Dual-Layer (Double Layer) DVD Drives and Media
Understanding the Newest Rewritable Technology
For several years, DVD movie buffs have appreciated the benefits of pre-recorded (pressed) dual-layer DVD media. Dual-layer DVD media enables movies of up to 4 hours (240 minutes) in length to be played without disc-swapping, and provides nearly twice the room on each disc for bonus features. However, until the development of double layer rewritable DVD drives and media in 2004, PC users could read, but could not create, dual-layer discs. In this article, you learn how double layer DVD rewritable drives work, how to enable older single-layer DVD drives to read double layer media, and which vendors produce double layer DVD drives and DVD media.
The first (and so far, only) double layer rewritable standard comes from the DVD+RW Alliance (www.dvdrw.com): DVD+R DL, although all drives capable of writing the DVD+R DL format will also be able to write the future DVD-R DL format when it is eventually released. Sony was the first to introduce a dual-layer drive (the DRU-700A) in March 2004, however many manufacturers including BenQ, LiteON, Philips, Pioneer, NEC, and others have since followed suit.
MediaTek is the world's largest supplier of optical drive chipsets, and supplies the chipsets driving most of the DVD burners on the market, including double layer models. Other companies including NEC, Philips, Sanyo and TDK also provide chipsets for double layer drives.
It is interesting to note that the DL in DVD+R DL actually stands for Double Layer and not Dual-Layer, however the terms are often interchanged, and some media is advertised using both terms. The DVD+RW Alliance uses the Double Layer terminology to help prevent confusion with the fact that most DVD burners are also capable of reading and writing Dual Format (+ and -) media. Since the term Dual-Layer was originally used with pressed DVD-9 discs, it seems that many are maintaining the use of that terminology for pressed DVD-9 discs, while reserving the Double Layer designation for writable or rewritable discs. In the end, the only important thing to note is that for all intents and purposes the terms are identical, and are often used interchangeably.
How Dual-Layer DVD Media Works
Dual-layer DVD-Video media has been used for several years by the DVD movie industry. Dual-layer DVDs have a thin substrate layer between the first and second layers of DVD data. Single-sided dual-layer DVD-Video media is known as DVD-9. Note that DVD+R DL is also referred to as DVD+R9. It has a capacity of 8.5GB (slightly less than twice the 4.7GB capacity of standard DVD-5 media). It's also possible to have double-sided dual-layer DVD-Video media, known as DVD-18; some movie vendors use this technology to place widescreen (letterbox) and fullscreen (4:3 TV format) versions of a film on a single DVD. The user inserts the media into the DVD player or drive with the widescreen side up to view the film in its original format. By inserting the media with the fullscreen side up, the film can be viewed in its modified TV format. Almost all DVD set-top boxes and DVD-ROM drives except for possibly some very old models can read DVD-9 and DVD-18 media. Inside the drive, a single laser is refocused when switching between layers, accounting for a slight delay when moving from the top to the bottom layer. See Chapter 13 my new book Upgrading and Repairing PCs, 16th Edition , for complete technical specifications and diagrams of DVD media.
How Double Layer DVD Rewritable Drives Work
DVD+R DL discs use a single refocusable laser to write both layers. The top layer (Layer 0; L0) is written first. The metal reflector used by L0 is semi-transparent, enabling the refocused laser to write to the second layer (Layer 1; L1). Because the L0 layer absorbs some of the laser's energy, only about half the laser power reaches the L1 layer.