Compact Disc Recording Technology Extended

1
Compact Disc Recording Technology - Extended

• Simon So
• 04 November 2003

2
Objective Statement

• We can understand how a CD work and apply the
  same concept in molecular screening technology.
• My job is to understand how the CD and recordable
  CD work and try to assist in future project in
  molecular screening on a CD.

3
Progress

• Much of the effort was spent in trying to read
  and understand the paper on Molecular screening
  on a compact disc.
• This is an update to the last presentation.

4
Recap - 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 diffracted
  beams involved in a three-beam optical design)
• 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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Optical System the LASER

• AlGaAs (Aluminum Gallium Arsenide) laser is used
  in CD readers.
• The wavelength of this monochromic laser is 780nm
  in air, which is a standard in current compact
  disc system. (Some manufacturer would produce CD
  readers to have a wavelength up to 790nm.)
• Power of the laser 0.5mW.

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Correction 1 from last presentation

• The mechanism in reading a CD involves
  diffraction.
• When the laser is shone onto the bump, the
  reflected beam is in phase.
• When the laser is shone onto the land, the
  reflected beam is out of phase.

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In-Phase/Out-of-Phase Difference in Laser
Reflection

• The AlGaAs laser diffracts when it passes through
  a diffraction grating.
• A diffraction pattern occurs only the first two
  orders maxima are of our concern.

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In-Phase/Out-of-Phase Difference in Laser
Reflection

• The first-order maximum is used for reading data
  and focusing.
• The second-order maxima are used for tracking
  only.
• Let ? be the phase of the diffracted beams.

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In-Phase/Out-of-Phase Difference in Laser
Reflection

• Assume that the height of the bump is ?/4, and
  height to travel one-way to the bump by the laser
  beams is ?/2. (I can make that assumption because
  it is designed so that the phase difference of
  the bump and land is ?/4.
• The total height travelled to the bump is ?/2
  ?/2 ?.
• The total height travelled to the land is ?/2
  ?/4 ?/4 ?/2 ? ?/2 3?/2.

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Correction 2 from last presentation

• It is a 0 when the phase of the reflected beam
  does not change and 1 when the phase of the
  reflected beam is changed.

11
Polymer Layer

• The hard plastic polymer of a CD is a bisphenol A
  type of polycarbonate.
• It is produced by reacting bisphenol A with
  phosgene in an industrial scale.

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Polymer Layer

• Currently, I am still looking for more
  information on the molecular composition of
  different layers of a CD.

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Data Organisation Revisit Error Detection and
Correction

• Recall that pits width is about 0.5?m.
• With error correction/detection mechanism, we
  could avoid/minimise data loss due to small
  damage to the CD.

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Error Detection and Correction

• Error detection and correction is achieved by
  duplicating data stored on the disc.
• For example
• Chemistry is going to be my field.
• Chemistry Rocks! Chemistry Rocks!

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Error Detection - Parity

• Before we can correct an error, we have to first
  detect and locate the error.
• A basic way of detecting error is called parity.
• Parity is a or several binary digits (now shall
  be called bit) that is/are added to the end of a
  binary string to identify error status in such
  string.
• Two states of parity
• Even parity
• Odd parity

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Error Detection - Parity

• Even parity bits in concern will have an even
  number of 1s used to identify a normal
  condition.
• Odd parity bits in concern will have an odd
  number of 1s used to identify an abnormal
  condition.
• Bits in concern are the bits that we use to test
  for binary string validity.

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Error Detection - Parity

• For example
• 1011
• We can design a simple logic circuit that could
  generate 3 parity bits from any 4-bit binary
  string.
• We can use bit number 1,2,3 to generate the first
  parity bit 1,2,4 to generate the second and
  1,3,4 to generate the third.

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Error Detection - Parity

• For example
• 1011 001
• bit string parity bits
• In an initial stage, we want to establish a
  normal condition even parity in all cases.
• We can combine the bit string with the parity bit
  for storage. (ie. Store the bit string 1011001)

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Error Detection - Parity

• For example
• 1011 001
• bit string parity bits
• To detect if there is an error, we check every
  parity bit to see if they match.
• 101X 0XX -gt even
• 10X1 X0X -gt even
• 1X11 XX1 -gt even
• Normal condition everything even.

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Error Detection - Parity

• After a scratch
• 1111 001
• bit string parity bits
• Error detection process
• 111X 0XX -gt odd
• 11X1 X0X -gt odd
• 1X11 XX1 -gt even
• Odd cases bit 2 has an error because we detected
  an error in 1,2,3 and 1,2,4 case and no error in
  the 1,3,4 case.

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Error Detection - Parity
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Error Correction

• Once we found the location of error, we can flip
  the bit that is in error and it would be the
  right bit. (The formal terminology is
  complementation of the bit)

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Error Detection Limitation of Parity

• Let say that both bits 1 and 2 are in error, our
  even/odd parity method does not work since if two
  bits are flipped, it would still be the same
  parity, and thus error will not be caught or
  incorrectly caught.
• A more sophisticated way of error detection
  method shall be developed.

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Cross Interleave Reed-Solomon Code (CIRC)

• This is the actual format that CD-ROM uses to
  encode data and store onto the CD.
• The basic idea is the same with parity.
• I will need some time to find more information
  about it.