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Table Of Contents
- Compressor User Manual
- Contents
- An Introduction to Compressor
- Getting Started Quickly
- The Basic Transcoding Workflow
- The Compressor Interface
- Importing Source Media Files
- Creating, Previewing, and Modifying Settings
- Creating Jobs and Submitting Batches
- Assigning Settings to Source Media Files
- Assigning Destinations to Source Media Files
- Submitting a Batch
- About the History Drawer
- Resubmitting a Batch
- Saving and Opening a Batch File
- Submitting a Final Cut Pro Project for Transcoding
- Resubmitting a Final Cut Pro Project
- Transcoding Different Clips From One Source Media File
- Creating Dolby Digital Professional Output Files
- Creating H.264 DVD Output Files
- Creating MPEG-1 Output Files
- Creating MPEG-2 Output Files
- Creating MPEG-4 Output Files
- Creating QuickTime Movie Output Files
- Creating QuickTime Export Component, AIFF, and TIFF Files
- Adding Filters to a Setting
- Adding Frame Controls, Geometry, and Actions to a Setting
- Using the Preview Window
- Creating and Changing Destinations
- Using Droplets
- Customer Support
- Command-Line Usage
- Index
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