ZIFFPAGE TITLEDisc Anatomy
Disc Anatomy A CD consists of a piece of polycarbonate "substrate" into which up to 6 billion tiny pits that represent stored data have been molded. This pattern is coated with a reflective material like aluminum, which is covered by an ultrathin protective coating. When a CD player aims its laser through the clear substrate, the aluminum reflects the beam back. The drive reads data by detecting changes in the reflected beam as it passes across the molded pits.Unlike a CD, which is a solid piece of plastic, a DVD is composed of two polycarbonate discs that are bonded together. This helps protect the coatings, as it sandwiches them between plastic layers. But it also makes DVDs more vulnerable to stresses caused by severe bending and flexing, especially at the discs inner hub.
Mass-produced DVDs employ similar technology, but with a few important differences. Most replicated DVD-Video discs store two layers of data and use a semireflective material like gold to coat the layer closest to the laser (see http://www.pcmag.com/article2/0,1759,1573655,00.asp). This allows DVD drives to select which layer to play by simply adjusting laser power and focusing distance. DVDs also boast greater data density and can store up to 4.7 billion bytes per layer, far more than the 650800MB that can be squeezed onto a CD. This means that, despite a more robust error-checking and correction mechanism, each bit of DVD data is smaller and more easily obscured by tiny scratches and debris on the discs surface.