# How Chapters Work

#### How precisely can I "mark" a chapter?

The smallest addressable unit in a DVD-Video is the VOBU (Video OBject Unit). It is about 0.5 seconds of video, audio, and subpictures. The entire contents of a VOBU belongs to the same VOB and CELL, so the VOBU has just one VobID and CellID. Since one requirement is that a VOBU contain one or more complete GOP (Group Of Pictures), and that each GOP starts with an I frame, a VOBU can be jumped into without any little green blocks appearing in the first 1 or 2 frames.

#### How does one VOBU get "marked" as the "chapter point"?

I know it's a nice paradigm to think there are marks, but there are not. A chapter can begin at the beginning of a CELL only. Some programs call this a VOB, it is all the VOBUs having the same VobID and CellID. Therefore, you have to decide while authoring where the chapters will be, and then the authoring program will assign VobIDs and CellIDs to accommodate you.

#### How, then, are chapters located?

Each title has a table (VTS_PTT - Video Title Set Part-of-Title Table) that contains 2 pieces of information. This is the PGCN (ProGram Chain Number) and PGN (ProGram Number) where the chapter starts. Please note - we said the Program number, not the CELL.
Now we move to the PGC, in it is a list of programs. Each program begins on a CELL.
In Summary:
Chapter (PTT) -> PGC/Program -> CELL

#### Is every CELL or Program a chapter?

NO. A chapter can encompass more than one program. Likewise, a program can encompass more than one CELL.

#### A little Off Topic, but what's the advantage of that?

Multiple angles require each angle to not only be in its own CELL, but to have a unique VobID. Yet they must be the same program, therefore multiple cells in a program.
Having more than one program in a chapter allows the author to break up the chapter into smaller parts, possibly scenes, that can be skipped using "next" and "prev" on the DVD remote. This happens because skipping is done by program, not chapter.

#### How can I move a chapter on an existing DVD?

With much difficulty! First, the VobID/CellID of the VOBUs that you want to move to another chapter must all be changed. This mucks up the trick-play pointers, which must never point from one cell to another. And finally several tables pointing to the first VOBU of the cell must be updated.

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