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DVD and CD optical drive
 
Most data storage relies on magnetism. Optical media like CDs and DVD instead store data using microscopic variations in the layer of a plastic disk.
 
The variation on the DVD or CD optical media are stamped ( pressed or molded ) onto the surface of the disk or burned into it using a laser , in a spiral pattern that runs from inside to the outside of the disk.
 
CDs consist of a 1.2mm clear polycarbonate plastic disk, with a reflective metal coating on one side. A standard 120mm CD holds about 700 –750Mbytes of data.
 
DVD consist of two half thickness CD like glued back to back , with the reflective coating in the middle. Using a shorter wavelength laser allows higher data densities , so that a single side of a DVD can hold about 4.7Gigabytes. Recording on both sides of the DVD doubles this.
 
 
DVD and CD comes in many formats:-
¨ The most basic form of data holding CD or DVD is one that can only be read.    Formally it is called CD Rom or DVD Rom. ROM stands for “read only memory” , which translates as non writeable.
 
¨ A CD –R , DVD-R or DVD+R may be recorded but only a single time. This write once process uses a light sensitive dye recording layer to which information is irreversibly written by means of a laser heating and altering it to create a pattern.
 
¨ A CD-RW , DVD-RW or DVD+RW may be recorded many times. The RW stands for “rewriteable” . The rewritng process employs a metallic phase change recording layer that can be repeatedly altered and restored by the writing laser.
 
¨ DVD Ram uses phase chage change technology that can be rewritten 100,000times. With its hard sector , DVD ram more closely resembles traditional hard disk storage media than do the DVD-RW and DVD+RW.
 
Almost all computers now come with at least an optical drive capable of reading DVDs.
 
DVD writing drives can be used for data backup , storage and transfer.All optical drives are rated by a multiplier that refers to to the maximum speed the drives operates when compared to the original CD or DVD standard.