Hardware

Compact disk – CD-R, CD-RW, CD-ROM — learn about digital optical data storage

Co-developed by Philips and Sony and released in 1982, a compact disk is a digital optical data storage device, basically a thin circular disk. At the time of their development Compact Disks(CD) stored more data than the hard drives of personal computers, this created a huge demand for the CDs at that time. The data in a CD is stored in the form of pits and lands. A CD player reads data with the help of these pits and lands and convert it into a digital binary system, hence it is a digital storage.

Pits – It does not reflect the laser beam and represents a ‘0’

Lands – It reflects the laser beam and is represented as ‘1’

A compact disc has a very large number of microscopic pits and lands to store the data.

Material used in a Compact disk

A compact disk has 120mm diameter and is of 1.2mm thickness. The material used in a compact disk is –

  1. Polycarbonate base layer – pits and lands are formed here
  2. Aluminium coating over the base layer
  3. A clear acrylic layer over the aluminium layer for protection

Types of Compact disks

  1. CD-R
  2. CD-RW
  3. CD-ROM

CD-R (Compact Disk Recordable)

This type of Compact disk is written once and read many times. The data is once written on CD-R and then it can be read many times, but cannot be erased or changed. Based on the quality of the disk and if handled carefully, CD-R can be read many times for a long period of time.

  • The polycarbonate layer is covered with an Organic dye,
  • This is then covered by a reflecting layer of silver, silver alloy or gold (very rarely).
  • And for protection a coating of photo polymerizable lacquer is done.

CD-R was used for recording audio songs

CD-RW (Compact Disk ReWritable)

A Compact Disk that is writable, then used many times and on need, can be erased and rewritten. For this reason, CD-RW cannot be played by simple CD-R readers, laser readers that are specially made for CD-RW are used for this purpose.

It has the same layers as that of CD-R, but differs in reflective layer. Reflective layer in CD-RW is made up of silver-indium-antimony-tellurium alloy. This allow is crystalline in normal state, but when heated by a laser beam, it gets liquefied and converts into amorphous state.

  • Amorphous state loses reflectivity and represents ‘0’
  • Where crystalline solid is present, light is reflected and it represents ‘1’

CD-RWs were used to temporarily store backup files.

CD-ROM (Compact Disk Read Only Memory)

CD-ROM has same structure as that of CD-R. But it cannot be written. It can only be read. CD-ROM was used by software companies to distribute software and also by gaming industry to distribute computer games.

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You may also like to read

Click here to learn about hard disk drives

Click here to learn about Solid state drives

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