What is a dual layer DVD?
"Understanding Dual Layer DVD Burning"

While consumers around the world have enjoyed burning their own DVDs for a few years now, the inevitable question of “what’s next” is now upon the industry. Certainly, blue laser technologies such as Blu-Ray will one day become affordable and prevalent; however, consumers today are still very much in love with DVD. The DVD format is mature, high-quality content is widely available, and players are low cost and now installed in approximately 53 percent of U.S. households.

Dual layer DVD technology is not new. Commonly called “DVD9,” Hollywood has been churning out major motion pictures on stamped dual layer DVD Discs for years. How else could they include the full length movie plus all the bonus materials commonly found on today’s DVDs? Because dual layer technology has always been part of the DVD specifications, dual layer DVD recording on the desktop is the natural progression of single layer 4.7GB recordable technology.

Dual layer DVD recordable discs offer up to four hours of high quality MPEG-2 video, or up to 8.5GB of data on a single-sided disc with two individual recordable “layers.” Dual layer capable recorders will have the ability to record on the new dual layer DVD recordable discs, as well as on traditional single layer DVD discs and CDs too. Want more? Because a recorded dual layer DVD disc is compliant with the DVD9 specification, the discs are compatible with most consumer DVD players and computer DVD-ROM drives already installed in the market.

How Are Dual Layer Discs Recorded?

Single-sided dual layer recordable discs are constructed by one dummy polycarbonate platter base and the other one that contains a single organic recording layer. Dual layer recordable discs contain two organic dye recording layers (termed L0 and L1, respectively) between dual polycarbonate bases and semi-reflective metal layers separated by a transparent spacing layer. Single layer DVDs have a wobbled pre-groove molded into the polycarbonate base that controls the rotation speed of the disc and provide the addressing scheme for the disc. In a dual layer recordable DVD, each recording layer has its own wobbled pre-groove that controls rotation speed and addressing for that layer. However, the entire “table of contents” and system area of a dual layer recordable disc is contained only on the first recordable layer (L0).

When a dual layer recordable disc is inserted into a dual layer-compliant recorder, the optics will focus the laser at one of the dual layers to try and detect an “Address In Pre-groove” (ADIP) signal. From the ADIP signal, the recorder can detect whether the disc is dual layer and which layer it’s focused on. Once the media type and the layer are detected, the laser will be able to move its range of focus down or up to access any one of the two recordable layers. The drive will then focus on the Lead-In area of the disc to determine whether the disc is completely blank, partially recorded in Multi-session format, or Finalized (completed).

The two layers represent one contiguous address stream for recording as a Video Disc, a DVD-ROM, or even a packet recorded disc. When recording on dual layer media, the drive first records on the first recordable layer L0 from the inside hub area outward, just like a typical DVD recordable disc. When the end of information recorded in L0 is reached, Middle Zone 0 is added. Next, the drive focuses on the second recordable layer L1 to create Middle Zone 1 that over-wraps Middle Zone 0. The disc is then recorded from the outside rim inwards. Multi-session discs can be recorded with dual layer recordable media, so it’s possible to add data in “sessions” on a disc.

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