Slide 4.10: The cost of magnetic-tape accesses
Slide 4.12: DVDs (Digital Versatile Discs)
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CDs (Compact Discs)

CD is a polycarbonate with one or more metal layers capable of storing digital information.

A 4.72 inch disc can store still and/or moving images in monochrome and/or color; stereo or two separate sound tracks integrated with and/or separate from the images; and digital program and information files. There are three kinds of CDs:
  1. The CDs are those used by the music industry to store digital recordings.

  2. CD-ROMs used to store computer data: The same fabrication process is used to make both first and second kinds of CDs, the only difference is in the device used to read the CD (the player or drive). Both of these types of compact disc are read-only, which means that once the data has been recorded onto them, they can only be read, or played.

  3. CD-Rs and CD_RWs: They can have their data erased and overwritten by new data. Currently, erasable optical storage is too slow to be used as a computer's main storage facility, but as the speed improves and the cost comes down, optical storage devices are becoming a popular alternative to tape systems as a backup method.
The figure shows a cross section of a compact disc. It is built from a layer of polycarbonate plastic, covered in a color-dyed layer of aluminum, and followed by a protective layer of lacquer.