9. What is a dual-layer disc ?
Dual layer DVD technology is commonly called DVD9 to indicate its approximate 9-GB storage capacity. Hollywood has been producing major motion pictures on stamped dual layer DVD discs for years. Dual layer DVD recordable discs offer up to four hours of high quality MPEG-2 video, or up to 8.5 GBytes of data on a single-sided disc with two individual recordable layers. With dual layer DVD recording, consumers can create longer home movies and even consolidate many home movie projects onto a single disc. 8.5 GB of uncompressed DVD disc space provides large storage and back up media for computer applications. You can pack up to 12 CDs or more than 6,000 floppy disks’ worth of information onto a single dual layer DVD disc.

The DVD specifications require that compliant players and drives read dual-layer discs. Some discs are capable of "seamless layer change" which means that there is no noticable pause or break when the laser reads video materials continuing from one layer to another at the layer break.

A dual-layer DVD has two layers for storage of data. One of the layers is semi-transparent so that the laser can focus through it and read the second layer so that data on both layers can be read from the same side of the disc. A dual-layer disc can hold almost twice as much storage space as a single-layer disc. Since the laser can read both layers from one side, there is no need to flip the disc as in the case of double-sided single-layer discs. Dual-layer DVD discs are much more difficult to manufacture than the double-sided single-layer disc.

In a dual-layer disc, the second layer can use either a PTP (parallel track path) layout where both tracks run in parallel, or an OTP (opposite track path) layout where the second track runs in an opposite spiral. Advanced DVD authoring software such as Sonic Solutions Scenarist allows the DVD author to define this type of layout. The OTP layout is more suitable to provide continuous video playback across both layers. The layer change can occur anywhere in the video stream and does not have to be at the end of a chapter. However, it usually is specified at the end of a DVD cell. The layer change is invisible on most DVD players, but it can cause some noticeable delay during playback on some players depending upon the program material on the disc. Most full-length Hollywood titles are packaged in dual-layer DVD to provide high quality video at higher bit rate.

When a dual layer recordable disc is inserted into a compliant recorder, the laser optics focus the beam 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 is focused on. Once the media type and the layer are detected, the laser can adjust its range of focus down or up to access any one of the two recordable layers. The drive then focuses on the Lead-In area of the disc to know whether the disc is unformatted, partially recorded in Multi-session format, or has been finalized (no further recording is possible).

The two layers represent one contiguous address stream for recording. 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 single-layer 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 overwraps Middle Zone 0. The disc is then recorded from the outside rim inwards (toward the hub). Multi-session discs can also be recorded with dual layer recordable media.