Apple Compressor 2 - Manual (Page 142)

142 Chapter 10 Creating MPEG-2 Output Files
By definition, closed GOPs cannot contain any frame that refers to a frame in the
previous GOP. In contrast, open GOPs begin with one or more B-frames that reference
the last P-frame of the previous GOP. Closed GOPs created by Compressor always begin
with an I-frame.
Open GOPs generally provide slightly better compression than do closed GOPs of the
same structure and size. The illustration above shows that a closed GOP contains one
more P-frame than does an open GOP of the same length. Since P-frames generally
require more bits than do B-frames, the open GOP achieves slightly better
compression.
There are limitations to using open GOPs for DVD-Video discs that are created using a
DVD authoring application such as DVD Studio Pro. One limitation is that only closed
GOPs are permitted within MPEG-2 streams that will be used for mixed-angle or
multi-angle DVDs.
The other limitation is that DVD chapter markers can only be set at the beginning of a
closed GOP. The best time to define chapter markers is before doing MPEG-2
transcoding. For example, if you specify your chapter markers in Final Cut Pro, you can
set Compressor to do MPEG-2 transcoding with open GOPs. Compressor will then force
a closed GOP to begin only at the specified chapter markers, and will make all other
GOPs open. You can also accomplish this by specifying “forced I-frames” in the
Compressor Preview window, and giving them a chapter name to be used by
DVD Studio Pro.
However, if you want to specify chapter markers at any GOP boundary after your video
has been transcoded in the MPEG-2 format, you should only use closed GOPs. This
freedom is limited, as it allows you to set chapter markers only at GOP boundaries,
rather than at any video frame.