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 –
- Polycarbonate base layer – pits and lands are formed here
- Aluminium coating over the base layer
- A clear acrylic layer over the aluminium layer for protection
Types of Compact disks
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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