cd-rom

1. COMPACT DISC-READ ONLY MEMORY
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CD-ROMCD-ROM

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CD-ROMCD-ROM
CD-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.

3. 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 DISC
THIS 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

5. 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 CD
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CD-ROM DRIVECD-ROM DRIVE

7. 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

8. •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-ROM THREE MAIN COMPONENTS OF A CD-ROM
DRIVEDRIVE

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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.

11. 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 WORKS?HOW CD PLAYER WORKS?

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

13. 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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