What differences between a CD and a DVD account for the larger capacity of the latter?

  1. 1. Bits are packed more closely on a DVD. The spacing between loops of a spiral on a CD is 1.6 μm and the minimum distance between pits along the spiral is 0.834 μm. The DVD uses a laser with shorter wavelength and achieves a loop spacing of 0.74 μm and a minimum distance between pits of 0.4 μm. The result of these two improvements is about a seven-fold increase in capacity, to about 4.7 GB.  

    2. The DVD employs a second layer of pits and lands on top of the first layer A dual-layer DVD has a semireflective layer on top of the reflective layer, and by adjusting focus, the lasers in DVD drives can read each layer separately. This technique almost doubles the capacity of the disk, to about 8.5 GB. The lower reflectivity of the second layer limits its storage capacity so that a full doubling is not achieved. 

    3. The DVD-ROM can be two sided whereas data is recorded on only one side of a CD. This brings total capacity up to 17 GB.


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