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					                               Digital Media Discs
                               By Greg Galloway




                  Above is the image of a dirty disc. Never touch the shiny
                  surface unless you want to invoke the wrath of the owner.




Summary:
        Digital media discs are used every day, but not everyone knows how information
is stored on them. DVDs and CDs store immense amounts of music, video, and data, yet
as close as you look, you cannot see any pictures. This paper aims at expounding upon
the details intrinsic in these storage devices that are used daily.

Brief Physical Description:
        Both types of disc have the same general measurements. Both are 120mm
diameter discs of primarily polycarbonate. The center hole is 15mm in diameter. The
clear area does not hold data, and ranges from a diameter of 15mm to 46mm. The data
region begins at 46mm and ends at 117mm. The area from 46-50mm is the start, or lead
in, region. The 116-117mm range is the end, or lead out, region.
What Do They Do?
        Optical discs store information as a continuous line of bumps or dents (depending
on perspective, explained later). The devices that read this information direct a laser that
spirals outward counter-clockwise from the middle of the disc and determines what has
been stored on the surface.

                                                    For a CD, the tracks of the spiral are 0.5
                                                   microns wide, and separated by a distance of
                                                   1.6 microns1. For a DVD, the tracks are
                                                   separated by 0.74 microns, with the tracks
                                                   being 0.32 microns wide.




    Disc information is stored spiraling out
               from the center.




                              A comparison of the surfaces of CDs and DVDs.




1
 A micron is a millionth of a meter. For comparison, a human hair is 50 to 100 microns in diameter.
(nanopedia.case.edu)
Pits Or Bumps?
       From the perspective of the laser, the surface is covered in bumps. On the surface
of a CD, the bumps are 125 nanometers2 high, for a DVD the bumps are 120 nanometers
high. The minimum length of the bumps is 830 nanometers for a CD, and 400
nanometers for a DVD. They are sometimes referred to as pits due to the way audio CDs
and video DVDs are mass produced. (See references)

                                                                                The surface of a compact disc,
                                                                                or CD. CDs are used
                                                                                primarily for music and data.
                                                                                The spacing between tracks
                                                                                on a CD is 1600 nanometers.
                                                                                On a DVD there are 740
                                                                                nanometers between tracks.




      A DVD uses bumps that are only 400 nanometers in length, however a
      CD has bumps that are 830 nanometers in length (Over twice as long).




Data Marking:
        The information is stored as bits3. Each bump stores information, where a change
        4
in state signifies a one. The values to be stored are decided through a process called
eight-to-fourteen modulation. This modulation gives the data more resilience to damage
and mishandling, such as fingerprints and scratches. (See references for more information.)




2
  A nanometer is one billionth of a meter, or 1/1000 th of a micron (Abbreviated nm).
3
  A bit is a Binary digit.
4
  A state is a bump or a land (flat spot). A change in state would be going from one to the other.
                                                                               Note: Since the label and
                                                                               acrylic are so close to the
                                                                               reflective layer, it is easier to
                                                                               damage the CD with scratches
                                                                               from the top. This is also why
                                                                               care should be taken when
                                                                               writing on CDs.




                                CD cross section, not to scale.



Layer Materials:
A compact disc is composed of layers of various materials, listed from top down:

Label: This is the visual section for human readable information. For professional
productions this is silk-screened onto the top. The thickness of this layer is 20 microns.
Acrylic: This is a protective layer to help prevent scratches on the label side from
corrupting the data patterns on the other side.
Aluminum: This is the reflective layer that allows the laser to register the height
changes. The reflective layer varies from 50nm to 125nm in thickness. Gold and silver
are sometimes used instead of aluminum.
Polycarbonate plastic: This layer forms the mold that the aluminum is impressed upon.
This Polycarbonate layer is 10 to 30 microns thick. It also provides a protective barrier
limiting the effects of scratches and lending strength to the CD as a whole.

Dual Layer DVD Technology:
            Dual layer DVD players use laser focusing to read the subsurface layer. The
laser can travel through the polycarbonate layers without significant loss. The first
reflecting layer is semi-transparent, allowing the laser to be refocused to ‘see through’ to
the second layer. This is analogous to a one-way mirror. The light on one side, if bright
and focused, reflects almost entirely. But if diffused, it will be reflected back from the
focus point farther in.


                                                                   Layer Translations:
                                                                   Label

                                                                   Polycarbonate Base

                                                                   Metal Reflecting Layer
                                                                   Second Recording Layer

                                                                   Polycarbonate Spacer
                                                                   Semi-transparent Reflector
                                                                   First Recording Layer

                                                                   Polycarbonate Base
Storage Comparison:
           DVD                                    Capacity                Movie Duration
                                              5
Single-sided/single-layer            4.38GB                         2 hours
Single-sided/double-layer            7.95GB                         4 hours
Double-sided/single-layer            8.75GB                         4.5 hours
Double-sided/double-layer            15.9GB                         8-9 hours

            CD                               Capacity                    Music Duration
Single-sided/single-layer            650-754MB                      64-75 minutes




Further Information6 and References:
General DVD information: http://electronics.howstuffworks.com/dvd.htm
General CD information: http://electronics.howstuffworks.com/cd.htm
Modulation information: http://en.wikipedia.org/wiki/Eight-to-Fourteen_Modulation
More on pits and bumps: http://www.kitchenmastering.com/database/1140477795.html
In-Depth CD information: http://www.chipchapin.com/CDMedia/index.php3


Images in this document were drawn from the above sources.




5
    One GB (gigabyte) is 1,000 Megabytes, or 1,000,000,000 bytes.
6
    These web pages are valid as of 02/25/2007.

				
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