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COEN 180

COEN 180. Optical Storage. Optical Storage. Store data based on the optical properties of a device. Strong, established market for removable media. Small market for archival storage. WORM devices. Magneto-Optical Harder to erase data accidentally.

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COEN 180

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  1. COEN 180 Optical Storage

  2. Optical Storage • Store data based on the optical properties of a device. • Strong, established market for removable media. • Small market for archival storage. • WORM devices. • Magneto-Optical • Harder to erase data accidentally. • Storage density limited by wavelength of light used. • Shift from red to blue laser.

  3. Optical Removable Storage • Video Disk 1970s • 1982 CD-DA (Phillips, Sony) • 1985 CD-ROM (Phillips, Sony) • 1995 DVD (Phillips, Sony, Toshiba, Time Warner)

  4. CD-ROM CD-ROM: Standard Example for Optical Storage

  5. CD-ROM ... • Spiral track (spiraling outwards) • CD pit 0.5 * 2 m • Dust 40 m • Human Hair 70 m

  6. CD-ROM ... • Disk Geometry • 12 cm diameter • 1.2 mm thick • 1.5 cm center hole • Spins at • Constant linear velocity (CVL) • Constant angular velocity (CAL) • Areal density is 2Mb/mm2

  7. CD-ROM ... • 74 minutes of music • 2 channels • 44,100 samples/channel/second • 2 B / sample • 74 minutes • 60 sec/minute • = 783,216,000 B

  8. CD-ROM ... • X-rating • 1X = 150 KB/sec • Up to 12X use CLV • Adjust speed so that tracks are read at constant speed. • After 12X use CAV • Easier to build faster motors. • Data rate now varies. • X-rating is then an average.

  9. CD-ROM ... • Focus laser beam on land area • Sharp reflection from land. • Dispersion from pit.

  10. CD-ROM ...

  11. CD-ROM ...

  12. CD-ROM ...

  13. CD-ROM ...

  14. CD-ROM ... Mistracking detection: Main beam reads data, side beams tell whether there is a misregistration.

  15. CD-ROM ... • Data encoding • In principle, pits could be ones, lands zero, or vice versa. • But long sequences are hard to detect. • Use EFM modulation

  16. CD-ROM • EFM • 8-14 modulation • code words of length 14 bits that consists of single ones separated by two to nine zeroes • 0000 1010  1001 0001 0000 00. • The pits range in length from 3 bits to 11 bits. • http://www.physics.udel.edu/~watson/scen103/efm.html

  17. CD-ROM ... Designation Size Byte Numbers Synchronization 12B 0-11 Header 4B 12-15 User Data 2048B 16-2063 Error Detection 4B 2064-2067 Space 8B 2068-2075 Error Correction 276B 2076-2351 Table 2: ECMA-130 Media Standard for Block Layout

  18. CD Formats

  19. CD-R CD-R disks have a layer of dye over the reflecting aluminium layer. The dye is photo-sensitive. In normal state, the dye is translucent, but after heating at a given frequency of light, the dye turns opaque, allowing us to store the data. A CD-R optical heat needs two laser light sources, to generate the write and the less intense read light. A CD-R disk has grooves pressed in to enable tracking with the read laser.

  20. CD-RW

  21. CD-RW • Can erase, i.e. undo the effect of a write. • Use phase shift compounds • Two phases of different reflectivity. • Compound of • Silver • Antimony • Tellurium • Indium. • Melts at 600o C • If it cools rapidly, it remains in a fluid, amorphous state that is quite opaque. • If the cooling is slower, then the compound crystallizes into a quite translucent form at around 200oC.

  22. CD-RW • The read layer in a CD-RW burner has not enough power to melt the compound. • The higher power erase layer can heat the compound to the crystallization point and restore the original crystallization of the compound.

  23. DVD • Same basic technique as CD • DVDs can use double layering • Two reflective layers on a single surface. • The inner one is made of aluminum (as for CD-ROM) • The outer one is a semi-reflective gold layer through which the laser can focus on the inner layer. • Maximum data capacity of 17GB, • DVDs can be in a double sided, double layered format, where a total of four surfaces stores data.

  24. DVD • DVDs achieve higher data density per layer by halfing the dimensions of the pits / bumps on a CD: • track pitch 740nm vs 1600 nm • minimum pit length is 400 nm (440 nm on a double layer DVD) vs 830 nm. • Since data is stored on a surface, this effectively quadruples the data density. • Use of higher frequency light with a wave length of 640 nm as opposed to 780 nm for the laser used in CD-ROM.

  25. DVD • The error correction scheme on a DVD is much more sophisticated and powerfull than the one used for the CD-ROM standards, • Progress in the theory of ECC • Progress in the processing of data.

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