The Medium

In the article on CD-ROMs, I discussed how the medium of the CD is structured and designed so as to hold data. DVD works on the exact same principle, with a few advances to allow increased capacities. First, the pits and landings in the DVD surface are a lot closer together than in CDs. This means that DVDs are much higher density mediums. The pits and landing are smaller than on CDs, hence more of them can fit on the medium. In order to work with these denser discs, the laser of DVD drivers operates on a much shorter wavelength, thereby allowing it to work in closer quarters on smaller units.

Packing the data closer together, though, does not in itself provide enough room for the high capacities we see on DVD. So, DVD can also employ multiple layers of pits and landings on the same disc. Each layer has its own reflective layer, and the laser can focus on any one of the layers. Additionally, DVDs can have data written to them on both sides of the disc. So, when you combine both two sides with two layers on each side (the max number of layers), you have a total of four layers. These double-sided, double-layer DVDs provide the highest capacity of DVDs on the market, 17 GB.

DVD5 is a single side single layer version, meaning that data is only on one side of the CD and comes in one layer. DVD5 has a capacity of 4.7GB. DVD9 is a single side, double layer and has a capacity of 8.5GB. DVD10 is dual side single layer and has a capacity of 9.4GB. Finally, we have DVD18 with a dual side and layer for a total of 17GB. With double sided discs, one has to actually eject the disk and flip it over when the software directs the user to do so. Hopefully, in the future, we will see double sided drives, but this is probably a way off of the mainstream market because, in essence it would be two drives in one.

The Hardware

A DVD drive looks identical to a CD-ROM, other than the fact that it will sport a DVD logo on it rather than a CD-ROM logo. All DVD-ROM drives are backward compatible to CD-ROMs, so you can read normal CDs in your DVD drive. On most modern PCs, the CD-ROM is actually a DVD-ROM drive, and the drive is used for both purposes. Structurally, DVD drives are almost identical to CD drives, the primary differences being in the optics. With advances in video card technology, many manufacturers have built DVD-decompression right into the video card, meaning all you would need to buy is the CD drive itself. Some computers are set up with DVD decoder software. This software acts as the MPEG decoder card would, doing all the decompressing purelu through software. This option works, but geneally is slower. Hardware decoding is recommended. Since DVDs are mostly used for movies, the demands placed on a PC when playing movies have to be dealt with. The video and audio of a movie are separate signals, and each uses high compression MPEG standards. When playing a movie, this has the capacity to really bog down a PC as it tries to decode the data into a movie. Many earlier DVDs came packaged with stand-alone MPEG decider cards. They work independently of the drive controller, video system and sound system, so they are completely dedicated to handling the load of DVD video. For such a kit, you basically install the MPEG decoder card into a PCI slot, connect the DVD drive to an EIDE connector, and connect the MPEG card to your sound card. Some kits also require that you install a loopback cable to place the decoder card in-line between your monitor and your video card. This step adds a few minutes to the install time, but usually contributes to better video performance.