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How DVD Works

-- by John D. Ruley

Say goodbye to those multi-disc games. Say hello to discs that can hold more than one movie (or the same movie in two languages). If consumers buy into it, the DVD, or Digital Versatile Disc, could substantially change personal computing, entertainment and education. Essentially a second-generation CD, DVD exploits several new technologies to provide increased capacity and high performance. A DVD stores more than four times as much data per disc as a CD, and the drive delivers performance equivalent to at least an 8X CD-ROM, impressive for a first-generation device. To understand what makes DVD tick, let's look at four key technologies.

Bonded Substrates. One quality that distinguishes DVD is its two-sided capability, a feature made possible by bonding. Two thin (0.6mm) substrates are formed into a single disc that's the same thickness (1.2mm) as a regular CD. Bonding provides two advantages. First, it creates a disc with two distinct sides, making possible both single-sided and double-sided configurations. Since each side is only half as thick as a CD, it's possible to use smaller pits to represent the data, resulting in higher data density. And while a single substrate only 0.6mm thick would give new meaning to the word "floppy," bonding two such substrates together results in a disc that's actually more rigid than a CD.

High Data Density. Data is represented on a DVD in exactly the same way as it is on a CD-with physical "pits" on the disc. But the thinner DVD substrates (and short-wavelength light source) permit the pits to be smaller. In fact, they're roughly half the size, which in turn allows them to be placed closer together. The net effect is that DVDs have the capacity for over four times as many pits per square inch as CDs, totaling some 4.7GB in a single-sided, single-layer disc. As a result, four times as much data passes the detector per second on a DVD. This feature, combined with the rigidity of the bonded substrates-which permits DVDs to spin faster-lets DVDs easily achieve high data rates.

Visible-Light Laser. Imaging the smaller data pits on a DVD requires short-wavelength light. Recent advances in semiconductor laser technology have produced red-light lasers that are ideal for this purpose. The lasers used in DVD players are also more powerful than those used in CDs, because they must focus on more than one layer within each substrate.

Multilayered Sides. A DVD's capacity is further increased when more than one layer is placed on each side. The inner layer reflects light from the laser back to a detector through a focusing lens and beam-splitter. An optional outer layer can be partially reflective and partially transmissive-passing some light on to the inner layer and reflecting some light back. DVD player designs incorporate novel dual-focus lenses to support two-layer operation, yielding 8.5GB in a single-sided DVD, or 17GB in a double-sided disc.

DVD Video Player or DVD-ROM Drive?

DVD devices are currently available in two substantially different variants. DVD video players look like video or laser disc players (indeed, a DVD is often referred to as a Digital Video Disc), and are designed for use in applications like home theater. DVD-ROM drives look like multimedia CD-ROM drives and are designed for use in PCs.

While there is much talk of a convergence between the two, for the time being you're pretty much forced to pick one or the other. This may be changing, though-Microsoft has made public a specification for how DVD video players can be made compatible with systems running Windows 98.

A. Laser: CD players and CD-ROM drives use infrared lasers. But the visible-light lasers used in DVD players provide the light source needed to image a DVD's smaller pits. The DVD's laser can also be focused on either of the layers on a DVD side.

B. Pits: The use of smaller pits (0.4 microns for the DVD vs. 0.83 microns for the CD) and narrower track-to-track spacing (0.74 microns for the DVD vs. 1.6 microns for the CD) means there's room on the DVD for nearly four times as much information-per layer-than on a CD. Even in a single-sided, single-layer configuration, DVDs can store some 4.7GB of data-compared to less than 1GB on a CD. The tighter spacing also produces higher throughput-first-generation DVD-ROM drives deliver data at 11MB per second, equivalent to an 8X CD-ROM drive.

C. Layers: A DVD consists of two separate, thin (0.6 mm) plastic substrates that are bonded together, creating a single disc that's the same size as-but much more rigid than-a CD. Each of the substrates becomes the side of a DVD, and contains one or two layers of data-an inner, reflective layer and an optional semitransparent outer layer. The DVD's laser can focus on either. By combining double sides and two layers per side, a single DVD can store up to 17GB.

Windows Magazine, October 1997, page 253.

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