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It seems like our quest for greater storage capacity never ends. I can vividly remember when CD-ROM drives first appeared, and computer users wondered how anyone would fill such capacious discs. Well, it didn’t take long before software was shipping on two, three, or even more CD-ROM discs and the need for a higher capacity, distribution medium was required.
When DVD launched, it came in several flavours – single layer single sided, single layer double sided, dual layer single sided and dual layer double sided. It was the dual layer discs that were the impressive ones, holding massive amounts of data without the need to turn the disc over. In fact the demand for dual layer discs was so high, that there simply wasn’t the production capacity available to meet it. This is why, as I’m sure any movie buff will remember, many Region 2 DVD movies shipped on single layer double sided discs in the early days. This meant that halfway through the movie you needed to get up, eject the disc, turn it over and press play again.
It was issues like this that highlighted the need for dual layer DVD discs, where whole films could be put on a single side, and there was no need to flip discs. Now, creating a dual layer disc involved actually bonding two pressed layers together. The top layer would be semi-transparent, allowing the laser in your DVD player or DVD-ROM drive to either focus on it, or through it. Focusing through the top layer, meant that the laser could then read the lower layer and access the data stored on it.
DVD writers obviously don’t press discs, instead they burn “pits and lands” (the equivalent of 0s and 1s in binary format) into a special dye that’s sealed between a reflective layer and polycarbonate layer. On a blank DVD disc the dye is in a translucent state, so light can pass through it and bounce back from the reflective layer behind it. But when the laser heats the dye it becomes opaque and light can no longer pass through it. It’s these translucent and opaque parts that form the binary states that make up the data.
DVD writers have proved to be a huge success, holding 4.7GB of data on a single side, but the holy grail has always been dual layer DVD writers. Obviously, the method used for mass produced DVDs is not an option, since you need a pressing factory for that and few PC owners could afford such a luxury. So, the main hurdle has always been finding a way of producing a DVD burning method that could work with dual layer blank media.
Well, I’m very glad to say that all the R&D has paid off and sitting in front of me is an NEC ND2510 Dual Layer DVD writer. This drive can write up to 8.5GB of data to a single sided dual layer disc. The disc in question has two layers of dye bonded together, with a reflective surface behind them. Now, the key to dual layer technology is getting the laser to write to both layers, without corrupting the data on either. To get around the data corruption or “cross-write” problem, DVD+R DL discs have a spacer between the two write layers. As with pressed dual layer DVD discs, the top layer on a DVD+R DL disc is semi-transparent, allowing the laser to pass through it completely in order to write to the second layer. Switching between the two layers is achieved by varying the laser frequency, in order to focus on, or through the top layer.
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