.m. this means an average linear speed of the sledge of 3 mm/m.
The scanning light beam has to remain focused on the track with a radial accuracy of 0.1 μm, a requirement that cannot be met by a purely mechanical guidance system. By varying the speed of the drive motor by incorporating it in a servo control system certain slow corrections are possible. However, to cope with the effects caused by ...
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