Secondary Storage

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  • Topic: DVD, Compact Disc, Blu-ray Disc
  • Pages : 6 (2243 words )
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  • Published : March 25, 2011
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Secondary storage (optical)

Table of contents
Why was secondary storage needed?1
Different types of optical discs:1
1, CD (compact disc):1
2, DVD (digital versatile storage)2
3, Blu-Ray Disc (BD):3
Which type of recordable disc should I choose:3
Understanding the difference between the Live File System and Mastered disc formats:3
Discs formatted with the Live File System option:3
Discs formatted with the Mastered option:4
Questions from students:4

Why was secondary storage needed?
There was a need for cheaper, more compact, more versatile storage devices with greater capacity. This lead to the invention of secondary storages, which offers the advantages of nonvolatility, greater capacity and greater economy. The selection of secondary storage devices requires the understanding of their characteristics: access method, capacity and portability. The most common forms of secondary storage include magnetic tapes, magnetic disks and optical discs. This essay deals with optical secondary storage devices. Different types of optical discs:

1, CD (compact disc):
There are three types of CDs: CD-ROM, CD-R, CD-RW.
CD-ROM stands for compact disc-read only memory. After data has been recorded on a CD-ROM, it cannot be modified so it is mainly used by software suppliers. CD-recordable (CD-R) discs allow personal computer users to write data up to 650 or 700 MB on them. Moreover, they offer an inexpensive and convenient way of storage. CD is a piece of plastic. During manufacturing, it is impressed with little bumps, called pits, and then it is covered with an aluminium layer. The bumps make up a long spiral track of data, which would be five kilometres long, if it was lifted. The other parts of the aluminium layer are called lands. The CD player focuses the laser on the pits which reflect the light differently than the lands. While reading, a change from pit to land or land to pit indicates a one, while no changes indicate a zero. Pits are closer to the label side of the CD so CDs suffer more scratch damage on this side, whereas scratches can be repaired on the other side by careful polishing or refilling with refractic plastic.

Cross-section of a CD Spiral track of data CD-R discs were originally named CD-WO (write-once) discs and were first published in 1988 by Philips and Sony. CD-R has a life expectancy of 30 or more years. They work the following way according to Brain: CD-Rs have a smooth reflective metal layer, which rests on top of a layer of photosensitive dye. When the disc is blank, the dye is translucent. The laser can modify the greenish, photosensitive layer. In a normal CD, you have a plastic substrate, covered with a reflective aluminium or gold layer. In a CD-R, you have a plastic substrate, a dye layer and a reflective gold layer. When you burn data to a CD-R, the laser heats up the dye layer and changes its transparency, so the disc becomes opaque. The change in the dye creates the equivalent of a non-reflective bump. This is a permanent change, and both CD and CD-R drives can read the modified dye as a bump later on. The dye is sensitive to light, so you should avoid exposing your CD-R disc to sunlight. CD-RW discs are rewritable and they have a shorter life expectancy which is 25 years. CD-RW (compact disc hyphen rewritable) discs must be closed before they can be read in CD drivers or players. On this kind of CD, the dye layer must be changed back and forth between opaque and translucent. The material has the property that it can change its transparency depending on temperature. Heated to one temperature, the material cools to a transparent state; heated to another temperature, it cools to a cloudy state. By changing the power (and therefore the temperature) of the writing laser, the data on the CD can be changed, or "rewritten." Unfortunately, lots of older CD players cannot read CD-RW discs. 2,...
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