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How Much Space is on a DVD?
It seems like there should be a simple answer to the question of how much space there is on a DVD. Unfortunately, there are several types of DVDs (and more on the way!), so there isn't a single answer to this question -- it depends on what kind of DVD you're talking about.
How Much Space is on a DVD-ROM?
DVD-ROM is the silvery-looking DVD that consumers generally get when they go to rent a movie. If you have ever tried to make a backup copy of your favorite movie from a DVD-ROM to a recordable DVD, like DVD-R or DVD-RW, you probably already have discovered that DVD-ROMs seem to be able to hold more data.
DVD-ROMs are the same overall physical size as recordable DVDs, so they must have some way of storing more bits in the same space. In fact, DVD-ROMs come in multiple types. The DVD-ROM type known as DVD-5 can hold only 4.7GB -- about the same amount of data as a DVD-R/W or DVD+R/W. But another flavor of DVD-ROM known as DVD-9 uses a neat trick: on top of one 4.7GB layer of data is overlaid another layer of data that is translucent. A compatible DVD-ROM player (like the one hooked up to your TV) can read the topmost layer, but can also "peek through" that layer to read the other layer of data beneath it.
For technical reasons, each of the tiny pits that represent 1's and 0's on a dual-layer DVD-ROM take up a bit more space than on a single-layer DVD-ROM. As a result, the capacity of a dual-layer DVD-ROM (DVD-9) is not quite twice the capacity of a single-layer DVD-ROM (DVD-5). Instead of 9.4GB (2 x 4.7GB), a dual-layer DVD-ROM (DVD-9) has a capacity of 8.5GB.
To complicate things further, it's possible to master a DVD-ROM with data on both sides -- a "double-sided" DVD-ROM. This sounds like great deal, getting twice as much data on the same-sized disc. Unfortunately, most DVD players do not have two sets of lasers for reading from both sides, or the ability to automatically flip the DVD-ROM over to the other side. Thus, while a dual-sided DVD-ROM gets you twice as much space on the same disc, from a user convenience perspective, it's often not much better than just having two separate discs.
Why undelete utilities may fail just when you need them most!
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