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Optical Storage Types: CD-R and CD-RW

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CD-Rs and CD-RWs maintain a lot of the features of CDs and CD-ROMs (as discussed in the previous article); that’s why they’re still called compact discs in the first place! This pair highlights the improvements in optical storage technology.

CD-R (CD-Recordable)

CD-Rs are WORM (Write Once, Read Many) discs that give people a cheaper means of storing data in compact discs. Unfortunately, the arrival of these low-cost, easy to use CD-Rs also helped spread software, music, and movie piracy.

CD-Rs look like regular CD-ROMs except that they are gold-colored instead of silver. This is due to the use of gold instead of aluminum, which is used in regular CDs. These discs contain 0.6mm-wide grooves that guide the laser for writing data.

If the regular compact discs have the Red, Yellow, and Green Books, CD-Rs have their own Orange Book, which documents the medium’s technical specifications and standards.

CD-RW (CD-Rewritable)

CD-RWs represent the next step in CD technology after the CD-R. These discs use the same standard size of CD-Rs but are more expensive than them. CD-RWs allow many writes onto the compact disc, in contrast to the WORM CD-R.

Keep in mind that CD-Rs aren't rendered obsolete with the availability of CD-RWs. CD-Rs can still be used as cheaper backup media, as well as more secure storage – unlike CD-RWs, CD-Rs can only be written once, which means you can't accidentally erase the stored data with another write process!

In CD-RWs, an alloy of silver, indium, antimony, and tellurium replaces the cyanine/pthalocyanine dye for the recording layer. This alloy provides two stable states that have different ‘reflectivities’:

  • Crystalline – High reflectivity, represents land (see definition in the first CD article [link])
  • Amorphous – Low reflectivity, represents pit (see definition in the first CD article [link])

Reading and writing processes for CD-RWs use different magnitudes for the laser:

  • High power – Converts alloy from crystalline to amorphous state
  • Medium power – Melts alloy in order to bring it back to its crystalline state
  • Low power – Used in reading the CD-RW (that is, to sense the alloy’s state)

Next: DVD



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