Compact Disc Recording Technology Overview

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Compact Disc Recording Technology Overview

• Simon So
• 23 September 2003

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What is a Compact Disc?General Specification

• A thin piece of flat circular donut-shaped
  plastic that can store data coded in binary (0s
  and 1s).
• The disc itself is 12.0 cm in diameter with a 1.5
  cm diameter hole in the centre.
• The data is stored starting from a 4.6 cm
  diameter location up to 11.7 cm diameter.

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What is in the Compact Disc?A Microscopic
Overview
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What is in the Compact Disc?A Microscopic
Overview

• On the surface of a CD, there are spots that are
  dented (either truly or imaginarily).
• The length of the dented area may range in sizes
  depending on data contained in them.

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What is in the Compact Disc?A Microscopic
Overview

• The dented areas (from the label-side
  perspective) are called pits and the rest are
  called land (kind of like digging pits on flat
  land).
• The width of the pits is 0.5 micron where width
  between pits is 1.6 micron, as shown on the
  diagram below.

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What is in the Compact Disc?Data Organisation

• Pits are arranged in a single spiral track, much
  like the phonographs that people used to have in
  the old days.
• The reader has to be able to follow and read from
  the spiral tracks quickly.
• As the the image on the left imply, the CD is
  read from the inner tracks to the outer tracks.

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What is in the Compact Disc?Data Organisation

• What I know about data organisation of a compact
  disc is up to this point.
• This is the most difficult and time-consuming
  part of information to find, so please give me
  feedback on how much you want to know.

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What is in the Compact Disc?Error
Correction/Detection

• Sometimes when we handle CDs, we might damage
  the surface of it.
• Recall that the pits on a CD is only 0.5 micron
  in width even if we touch the recording surface
  with our greasy finger would completely destroy a
  big chunk of data.
• Error Correcting Code (ECC) is used when a CD is
  written and read to ensure data can be recovered
  (to a maximum of 30) when parts of the data are
  destroyed.

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How to read a CD?
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How to read a CD?

• Reading a CD involves shining a laser beam to it
  and detect the beam that is reflected by the
  optical sensor. (actually, there are 3 beams
  involved)
• The optical sensor then outputs high and low
  voltage levels and is interpreted by a computer
  whether it is a 0 or a 1.

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How to read a CD?

• The actual mechanism in reading a CD involves
  diffraction.
• When the laser is shone onto the land or on a
  bump, the reflect beam is in phase.
• When the laser is shone onto the edge of a bump,
  the reflect beam is out of phase.

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How to read a CD?

• It is a 0 when the reflected beam is in phase and
  1 when the reflected beam is out of phase.

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How to read a CD?

• From our understanding of circular motion, with
  constant angular speed ?, v r?, where v is the
  tangential speed and r is the distance from
  centre, we know that as the an object move away
  from the centre, the tangential speed is faster.

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How to read a CD?

• Similarly, as the reader component follows the
  spiral track, the speed which the data is read is
  increasing as the spiral moves away from the
  centre.
• A disc drive system has to decrease the angular
  speed so that the laser system could detect
  signals at a constant rate.

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The CD-ROM Cross Section
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Three Common Types of CD

• Conventional CD-ROM
• As the acronym implies Compact Disc Read
  Only Memory (CD-ROM)
• Manufacturer stamps the polycarbonate plastic
  to create pits on the surface of polymer.

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Three Common Types of CD

• Conventional CD-ROM
• A thin coat (50-100 nm) of reflective metal
  layer, usually made of aluminum or silver, covers
  the dented side of the stamped polycarbonate
  plastic.
• Gold is not frequently used in this type of CD
  because it is expensive.
• Copper gives a strange appearance when it is
  coated and thus is not used often.

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Three Common Types of CD

• Conventional CD-ROM
• A coat of acrylic lacquer covers the top of the
  CD to protect the reflective metal layer.
• Usually on a commercial CD-ROM, there would be a
  label embedded on top of the lacquer layer for
  identification.

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Three Common Types of CD

• Conventional CD-ROM
• As it is a mechanically stamped CD, it physically
  has dents (or bumps from the polycarbonate-layer
  prospective) to be recognised by the reader.

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Welcome to the Napster Generation!

• We can write our own CDs and be able to play it
  on a standard CD player.
• Here comes CD-Recordable (CD-R) and CD-ReWritable
  (CD-RW)

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Three Common Types of CD

• CD-Recordable (CD-R)
• The composition of a CD-R is similar to a
  conventional CD-ROM except that they do not
  physically stamp CD-R.
• A coat of permanent photosensitive dye is covered
  on the polycarbonate surface so that it could be
  darken by a high power laser in the burner.

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Three Common Types of CD

• CD-Recordable (CD-R)
• The darkened area on the disc is similar to those
  bumps on the conventional CD-ROM the edge of
  the darkened spots represents a 1.
• The lacquer layer is not present in the CD-R
  since it has a smooth metal layer.

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Three Common Types of CD

• CD-ReWritable (CD-RW)
• Instead of a permanent dye coated between the
  reflective layer and the polycarbonate, on a
  CD-RW, a phase changing compound (composes of
  silver, antimony, tellurium, and indium).

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Three Common Types of CD

• CD-ReWritable (CD-RW)
• This phase changing compound has two forms
• Crystalline form, which is translucent and,
• Amorphous fluid form, which absorbs most lights.

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Three Common Types of CD

• CD-RW
• The phase-change compound of a new CD-RW is in
  crystalline phase.
• The high power burning laser heats the compound
  to its melting point and quickly cools it off so
  that crystals could not be formed ? it then is a
  amorphous fluid.

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Three Common Types of CD

• CD-RW
• If time is given to cool after the compound is
  heated, crystals are allowed to form ? it then is
  in crystalline form.

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Summary

• A CD contains different thin layers, each
  performing different tasks.
• The real or imaginary dents on the CD are
  arranged in spiral tracks.
• Due to the microscopic feature of the dents, an
  error correction/detection method is required to
  ensure data protection on the CD.

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Summary

• Reading of a CD is achieved by shining a laser to
  the CD and detect the property of reflected beam.
• There are three well-known types of CD in the
  market conventional CD-ROM, CD-R, and CD-RW.
• Writable and Rewritable CDs contains a certain
  compound that changes property when a strong
  laser shone through.