Hardware Reference
In-Depth Information
First-generation optical disks were invented by the Dutch electronics conglom-
erate Philips for holding movies. They were 30 cm across and marketed under the
name LaserVision, but they did not catch on, except in Japan.
In 1980, Philips, together with Sony, developed the CD (Compact Disc), which
rapidly replaced the 33 1/3 RPM vinyl record for music. The precise technical de-
tails for the CD were published in an official International Standard (IS 10149),
popularly called the Red Book , after to the color of its cover. (International Stan-
dards are issued by the International Organization for Standardization, which is the
international counterpart of national standards groups like ANSI, DIN, etc. Each
one has an IS number.) The point of publishing the disk and drive specifications as
an International Standard is to allow CDs from different music publishers and
players from different electronics manufacturers to work together. All CDs are 120
mm across and 1.2 mm thick, with a 15-mm hole in the middle. The audio CD was
the first successful mass-market digital storage medium. Audio CDs are supposed
to last 100 years. Please check back in 2080 for an update on how well the first
batch did.
A CD is prepared by using a high-power infrared laser to burn 0.8-micron
diameter holes in a coated glass master disk. From this master, a mold is made,
with bumps where the laser holes were. Into this mold, molten polycarbonate is
injected to form a CD with the same pattern of holes as the glass master. Then a
thin layer of reflective aluminum is deposited on the polycarbonate, topped by a
protective lacquer and finally a label. The depressions in the polycarbonate sub-
strate are called pits ; the unburned areas between the pits are called lands .
When a CD is played back, a low-power laser diode shines infrared light with
a wavelength of 0.78 micron on the pits and lands as they stream by. The laser is
on the polycarbonate side, so the pits stick out in the direction of the laser as
bumps in the otherwise flat surface. Because the pits have a height of one-quarter
the wavelength of the laser light, light reflecting off a pit is half a wavelength out
of phase with light reflecting off the surrounding surface. As a result, the two parts
interfere destructively and return less light to the player's photodetector than light
bouncing off a land. This is how the player tells a pit from a land. Although it
might seem simplest to use a pit to record a 0 and a land to record a 1, it is more
reliable to use a pit/land or land/pit transition for a 1 and its absence as a 0, so this
scheme is used.
The pits and lands are written in a single continuous spiral starting near the
hole and working out a distance of 32 mm toward the edge. The spiral makes
22,188 revolutions around the disk (about 600 per mm). If unwound, it would be
5.6 km long. The spiral is illustrated in Fig. 2-25.
To make the music play at a uniform rate, it is necessary for the pits and lands
to stream by at a constant linear velocity. Consequently, the rotation rate of the
CD must be continuously reduced as the reading head moves from the inside of the
CD to the outside. At the inside, the rotation rate is 530 RPM to achieve the de-
sired streaming rate of 120 cm/sec; at the outside it has to drop to 200 RPM to give
 
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