OPTICAL MEMORY

CD – Compact Disk 

• 1983
• Non-erasable
• Stores digitized audio
• 12 cm disk
• 60 minute uninterrupted playing time

CD-ROM – Compact Disk Read-Only Memory 

• Non-erasable
• 12 cm
• 650 Mbyte capacity
• Disk made of polycarbonate resin
• Stores computer data as a series of microscopic pits on the surface of a
polycarbonate resin using a highly focused laser
• Polycarbonate coated with highly reflective material such as aluminum
• Acrylic coating added for protection – label printed on acrylic coating
• Data read using reflecting laser – reflection intensity determines value
• Disk contains a single spiral track – starts at the center ends at the edge
• Constant linear velocity – disk rotates slower towards the edge


CD-R – CD Recordable 

• Similar to CD-ROM
• Can be written to once
• Medium includes a dye layer – high intensity laser used to changed reflectivity
• Provides a permanent record of large volumes of user data

CD-RW – CD Re-writable

• Similar to CD-ROM
• Can be written to and erased multiple times
• Phase change – two different reflections in two different phase states – laser
used to change state
o Amorphous state – molecules have random orientation – poor reflections
o Crystalline state – smooth surface – good reflections
• The materially eventually and permanently loses its properties – currently
500,000 to 1,000,000 erase cycles
• Can be used as a true secondary storage
• Optical disk engineering tolerances much less severe than magnetic disk –
higher reliability and longer life


DVD – Digital Versatile Disk 

• Originally for producing digitized, compressed representations of video
information
• Also used for storing large volumes of other digital data
• 8 and 12 cm disks
• 17 Gbyte capacity
• Basic DVD is read-only (DVD-ROM)


CD-ROM and DVD-ROM 

• Bits packed more closely on a DVD
o Spiral spacing on a CD is 1.6 um – distance between pits is 0.834 um
o Spiral spacing on a DVD is 0.74 um – distance between pits is 0.4 um
o Shorter wavelength laser
o Seven-fold increase in capacity over CD-ROM – 4.7 Gbyte

• DVD uses two layers of lands and pits
o Semi-reflective top layer - reflective under layer
o Laser can adjust focus to read each layer separately
o Double the capacity – 8.5 Gbyte

• DVD-Rom can be two sided 
o Double the capacity again – 17 Gbyte


DVD-R – DVD Recordable 
• Similar to DVD-ROM
• Can be written once
• One-sided disks only

DVD-RW – DVD Rewritable 
• Similar to DVD-ROM
• Can be written to and erased multiple times
• One-sided disks only


POSTED BY;

HAZYANTY SHAHIRA ABD. HALIM