DVD Region Free - Kylie Minogue: Showgirl - The Greatest Hits Tour - Video & DVD

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1. DVDs are made from a 0.6 mm thick disc of polycarbonate plastic coated with a much thinner (reflective) aluminium layer. Two such discs are glued together to form a 1.2 mm double-sided disc. The substrates are half as thick as a CD to make it possible to use a lens with a higher numerical aperture and therefore use smaller pits and narrower tracks.

2. A CD is read by focusing a 780 nm wavelength semiconductor laser through the bottom of the polycarbonate layer. The difference in height between pits and lands is a sixth of the wavelength of the laser light, leading to a third-wavelength phase difference between the light reflected from a pit and from its surrounding land. The sixth, 125 nm, (and not a quarter) of the wavelength was chosen to have a good trade-off between the push-pull radial tracking signal and the full-aperture read-out signal. The interference reduces the intensity of the reflected light compared to when the laser is reflected by just a land. By measuring this intensity with a photodiode, one is able to read the data from the disc. The pits and lands themselves do not represent the zeroes and ones of binary data. Instead a change from pit to land or land to pit indicates a one, while no change indicates a zero. This in turn is decoded by reversing the Eight-to-Fourteen Modulation used in mastering the disc, finally revealing the raw data stored on the disc.