Apple Compressor (4.0) - Manual (Page 94)

Chapter 5 Custom settings and output formats 94
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Two pass VBR: In this mode, Compressor reads through the entire source video file twice.
In the first of these passes, Compressor analyzes the entire source video stream prior to
transcoding it to determine the degree of encoding difficulty of each scene. It then creates
a bit rate allocation plan that gives a higher bit rate to complex scenes and a lower bit rate
to easy scenes, in order to achieve the specified average bit rate without exceeding the
specified maximum bit rate. In the second pass, Compressor does the actual compression,
creating the MPEG-2 output video file according to its bit rate allocation plan. As with “One
pass VBR,” the goal of “Two pass VBR” is constant quality rather than constant bit rate. Unlike
“One pass VBR,” “Two pass VBR” is able to establish an overview of where best to spend its
bits across the entire source media file, before beginning the actual encoding. The result of
this process is that transcoding time is almost twice that of “One pass VBR.” Although the
benefit is not twice the quality, “Two pass VBR” provides more consistent overall quality than
“One pass VBR,” especially in source media files with a substantial difference between the
most and the least complex scenes.
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Two pass VBR Best: In this mode, Compressor devotes more effort to its internal decision-
making process than it does for “Two pass VBR.” Encoding time for “Two pass VBR Best” is
slower than for “Two pass VBR,” but it provides the best possible quality that the Compressor
MPEG-2 encoder has to offer. This mode provides outstanding quality at bit rates of 3 to 3.5
Mbps and above for SD video. For information about using the two-pass modes with job
segmenting, see
Job segmenting and two-pass or multi-pass encoding on page 249.
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Average Bit Rate: Set the average bit rate by dragging the slider or entering a number in the
accompanying field. The available range depends on the Stream Usage setting. You can
also click the Automatic button to the right of the field to have Compressor determine the
appropriate average and maximum bit rates.
Note: The average bit rate setting directly controls the size of the encoded file. The maximum
bit rate setting does not affect the encoded file’s size.
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Maximum Bit Rate: Set the maximum bit rate by dragging the slider or entering a number in
the accompanying field. The available range depends on the Stream Usage setting.
Note: This slider is available only with the VBR modes and cannot be set lower than the
average bit rate. The slider is also not available if the Automatic button is active.
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Motion Estimation: The items in this pop-up menu provide trade-offs between image quality
and processing time, especially if there is a lot of motion in the source file.
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Good: The fastest motion estimation setting. This mode does well even with significant
amounts of motion between frames, if the motion has minimal interfield motion within
frames. For example, footage that has been exposed to frame rate conversion or other
effects processes tends to have little interfield motion. In general, use Good with the one-
pass encoding modes.
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Better: A good general-purpose motion estimation setting. The Better mode provides
good results even with complex interlaced motion. Better works well for almost all types
of interlaced video sources, even shaky footage from handheld consumer camcorders. In
general, use Better with “One pass VBR” and “Two pass VBR.” If you’re using a progressive
video format, the Better mode does only frame-based motion estimation, and not field-
based motion-vector searching, which isn’t relevant for progressive source files. In this case
the Better mode will run somewhat faster.
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Best: This is the highest-quality motion estimation setting and should be used for the most
demanding and complex motion for interlaced sources. It’s slower than the Better mode.
In general, use the Best mode to maximize quality when using “One pass VBR Best” or “Two
pass VBR Best.”