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Understanding Optical Storage: From CDs to DVDs and Beyond

Optical storage technology, utilizing the unique properties of light, has evolved significantly since the introduction of the Video Disk in the 1970s. This includes the development of CDs, CD-ROMs, and DVDs, which have all established a strong market for removable media. Optical storage allows for reliable data storage with advantages like resistance to accidental data erasure and compact design. Despite a small archival storage market, advancements in technology, including blue laser applications, have dramatically increased storage density and data retrieval efficiency, driving innovation in this field.

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Understanding Optical Storage: From CDs to DVDs and Beyond

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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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