Apple Compressor 2 Bedienungsanleitung PDF herunterladen (Seite 141)

Chapter 10 Creating MPEG-2 Output Files 141

In principle, the less average motion there is from one frame to the next, the farther apart

P-frames can be spaced, and the greater the compression can be. For most video

material, the IBBP structure is a good choice. Material with unusually fast motion

throughout the entire sequence may benefit from an IBP or IP structure, but in such cases

a relatively high bit rate (6 to 8 Mbps for SD video) may be required for good quality.

GOP size

This setting specifies the number of frames within a GOP. Because exactly one I-frame

exists per GOP, longer GOP sizes generally provide greater compression, because B- and

P-frames are smaller than I-frames.

For most media, spacing I-frames about 1/2 second apart gives good results. This

equates to a GOP size of 15 frames for NTSC and 12 frames for PAL. The DVD-Video

specification prohibits GOP lengths from being much longer than this. Generally, only

material with scene changes occurring less than 1/2 second apart frequently

throughout the video will benefit from shorter GOP sizes.

Open and closed GOPs

Open GOPs are most efficient because they allow B-frames from one GOP to look at the

last P-frame from the preceding GOP as well as the first I-frame of their own GOP. Open

GOPs don’t have to start with I-frames as shown below:

:06 :07 :08 :09

:09

:10 :11 :12 :13 :14 :15 :16 :17 :18 :19 :20 :21:04 :05

Open GOP

(IBBP, 15 frames)

I BBPBBPBBPBBPBB IBBP

:03

:06 :07 :08 :10 :11 :12 :13 :14 :15 :16 :17 :18:04 :05

Closed GOP

(IBBP, 15 frames)

PBBPB PB PBPIB BBB

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Apple Compressor 2 Bedienungsanleitung Seite 141