DVD Construction and Technology

DVD technology is similar to CD technology. Both use the same size (120mm diameter, 1.2mm thick, with a 15mm hole in the center) discs with pits and lands stamped in a polycarbonate base. Unlike a CD, though, DVDs can have two layers of recordings on a side and be double-sided as well. Each layer is separately stamped, and they are all bonded together to make the final 1.2mm-thick disc. The manufacturing process is largely the same, with the exception that each layer on each side is stamped from a separate piece of polycarbonate plastic and are then bonded together to form the completed disc. The main difference between CD and DVD is that DVD is a higher-density recording read by a laser with a shorter wavelength, focused more closely to the disc, which enables more information to be stored. Also, whereas CDs are single-sided and have only one layer of stamped pits and lands, DVDs can have up to two layers per side and can have information on both sides.

As with CDs, each layer is stamped or molded with a single physical track in a spiral configuration starting from the inside of the disc and spiraling outward. The disc rotates counterclockwise (as viewed from the bottom), and each spiral track contains pits (bumps) and lands (flat portions) just as on a CD. Each recorded layer is coated with a thin film of metal to reflect the laser light. The outer layer has a thinner coating to allow the light to pass through to read the inner layer. If the disc is single-sided, a label can be placed on top; if it's double-sided, only a small ring near the center provides room for labeling.

Just as with a CD, reading the information back on a DVD is a matter of bouncing a low-powered laser beam off one of the reflective layers in the disc. The laser shines a focused beam on the underside of the disc, and a photosensitive receptor detects when the light is reflected back. When the light hits a land (flat spot) on the track, the light is reflected back; when the light hits a pit (raised bump), the phase differential between the projected and reflected light causes the waves to cancel and no light is reflected back.

The individual pits on a DVD are 0.105 microns deep and 0.4 microns wide. Both the pits and lands vary in length from about 0.4 microns at their shortest to about 1.9 microns at their longest (on single-layer discs).

DVD uses the same optical laser read pit and land storage that CDs do. The greater capacity is made possible by several factors, including the following:

• A 2.25 times smaller pit length (0.90.4 microns)

• A 2.16 times reduced track pitch (1.60.74 microns)

• A slightly larger data area on the disc (8,6058,759 square millimeters)

• About 1.06 times more efficient channel bit modulation

• About 1.32 times more efficient error correction code

• About 1.06 times less sector overhead (2,048/2,3522,048/2,064 bytes)

The DVD disc's pits and lands are much smaller and closer together than those on a CD, allowing the same physical-sized platter to hold much more information.

DVD drives use a shorter wavelength laser to read these smaller pits and lands. A DVD can have nearly double the initial capacity by using two separate layers on one side of a disc and double it again by using both sides of the disc. The second data layer is written to a separate substrate below the first layer, which is then made semireflective to enable the laser to penetrate to the substrate beneath it. By focusing the laser on one of the two layers, the drive can read roughly twice the amount of data from the same surface area.