Peripherals: Optical (compact) discs

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Optical (compact) discs

Compact discs are the same size as standard 5.25 magnetic disks, but use laser light technology to store data. Because a laser light beam can be focused with a high degree of precision, the tracks on these discs can be much closer together than is the case with magnetic disks. As a result they have very high capacities, measured in hundreds of Mbytes (typically, 600 Mbytes). The same technology used for recording and playing music COs is used in computer compact discs. Another advantage is the fact that data is more secure on compact discs, as it can't be corrupted by magnetic fields.

Data is encoded on an optical disc by burning tiny pits in its surface with laser light. A pit represents a binary 1, the absence of a pit represents 0. Laser light is also used to read data from the disc, the pits and non-pits setting up different light-interference patterns that can be detected by the reading head. Retrieval times are faster than floppy drives, though not as fast as hard disk systems.

Several types of compact disc systems are available for computers. These are:

• CD-ROM. Like music CDs, these are 'pressed' by the manufacturer with whatever data he wishes to supply. Large databases of information can be supplied in this way, as can software.

• CD-WO. This is the WORM drive ('Write Once, Read Many Times'), which the user can write to, but can't erase. WORM drives are an excellent medium for archiving data.

• CD-I. This is interactive CD, under development by Philips for multimedia applications. This is described on page 169.

• Erasable optical discs. These use magneto-optical (MO) technology, and like ordinary magnetic disks allow you to record and erase data as often as you like. Unlike ordinary optical discs, the active layer of these discs is magnetized. To write a binary 1to a spot on this layer, a pulse of laser light is focused on it, heating it up to several hundred degrees. At this temperature its magnetic polarity can be altered by a magnet which is activated on the opposite side of the disc. (The remain­ing cold spots will be unaffected by this magnet.) Erasa­ble optical drives are currently very expensive, but prices will no doubt fall in the future.

Output devices- monitors

The main output devices are monitors and printers. We'll deal with monitors in this section, and printers in the next.

The resolution (or clarity) of the picture that monitors achieve is determined by the number of pixels, or 'picture elements', on the screen. High resolution monitors have resolutions of 2000 by 2000 pixels, though few present-day computers are able to provide images which take advantage of this degree of clarity. The old-fashioned IBM PC, for example, with a Colour Graphics Adaptor (CGA), can only output images with a resolution of 640 pixels horizontally by 200 vertically. This was adequate for character-based dis­ plays, but for graphics it was hopelessly inferior to the displays of more advanced machines such as the Apple Macintosh.

In the mid-1980s IBM introduced the Enhanced Graphics Adaptor (EGA), which offered a higher resolution. This gave a much better clarity, though it was still not as good as the Mac. Then, in the 1987, IBM brought out the Video Graphics Array (VGA) on its PS/2 range of microcompu­ters, with a resolution of 640 by 480. This was quickly taken up by the PC-compatible world, and is now the standard. In 1990 IBM brought out its Extended Graphics Array (XGA), with a resolution of 1024 by 768 pixels and support for 65,000 colours, and 'Super VGA' with the same resolution also became available from competing manufacturers, so perhaps these will become the standards in the future.

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