Compact Disk & DVD-ROM
Compact Disk-ROM
» Originally, Compact Disk (CD) was a popular medium for storing music. Now, it is used in computers to store data and is called Compact Disk-Read Only Memory (CD-ROM).
» As the name suggests, CD-ROM is an optical disk that can only be read and not written on. CD-ROM is written on by the manufacturer of the CD-ROM using the laser light.
» A CD-ROM drive reads data from the compact disk. Data is stored as pits (depressions) and lands (flat area) on CD-ROM disk. When the laser light is focused on the disk, the pits scatter the light (interpreted as 0) and the lands reflect the light to a sensor (interpreted as 1).
» As CD-ROM is read only, no changes can be made into the data contained in it.
» Since there is no head touching the disk, but a laser light, CD-ROM does not get worn out easily.
» The storage density of CD-ROM is very high and cost is low as compared to floppy disk and hard disk.
» Access time of CD-ROM is less. CD-ROM drives can read data at 150Kbps. They come in multiples of this speed like—2x, 4x, 52x, 75x, etc.
» It is a commonly used medium for distributing software and large data.
DVD-ROM
» Digital Video Disk-Read Only Memory (DVD-ROM) is an optical storage device used to store digital video or computer data.
» DVDs look like CDs, in shape and physical size.
» It improves on CD technology.
» It is a high-density medium with increased track and bit density.
» DVD-ROM uses both sides of the disk and special data compression technologies. The tracks for storing data are extremely small.
» A full-length movie can be stored on a single disk.
» Each side of DVD-ROM can store 4.7 GB of data, so a single DVD can store 9.4 GB of data.
» New DVD-ROMs use layers of data track, to double its capacity. Such dual layer disks can store 17 GB of data.