Digital Video Discs (also known as Digital Versatile Discs, depending on who you want to listen to) are actually a different technology all together. The disc looks the same; it’s the same size, refracts light into pretty colors, and all that. But there, the resemblance ends.
DVD didn’t get off to the most auspicious of starts. There was a rather bitter battle over control of the technology among several different companies, each with its own idea of how it should be done. The end result was five different file formats. See Table 1 for an overview.
A major technological difference is that DVD can be pressed in layers, and both sides of the disc can contain data. Combine that with a file format that fits more information onto a single layer, and you have a disc that can store several gigabytes of data. This is done by creating the first layer on one side, coating it with a microscopically thin, transparent gold layer, and then pressing the second layer. If you do that to the second side you can have four layers of data. Multilayer discs are easily discernible from regular ones by their gold color. Discs are made as shown in Table 17.2.
DVD File Formats
|DVD-ROM||High capacity data storage|
|DVD Video||Use for full length motion pictures|
|DVD Audio||Audio-only content|
Table 1: DVD actually needs to support five different file formats.
DVD Disk Formats
|DVD 5||Single sided, single layer||4.7GB|
|DVD 8||Single sided, double layer||8.5GB|
|DVD 10||Double sided, single layer||9.4GB|
|DVD 18||Double sided, double layer||17GB|
Table 2: DVD capacities vary with the number of layers embedded.
The way capacity was increased so dramatically was managed on several levels. To start with, the lands created during the creation of masters were reduced in width from 1.6 microns to about .75 microns. By itself, this change allowed for double the number of tracks. The relative lengths of the pits were also cut by half. Now manufacturers can fit twice the number of pits onto four times as many tracks.
DVD 8 and DVD 18 further increase storage by allowing for two separate data storage layers on the medium. A microscopically thin layer of semireflecting material separates these layers. When the optical stylus needs to read the lower layer, it simply changes its focus to that layer. The bottom layer can’t hold quite as much data as the top. Therefore, capacity isn’t quite doubled.
It’s interesting to note that double-sided discs are actually two separate discs bonded together by adhesive. The fact is, all DVDs are two layers. The layers actually required by a single-sided DVD are only half as thick as a standard CD. Manufacturers were concerned about the durability and stability of such thin material and bonded a layer of polycarbonate plastic to the back of the disc to make it the same thickness as a CD. By bonding two DVD discs, back-to-back, you get the double-sided variety.
As you might imagine, new file formats had to be developed to support this medium. As I mentioned earlier, the race to develop DVD resulted in five different formats. The call to arms led the Optical Storage Technology Association (OSTA) to develop the Universal Disc Format (UDF). This format merges video, audio, and data, allowing a file to support any of the three, or to mix any two or even all three into a single file. UDF also permits any type of optical disc to access any of these types of file, even if made by another medium.
Most DVD drives double as CD-ROM drives. In other words, they’ll read either format. Manufacturers do this by mounting two separate optical styli on a swivel mount. Once the drive has detected the type of medium that has been inserted, the proper head moves into place and the fun begins. However, to take advantage of the motion picture capabilities of DVD, you need to have a video card capable of MPEG II standards. Which these days, is pretty much all of them.