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.

Storage density limited by wavelength of light used.

Shift from red to blue laser.

Optical Removable Storage

Video Disk 1970s 1982 CD-DA (Phillips, Sony) 1985 CD-ROM (Phillips, Sony) 1995 DVD (Phillips, Sony, Toshiba,

Time Warner)

CD-ROM

CD-ROM: Standard Example for Optical Storage

CD-ROM ... Spiral track (spiraling outwards) CD pit 0.5 * 2 m

Dust 40 m Human Hair 70 m

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

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

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.

CD-ROM ... Focus laser beam on land area

Sharp reflection from land. Dispersion from pit.

CD-ROM ...

CD-ROM ...

CD-ROM ...

CD-ROM ...

CD-ROM ...

Mistracking detection:

Main beam reads data, side beams tell whether there is a misregistration.

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

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

CD-ROM ...Designation Size Byte NumbersSynchronization 12B 0-11Header 4B 12-15User Data 2048B 16-2063Error Detection 4B 2064-2067Space 8B 2068-2075Error Correction 276B 2076-2351

Table 2: ECMA-130 Media Standard for Block

Layout

CD Formats

CD-Audio ICE 908 Red Book

CD-ROM ISO/IEC 10149 Yellow Book

CD-I   Green Book

Video CD   White Book

CD-recordable ISO/IEC 11172/1/2/3 Orange Book

Logical Format

CD-ROM ISO-9660 High Sierra

CD-Recordable ECMA 168 / IS 13940 Frankfurt Proposal

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.

CD-RW

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.

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.

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.

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.

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.