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By definition, closed GOPs cannot contain any frame that refers to a frame in the previous
or next 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. 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 a DVD authoring application.
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 use only closed GOPs. This freedom
is limited, as it allows you to set chapter markers only at GOP boundaries, rather than at
any video frame.
About 24p (23.98p)
For DVD authoring and encoding, 24p refers to a video sequence that contains 24
progressive (non-interlaced) frames per second, with NTSC-related standard definition
frame dimensions (720 x 480 for MPEG-2). Film-based movies have a native frame rate of
24 fps, and because the MPEG-2 format is able to represent 24 fps video internally, many
commercial movie DVDs are encoded in this way. But any time you use NTSC video in
your project, the frame rate of film-transferred material will be slowed down from 24 fps
to 23.976 fps (rounded to 23.98) and a 2:3:2:3 pull-down is added. So, the more accurate
term is actually 23.98p.
Compressor can also do this for 24p source video files. For such material, the 23.98 frame
rate option (in the Video Format tab) compresses each source frame one-for-one, without
compressing repeated frames or fields in order to achieve a 29.97 fps display rate. This
results in higher quality at a lower compressed bit rate than would be possible if the 24p
material were converted to 29.97 fps prior to transcoding. Compressor also sets internal
MPEG-2 frame flags correctly, so DVD players will properly apply the 3:2 pull-down process
for display on 29.97 fps interlaced NTSC TV sets.
211Chapter 18 Creating MPEG-2 Output Files