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External Memory (2)

Chapter4: Memory. External Memory (2). Optical Storage CD-ROM. Originally for audio 650Mbytes giving over 70 minutes audio Polycarbonate coated with highly reflective coat, usually aluminium Data stored as pits Read by reflecting laser Constant packing density Constant linear velocity.

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External Memory (2)

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  1. Chapter4: Memory External Memory (2)

  2. Optical Storage CD-ROM • Originally for audio • 650Mbytes giving over 70 minutes audio • Polycarbonate coated with highly reflective coat, usually aluminium • Data stored as pits • Read by reflecting laser • Constant packing density • Constant linear velocity

  3. CD Operation • laser shines thru the clear polycarbonate while a motor spins the disk pass it • intensity of reflected light changes as it encounters pits • smooth surface reflects back at high density • area between pits called land. Change between pits and lands detected by photo-sensor and converted into a digital signal

  4. CD-ROM Drive Speeds • Audio is single speed • Constant linier velocity • 1.2 ms-1 • Track (spiral) is 5.27km long • Gives 4391 seconds = 73.2 minutes • Other speeds are quoted as multiples • e.g. 24x • Quoted figure is maximum drive can achieve

  5. CD-ROM Format • Consists of Sync, header, data & auxiliary fields • Header contains block address & mode byte • Mode 0=blank data field • Mode 1=2048 byte data+error correction • Mode 2=2336 byte data

  6. Random Access on CD-ROM • Difficult • Move head to rough position • Set correct speed • Read address • Adjust to required location • (Yawn!)

  7. CD-ROM for & against • Large capacity (?) • Easy to mass produce • Removable • Robust • Expensive for small runs • Slow • Read only

  8. Other Optical Storage • CD-Recordable (CD-R) • Write once, read many • WORM • Now affordable • Compatible with CD-ROM drives • CD-RW • Erasable • Getting cheaper • Mostly CD-ROM drive compatible • Phase change • Material has two different reflectivities in different phase states

  9. DVD - what’s in a name? • Digital Video Disk • Used to indicate a player for movies • Only plays video disks • Digital Versatile Disk • Used to indicate a computer drive • Will read computer disks and play video disks • Dogs Veritable Dinner • Officially - nothing!!!

  10. DVD - technology • Multi-layer • Very high capacity (4.7G per layer) • Full length movie on single disk • Using MPEG compression • Finally standardized (honest!) • Movies carry regional coding • Players only play correct region films • Can be “fixed”

  11. DVD – Writable • Loads of trouble with standards • First generation DVD drives may not read first generation DVD-W disks • First generation DVD drives may not read CD-RW disks • Wait for it to settle down before buying!

  12. CD and DVD • DVD – bits are packed more closely compared to CD • DVD – employs 2nd layer of pits and lands on top of the 1st layer. Almost double the capacity • DVD- ROM can be 2 sided. CD only one side.

  13. High Definition Optical Disks • Designed for high definition videos • Much higher capacity than DVD • Shorter wavelength laser • Blue-violet range • Smaller pits • HD-DVD • 15GB single side single layer • Blue-ray • Data layer closer to laser • Tighter focus, less distortion, smaller pits • 25GB on single layer • Available read only (BD-ROM), Recordable once (BR-R) and re-recordable (BR-RE)

  14. Optical Memory Characteristics

  15. Magnetic Tape • Serial access • Slow • Very cheap • Backup and archive • Linear Tape-Open (LTO) Tape Drives • Developed late 1990s • Open source alternative to proprietary tape systems

  16. Linear Tape-Open (LTO) Tape Drives

  17. END OF EXTERNAL MEMORY

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