OPTICAL MEMORY

OPTICAL MEMORY

• ·       The direct storage of data as bits in memory using optical storage and properties.

1)   Compact Disk :

a)   CD –ROM

• ·       The disk is formed from polycarbonate.
• ·       The data on a compact disc (CD) is stored on the disc as a series of         tiny, almost microscopic pits and lands.
• ·       The pit is an indicator of data, roughly equivalent to a "1" in binary       code.
• ·       The lands, or flat surfaces on the CD, are considered to be the "0" in       binary code.
• ·       When data is read from a CD, a laser directs an extremely fine                stream of light onto the surface of the disc. 
• ·       The laser follows the data stream of  pits and lands from the inside         center of the disc outward in a spiral direction.
• ·       As the laser light shines on the CD's data track, it reflects one              pattern of light off of a pit and a different pattern off of a land          area.
• ·       The resulting reflections equate to a series of ones and zeros by a           photo sensor.

          

                a)   CD Recordable  / CD-R :

·       CD-R is a type of write once read many (WORM). 

·       Allows one-time recording on a disc.

·       Composed of a polycarbonate plastic substrate, a thin reflective metal coating, and a protective outer coating.

·       A layer of organic polymer dye between the polycarbonate and metal layers serves as the recording medium.

·       The dye is permanently transformed by exposure to a specific frequency of light. 

·       The laser creates marks in the dye layer that mimic the reflective properties of the pits and lands (lower and higher areas) of the traditional CD. 

·       The distinct differences in the way the areas reflect light register as digital data.

b)   CD Rewritable :

·       CD-RW is a compact disc (CD) format that allows repeated recording on a disc.

·       CD-Rs and CD-RWs are composed of a polycarbonate plastic substrate, a thin reflective metal coating, and a protective outer coating.

·       In a CD-RW, the dye is replaced with an alloy that can change back and forth from a crystalline form when exposed to a particular light.

·       By controlling the temperature of the laser, crystalline areas and non-crystalline areas are formed.

·       The crystalline areas will reflect the laser, while the other areas will absorb it.

The differences will register as digital data.

c)    Digital versatile disk /DVD :

·       A DVD is based  on CD technology with increased storage density.

·        It has a much larger data capacity.

·       DVDs can be single- or double-sided.

·       A two-layered DVD will hold up to 17 gigabytes of video, audio, or other information. 

·       The DVD has a number of layers of plastic pieces with a total thickness of 1.2 millimetres.

·       Poly-carbonate plastic is basically used for making these layers.

             a)   Magnetic Tape  :

·       Uses the same reading and recording techniques as disk systems.

·       Medium is flexible polyester tape coated with magnetizable material.

·       Data on the tape are structured as a number of  parallel tracks running lengthwise.

·       Data are laid out as a sequence of bits along each track.

·       Data are read and written in contiguous blocks called physical records.

·        Blocks on the tape are separated by gaps referred to as inter-record gaps .

     

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HI FRIENDS I AM DHANESH. I ADD THIS BLOG FOR AN EDUCATION PURPOSE

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