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birka_adrian.jpg (2743 bytes) AUTHOR: Adrian Birka
E-MAIL: adbirka@MIT.EDU

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).


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 compact disc (known as a CD for short) is currently a de-facto standard medium for high-volume data storage.  It is used both for audio purposes (CD-AUDIO) and for computer data purposes (CD-ROM).  Other formats (e.g. video) are also available.  This page examines in brief the mechanism used to read the data off of a compact disc.


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).


 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.
 The circularly polarized light is then focused onto the disk by the objective lens. If the light strikes "land" it is simply reflected back into the objective lens. If it strikes a bump-like feature, only the center beam (of the three beams) is reflected by the feature, and the rest are still reflected by the land.  The features are 1/4 of wavelength high (DIAG_CDPL2), and therefore the reflected light rays have a 180 degrees phase difference; and hence interfere destructively completely.  No light is received back in that case.
 If any light is received back, it is passed through the wave plate again. Since it is going the reverse direction, it will be polarized vertically.  When the vertically polarized light hits the polarizing filter this time, it will be reflected (since the filter reflects all non-horizontally polarized rays). Thus,  it will reflect though the focusing lens and be imaged on the photodetector array.  The array treats this as a 1 bit, and if it receives no light due to destructive interference due to a hit of a feature, it reads a 0 bit.  The digital information thus gathered is then sent to the Digital to Analog converter and is played through the speaker systems (or headphones).


 A compact disc reader is an optical system, using light waves to read the CD’s 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 Snell’s 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 light’s 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 reader’s 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.


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A CD, due to the small dimensions of its features, can hold a lot of data (about 650 MB or 72 minutes of music).  However DVD technology, which uses a similar principle with much smaller features, can hold 4.7 GB per side, and as such can be used to hold entire movies (133 minutes of motion video at 40:1 compression (MPEG-2)).  Thus, the optical disc technology might have an even brighter future in video industry than it has had in audio.


Portable music players, computers, car stereos, data storage devices


 Brain, Marshall.  "How Compact Disks (CD’s) Work."  How Stuff Works.

 Kuhn, Kelin J.  "Audio Compact Disk - An introduction".

 Nakajima, Heitaro and Ogawa, Hiroshi.  Compact Disc Technology.  Translation by Charles Aschmann.  Washington:  IOS Press, Inc., 1992.

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