B.TECH (MBA) ECE

ROLL NO. :- 34

REG. NO:- 10808679

SECTION :- B6803(G2)

COMPACT DISK READ ONLYMEMORY

Compact Disc Read Only Memory.A compact disc format that is used to hold text,graphics, and hi-fi stereo sound.The disc is almost the same as the music CD,but uses different tracks for data.The music CD player cannot play CD-ROM disc but CD-ROM players may be able to play music CD discs and have jacks for connection to an amplifier and/or earphones.A CD-ROM player is cabled to and controlled by a card that is plugged into one of the PCs expansion slots.

• Sony of Japan and Philips of the Netherlands developed the compact disc (CD), a small hard plastic disc that could be economically produced in large quantities. CD-Audio was the first application for this new technology. CD-Audio was very successful, due in large part to the producers adherence to a set of specifications that later became standards. The adherence to these standards lead to the worldwide growth of the CD-Audio industry. When CD-ROM was designed, the industry recognized the need for standards and developed ISO 9660 as the base standard for all CD-ROMs.

• Physically, the standard Physically, the standard compact disc is a disc made of compact disc is a disc made of clear polycarbonate plastic, clear polycarbonate plastic, coated with a reflective metal, coated with a reflective metal, and a protective coat of clear and a protective coat of clear lacquer. Data is placed on the lacquer. Data is placed on the CD-ROM in the form of small CD-ROM in the form of small pits recorded in a spiral track pits recorded in a spiral track starting at the center of the starting at the center of the CD- ROM and working to the CD- ROM and working to the outer edge. If the data track of outer edge. If the data track of a CD-ROM could be stretched a CD-ROM could be stretched out it would be about 4.5Km out it would be about 4.5Km long.long.

The standard CD-ROM can hold up to 74 minutes (about 680 Megabytes) of data, uncompressed. This is roughly equivalent to 300,000 typewritten pages. Along with the data, error detection and correction codes are also recorded on the disc. This accounts for the incredibility low error rate when reading CD-ROMs.

Since CD-ROMs are replicated in large quantities, the process requires producing a "master" disc. To produce the glass master, the encoder's high power laser beam burns the pits onto a glass disc that has been coated with photoresist. Once "burned", the glass disc is then coated with an ultrathin metal coating (usually a nickel alloy). Finally, it is used to produce the metal stampers that are fitted into the CD-ROM replication machines to press the final CD-ROMs. After stamping the CD-ROMs, they are coated with the reflective layer (usually aluminum), the protective lacquer, and finally the label is printed on it.

Data is read from the CD-ROM with the help of Data is read from the CD-ROM with the help of CD-ROM drive. While reading a CD-ROM, a low CD-ROM drive. While reading a CD-ROM, a low power laser beam is focused on the rotating CD-power laser beam is focused on the rotating CD-ROM and its reflection is viewed by the read ROM and its reflection is viewed by the read head. When the beam reflects back from the CD- head. When the beam reflects back from the CD- ROM, it's intensity changes as it moves from ROM, it's intensity changes as it moves from "land" to "pits". These variations in the laser beam "land" to "pits". These variations in the laser beam are decoded as data by the CD-ROM drive. It are decoded as data by the CD-ROM drive. It should be noted, that unlike hard discs which should be noted, that unlike hard discs which rotate at a constant angular velocity (CAV), CD-rotate at a constant angular velocity (CAV), CD-ROMs rotate at a constant linear velocity (CLV) of ROMs rotate at a constant linear velocity (CLV) of about one meter per second. This requires that about one meter per second. This requires that the drives servo mechanism makes the CD-ROM the drives servo mechanism makes the CD-ROM turn slower as the read head moves to the outer turn slower as the read head moves to the outer edge of the disc.edge of the disc.

Data Access Data Access Unlike tape that must be accessed Unlike tape that must be accessed

sequentially, CD-ROM can be accessed sequentially, CD-ROM can be accessed randomly. Random access reduces the time randomly. Random access reduces the time required to retrieve data. It also allows required to retrieve data. It also allows retrieval software to quickly retrieve related retrieval software to quickly retrieve related data items across the entire database.data items across the entire database.

Longevity and IntegrityLongevity and Integrity Testing suggests that a properly produced CD-Testing suggests that a properly produced CD-

ROM can last more than 100 years. CD- ROM ROM can last more than 100 years. CD- ROM is a read-only medium, which insures the is a read-only medium, which insures the integrity of the data for the life of the CD- integrity of the data for the life of the CD- ROM.ROM.

Compared to other storage disks or diskettes, a CD-ROM is a very rugged, long- lasting, secure medium for storing large amounts of read-only information. CD-ROM is the most cost effective way to produce and distribute large quantities of data.