MAIN FUNCTIONAL REQUIREMENT: Read data encoded in the features of the aluminum side of a CD and convert this data to output for speakers (or other uses).
DESIGN PARAMETER: CD Player/Drive
It uses optical systems with a high precision laser to read the data. It relies on the aluminum layer of the CD to reflect the laser beam, and uses the reflected beam to determine the position of the features.
A typical CD is a polymer disc 6 cm (2.36 in) in radius, with a 7.5 mm (0.30 in) radius hole in the center. It is about 1.2 mm (0.047 in) thick (see DIAG_CD1). The CD consists mostly of clear polycarbonate plastic, with small bump-like features on its top side to encode data. This is covered with a thin aluminum level (about 30 microns, or 0.003 in), then with an acrylic level (approximately the same thickness as aluminum level) and a label printed on top of that.
The features which store the data are arranged in a long, extremely thin, spiraling track, which is read by the CD reading mechanism starting from the center outwards (See DIAG_CD2). This track is 0.5 microns (0.00002 in) wide, with 1.6 microns (0.00006 in) separating the adjacent tracks. The features on the track are at 0.125 microns (0.000005 in) high (see DIAG_CD1).
EXPLANATION OF HOW IT WORKS/ IS USED:
In order to read the CD, light is first emitted by a laser
(DIAG_CDPL1). It passes through a diffraction grating, which converts the light into a
central peak plus side peaks, but, as was stated before, only the three central beams are
strong enough to matter in the mechanism. The three beams go through a polarizing
beam filter, which reflects all but the horizontal polarization. The emerging light (now
horizontally polarized) is then collimated by a lens, and is passed through a plate that
converts it into circularly polarized light.
A compact disc reader is an optical system, using light waves to read the CDs data. Therefore, for better understanding of its operation, some knowledge of optics is required:
Reflection: light reflects according to the well known "angle of incidence is equal to angle of reflection" law (DIAG_OP1).
Refraction: when light enters a different medium, its direction, speed and wavelength usually change. The direction change (DIAG_OP2) is given by Snells law (the ratio of the sines of angles of incidence and refraction is equal to the ratio of refractive indices of media), while both the lights speed and wavelength change proportionally to the indices of refraction. Refraction is often used in lenses to collect or disperse light rays (DIAG_OP3).
Diffraction: When a wave passes through an opening smaller than or of comparable size to its wavelength, it diffracts, producing a wider wave than the original (DIAG_OP4). If diffraction is combined with interference (see paragraph below), as in a passage of a wave through two holes simultaneously, a distinctive diffraction pattern is seen. The diffraction grating used in CD reading systems can be viewed as simply splitting a light ray into three parts (the other parts of the diffraction pattern are not intense enough to be noticed).
Interference: When two identical waves meet, they can superimpose and interfere with each other (DIAG_OP4). If they are 180 degrees out of phase, they will cancel each other out. If they are in phase, they will reinforce each other. In general, though, the interference will be somewhere in between.
Polarization: Light can be polarized in a few different ways. The ones used in the CD mechanism are vertical polarization, horizontal polarization and circular polarization. For the purposes of understanding CD readers function it is enough to know that polarizing filters exist which let only light of certain polarization through, reflecting all the rest. Polarization converters also exist, which convert between different polarizations.
WHERE TO FIND CD PLAYERS:
Portable music players, computers, car stereos, data storage devices
Brain, Marshall. "How Compact Disks (CDs) Work." How Stuff Works. http://www.howstuffworks.com/cd.htm.
Kuhn, Kelin J. "Audio Compact Disk - An introduction". http://www.ee.washington.edu/conselec/CE/kuhn/cdaudio/95x6.htm.
Nakajima, Heitaro and Ogawa, Hiroshi. Compact Disc Technology. Translation by Charles Aschmann. Washington: IOS Press, Inc., 1992.
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