CD-ROM

1. CD-ROM

2. The CD Family • Standards • Red Book (1982) covers CD & CD-DA • Yellow Book (1985) covers CD-ROM • extended to cover CD-ROM/XA • Green Book (1988) covers CD-I • Orange Book (1991) covers CD-R • part II covers Photo-CD • White Book (1993) covers Video CD • HDCD currently under development

3. Compact Disc - History • Compact Discs started with CD-DA • Compact Disc - Digital Audio • Standard music CDs • Became CD-ROM • 650 MB of storage on 12cm optical disc • CD-I (Compact Disc Interactive) • Home entertainment system with built-in processor, can also play music CDs

4. Compact Disc - History (cont.) • CD-ROM/XA (eXtended Architecture) • extends CD-ROM with some of the compressed audio capabilities of CD-I (ADPCM) • permits interleaving of audio and video • All of the above are “mastered” rather than burned, so thousands of copies can be pressed - much cheaper than burning for copies above a dozen or so

5. Structure of CD-ROM • Since CD-ROMs are based on the music CD structure they are 10-20 times slower than hard discs • in order to fit as much music as possible on the disc, the standard was defined on the basis of storing data at the same linear density at the outer edge as at the center, thus there is more data on the outside tracks than the inside tracks.

6. Structure of CD-ROM (cont.) • In order to deal with this, the angular velocity of the disc decreases when the head moves from the center to the outside tracks. Thus for random access to the CD-ROM. The need to accelerate and decelerate the disc is the biggest obstacle to increasing the speed. Most hard discs spin at a constant angular velocity, so data density decreases towards the outside of the disc, but seek time is significantly faster.

7. Structure of CD-ROM (cont.) • Standard CD-ROM • 120mm in diameter, 1.2mm thick, hole 15mm across in center • Data is represented by a spiral of small pits, coated with a reflective metal layer, coated with a protective lacquer • Pits are 0-12m deep and about 0.6 m wide, neighbouring turns of the spiral are 1.6 m apart, giving a track density of 16000 tpi

8. Structure of CD-ROM (cont.)

9. Structure of CD-ROM (cont.) • The transition from pit to land and from land to pit corresponds to a coding of 1 in the digital data stream, 0 is no transition. Land Pit 10000100010000100001000100001000010000

10. Structure of CD-ROM (cont.) • Recording • process is called mastering • waveform carrying the encoded information is transferred to a modulator, controlling a powerful short-wavelength laser beam as it passes through a lens, forming a spot on the photoresist coating of a glass master disc. • Physical negative “stampers” are then developed from the glass master.

11. Structure of CD-ROM (cont.) • The 0s and 1s of the pits and lands do not correspond to the actual user data, as bit resolution is limited by the wavelength of laser light and the lens aperture. Thus adjacent transitions can be too close together to be read. • As a result each user data byte is represented by 17 channel bits • 14 modulation bits from a lookup table • 3 merge bits to separate each symbol

12. Structure of CD-ROM (cont.) • A set of 24 of these 17 bit symbols is combined with a sync pattern (another 24 channel bits with 3 merge bits), a control and display symbol, and 8 error correction symbols to form a frame - the basic unit of storage on a CD-ROM • Frames are grouped in blocks of 98, which occur 75 times per second, giving a CD-ROM data rate of 1.17Mbps (or 1.2288 in US)

13. Recordable CD • CD-R, CD-WO • WORM based • single or multisession • slightly different physical structure, but conforms to CD-ROM/XA • CD- RW • fully rewritable discs, using optical film technology