Adobe Premiere Elements 8 - Manual (Page 286)

281 USING ADOBE PREMIERE ELEMENTS 8 EDITOR
Saving and sharing your movies
Last updated 8/12/2010
display and motion-picture film. Choose Upper Field First or Lower Field First (the default) when exporting video for
an interlaced medium such as NTSC, PAL, or SECAM. DV footage is generally Lower Field First; however, some newer
nontape camcorders produce video with reverse field order, so make sure to check your camcorder’s documentation.
Pixel Aspect Ratio Specifies the ratio of each pixel’s width to its height, which determines the number of pixels
required to achieve a given frame aspect ratio. Some formats specify square pixels, while others use nonsquare pixels.
Keyframe Intervals (seconds) Specifies the number of seconds after which the codec will create a keyframe when
exporting video.
Bitrate Encoding Specifies whether the codec achieves a constant or variable bitrate in the exported file.
• CBR Constant Bitrate (CBR) keeps the data rate of the exported file constant within a fixed limit you specify. Since
the complex sections are held to the same bitrate as the simple, they are more likely to show the quality-degrading
artifacts of compression.
• VBR Variable Bitrate (VBR) allows the exported file’s data rate to vary within a range you specify, allocating higher
bitrates, and therefore less compression, to the more complex sections and lower bitrates to the less complex.
In general, a frame is complex and more difficult to compress efficiently if it contains great detail, or if it significantly
differs from surrounding frames, as it would in a scene containing motion.
Note: When comparing CBR and VBR files of the same content and file size, 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 you want the encoded file to have. This option only appears if you
select CBR as the Bitrate Encoding option.
The following options appear only if you select VBR as the Bitrate Encoding option:
Minimum Bitrate [Mbps] Specifies the minimum number of megabits per second you want the encoder to allow. The
minimum bitrate differs according to the format. For MPEG2-DVD, the minimum bitrate must be at least 1.5 Mbps.
Target Bitrate [Mbps] Specifies the number of megabits per second (Mbps) you want the encoded file to have.
Maximum Bitrate [Mbps] Specifies the maximum number of megabits per second you want the encoder to allow.
M Frames Specifies the number of B frames (bi-directional frames) between consecutive I frames (intra-frames) and
P frames (predicted frames). This option is available only for MPEG formats.
N Frames Specifies the number of frames between I frames (intra-frames). This value must be a multiple of the M
frames value. This option is available only for MPEG formats.
Closed GOP Every Specifies the frequency of each Closed Group of Pictures (Closed GOP), which can’t 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 either of the Multimedia Compatible presets (MPEG1 Multimedia Compatible or MPEG2 Multimedia
Compatible) from the Export MPEG dialog box, and then click Advanced.)
Automatic GOP Placement When selected, sets the placement of Group of Pictures (GOP) automatically. (This option
is available when you choose either of the MPEG Multimedia Compatible presets from the Export MPEG dialog box,
and then click Advanced.)
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 industry specifications for the MPEG-1 and MPEG-2 formats.