Apple Compressor 2 - Manual (Page 131)

Chapter 10 Creating MPEG-2 Output Files 131
Note: If you choose Automatic, the GOP size is limited to 12 or 15, with a closed IBBP
pattern. With Automatic selected, you cannot change any of the frame rate and field
dominance settings in this tab, or the GOP settings in the GOP tab. See
“
Understanding GOPs and Frame Types ” on page 140 for more information about
GOPs.
Note: Since MPEG-2 for DVD uses fixed video frame sizes (shown in the table below),
Compressor enters the output frame size in the fields in the Geometry pane based
on your video format selection.
Note: The normal frame dimensions of standard definition (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 have 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 Batch table to open the
Preview window.
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 NTSC frame rate is frequently mentioned as 29.97 fps. Similarly, the “NTSC
version” of 24 fps is mentioned as 23.98 fps. Both these numbers are actually
approximations to the true values 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 to 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
the 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.
Video format Frame size (pixels) Frame rate (fps) Interlaced source
NTSC 720 x 480 29.97 Y
PAL 720 x 576 25 Y
720p 1280 x 720 23.98, 50, 59.94 N
1080i (HDV res) 1440 x 1080 23.98, 25, 29.97 Y
1080i (Std. res) 1920 x 1080 23.98, 25, 29.97 Y