DVDs have become such a standard in today's computer and media playing devices that some of us are still trying to catch up with its speedy development and mass market distribution. Your local electronics store is stocked with burning drives of varying degrees and shelves are stacked with recordable media in multiple formats (CD-R/RW, DVD-R/RW, DVD+R/RW). There is no denying that recordable media are more mainstream now than in the past, allowing us to play and record our own music/data CDs, play DVD movies as well as create our own DVDs (even Blu-Ray) to share with family and friends on a variety of compatible home devices. DVD media in particular has totally captured the attention of the public not just for recording digital movies but also for its vast storage capability. DVD discs currently hold 4.7GB of space (about a 2-hour long movie).
Just as we were familiarizing ourselves with the "+" and "-" format of DVD recordable media (they are essentially the same with the exception of device compatibility) they introduced Dual-Layer DVD recordable media. Of course, this has been around for a while now, but you may not know that the movie industry was actually distributing their movies on dual-layer discs to include the film along with featurettes, deleted scenes, music videos and other bonus material, for several years before it became available for PC recoding use.
There are two types of Dual Layer DVD: Single-sided and Double-sided. Single-sided Dual-Layer DVDs contain all the data on one side of the disc and are read on one side only while Double-sided Dual Layer DVD utilizes both sides of the disc and both sides are readable. 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 double-sided dual layer DVD on one side to view the widescreen version of the film or flip it over to watch the fullscreen version.
What Is Dual Layer?
Specifically, Dual Layer DVDs hold just about double the data of a standard recordable "+" and/or "-" DVD recordable disc (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 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 and Double-sided dual-layer DVD-Video as DVD-18. 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. 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.
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.
First Layer Recorded Inside Hub To Outer Disc Rim
Second Layer Recorded Outside Rim Towards Disc Hub
Reflectivity of both recording layers of a dual layer recordable disc is the similar: greater than 18 percent. The reflectivity between the L0 and L1 layers, however, is greater than 50 percent because the upper (second) recording layer absorbs and reflects some of energy that is directed at the lower (first) recording layer L0 in order for organic dye to be recorded. As a result, the organic dye formulation and shape of the pre-groove in dual layer discs must be optimized to provide the appropriate reflectivity for both layers. The spacer layer separates the two recording layers and prevents cross layer recording. It is transparent to allow the laser to easily focus on either recording layer by simply changing the position of the laser's object lens.