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CDR Capacity and Gnomes

  1. Jan 31, 2007 #1
    What is the difference between a 74min and 80min CD?

    I was just recording to my scratch CD and noticed that it had passed the 645Mb limit of a 74min CD, therefore it must be 80min.
    I base a CD's capacity on how wide the unwritten stripe is, but I wondered how the CD Writer knows the difference?

    I assume the CDs are the same physical size, and the laser diode tracks and pulses at the same rate and the disk spins at the same speed. So there must be something else clever, like a small gnome that twiddles a small screw in the CD writer.

    Any clever people out there who can confirm my gnome theory, preferably with photographs; or alternatively any crack pots with ridiculous off the wall theories about how it could be done with technology?
  2. jcsd
  3. Jan 31, 2007 #2


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    Writeable CDs are manufactured with a spiral timing track already written on them. The CD writer follows that track to control the rotation speed and laser position.

    The basic difference between audio and data CDs is that data CDs have more error checking and correction data written on them. Therefore you can get 80 minutes of audio on an audio CD, but only 74 minutes of audio in a .wav file at 44.1Kz stereo on a data CD.

    Reading an audio CD, the player can interpolate some plausible-sounding data (pun intended) if it finds an uncorrectable error. That is not possible for a computer data file - the only option is to display an error message and stop.
  4. Jan 31, 2007 #3
    So a 700Mb (80min) CDR has a finer pitched timing track than a 650MB (74min) CDR.
    Obviously the surface coatings improved by about 10% at sometime, I think even budget CDRs are 700Mb now.

    So where does the Gnome come into it?
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