COMPACT DISC-READ ONLY MEMORY

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CD-ROMCD-ROM

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CD-ROMCD-ROMCD-ROM:-Compact Disc read-only memory is a Compact Disc that contains data accessible by a computer. •Firstly the Compact Disc format was designed for music storage and playback, •Later the format was adapted to hold any form of binary data. •CD-ROMs are popularly used to distribute computer

software, including games and multimedia applications, though any data can be stored (up to the capacity limit of a disc). Some CDs hold both computer data and audio with the latter capable of being played on a CD player, whilst data (such as software or digital video) is only usable on a computer (such as PC CD-ROMs. These are called Enhanced CDs.

MANUFACTURING OF A CDMANUFACTURING OF A CD

•IT IS MADE UP OF POLYCARBONATE PLASTIC ON WHICH A REFLECTING SHEET OF ALUMINIUM IS PASTED•DIAMETER OF NORMAL DISC:-120mm•DIAMETER OF MINI DISC :-80mm

THEN THE DATA IS WRITTEN ON THE DISC BY THE ACTION OF LASER WHICH MAKES PITS & BUMPS ON THE DISCTHIS PROCESS IS OFTEN KNOWN AS BURNING PROCESS.

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CD sector contentsCD sector contents

•A standard 74 min CD contains 333,000 blocks or sectors. •Each sector is 2352 bytes, and contains 2048 bytes of PC Data(MODE1), 2336 bytes of VCD Data (MODE2), or 2352 bytes of AUDIO.

The difference between sector size and data content are due to the Headers info and the Error Correction Codes, that are big for Data (high precision required), small for VCD (standard for video) and none for audio

Data is stored on the disc as a series of microscopic indentations (“bumps", with the gaps between them referred to as "lands").

HOW DATA IS STORED ON A HOW DATA IS STORED ON A DISCDISC

BUMPS ON A SECTION OF A CDBUMPS ON A SECTION OF A CD04/10/23AMRIT BRAR5

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CD-ROM DRIVECD-ROM DRIVE

CD-ROM discs are read using CD-ROM drives, which are now almost universal on personal computers.

A CD-ROM drive may be connected to the computer via an IDE (ATA), SCSI,

S-ATA, Firewire, or USB interface or a proprietary interface, such as thePanasonic CD interface. Virtually all modern CD-ROM drives can also play audio CDs as well as Video CDs and other data standards when used in conjunction with the right software

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CD-ROM drivesCD-ROM drives

•A drive motor spins the disc. This drive motor is precisely controlled to rotate between 200 and 500 rpm depending on which track is being read. •A laser and a lens system focus in on and read the bumps. •A tracking mechanism moves the laser assembly so that the laser's beam can follow the spiral track. The tracking system has to be able to move the laser at micron resolutions.

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THREE MAIN COMPONENTS OF A CD-THREE MAIN COMPONENTS OF A CD-ROM DRIVEROM DRIVE

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A view of a CD-ROM drive's disassembled laser system.

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Laser and OpticsLaser and Optics

CD-ROM drives employ a near-infrared 780 nm laser diode. The laser beam is directed onto the disc via an opto-electronic tracking module, which then detects whether the beam has been reflected or scattered.

The fundamental job of the CD player is to focus the laser on the track of bumps. The laser beam passes through the polycarbonate layer, reflects off the aluminum layer and hits an opto-electronic device that detects changes in light. The bumps reflect light differently than the "lands" (the rest of the aluminum layer), and the opto-electronic sensor detects that change in reflectivity. The electronics in the drive interpret the changes in reflectivity in order to read the bits that make up the bytes.

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HOW CD PLAYER HOW CD PLAYER WORKS?WORKS?

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HOW CD IS BEING READ BY HOW CD IS BEING READ BY A LASER A LASER ??

The size of the pit or the bump is

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approximately one-quarter to one-sixth of the wavelength of the laser light used to read the disc, the reflected beam's phase is shifted in relation to the incoming beam, causing destructive interference and reducing the reflected beam's intensity. This pattern of changing intensity of the reflected beam is converted into binary data.

HOW LASER READ THE CDHOW LASER READ THE CD

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The rate at which CD-ROM drives can transfer data from the disc is gauged by a speed factor relative to music CDs: 1x or 1-speed which gives a data transfer rate of 150 kilobytes per second in the most common data format. By increasing the speed at which the disc is spun, data can be transferred at greater rates. For example, a CD-ROM drive that can read at 8x speed spins the disc at up to 4000 rpm, giving a transfer rate of 1.2 megabytes per second. Above 12x speed, vibration and heat can become a problem.

TRANSFER RATETRANSFER RATE

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