Now , it might occur to someone ... "why is it called a dual layer!! its just a single disk with more memory.."..
well a few years back I also thought so...
I came across this fine little info on dual-layer break points which clears all doubts ....
The layer break point is the place where a dual layer DVD switches from the first layer (layer 0) to the second layer (layer 1). DVDs are laid out in sectors (each sector contains 2048 bytes of content), starting with sector 0 (zero). The break point is the sector address of the first sector on the second layer. This number also represents the number of sectors on layer 0.
It is important to keep in mind that although your DVD-9 title has 2 layers, it is not 2 discs. The DVD player software will see only one disc, with one ISO/UDF file system, and one set of files. The DVD drive that plays back the disc will have to know where the layer break occurs, so it can refocus to layer 1 when it reaches the break point, or whenever any content is requested that is beyond the break point. The drive will read the layer break point in the control data, a small file at the start of the disc (in the lead-in).
The position of the layer break is important. It can also be called the layer split point, because it splits the contents of the disc into 2 layers. So, the position of the layer break determines how the contents of the whole disc is divided into 2, and how much content ends up on each layer. A dual layer DVD can only hold about 4 Gigabytes of data on each layer. If the total size of the DVD is 7 Gigabytes, for instance, and the layer break was set too far from the halfway point, one layer might end up with more data than the disc can hold. GEAR checks the layer break point that you enter, to make sure that it doesn't cause this problem.
At this time, all dual layer recordable DVDs are oriented with Opposite Track Path. This means that the first layer (0) is recorded in a spiral track from the inside to the outside of the disc, and the second layer (1) is recorded in a spiral track from the outside (at the same diameter where the first layer finished recording), back towards the inside of the disc. For all Opposite Track Path DVDs, the layer break point must be at a sector address that is at least halfway through the total number of sectors. This insures that layer 0 is larger than layer 1. If layer 0 was not larger than layer 1, the disc would run out of room when it records layer 1, since it starts at the end of layer 0 and records back to the inside.
DVDs are encoded with a sophisticated scheme that scrambles groups of 16 sectors, in order to minimize the visibility of any errors that occur during playback. These groups are called ECC blocks (Error Checking and Correction). The layer break must occur at the start of a new ECC block, so that an ECC block is not written on 2 separate layers. So, the layer break must occur at a sector address that is evenly divisible by 16.
When the DVD is being played back, it will reach the end of layer 0 at the layer break point. The player will refocus the laser to layer 1, which is behind layer 0. While this layer transition usually takes only a fraction of a second, the player's data buffer may run dry during this time, and the video and audio will appear to freeze for a moment or two. This is normal, but DVDs are designed to try to minimize this problem.
DVD Video titles consist of up to 99 Video Title Sets (a VTS). The audio and video content of each program is arranged in Video Objects (VOBs). Each Video Object is broken up into files that are 1 Gigabyte or less, but really the video object itself can be as large as 8 Gigabytes. DVD Video discs have an Information file (IFO file) for each Video Title Set. A spare backup (BUP) copy of this information file is also stored on the disc, in case the information file is unreadable.
The content within each VOB is organized into cells. Each cell is a segment of video (or a still picture), along with the audio. In order for the video decoder on a DVD player to function correctly, the layer break for DVD Video titles must occur at the start of a cell. To find the location of cells, you can use a tool called IFOEdit.
To help you calculate your DVD layer break point, download our Layer Break Calculator spreadsheet.
To summarize, you can find an appropriate layer break point using the following rules...
For all DVDs, the layer break point:
must be less than 2,074,496 sectors (so that layer 0 does not exceed 2,074,496 sectors, the maximum capacity of a DVD-9 layer)
can not be within a 16-sector ECC block (must be on a sector that is evenly divisible by 16)
For Opposite Track Path (OTP) discs (all DVD-Video titles, and all titles written to Double Layer recordable DVDs) the layer break:
must be greater than 1/2 the total sectors of the disc (insuring that layer 0 is bigger than layer 1)
For DVD-Video titles, the layer break:
must point to the start of an IFO File, or the start of a program cell within a VOB.
ideally, it should be at a cell that is flagged as non-seamless (seamless playback linked in PCI = no). This rule can be broken without negative consequences.
Generally, the ideal layer break point is halfway through the title, or as near to this point as possible (given the conditions described above). When one or both layers is filled to near capacity, the content is written close to the outer radius of the disc, where there is a greater possibility of problems in manufacturing either replicated DVDs or DVD recordable discs. This is why it is not a good idea to push the capacity limits of a DVD.
Also, you will notice that some of the rules that must be followed specify the minimum layer break point and some specify the maximum layer break point. These two points give you a range of acceptable values, or a "window" where you can place your layer break. As the content of your title gets larger, this window will get smaller. If you attempt to fill a DVD-9 to it's maximum capacity, the window for your layer break point will narrow to a single sector, exactly halfway through the title (at 2,074,496 sectors).
good to know.. right!!!