Rivergate Software Inc. - makers of DVDAfterEdit

Physical Structure

All video DVD's consist of several different types of file - for example for DVDs these are - Information or .IFO files, Video Object or .VOB files and Backup or .BUP files - all contained in a folder named VIDEO_TS on an optical disc in UDF format. The layout of these files is referred to as the Physical Structure - meaning where the files are actually located on the disc's surface. For example on a DVD-9, the files in the first VTS will be closest to the center of the disc in the first layer, whereas later files will be in the second layer.

Logical Structure

By contrast, Logical Structure is a term used to distinguish the way that the DVD "appears" to a DVD player. It tells the player what order to play the video in, how the user can interact with the disc and so on. This is probably easiest to understand through an example.

Imagine a DVD with a single piece of 30-minute video on it. This video footage will be contained in a video object (VOB) file, and will probably be divided into chapters - three, for example. These are usually encoded in the VOB in simple numerical order, ie. if you could look at the disc you would see:

• Chapter 1 > Chapter 2 > Chapter 3

This is the "physical structure" of this VOB. However, it is very simple to get the player to play these chapters in a different order - for example:

• Chapter 3 > Chapter 2 > Chapter 1

- in which case we would say that the "logical structure" ( DVD plays Chapters in the order 3, 2, 1 ) is different from the "physical structure" ( DVD contains video data in the order Chapter 1, 2, 3 ). As another example, it's quite possible to have a disc which physically contains pieces of video which aren't contained in the logical structure at all - in which case the person watching the disc will never see them.

The distinction between physical and logical structure is made possible because a DVD Player doesn't simply play a VOB file blindly from beginning to end - it looks at an information (IFO) file to get it's instructions as to how to play the video contained in that VOB. But IFO files also contain a mass of other information relating to more complex logical information about the DVD - for example Prohibited User Operations (UOPs), pre and post commands, cell commands and so on.

HDAE and Logical Structure

In general the physical structure of a DVD is relatively straightforward, whereas the logical structure can be mind-numbingly complex. This is almost inevitable given the nature of the DVD Specification, and luckily most authoring applications hide most of it from the user. HDAfterEdit is an exception to this rule in that almost any aspect of the structure is accessible, but the genius of it's interface is to actually express the logical structure in it's interface. For example, the way unfolding a PGC twirly shows that a PGC "contains" the different cells from a VOB(*), but these cells can be re-ordered at will using the Program Map Editor. And, in turn the way a PGC "contains" programs, which "contain" cells(**), which contain nav-packs... Notice that cells really do contain nav-packs, because below the cell level DVDAE is actually looking into the VOB files, ie. the physical structure.

Conclusion

It's actually possible to use HDAE extremely effectively without fully understanding the concept of logical vs. physical structure, but understanding how the two relate can be very useful. Navigation speed can be improved by making sure there isn't excessive leaping around the logical structure to access assets which are physically close together, for example. DVDs authored using abstraction layer methods tend to have a "one-size-fits-all" logical structure and as a result have much more sluggish navigation than those programmed in spec commands directly.

Footnotes

<ref>PGCs don't really contain anything, since they are purely logical structures defined by .IFO files. So, the .IFO file directs the player to play the cells in a particular order, as in the simple example above - the player "sees" a PGC "containing" those cells.</ref> <references/>