DUAL LAYER DVD
Dual Layer recording allows DVD-R and DVD+R discs to store significantly more data, up to 8.5 Gigabytes per disc, compared with 4.7 Gigabytes for single-layer discs.
A Dual Layer disc differs from its usual DVD counterpart by employing a second physical layer within the disc itself. The drive with Dual Layer capability accesses the second layer by shining the laser through the first semi-transparent layer. The layer change mechanism in some DVD players can show a noticeable pause, as long as two seconds by some accounts. More than a few viewers have worried that their dual layer discs were damaged or defective.
Understanding Dual Layer DVD Recording
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.
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.
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Dual Layer CD/DVD
Re: Dual Layer CD/DVD
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