Problem with "Memory with Moving Parts"

Discussion in 'Feedback and Suggestions' started by Chris01720, Mar 25, 2008.

  1. Chris01720

    Thread Starter Guest

    In Chapter 5 of Volume 4, under "Memory with moving parts" it is stated that with CD's "Binary bits are 'burned' into the aluminum as pits by a high-power laser". This is factually incorrect because commercial CD's (the ones with pits) are pressed in a stamp. There is no laser or burning involved in the creation of the finished CD. The only CD that is "burned" are CD-Rs. CD-R's don't have pits. With CD-R's, a laser burns into a dye causing it to either let light pass through the dye (to the reflective aluminum) or not let light pass through the dye.
     
  2. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    Thanks for the feedback Chris.

    You are right that commercial CDs are not burnt like CDs written to by CD-burners are. I was always under the impression that the tracks were moulded or pressed on as data layers - is this the stamping process to which you refer?

    Dave
     
  3. Dcrunkilton

    E-book Co-ordinator

    Jul 31, 2004
    416
    11
    Thanks Chris por pointing out the error.

    This text referenced here:
    http://www.allaboutcircuits.com/vol_4/chpt_15/6.html

    Has been changed at ibiblio.org

    From:
    This technology is a read-only type, the media being a thin film of aluminum foil embedded in a transparent plastic disk. Binary bits are "burned" into the aluminum as pits by a high-power laser.

    To:
    This technology is a read-only type, the media being a transparent plastic disk backed by a thin film of aluminum. Binary bits are encoded as pits in the plastic which vary the path length of a low-power laser beam.

    This link has some diagrams showing the process of making the stampings from a glass master.

    http://www.softpack.com/CDROMmanufac.htm
     
  4. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    I concur with the corrections. Thanks Dennis.

    Dave
     
  5. Chris01720

    Thread Starter Guest

    Dave, that is what I was referring to. My understanding is that the pits are pressed using a pre-made stamp. I agree with the changes that were made to the article. The only thing I would change is the word "disk". I believe when referring to optical media the correct spelling is "disc".
    I love this site and can't wait for Volume III to be finished. This has been a much better resource than my EE professor. Unfortunately, over the past couple years we've covered material that wasn't always in All About Circuits at the time. Keep up the good work everyone.
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Since we have been discussing the difference between levels and transitions for signalling purposes in another thread, this is a good place to illustrate the distinction.

    The signal is not the light or dark as seen by the receiver but the transition from one to the other which is translated into 1 bits. This is interesting as both light/dark and dark/light transitions are seen as ones, with no change seen as zeros.
    This is similar to the DDR process in memory using both rising and falling edges to signify data.

    Light is reflected from both pits and lands but, the height of the land is exactly 1/4 wavelength (of the laser light) above the pit so the light travels an extra 1/4+1/4=1/2 wavelengths from a pit compared to light from a land.
    The 1/2 wavelength means that the light is exactly 180 out of phase so destructively interferes with the incident light and so the pit is seen as dark.

    With computer recordable disks (CD-R, DVD-R etc) there is one long groove and thefore adjacent land pressed into the disk at manufacture. This is coated with an organic dye which is transparent to the laser beam. So the virgin disk is seen as one long land or series of 1s.
    When data is written to the disk with the write laser, the frequency is just correct to cause a chemical change to (partial) opacity in the dye layer at the spots the laser hits.
    Thus less or non reflective spots are created on the disk, corresponding to the pits on a stamped CD.


    Please feel free to push this ramble into literary shape or ask for any more details if you feel it is a useful addition to your info.
     
  7. Dcrunkilton

    E-book Co-ordinator

    Jul 31, 2004
    416
    11
    Thanks for the correction. I have entered the correction in the copy at ibiblio.org
     
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