24 September 2009

Optical disk




The invention:Anonmagnetic storage medium for computers that
can hold much greater quantities of data than similar size magnetic
media, such as hard and floppy disks.
The people behind the invention:
Klaas Compaan, a Dutch physicist
Piet Kramer, head of Philips’ optical research laboratory
Lou F. Ottens, director of product development for Philips’
musical equipment division
George T. de Kruiff, manager of Philips’ audio-product
development department
Joop Sinjou, a Philips project leader
Holograms Can Be Copied Inexpensively
Holography is a lensless photographic method that uses laser
light to produce three-dimensional images. This is done by splitting
a laser beam into two beams. One of the beams
is aimed at the object
whose image is being reproduced so that the laser light will reflect
from the object and strike a photographic plate or film. The second
beam of light is reflected from a mirror near the object and also
strikes the photographic plate or film. The “interference pattern,”
which is simply the pattern created by the differences between the
two reflected beams of light, is recorded on the photographic surface.
The recording that is made in this way is called a “hologram.”
When laser light or white light strikes the hologram, an image is created
that appears to be a three-dimensional object.
Early in 1969, Radio Corporation of America (RCA) engineers
found a way to copy holograms inexpensively by impressing interference
patterns on a nickel sheet that then became a mold from
which copies could be made. Klaas Compaan, a Dutch physicist,
learned of this method and had the idea that images could be recorded
in a similar way and reproduced on a disk the size of a phonograph
record. Once the images were on the disk, they could be
projected onto a screen in any sequence. Compaan saw the possibilities
of such a technology in the fields of training and education.
Computer Data Storage Breakthrough
In 1969, Compaan shared his idea with Piet Kramer, who was the
head of Philips’ optical research laboratory. The idea intrigued
Kramer. Between 1969 and 1971, Compaan spent much of his time
working on the development of a prototype.
By September, 1971, Compaan and Kramer, together with a handful
of others, had assembled a prototype that could read a blackand-
white video signal from a spinning glass disk. Three months
later, they demonstrated it for senior managers at Philips. In July,
1972, a color prototype was demonstrated publicly. After the demonstration,
Philips began to consider putting sound, rather than images,
on the disks. The main attraction of that idea was that the 12-
inch (305-millimeter) disks would hold up to forty-eight hours of
music. Very quickly, however, Lou F. Ottens, director of product development
for Philips’ musical equipment division, put an end to
any talk of a long-playing audio disk.
Ottens had developed the cassette-tape cartridge in the 1960’s.
He had plenty of experience with the recording industry, and he had
no illusions that the industry would embrace that new medium. He
was convinced that the recording companies would consider fortyeight
hours of music unmarketable. He also knew that any new
medium would have to offer a dramatic improvement over existing
vinyl records.
In 1974, only three years after the first microprocessor (the basic
element of computers) was invented, designing a digital consumer
product—rather than an analog product such as those that were already
commonly accepted—was risky. (Digital technology uses
numbers to represent information, whereas analog technology represents
information by mechanical or physical means.) When
George T. de Kruiff became Ottens’s manager of audio-product
development in June, 1974, he was amazed that there were no
digital circuit specialists in the audio department. De Kruiff recruited
new digital engineers, bought computer-aided design
tools, and decided that the project should go digital.
Within a few months, Ottens’s engineers had rigged up a digital
system. They used an audio signal that was representative of an
acoustical wave, sampled it to change it to digital form,
and encoded it as a series of pulses.
On the disk itself, they varied the
length of the “dimples” that were used to represent the sound so
that the rising and falling edges of the series of pulses corresponded
to the dimples’ walls. A helium-neon laser was reflected from
the dimples to photodetectors that were connected to a digital-toanalog
converter.
In 1978, Philips demonstrated a prototype for Polygram (a West
German company) and persuaded Polygram to develop an inexpensive
disk material with the appropriate optical qualities. Most
important was that the material could not warp. Polygram spent
about $150,000 and three months to develop the disk. In addition, it
was determined that the gallium-arsenide (GaAs) laser would be
used in the project. Sharp Corporation agreed to manufacture a
long-life GaAs diode laser to Philips’ specifications.
The optical-system designers wanted to reduce the number
of parts in order to decrease manufacturing costs and improve
reliability. Therefore, the lenses were simplified and considerable
work was devoted to developing an error-correction code.
Philips and Sony engineers also worked together to create a standard
format. In 1983, Philips made almost 100,000 units of optical
disks.
Consequences
In 1983, one of the most successful consumer products of all time
was introduced: the optical-disk system. The overwhelming success
of optical-disk reproduction led to the growth of a multibillion-dollar
industry around optical information and laid the groundwork
for a whole crop of technologies that promise to revolutionize computer
data storage. Common optical-disk products are the compact
disc (CD), the compact disc read-only memory (CD-ROM), the
write-once, read-many (WORM) erasable disk, and CD-I (interactive
CD).
The CD-ROM, the WORM, and the erasable optical disk, all of
which are used in computer applications, can hold more than 550
megabytes, from 200 to 800 megabytes, and 650 megabytes of data,
respectively.
The CD-ROM is a nonerasable disc that is used to store computer
data. After the write-once operation is performed, a WORM becomes
a read-only optical disk. An erasable optical disk can be
erased and rewritten easily. CD-ROMs, coupled with expert-system
technology, are expected to make data retrieval easier. The CD-ROM,
the WORM, and the erasable optical disk may replace magnetic
hard and floppy disks as computer data storage devices.

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