Apple Compressor (4.0) - Manual (Page 100)

Chapter 5 Custom settings and output formats 100
Open and closed GOPs
Open GOPs are most efficient because they allow an extra B-frame in the GOP pattern. Open
GOPs start with a B-frame that is able to look at the last P-frame from the preceding GOP as well
as the first I-frame of its own GOP.
Open GOP
(IBBP, 15 frames)
:06 :07 :08 :09 :10 :11 :12 :13 :14 :15 :16 :17 :18 :19 :20 :21 :04 :05 :03
I B B P B B P B B P B B P B B I B B P
Closed GOP
(IBBP, 15 frames)
:09 :06 :07 :08 :10 :11 :12 :13 :14 :15 :16 :17 :18 :04 :05
P B B P B P B P B P I B B B B
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. Because 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 be set only 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 then forces a closed GOP to begin only at the specified
chapter markers and makes 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.
AIFF files
AIFF overview
The Audio Interchange File Format (AIFF) was developed by Apple for storing high-quality audio.
The format was intended for DVD or CD authoring and is now one of the most commonly used
audio file formats for the Mac OS.
Note: Choose the AIFF option from the File Format pop-up menu only if you need to create
a specialized AIFF file. If you’re creating a standard AIFF file, choose the Common Audio
Formats option.