Adobe After Effects CS3 - Manual (Page 620)

AFTER EFFECTS CS3
User Guide
615
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 setting 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
increase the time it takes to encode the file, but generally result in more efficient compression and higher image
quality. (Adobe After Effects doesn’t 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 if 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 if 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.
Audio options for encoding
In the Export Settings dialog box, the options available in the Audio tab depend on the format you’ve specified. Some
common audio options include the following:
Codec Specifies the codec used to encode the audio:
• AAC (Advanced Audio Coding) A high-quality encoding format supported by many mobile devices. This codec is
the default for the H.264 format.
• SurCode for Dolby Digital 5.1 A high-quality encoding format developed for multichannel digital sound and the
most common encoder for DVD-video. (This codec is available only in Adobe Premiere Pro.)
• MainConcept MPEG Audio A high-quality encoder developed by MainConcept media technologies, and included
with Adobe Premiere Pro, After Effects, and Soundbooth.
• PCM (pulse-code modulation) Audio A lossless audio format. Files of this format tend to be larger than files of the
other formats.