Optical disks

All the preceding technologies for improving the capacity of disks are pushing magnetic media to their density limitations. However, the optical disk continues where the high-density magnetic disk stops. An optical disk is a disk on which data are encoded for retrieval by a laser. Optical disks for computer applications store their information on concentric tracks, like their magnetic cousins. One major disadvantage of optical disks is their slow access time (300 milliseconds average), versus the access speed of a magnetic hard disk (10 to 20 milliseconds average).

Currently, three versions of optical disk technology are competing for the mass-storage market: (1) read-only optical disks, (2) write-once optical disks, and (3) erasable optical disks.

Read-only optical disks, also called CD ROMs (compact disk read-only memories) cannot be written on; thus, they have the functional equivalency of read-only memory (ROM). The most popular version of read-only optical disks employs the same technology as the compact disk (CD), which has become popular for audio recording.

Although a CD ROM can hold massive amounts of text, the most compelling area of CD ROM applications is in the storage and retrieval of multimedia images and sound along with text. CD-ROM is the most economical way of sharing information.

Erasable optical disks use lasers to read and write information to and from the disk, but also use a magnetic material on the surface of the disk and a magnetic write head to achieve erasability. To write on such a disk, a laser beam heats a tiny spot on it; then a magnetic field is applied to reverse the magnetic polarity of the spot. Because of this property, they are called magneto-optical disks. They offer the same storage capabilities of the non-erasable optical disks along with the same reusability capabilities of conventional magnetic disks; however, write and erase times are usually slow. EODs have two important advantages over hard disks: they are not affected by magnetic fields, and they have a longer data life.

Write-once optical disks (also called write-once, read-mostly, or WORM) arc blank disks that are recorded on by the user. To write data, a powerful beam of laser light burns tiny spots or pits into the coating that covers the surface of these disks. Once burned in, the spots are not erasable. To retrieve the data, a less powerful laser is used to read the pattern of spots.

Like their CD ROM cousins, write-once optical disks can store video, images, and sound, as well as text. Being indelible, WORM disks can last 100 years.

	Pros	Cons	Use/Purpose
WORM
CD-ROM
Erasable optical disks

4.4 Translate into English the following word-combinations so that you could form compound nouns:

Обмеження за щільністю, метод/техніка ущільнення, проблема несумісності, щільність доріжок, тривалість доступу, швидкість доступу, файлова система, промінь лазера, споживання енергії, дискова пам’ять, файловий процесор

Дата добавления: 2015-01-03