This feature provides an overview of the various optical disk formats. Much information has been provided by Imation which has a highly informative web site and has just announced an 8.5GB capacity dual layer DVD, the DVD-DL.

CD formats
CDs- compact disks - hold either 680 or 700MB of data. That's sufficient for 80 minutes of music. They are used for music and also for distributing software or storing digital data such as images. An overview of the various formats is:-

CD: Compact Disc or CD-Audio, a digital storage medium formed of a 12cm diameter polycarbonate foundation layer coated with a reflective metalized layer, and a protective lacquer coating. The physical format of CDs is described by the ISO9660 industry standard.

CD-R: Compact Disc-Recordable. Recordable music CDs.

CD-R Music: This refers to CD-R Digital Audio (CD-R DA). This write-once recordable media is specifically designed to create audio CDs using consumer/home audio recorders bearing the "Compact Disc Digital Audio Recordable" logo. These recorders are not generally connected to a computer, like CD-R and CD-RW burners; instead, they are stereo system components that require the use of CD-R Music discs.

CD-ROM: CD-read-only memory. A format for storing large digital files.

CD-ROM XA: A multimedia version of CD-ROM used as the basis for CD-I, Video CD and Photo CD. CD-i Bridge allows the last two formats to play on CD-I players.

Photo CD: A special CD format developed by Eastman Kodak and Philips. The idea was to provide digital storage for consumer photographs.

It rapidly became apppparent in the late 90s that the CD with its 680M or slightly more capacity was insufficient. The consumer drive was for a format that could hold more digital photos with the holy grail being the storage of a whole movie. So digital video disks were developed.

DVDs are a big advance in capacity over CDs, offering 4.7GB. Like CDs, DVDs store data in microscopic grooves running in a spiral around the disc. All DVD drive types use laser beams to scan these grooves: Minuscule reflective bumps (called lands) and nonreflective holes (called pits) aligned along the grooves represent the zeros and ones of digital information.

DVD technology writes in smaller 'pits' to the recordable media than CD technology. Smaller pits mean that the drive's laser must produce a smaller spot. DVD technology achieves this by reducing the laser's wavelength from the 780nm infrared light used in standard CD drives to 625nm to 650nm red light.

Smaller data pits allow more pits per data track. The minimum pit length of a single layer DVD-RAM is 0.4 micron as compared to 0.834 micron for a CD. Additionally, DVD tracks are closer together, allowing more tracks per disc. Track pitch-the distance from the center of one part of the spiral information or 'track' to the adjacent part of the track-is smaller. On a 3.95GB DVD-R, track pitch is 0.8 microns; CD track pitch is 1.6 microns. On 4.7GB DVD-R media, an even smaller track pitch of 0.74 microns helps boost storage capacity.

These narrow tracks require special lasers for reading and writing — which can't read CD-ROMs, CD-Rs, CD-RWs, or audio CDs. Most drive makers have solved this problem by putting two lasers in their drives: One for DVDs, the other for CDs.

Data access speeds
DVD accesses data faster than CD and uses more robust error correction. In fact, the speed of DVD demands a new unit of measure. CD drive speeds are expressed as multiples of that format's original data transfer rate 'X,' or 150KB per second. A 32X CD-ROM drive reads data at 32 times 150KBps or 4MBps. DVD's 1X is 1.38MBps. That's faster than an 8x CD drive.

DVD formats
The various DVD Formats are:-

DVD-R: The most universal of recordable DVD formats used by DVD burners and many DVRs. DVD-R is a write-once format, much like CD-R, and discs made in this format can be played in most current DVD players.

DVD-RW: Recordable and rewriteable format (like CD-RW). Discs are playable in most DVD players, provided they are recorded in the straight video mode and finalized.

DVD+RW: Recordable and rewriteable format. Is claimed to offer a greater degree of compatibility with current DVD technology than DVD-RW.

DVD+R: A record-once format introduced recently that is claimed to be easier to use than DVD-R, while still playable in most current DVD players.

DVD-RAM: Recordable and rewriteable format which is not compatible with current DVD technology and is not compatible with most DVD-ROM computer drives.

Towards higher DVD capacity
DVDs can have two recording layers. The Double Layer disc is referred to by several names in the industry. Initially the Alliance (made up of Philips, Sony, Dell, HP, Yamaha, Mitsubishi Chemical-Verbatim, Ricoh, and Thomson) referred to the disc as the Dual Layer disc. Drive manufacturers began to market their Dual Layer drives, which were quickly confused with the drives capable of reading and writing Dual Format (+ and -) media. The Alliance quickly converted to calling the media Double Layer and some of the drive manufacturers followed suit with their drive nomenclature. However, you will still find the Double Layer and Dual Layer nomenclature interchanged.

The Imation product is called DVD-DL.

Regardless of what it is called, one fact remains constant - it is the highest capacity DVD media available and nearly doubles the storage capacity of a DVD recordable disc from 4.7GB to 8.5GB (equivalent to twelve CD-Rs) on a single side.

And after DVDs
There are three optical formats that offer more than 8.5GB. These are Plasmon's UDO, HD-DVD and Blu-ray. All three used blue laser light. This has a narrower beam than red laser light. In turn this means smaller data areas, meaning more densely-packed data delivering higher capacity.

UDO is positioned as a business data storage format. The others are primarily consumer-focused with a secondary business storage use, just like CDs and DVDs.

UDO is developed from the earlier MO - magneto-optical - disks.

Blu-ray: Blu-ray is a Sony-supported format and holds 25GB in single layer form, but with two layers and double-sided recording, could hold 100GB. Such capacities mean high-definition movies could be stored. It needs a cartridge to hold it and the disk can't be played in DVD players.

HD-DVD: High-definition DVD. HD DVD is a 20GB capacity format which could, via multiple layers hold more than 50GB. Disks don't need catridges.

MO: An older format with disks providing 9.1GB and were used for business data archiving purposes.

UDO: A Plasmon-only format offering 30GB capacity for business data storage purposes.

Format wars
As ever with removable storage media there are competing post-DVD format adoption efforts. Some movie studios have plumped for Blu-ray, others for HD-DVD. Most are non-exclusive arrangements. Most business software is distributed on CD and there isn't any real pressure to move to DVD. By the time there is a winner will probably have emerged in the post-DVD consumer/business optical disk format war. Thank goodness.