Differences Between Single & Double-Layer DVD Discsby David Lipscomb
The Digital Versatile disc standard revolutionized digital storage, allowing over ten times the amount of data possible on a compact disc. Advances in the technology eventually brought about dual-layer capacity discs, increasing capacity from 4.7 gigabytes to 8.5GB. DVD burners and players use specific optics and techniques to read dual-layer discs, allowing longer movies to stay on one disc and allowing even more capacity for your photos and home movies.
A single-layer DVD or DVD-5 disc consists of a reflective layer encased in a polycarbonate sandwich. Data is encoded on this layer via a series of pits, representing digital information. These pits are roughly half the size of those found on a compact disc, requiring a special laser to read this data. Dual-layer discs or DVD-9 media are effectively two single-layer DVDs fused back-to-back, although are the same overall thickness. This increase in data is achieved by having the reading laser focus on a semi-transparent reflective layer. The laser then shifts focus as needed, reading past the semi-transparent layer to the second. You can see this happen when watching a dual-layer DVD, with the movie pausing briefly as the laser switches layers.
Ripping and Archiving
Not all DVD burners are dual-layer compatible. When ripping data that would normally exceed 4.7GB in these instances, increased data compression is required. If you're archiving video, you can encode the data at a higher quality level with less compression using dual-layer media. For those computer drives that do support dual-layer burning, you can purchase slightly more expensive dual-layer media that also prevents potentially having to use multiple discs for a single application. These drives are labeled accordingly.
DVD Media Types
When looking at single and dual-layer DVD media, you will encounter different designations. DVD-R is an older standard, more compatible with the majority of DVD drives and players. DVD+R is a competing format, and is the primary version offering dual-layer blank media. The technical difference for the designation refers to the manner in which data is stored. Like CD, DVD-R data begins toward the center of the disc, spiraling outward. DVD+R media is encoded in the opposite direction. Dual-layer DVDs use both, with the reverse spiral typically on the second layer. Plus- or minus-RW refers to the ability to erase and re-record data, although these discs are normally not as durable long-term and not recommended for critical data archiving.
Single and dual-layer DVDs have another iteration, dual-sided, dual-layer. Designated DVD-18, these discs house nearly 16GB of data, good for backing up entire smaller hard drives or placing a movie on one side and special features on another. DVD-14 is another version rarely encountered today, which is dual-sided but having one layer on one side and two layers on the other. DVD-10 is familiar to those possessing some older DVD releases, consisting of a single layer on two sides. This format was widely used to house a widescreen and full-screen movie or special features, each on its own side.
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