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Erasable Optical DiskA disk that uses optical technology but that can be easily erased and rewritten. A typical capacity is 650 Mbytes. Both the audio and the CD-ROM share similar technology. The main difference is in the formats of data presentation. Optical Storage CD-ROM à Originally for audio à 650 (775)Mbytes giving over 70(73.2) minutes audio à Poly-carbonate coated with highly reflective coat (aluminum) à Data stored as pits à Reads by reflecting laser à Constant packing density à Constant linear velocity (1.2 m/s)
CD-ROM block Format
12 bytes 4 bytes 2046 bytes 288 bytes Sync Id
z Mode 0 = blank data field z Mode 1 = 2048 bytes data+ error correction z Mode 2 = 2336 bytes data
CD-ROM block Format consists of the following fields: 1. Sync: identifies the beginning of a block; 2. Header: contains the block address and the mode byte; 3. Data: User?s data; 4. Auxiliary: additional user?s data in mode 2. In mode 1, this is 288-bytes error-correcting code.
Random Access on CD-ROM à Difficult à Move head to rough position à Set correct speed à Read address à Adjust to required location
Other Optical Storage q CD-Writable ? WORM ? Now affordable ? Compatible with CD?ROM drives q CD-RW ? Erasable ? Getting cheaper ? Mostly CD-ROM drive compatible
DVD Storage q Digital Video Disk ? Used to indicate a player for movies ? Only plays video disks q Digital Versatile Disk ? Used to indicate a computer drive ? Will read computer disks and play video disks
DVD technology à Multi-layer à Very high capacity (4.7 G per layer) à Full length movie on single disk ? Using MPEG compression à Finally standardised (honest!) à Movies carry regional coding à Players only play correct region films à Can be ?fixed?
DVD-Writable à Loads of trouble with standards à First generation DVD drives may not read first generation DVD?W disks à First generation DVD drives may not read CD-RW disks
Magnetic Tape q Serial access q Slow q Very cheap q Backup and archive
Date: 2016-06-13; view: 53
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