Monday, April 30, 2001		On Hardware

DVD—the mammoth disk
By Jasjot Singh Narula

TECHNICAL innovations have extremely short spans of life—a product that appeared king till yesterday is probably barely acceptable today and might be obsolete tomorrow.

Graduating from 1.44 floppy disk drives to DVDs (digital versatile disk) has given a new dimension to computer users, what with bulky databases, ever-increasing multimedia file sizes (audio, graphics and video) in computer games, educational software and electronic encyclopaedias. In the process, CDs might soon be not as hot as they are today.

A DVD (initially called digital video disk and now digital versatile disk) is an optical medium with higher capacity and bandwidth as compared to a CD. Like CDs, DVDs too were initially used for entertainment.

A layman may call it an enhanced version of CDs. To a naked eye it looks essentially like a CD, but has data storage capacities four to 17 times higher than the CD, depending upon the kind.

A DVD has the same diameter as a CD, i.e. 120 mm, and is 1.2mm thick and both rely on a laser to read data from microscopic pits in a spiral track.

DVDs come in different formats—DVD-video; DVD-audio; and DVD-ROM.

DVD-videos were initially launched for high-end movies to give an extra technological touch to them. They can hold a full movie—133 minutes of high-quality video—in MPEG-2 format (Motion Picture Expert Group and used for extended video).

A DVD-audio is for audio alone and works like an ordinary CD. The good part is it can hold 74 minutes of music (like CDs), but at its highest quality level, i.e. 192 Khz/ 24bit, as compared to the CD’s 44.1Khz/16bit. Lower the specification, and a DVD-audio can hold almost seven hours of CD-quality audio tracks.

A DVD is composed of several layers of plastic, each layer is created by injection moulding polycarbonate plastic. The storage system of a DVD is the same as CDs—it relies on pits and lands in a spiral track. In a DVD these tracks are placed closer together, thereby allowing more tracks per disk. The spiral track of a DVD is reduced to 0.74 micron, less than half of a CD’s 1.6 micron. The pits that are interpreted as data are also a lot smaller, thus allowing more pits per track. The minimum pit length of a single-layer DVD is 0.4 micron as compared to 0.834 micron for a CD. These characteristics give the DVD its extra storage capacity.

As DVDs use layers, the innovative idea is to make two layers of pits. The lower pitted surface is semi-transparent, allowing the laser to read both the surfaces from below. This nearly doubles the amount of data that can be stored on one layer, i.e. 4.7 GB. An interesting feature of the DVD is that the second layer can be read in reverse too. In a standard CD, the initial data is stored from the centre to the edge. The procedure is same for the DVD, but the second layer can contain data recorded in a reverse spiral track, i.e. from the edge to the centre.

To get the maximum out of the DVD, the industry created another version in which the upper side is also used, which is a pioneering idea in the field of storage media. The thickness of a DVD was reduced to 0.6mm—half the thickness of a CD-ROM. However, these thinner disks were too thin to remain flat, so the manufacturers bonded them back-to-back, increasing the thickness to 1.2mm. This cohesion resulted in two sides of a disk, doubling the storage capacity from 8.5 GB to 17 GB.

DVDs are divided into the following categories according to their capacity: DVD-5—a single-layer disk with a capacity of up to 4.7GB; DVD-9—a single-sided, but double-layered disk with a capacity of up to 8.5GB; DVD-10—a dual-sided, single-layer disk offering 9.4 GB storage capacity; and DVD-18—it has a 17 GB capacity on a dual-sided, dual layered disk.

To save the DVD from piracy, a non-profit organisation, DVD-CCA (DVD Copy Control Association), had come up with CSS technology (content scramble system) to protect the movie and audio industries. However, this system too has been hacked and ways found to bypass the encryption—a bane of all intellectual property.