Compact Discs (CD)

Compact Disc (CD) is made of several layers of a type of plastic usually Polycarbonates. The outer layers are protection layers, which simply absorb scratches so that the inner layer holding data remains intact. The inner data layer has small tracks which spiral out from the centre of the disc towards the ends. These tracks ate such tightly wound that the entire length of the track if stretched out in a straight line would stretch over five kilometers.

Data is written to the CD-ROM by burning microscopic pits into the reflective surface of the disk with a powerful laser. These spiral tracks have flat reflective areas and non-reflective bumps. A flat reflective area represents a binary 1, while a non-reflective bump represents a binary 0. The data is stored in a spiral pattern that originates from the centre of the disc and spirals out toward the outer edge.

Data is read from a CD-ROM with a low power laser contained in the drive that reflects infrared light off of the reflective surface of the disk and back to a photo detector. The pits in the reflective layer of the disk scatter light, while the land portions of the disk reflect the laser light to the photo detector. The photo detector then converts these light and dark spots to electrical impulses corresponding bits. Electronics and software interpret this data and accurately access the information contained on the CD-ROM. A standard 74 min. CD contains 333,000 blocks and each block is 2,352 bytes.

Thus, with proper devices and software, one can record and read the data from a CD ROM. It’s worth note that CDROM is manufactured with an organic dye layer that is permanently changes by the laser of the recorder in a pattern that represents the bits and bytes of the data. These are called Write once discs. On the other hand, CD-RW (CD-read/write) are discs, which can be erased and re-recorded with data.

Please note that underside of the CD-ROM disk is coated with a very thin layer of aluminum that reflects light


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