ADOBE AFTER EFFECTS 7.0
User Guide
615
Bitrate Encoding
Specifies whether the codec achieves a constant or variable bitrate in the exported file:
•
Constant Bit Rate (CBR)
Compresses each frame in the source video to the fixed limit you specify, producing a file
with a fixed data rate. Therefore, frames containing more complex data are compressed more, while less complex
frames are compressed less.
•
Variable Bit Rate (VBR)
Allows the exported file’s data rate to vary within a range you specify. Because a given
amount of compression degrades the quality of a complex image more than it degrades the quality of a simple image,
VBR encoding compresses complex frames less and compresses simple frames more.
In general, an image is complex and more difficult to compress efficiently if it contains great detail or if it differs
significantly from previous frames, as it would in a scene containing motion.
Note:
When comparing CBR and VBR files of the same content and file size, you can make the following generalizations:
A CBR file may play back more reliably over a wider range of systems, because a fixed data rate is less demanding on a
media player and computer processor. However, a VBR file tends to have a higher image quality, because VBR tailors
the amount of compression to the image content.
Bitrate
Specifies the number of megabits per second of playback for the encoded file. (This is available only if you
select CBR as the Bitrate Encoding option.)
The following options appear only if you select VBR as the Bitrate Encoding option:
Encoding Passes
Specifies the number of times the encoder will analyze the clip before encoding. Multiple passes
increases the time it takes to encode the file, but generally results in more efficient compression and higher image
quality. (Adobe After Effects does not support multiple encoding passes.)
Target Bitrate
Specifies the number of megabits per second of playback for the encoded file.
Maximum Bitrate
Specifies the maximum number of megabits per second of playback you want the encoder to
allow.
Minimum Bitrate
Specifies the minimum number of megabits per second of playback you want the encoder to allow.
The minimum bitrate differs according to the format. For MPEG-2-DVD, the minimum bitrate must be at least 1.5
Mbps.
M frames
Specifies the number of B frames (Bi-directional frames) between consecutive I frames (Intra-frames) and
P frames (Predicted frames).
N frames
Specifies the number of frames between I frames (Intra-frames). This value must be a multiple of the M
frames value.
Closed GOP Every
Specifies the frequency of each Closed Group of Pictures (Closed GOP), which cannot reference
frames outside of the closed GOP. A GOP consists of a sequence of I, B, and P frames. (This option is available when
you choose MPEG-1 or MPEG-2 as the format.)
Automatic GOP Placement
When selected, sets the placement of Group of Pictures (GOP) automatically. (This
option is available when you choose MPEG-1 as the format.)
Note:
MPEG-1 and MPEG-2 formats include numerous advanced options not listed here. In most cases, selecting a
format or preset designed for your target output sets the appropriate options automatically. For detailed information on
options not listed, consult the specifications for the MPEG-1 (ISO/IEC 11172) and MPEG-2 (ISO/IEC 13818) formats.
