Mass Produced CD/DVD Discs

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A mass-produced or replicated CD or DVD, for example an Audio CD or a CD-ROM, is a clear plastic disc which is used to store digital information. A spiral track is molded onto one side of this disc with tiny indentations, which are called pits. A thin reflective layer of aluminum (for CDs and DVD-5s) or gold or silver (for dual layer DVDs) is "sputtered" onto the information surface of the plastic disc. For CDs, this surface is coated with a protective lacquer onto which printing is applied. For DVDs, the plastic disc is bonded to another disc (each disc or "substrate" is half the thickness of a CD), with the information layers bonded together on the inside. Discs are read by focusing a laser through the plastic to the information layer. In this way, defects on the surface of the disc, such as scratches or dust, are out of focus with the reading laser beam. This greatly minimizes the effect of these problems (although you should try to keep your discs free from dirt, fingerprints, or scratches).

In a DVD/CD drive the information is read by focusing a laser beam on this spiral track, and scanning along the track at a constant speed (a constant linear velocity). Because the pits scatter some of the reflected light from the laser, the returning laser beam will have a reduced intensity whenever it encounters a pit. When the laser light is converted into an electrical signal using photodiodes, the drive can interpret the pattern of pits as binary information; ones and zeros. The information on such a replicated disc can only be read; no data can be added (or deleted) afterwards.

What makes a recordable disc, such as a CD-R or DVD-R, different from an ordinary DVD/CD is the fact that the recordable disc starts out without any pits. A recordable disc is molded with a continuous spiral groove, instead of pits. A layer of organic dye is coated onto the plastic substrate. This is followed by a reflective metal layer, which is then coated with lacquer (CD-R) or bonded to another substrate (DVD-R). In the DVD/CD recorder a laser beam is used to burn a pattern in the organic dye. When you place the disc in the DVD/CD-drive, these burns cause changes in the amount of laser light absorbed by the dye layer, causing the same effect to the returning laser beam as do the pits on a manufactured disc. Any DVD/CD-drive can therefore read the information you write on a recordable disc. Once written, these burns cannot be deleted. Each part of the disc can therefore be used only once. You can, for instance, use recordable discs to create premaster discs that can be used to mass-produce DVDs/CDs in a mastering and replication factory.

The information on a Rewritable disc (CD-RW or DVD-RW) can be erased to make place for new data. The surface of such a disc can be reset to its original condition by means of a separate, high intensity laser beam in the recorder, preparing the disc for reuse. In this way a CD-RW or DVD-RW can be used many times. While CD-RW discs may be used in a multisession mode as regular CD-R discs, when erasing data from a RW disc it is necessary to erase all contents from the disc. Before a rewritable disc can be reused, it must be made blank again via the erase command.

DVD+RW and DVD-RAM are media that can be randomly overwritten, without the need for erasing the media first.

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