Lec 5_CDROM

8/9/2019 Lec 5_CDROM

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ONFIGURATION OF ONFIGURATION OFCDROMDROM

Er. Sakshi Dogra


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Contents

Introduction Make of a CD CD Player Components Laser Focus


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introductionntroduction


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CD-ROM (compact disc read-only memory) is a pre-pressed compact disc that contains data accessible to, butnot writable by, a computer for data storage and musicplayback

Some CDs hold both computer data and audio with the lattercapable of being played on a CD player, while data (such assoftware or digital video) is only usable on a computer

These are called enhanced CDs


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A CD is a fairly simple piece of plastic, about four one-hundredths (4/100) of an inch (1.2 mm) thick. Most of a CDconsists of an injection-molded piece of clear polycarbonateplastic. During manufacturing, this plastic is impressed withmicroscopic indentations (pits) arranged as a single,continuous, extremely long spiral track of data

Once the clear piece of polycarbonate is formed, a thin,reflective aluminum layer is sputtered onto the disc,covering the pits

Then a thin acrylic layer is sprayed over the aluminum toprotect it

The label is then printed onto the acrylic


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cross section of a complete CD


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ake of a CD ake of a CD


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he Spiral

A CD has a single spiral track of data, circling from the inside

of the disc to the outside The data track is approximately 0.5 microns wide, with 1.6microns separating one track from the next


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Pits The elongated pits that make up the track are each 0.5

microns wide, a minimum of 0.83 microns long and 125nanometers high

The incredibly small dimensions of the pits make the spiraltrack on a CD extremely long

(If you could lift the data track off a CD and stretch it outinto a straight line, it would be 0.5 microns wide and almost 5km long)


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D sector contents

A standard 74 min. CD contains 333,000 blocks or sectors. Each sector is 2,352 bytes, and contains 2,048 bytes of PC

(mode 1) data, 2,336 bytes of VCD (mode 2) data, or 2,352bytes of audio

The difference between sector size and data content arethe header information and the error-correcting codes, thatare big for data (high precision required), small for VCD

(standard for video) and none for audio


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Layouttype

2,352 byte block

CDdigitalaudio :

2,352Digital audio

CD-ROM(mode1):

12Sync.

4Sector id.

2,048Data

4Error detection

8Zero

276Error correction

CD-ROM

(mode2):

12Sync.

4Sector id.

2,336Data


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D Player Components D Player Components


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The CD player has the job of finding and reading the datastored as pits on the CD. Considering how small the pitsare, the CD player is an exceptionally precise piece of equipment

The drive consists of three fundamental components:

A drive motor spins the disc. This drive motor is preciselycontrolled to rotate between 200 and 500 rpm dependingon which track is being read

A laser and a lens system focus in on and read the pits A tracking mechanism moves the laser assembly so that

the laser's beam can follow the spiral track


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nside a CD player


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hat the CD Player hat the CD Player:

oes Laser Focus

oes Laser Focus


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Inside the CD player, there is a good bit of computertechnology involved in forming the data intounderstandable data blocks and sending them either to theDAC (in the case of an audio CD) or to the computer (in thecase of a CD-ROM drive)

The fundamental job of the CD player is to focus the laser onthe track of pits. The laser beam passes through thepolycarbonate layer, reflects off the aluminum layer andhits an opto-electronic device that detects changes in light.

The pits reflect light differently than the "lands" (the rest of the aluminum layer), and the opto-electronic sensor detectsthat change in reflectivity. The electronics in the driveinterpret the changes in reflectivity in order to readthe bits that make up the bytes


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hank youhank you

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