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How DVD Works

Reviews

How DVD Works
Page 1 of 1
Author: Chua A.P.
Date of review: 11-October-1998
Type Of Review: Articles/Editorials


Introduction
Say goodbye to those multi-disc games. Say hello to discs that can hold more than one movie (or the same movie in 2 languages). If consumers buy into it, the DVD, or Digital Versatile Disc, could substantially change personal computing, entertainment and education.

"A DVD stores more than 4 times as much data per disc as a CD, and the drive delivers performance equivalent to at least an 8x CDROM, impressive for a 1st-generation device."

Essentially a 2nd-generation CD, DVD exploits several new technologies to provide increased capacity and high performance. A DVD stores more than 4 times as much data per disc as a CD, and the drive delivers performance equivalent to at least an 8x CDROM, impressive for a 1st-generation device. To understand what makes DVD tick, let's look at 4 key technologies.

Bonded Substrates
One quality that distinguishes DVD is its 2-sided capability, a feature made possible by bonding. 2 thin (0.6 mm) substrates are formed into a single disc that's the same thickness (1.2 mm) as a regular CD.

Bonding provides 2 advantages. First, it creates a discs with 2 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 0.6 mm thick would give new meaning to the word 'floppy', bonding 2 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.

"...4 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."

The net effect is that DVDs have the capacity for over 4 times as many pits per square inch as CDs, totaling some 4.7 GB in a single-sided, single0layer disc. As a result, 4 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.

Multi-layered 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.

"DVD player designs incorporate novel dual-focus lenses to support 2-layer operation, yielding 8.5 GB in a single-sided DVD, or 17 GB in a double-sided disc."

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 2-layer operation, yielding 8.5 GB in a single-sided DVD, or 17 GB in a double-sided disc.

DVD Video Player or DVD-ROM Drive?
DVD devices are currently available in 2 substantially different variants. DVD video players look like video or laser disc player (indeed, a DVD is often referred to as a Digital Video Disc), and are designed for use in applications like home theatre. DVD-ROM drives look like multimedia CDROM drives and are designed for use in PCs.

"Microsoft has made public a specification for how DVD video players can be made compatible with systems running Windows 98."

While there is much talk of a convergence between the 2, 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.




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