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Tech 101 | How Stuff Works


November 23, 2000

Digital Video Discs or Digital Versatile Discs--are similar to traditional compact discs. They are the same diameter and thickness and made with some of the same materials and methods. But DVDs have a much larger data capacity. A standard DVD holds about seven times more data than a CD.

DVD Design

* DVDs are made of several layers of clear polycarbonate plastic, about 1.2 millimeters thick.

* Each layer is impressed with microscopic bumps arranged as a single, continuous spiral track of data.

* A thin layer of reflective aluminum or semi-reflective gold is applied to the disc, covering the bumps.

* Two double layers can be bonded back to back, forming a double-sided, double-layer disc containing as much as 17 gigabytes of data.


The DVD player finds and reads the data stored as bumps on the disc. The player interprets the differences in reflectivity as either ones or zeros. It then converts the data to appropriate formats and sends them on to other components. In the case of a movie, the data are converted to a standard composite video signal.

DVD Facts

* DVD data bumps and track spacing are about half the size of their counterparts in a CD, allowing much more information to be crammed into the same space. A DVD's tracks are separated by just 740 billionths of a meter.

* The microscopic dimensions of the bumps make the spiral track extremely long. If you could lift the data track off a single layer of a DVD and stretch it out into a straight line, it would be almost 7.5 miles long. That means a double-sided, double layer DVD would have 30 miles of data.

* Bumps on a DVD are often referred to as "pits." They can be either pits or bumps depending on which side you're looking at them from, but from the viewpoint of the laser beam reading the disc, the pit is actually a bump.

* Movies are encoded in the MPEG-2 compression format before being put onto the disc. Compression basically streamlines video data by saving only enough information to enable reconstruction of the movie, for example, just the changes from frame to frame, rather than saving every frame in its entirety.

* The first DVD player hit the market in March 1997.

* There are about 8,000 DVD titles available and 9 million DVD players in the United States.

* Sony PlayStation 2 is the first video game system able to play DVDs.

* DVDs are durable, so you can watch a movie repeatedly without losing image or sound quality.


Researched by VICKI GALLAY/Los Angeles Times

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