CD, CDR, CDRW

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CD, CD-R, CD-RW

• David Turton
• Conestoga College
• Institute of Technology and Advanced Learning
• http//www.conestogac.on.ca/dturton
• Doon 1B43 X3610

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Compact Disks
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Commercial CD's

• Plastic disk is cast or etched with bumps flats
• Spiral track .5 microns wide, 5 km long
• Aluminum is vaporised onto marked surface
• Forming a reflective layer, viewed through the
  plastic
• Acrylic surface label protect the aluminum

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Bump Flat become 1 0

• CD spins over a read laser
• Light on bump reflects into receptor, becoming a
  1 bit
• Light on flat reflects away from receptor,
  becoming a 0 bit
• Laser tracks spiral from centre out
• Outside edge travels faster than centre
• CD spin slows as laser moves out
• Bits pass laser at constant rate

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CD-R

• CD-R better for home CD production
• transparent dye between clear plastic aluminum
• Light passes reflects, making a 1
• "write" laser heats dye, making it opaque
• Spot absorbs light, making a 0
• "read" laser not strong enough to affect dye
• Contrast good enough for most CD readers

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CD-RW

• Dye layer replaced with a crystalline phase
  change compound (silver, antimony, tellurium,
  indium)
• Erase laser heats to 200oC, forming a translucent
  crystal
• Write laser heats to 600oC, forming an opaque
  "liquid"
• Read laser not strong enough to affect it
• Reflect/non-reflect (1/0) contrast not strong
  enough for some CD readers

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Trailer Track

• Bit of track before location 0000 on CD-R's and
  CD-RW's
• PMA (power memory area)
• Temporary table of contents
• For partially recorded disks
• Used to create final TOC when you "complete" the
  disk
• PCA (power calibration area)
• Testing ground for burner
• Makes series of test marks
• Reads back to check intensity of reflection
• Used to set optimum laser setting for writes

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Minimising read errors (music CDs)Data CDs have
more complex standards cannot lose any bits

• Table of Contents start of disk (i.e. centre)
• describes song/file locations
• Track marked with timecodes
• Reader knows where it is
• Long sections without data
• Read laser could lose its place
• Filler data used in these places
• Error checking correction
• Extra bits to identify a missing bit recreate
  it
• Example Hamming Code 12 bits per byte
• Data is interlaced, not recorded sequentially
• Scratch on disk
• Lose bits of different parts of music
• Don't lose a whole segment
• Reader can correct or compensate

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DVD, compared to CD

• can have 2 layers of bumps flats per side
• Semi-reflective gold is used on top layer
• Part of laser passes through to aluminum layer
• can use both sides
• have smaller bumps
• .320 microns vs. .5 microns
• Tracks are ½ as wide 9x longer
• 12km4 vs. 5 km1

1 micro 1 millionth of a metre 1 nanometre 1
billionth of a metre