Article, Apr96: DVD sidebar: It's The Pits

To make a standard CD, data from the recording studio, producer or software supplier are sent to the replicator on tape or CD-R. This information is converted into a digital signal, which is used to modulate a laser beam that creates pits in a glass substrate. The glass substrate acts as a master to produce a stamper from which plastic replicas are made using an injection molding process.

The disc replicas are covered with a thin layer of metal to create a reflective surface and finished with a protective coating. Each pit represents part of a digital code. The pits are read by a red laser that reflects off the metallic surface of the disc. The amount of information that can be recorded to a disc is limited by the number of pits that can be placed on it.

To store a full-length movie on a disc, for example, the video must be compressed and digitized, and the disc's pits must be narrower, shorter and closer together than those of a standard-density CD. Players that read back these smaller pits will still use red lasers, but they will be narrower: Today's red lasers are too wide to read the pits on a high-density recording.

A 4.7 gigabyte DVD has about seven times the capacity of a conventional CD. The unified high-density format will obtain even more capacity via the bonding together -- back to back or front to back -- of two 0.6 millimeter, 4.7 gigabyte discs.

When the discs are bonded back to back, providing 9.4 gigabytes of data capacity, disc readers incorporating only one readback laser will turn the disc over after playing one side. Other player designs may use two readback lasers, with each reading one side of the disc.

For discs that are bonded front to back (which have a capacity of up to 8.5 gigabytes), one readback laser will read through one disc to the information on the disc below it. For discs with two layers on each side (8.5 gigabytes on each 0.6-millimeter side, bonded back to back, for a total capacity of 17 gigabytes), the readback laser will read each layer on each side of the discs separately.

The top layer on each side of these discs will be semi-reflective, allowing the laser to penetrate to the information below. This has been described using the analogy of a picket fence: Viewed from some angles, the fence will appear to be solid; if one looks through the slats, however, the scene behind the fence becomes visible.

Source: DVD information sites on the World Wide Web