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The Evolution of Optical Storage Media

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Retired Tech

Retired Tech

    Retired Staff

  • Retired Staff
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You have a lot to thank the laser for. Were it not for its shiny red (or latterly blue) glow, you’d still be reading magnetic data from utterly unstable discs in noisy, slow drives. We used to live in a world dominated by floppy disks. That 1.44MB of portable data was important to us, and the cripplingly slow read and write speeds didn’t bother us in the least. Programs would be sold on a raft of floppies, making installation a painstaking task of sitting and changing discs until
the noisy grinding was complete. Then it all changed. The compact disc that was already in use in the music industry was adapted for the PC, making the existing methods of storage (and indeed many hard drives of the time) pale in comparison.

CD was already a digital format, which is essential for compatibility with computers. However, the original CD-ROM drives were notoriously unstable, requiring caddies to balance the disc and reading at a fiftieth of the speed of today’s drives. Yet it seemed like such a luxury, and that the then-ludicrous capacity of 700MB would never be too small. Games could include video and thousands more textures, software would no longer be restricted by floppy disc costs. And it happened, too. Packages now grow to fit their surroundings and since optical media is so popular, it’s also a lot cheaper to produce. It’s not uncommon to see games that span five or six CDs, and we’d be surprised if Windows Vista appeared on anything less than a DVD.

DVD, with all its extra capacity, was supposed to herald a huge revolution, but the never ending stream of data isn’t stopping and the single-layer size of 4.7GB is already starting to feel pretty pokey. High-definition video and increasingly expansive games mean the DVD could already be on its last legs.

We’ve got a lot to look forward to, though: new formats won’t have the same agonising waiting period for writeable media, they’ll store at least 15GB (if not 200GB), and even then we’ll probably outgrow them. Thanks, laser, you’ve given us a lot, and there’s only more to come.

A silver CD starts life as an injection-moulded polycarbonate disc, etched with a series of incredibly tiny bumps – 88,200 for every second of CD audio, on a spiral track that would stretch over 3km if laid out. Aluminium is sprayed on to make the disc shiny, a layer of acrylic is applied to protect the data, and finally a label is applied. When a laser, synchronised with the speed of the motor, shines on the pits and peaks of the spinning CD, it reflects the digital information into a receiver, and – ta-da – data pours into your PC.

Instead of a stream of peaks and troughs, CD-Rs use an extra layer below the reflective layer to store their data. During the writing process an second laser is used, more powerful than the reading laser, which heats up the disc and creates microscopic dark patches around the spiral. Green CD-Rs use cyanine and gold use phthalocyanine, both organic dyes, to mimic the pits of a traditional CD. Be careful with your CD-Rs – even the best dye is light sensitive and you shouldn’t expect CD-Rs to have a life expectancy of more than ten years.

Once you know how a normal CD-R works, it’s not impossible to fathom the concept of rewriteable media. Instead of a dye that can only be changed once, CD-RWs have a layer, made of a special alloy of metals, that can basically be toggled: heating it to 700°C causes the dye to turn opaque, and 200°C makes it clear instead. CD-RW discs are generally more resilient than CD-Rs and can be re-written thousands of times, but all that complex technology costs money – CD-RW discs have remained a lot more expensive than other rapidly depreciating formats.

So, you’ve got a disc with a spiral of data, and you want to store more on it. No doubt it took clever engineers many years to come up with the solution – smaller bits, and a longer spiral. Apart from that, there’s little between a DVD-R and a CD-R, and the difference between +R and -R is merely one of standards. -R was developed by Pioneer, and is the closest in technology to the old CD-RW standard. +R was created by consortium of big names in optical media, and is slightly technologically superior because it can support dual-layer writeable disks.

A traditional silver DVD is a very clever device: it can store double its capacity by including a second reflective layer below the first. If the laser is focused appropriately and it can read tracks on either of these layers. The principle holds up for Dual Layer writeable DVDs – by using twin layers of ink, each of which respond to different frequencies of light, twice as much data can be stored on a single DVD. Naturally the laser needs to be incredibly accurate when writing to a Dual Layer disc, which means that only modern drives have the capability to do so.

DVD-RAM doesn’t seem far removed from standard DVD, but its method of storage borrows more from hard disks and floppies than its shiny brethren. Instead of using a single spiral track of data, DVD-RAM discs feature a series of concentric circles. This means that changes made to one part of the disc don’t affect those on other parts, that the disc can be read from and written to simultaneously, and that Windows can write to DVD-RAM media as if it were a hard drive. It also means extra expense, though – look for DVD-RAM discs to be twice the price of DVD-RW, if not more.

Where is optical media going next?
We’re heading towards what could be a six-way battle for storage domination. The main contenders are Sony’s Blu Ray and Toshiba’s HD-DVD, although neither has been officially released. Blu Ray is arguably superior technically, boasting high capacity discs (25GB per layer) and apparent advantages in read times. However, the cost of using a blue laser, as Blu Ray does, is currently so high that Sony has delayed its PS3 console.

HD-DVD may be trailing Blu-Ray by 10GB per layer, but the technology is far simpler. HD-DVDs are essentially extended versions of standard DVDs with increased density and greater laser accuracy. HD is less restrictive for its users, especially in the computing domain, eschewing the DRM that Sony built into its discs. You’ll find HD-DVD hitting the streets first, although only in movie form to begin with.

China has also been developing a format based on HD-DVD called Extended Video Disc. This is technologically similar to London-developed Versatile Multi-layer disc. It apparently offers 50GB capacity via ten 5GB layers. Another system, Versatile Compact Disc High Density, has come from a Europe-wide consortium of developers. It claims VCDHD discs will hold 4.7GB (the same as a DVD), be half as thin, and have manufacturing costs a third as high. The idea is fantastic, but the relatively low capacity is likely to see this format off before it arrives. Keep an eye out for holographic storage media. It’s not ready for the mainstream yet, but in ten years time 100GB will seem like a tiny capacity for a disc. The first holographic drives, as well as Blu Ray and HD-DVD, are expected towards the end of this year.
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