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The Compact Disc - Pure High Tech Fantastic Digital Sound

Bayer MaterialScience AG : 14 June, 2002  (Company News)
The audio CD celebrates its 20th birthday on Aug. 17, 2002. But even before it was born in 1982, Philips had already developed the basic technologies required to read digitally stored data with the help of a highly focused laser beam.
The original idea for CD technology, still in use today, emerged in 1974 in the research laboratories of the Dutch electronics corporation. The goal was to dispense with pick-up heads for playing audio signals and instead store them digitally on an optically readable medium.

An essential prerequisite for this pioneering invention was the development of a laser system to 'read' the digital information stored on the CD. At the same time, the search began for a suitable material to be used as the substrate. Several types of plastic were analyzed to determine whether they met the extremely high demands. The choice was high-purity Makrolon« polycarbonate from Bayer AG.

In the face of competing concepts from the United States and Japan, Philips looked for an international partner, finally reaching an agreement with Sony in 1979 to implement the project jointly and establish a uniform, worldwide standard for the compact disc. The result: the two corporations simultaneously shipped the first CD players to stores at the end of 1982.

Pits and Lands - The Language of Data Storage

A mass-produced compact disc consists of an ultrapure polycarbonate disc with a diameter of 12 centimeters, an aluminum layer 40 to 50 nanometers thick, and a protective coating 10 micrometers thick. The maximum playing time is 74 minutes. The label, usually applied by color screen-printing, describes the content of the CD.

Digital information is stored on the silver disc in the form of 'pits' and 'lands.' Pits are microscopic indentations with a width of 0.6 micrometers and a length of 0.83 to 3.5 micrometers. The areas in between are referred to as lands. The pits are arranged in a continuous, helical track from the inside out. This track consists of approximately four to five billion pits and is nearly six kilometers long. The distance between the individual 'grooves' of a track is a mere 1.6 micrometers. To compare, a CD the size of St. Peter's Square in Rome would have pits the size of a pinhead. These proportions indicate the high surface reproduction accuracy required of a plastic on which digital information is to be stored with flawless precision and constant accessibility.

How Does Digital Data Get Onto a CD?

The master disc for a CD consists of a glass plate made of flawless optical glass. The blank has to meet very high demands and must not display even the slightest unevenness or contamination. A thin adhesive coating is applied to the plate first, followed by a photosensitive emulsion, similar to that on photographic film. The surface is now ready for the recording process. Read from a mastering tape, the data is written into the photolayer by a laser. This exposure process is followed by photographic developing, fixing, stabilization and drying.

In order to produce a stamper that is later inserted into an injection mold, the glass master is given an electrically conductive coating. A nickel coating is then deposited in an electroplating bath and later separated mechanically, after which the glass master is useless. The result is referred to as the 'father' and is the master copy for all subsequent compression molds. This copy is subjected to another metallization step to produce the negative 'mother,' which in turn is transferred to the 'sons.'

The son stamper impresses the pit pattern on an initially transparent CD blank during the injection molding process, similar to the pressing of a record. A thin aluminum layer applied thereafter serves as a reflector for the laser and enables playback of the data.

Reading with a Laser

The digital information on a CD is read by a laser from the bottom of the disc. The laser beam penetrates the highly transparent polycarbonate disc, is reflected by the aluminum layer and hits a photo diode. An actuator positions the optical elements and a motor provides the necessary rotation of the CD, so that the highly focused laser beam can accurately follow the helical track. Because less information is stored on the inside tracks of a CD than on the outside tracks due to the smaller circumference, the rotational speed of the CD must be continuously adapted to the groove beingscanned.

The light is reflected differently by a pit and a land during the reading process. The smallest storage unit a computer can interpret is a bit (binary digit). A bit can only assume one of two values, namely 'binary zero' or 'binary one.' The various combinations of 'binary zero' and 'binary one' produce a string of information, the alphabet of digital language. In an audio CD player, the signals are transmitted to a digital/analog converter, which converts them directly into analog stereo signals. And that's when listening pleasure begins!
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