Optical discs

Discussion in 'General Science & Technology' started by pluto2, Jan 24, 2008.

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  1. pluto2 Banned Valued Senior Member

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    In optical discs, what are light and dark pits? Also what is a dye/polymer?
     
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  3. esp Registered Senior Member

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    I can only speak as a bog-standard electrical engineer....

    My understanding is (and it may be flawed)....


    In normal digital signal encoding / transmission;

    An industrial standard....

    Logical 0
    (or low, or 'off') is (depending upon the standard employed) 2, 2.3, 2.5 or 2.7 volts.

    Logical 1
    (or high, or 'on') is (again depending on the standard employed) 3.2, 3.5, 5 or 5.5 volts.

    This allows a constant current but variable voltage signal to transmit anything from audio to video to communication signal.

    Depending on the 'intelligence' of the partisan components, this protocol can control anything from audio to video output, speed reference from and to servo motors to closed loop controlled media valves.

    Sooo


    If there is a laser reading, say 0.5 to 0.08 mm high and lows, this can be translated to usable data as discussed.

    The light / dark pits are in fact different depths on the disc that a laser reads by virtue of the length of time it takes the laser pulse to returnto it's source, thus giving a logical 1 or 0.
     
    Last edited: Jan 27, 2008
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  5. kmguru Staff Member

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    From How CDs Work, you learn that a normal CD uses microscopic bumps to store data. The surface of the CD is a mirror, and the bumps disrupt the mirror's perfect surface. The laser that reads the CD can detect the difference between a perfect mirror and an imperfection caused by a bump because of the difference in reflectivity. By interpreting "perfect mirror" as a "1," and "bump" as a "0," it is easy to store digital information on a CD. The bumps on a CD are molded into the plastic when it is manufactured, so they are permanent.

    To create a writeable CD (CD-R), you need to modify the surface of a CD so you can burn data onto it, turning it into a CD-R. There are no bumps on a CD-R. A clear dye layer covers the CD's mirror. A write laser heats up the dye layer enough to make it opaque. The read laser in a CD player senses the difference between clear dye and opaque dye the same way it senses bumps -- it picks up on the difference in reflectivity.

    To create a rewriteable CD (CD-RW), you need a dye layer that can be changed back and forth between opaque and transparent. This page discusses the special material that CD-RW's use. The material has the property that it can change its transparency depending on temperature. Heated to one temperature, the material cools to a transparent state; heated to another temperature, it cools to a cloudy state. By changing the power (and therefore the temperature) of the writing laser, the data on the CD can be changed, or "rewritten."

    --from how stuff works.com
     
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  7. pluto2 Banned Valued Senior Member

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    But what are bumps?
     
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