Apple Compressor (4.0) - Manual (Page 91)

Chapter 5 Custom settings and output formats 91
If you click the Video Format Automatic button, the GOP size is limited to 12 or 15 (based on
whether the video is PAL or NTSC), with a closed IBBP pattern. With Automatic selected, you
cannot change the frame rate setting in this tab, or the GOP settings in the GOP tab. For more
information about GOPs, see
About GOPs (groups of pictures) on page 99.
Video Format
Automatic button
Note: Because MPEG-2 uses fixed video frame sizes, Compressor enters the output frame size
in the fields in the Geometry pane based on your video format selection.
The video format you choose determines the options for the associated characteristics such as
frame size and rate, aspect ratio, and field dominance.
Note: The normal frame dimensions of SD NTSC digital video are 720 x 486. If you want
to transcode a source file using the NTSC setting, Compressor crops the file to achieve the
required MPEG-2 frame size of 720 x 480, unless you’ve already specified cropping attributes
for your setting. If not, Compressor crops two rows of pixels from the top and four from the
bottom. This crop attribute is only temporary and is not saved in the setting. You can see the
effects of the crop by double-clicking the setting in the job to open the Preview window.
•
Frame Rate: Choose the intended frame rate for the MPEG-2 output file. Film and some video
camcorders create progressive material at 24 fps (or the NTSC variant 23.98 fps). Although the
term “24p” implies video with a frame rate of 24 fps, the frame rate is usually 23.976 fps (rounded
to 23.98 fps). To transcode these sources for DVD playback on NTSC-related TVs, choose the 23.98
setting. For more information, see
About GOPs (groups of pictures) on page 99.
Note: The NTSC frame rate is frequently referred to as 29.97 fps. Similarly, the “NTSC version”
of 24 fps is referred to as 23.98 fps. Both these numbers are actually approximations of the
true values of 30/1.001 and 24/1.001, or 29.97003 and 23.97602 shown to more decimal places.
These numbers show that 29.97 is a pretty good approximation of 30/1.001 (only 3 frames drift
in 100,000 seconds), but that 23.976 would be much better than 23.98. In fact, even though
23.98 is used as shorthand in Compressor Help and in the Compressor user interface, accuracy
of 23.976 or better is maintained internally in Compressor, and generally within QuickTime as
well. For more information, see
About GOPs (groups of pictures) on page 99.