Digital Video Disc Revolution

Inside the DVD and CD formats share the same basic optical storage technology - microscopic pits formed on the surface of the plastic disc when the material is injected into the mold, which represents information.

The pitted side of the disc is then coated with a thin layer of aluminum followed in the case of the CD by a layer of protective lacquer and a label. To read the data, the player shines a small spot of laser light through the disc substrate on to the data layer as the disc rotates.

The intensity of the light reflected from the disc's surface varies according to the presence or absence of pits along the disk information track. A pit reflects much less light than the flat part of a track. A photo-detector and other components inside the player translate this variation into Os and 1s, representing the stored information.

There are two essential physical differences between CD and DVD discs. First, the smallest DVD pits are only 0.4 micron in diameter; the equivalent CD pits are twice as large or 0.83 micron wide. Moreover, DVD data tracks are only 0.74 micron apart, where as 1.6 microns separate CD data tracks. Therefore, although DVD is the same size as a CD, its data spiral is upward of 11 km of a CD's data spiral.

The read-out beam of a DVD must achieve a finer focus than a CD player does. In order to do this, it uses a red semiconductor laser that has a wavelength of 635 to 650 nanometer. In contrast, CD players use infrared lasers with a longer wavelenjth of 780 nanometer. In addition, DVD players employ a more powerful focussing length - one having a higher numerical aperture than the lens in a CD player. These differences together with the additional efficiencies of DVD format described below account for the huge 4.7gigabyte capacity of each DVD information layer.A DVD's capacity can be doubled to 9.4 gigabytes and nearly doubled again to about 17 gigabytes, by two more innovations. Although DVDs and CDs have the same thickness, 1.2 millimeters, DVDs posses two substrates that can carry information, where as CDs have one. DVDs substrates are bounded together so that the pitted surface face each other in the center of the disc. This setup shields the surface from the damaging effects of dust particles and scratches. In the simplest DVD design, the second side is accessed by physically removing the disc from the player, turning it over and reinserting it. This is where another design comes into play.

Multi-layer design
Another variation - the multi-layer design - enables both information surfaces to be played from the same side. In a multi-layer player disc, the upper substrate is coated with a partially reflective layer. The reflectivity of the upper layer is sufficient to enable the laser beam to read the pits in the upper substrates. It also permits the laser beam to focus on the lower substrate and read the pits in that layer. (Source: www. pcwebopedia.com)

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