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Compact discs

 The most common way of storing data on a portable disk is to store it on a compact disc or CD. CDs can store an amazing amount of data in the form of words, music, animated cartoons and pictures.

Data is written onto a CD by a strong laser beam which burns tiny pits into its surface. A pit stands for binary 1 and the absence of a pit, called a land, stands for binary 0. The disc is read by a weaker beam which passes over the surface of the disc as it turns.

Personal computers can be equipped with CD drives and speakers. There are three different types of compact discs which are described below.

How a CD is read

1. A motor spins the disc.

2. A laser fires a beam of light onto the bottom of the disc

3. Light that hits a pit is scattered, but light that strikes a land is reflected back to a light-sensitive detector.

4.  Each beam of light that strikes the detector generates a pulse of electricity. The pattern of pulses and non-pulses becomes a stream of 1s and 0s which is sent to the CPU.

Read-only optical discs

Read-only optical discs

cannot be written on by the user. They can only be read. Data is encoded onto them when they are made and you cannot wipe it off or change it. The most popular read-only optical discs come as compact discs and are known as CD-ROMs

WORMs

Write-once, read-many optical discs (WORMs) are blank CDs on which you can write your own data. But only once. After you have stored the data you can read it from the disc, but you cannot change it or wipe it off

Erasable optical discs

Erasable optical discs can store as much as other CDs but, like magnetic disks, you can wipe them clean and use them again. They are probably the discs of the future. You can't buy them in all shops yet, because more research is needed to make them cheaper

Multimedia presentations

Multimedia means using computers to combine text, sound, graphics and video. Multimedia presentations are combinations of sound, text, still pictures and videos that are usually stored on a CD. You can also see them on the Net. Most multi-media presentations are interactive. This means you can take part in or control the program.

You can also buy CD-ROM stones in which you become one of the characters. You can find yourself trapped in a haunted house, investigating a murder or embarking on an around-the-world journey, and it's up to you to make it a happy ending3.

Fitting it in

Pictures take up lots of room on a CD, so a process called data compression is used to save space. Much of the data in a picture is the same. For example, large areas of sky may be the same blue. Using data compression, only one tiny dot of blue is recorded on disc. When the image is created on screen, an instruction is given to copy that dot in all the sky areas.

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[Programs and languages]   [Compact discs]   [Computer health]   [Communicating computers]