How a CD is spun in drive??

Discussion in 'Digital Audio' started by G.T., May 5, 2010.

  1. G.T. macrumors 6502a

    Joined:
    Jul 12, 2008
    #1
    This to me makes no sense as it just doesn't follow basic physic's etc.

    "Audio CD players read their discs at a constant 150 kB/s and thus must vary the disc's rotational speed from around 500 rpm (actually 8 Hz), when reading at the innermost edge, to 200 rpm (actually 3.5 Hz) at the outer edge.[1] CD-ROM drives’ maximum rotational speeds are rated in multiples of this figure, even though they do not hold to constant read speeds when reading from most disc formats."

    Surely if u drew a line from centre to out point on the disk and marked two points at either end, no matter how fast the disk is spun both points will complete a revolution at the same time??
     
  2. spinnerlys Guest

    spinnerlys

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    Sep 7, 2008
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    forlod bygningen
    #2
    Yep, but there is more data on the outer regions of a CD and DVD.
     
  3. ChrisA macrumors G4

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    Jan 5, 2006
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    Redondo Beach, California
    #3
    CDs are recorded at constant linear bit density. As the circumference gets larger that are more bits. The best way to think about it is that the CD is read as a constant linear rate. Think of "inches per second" as being held nearly constant
     
  4. G.T. thread starter macrumors 6502a

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    Jul 12, 2008
    #4
    OK thanks I did wonder if it was the density of bumps that allow information to be read, it was the only thing I could think of. So I thought it was a constant series of rises and drops, or is it and because u go centre out the circumference increase and so the disk needs to slow down.

    Sorry for lack of technical wording lol
     
  5. mstrze macrumors 68000

    Joined:
    Nov 6, 2009
    #5
    It is a constant density of rises and drops. The innermost 'lines' would hold maybe 1000 (arbitrary number) the larger-diameter outermost 'lines' would hold probably 2200. If one wanted to read 1000 a second, you can do that at 1RPM on the inner ring, but it would slow to 2.2RPM for the outer ring.

    The circumference of the inner circles is smaller than the outer circles and since the data rises and bumps are the same density on each circle, for equalized data transfer speeds, the CD drive would need to slow down on the outer rings since there is more data on them.

    (Just re-iterating)
     
  6. yg17 macrumors G5

    yg17

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    Aug 1, 2004
    Location:
    St. Louis, MO
    #6
    I remember back in the olden days of portable CD players (Am I dating myself here? It wasn't that long ago :p) a lot of them had windows so you could see the disc spin, and if you changed it from the first track to the last track, or vice versa, you could clearly see the CD change speeds. I remember an old friend of mine had a portable CD player where if you opened it and stopped the CD from spinning, it would still play for another minute until the anti skip buffer would run out, talk about a mind**** ;)
     
  7. Liquorpuki macrumors 68020

    Liquorpuki

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    Jun 18, 2009
    Location:
    City of Angels
    #7
    I got a Sony Discman somewhere in my closet.... next to my waterproof yellow Sony Walkman... which is at least 10 times the size of my iphone :D
     

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