Optical Storage





Optical Storage

Today the chief alternative to magnetic media in mass storage system is optical technology. Cheap and easy duplication has made computer optical disc storage an ideal distribution medium. Its versatility (the same basic technology stores data, sound, and video) came about because all engineers eagerly search for big, cheap storage. Both the Compact Disc (CD) and Digital Versatile Disc (DVD) have both of those virtues by design. In addition, because the discs both systems use for storage are removable and interchangeable, CD and DVD systems can serve as both online and offline storage.

Engineers made optical media into true mass storage systems by adding writing capabilities to both the CD and DVD designs. By altering the materials used in the discs, they were able to allow the same equipment that read data to write it as well.

One of the great virtues of any kind of optical storage is data density—little discs mean a lot of megabytes. By using light beams provided by lasers, optical storage earns several advantages. Lenses can focus a beam of light—particularly the coherent beam of a laser—to a tiny spot smaller than the most diminutive magnetic domain writable on a hard disc drive. Unlike the restricted magnetic fields of hard discs that have to be used within a range of a few millionth of an inch, light travels distance with ease. Leaping along some 5.9 trillion miles in a year, some beams have been traveling since almost the beginning of the universe, 10 to 15 billion years ago. A small gap between the source and storage medium is consequently no problem. The equipment that generates the beam of light that writes or reads optical storage need not be anywhere near the medium itself, which gives equipment designers more freedom than they possibly deserve.

The basic idea behind optical disc storage is that you can encode binary data as a pattern of black and white splotches just as on and off electrical signals can. You can make your mark in a variety of ways. The old reliable method is plain, ordinary ink on paper. The bar codes found universally on supermarket products do exactly that.

Reading the patterns of light and dark takes only a photo-detector, an electrical component that reacts to different brightness levels by changing its resistance. Light simply allows electricity to flow through the photo-detector more easily. Aim the photo-detector at the bar code, and it can judge the difference in reflected light between the bars and background as you move it (or move the product along in front of it). The lasers that read barcodes in the supermarket checkout line quicken the scan. The photo-detector watches the reflections of the red laser beam and patiently waits until a recognizable pattern—the bar code as the laser scans across it—emerges from the noise.

You could store the data of a computer file in one gigantic bar code and bring back paper tape as a storage medium—one long strip of bars that stretches past the horizon. Even if you were willing to risk your important data to a medium that turns yellow and flakes apart under the unblinking eye of the sun like a beach bum with a bad complexion, you'd still have all the joy of dealing with a sequential storage medium. That means renew your subscription to your favorite magazines because you'll have a lot of waiting to do.

The disc, with its random-access capabilities, is better suited as a storage system. Its two-dimensional storage makes randomly accessing a particular block of data a matter of milliseconds rather than miles. The choice of a fast-spinning disc was obvious even to the audio-oriented engineers who put the first compact disc systems together. They had a successful pattern to follow: the old black vinyl phonograph record. The ability to drop a needle on any track of a record had become ingrained in the hearts and minds of music lovers for over 100 years. Any new music storage system needed equally fast and easy access to any selection. The same fast and easy access suits computer storage equally well.

Optical storage brings another benefit. The optical patterns that encode data in both the CD and DVD systems can be formed mechanically. Both systems use physical pits that show up as small black holes against a shiny silver background as the dark and bright spots for optically coding data. The pits are mechanical features that can be duplicated mechanically. The process is inexpensive and fast. Machines can flawlessly mold multiple copies in a few seconds, and with multiple master molds, a factory can crank out a million copies in a few days. In other words, optical technology allows for duplicating megabytes, even gigabytes, en masse. Magnetic media, on the other hand, can duplicate data only byte by byte.


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