Apple Compressor 2 User Manual
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Chapter 3
The Compressor Interface
Job Segmenting and Two pass (or Multi pass) Encoding
If you choose the two-pass or the multi-pass mode, and you are using Compressor 2 or
higher with distributed processing enabled, you may have to make a choice between
speedier processing and ensuring the highest possible quality. The Apple Qmaster
distributed processing system speeds up processing by distributing work to multiple
processing nodes (computers). One way it does this is by dividing up the total amount
of frames in a job into smaller segments. Each of the processing computers then works
on a different segment. Since the nodes are working in parallel, the job is finished
sooner than it would have been on a single computer. But with two-pass VBR and
Multi-pass encoding, each segment is treated individually so the bit-rate allocation
generated in the first pass for any one segment does not include information from the
segments processed on other computers.
First, evaluate the encoding difficulty (complexity) of your source media. Then, decide
whether or not to allow job segmenting (with the “Allow Job segmenting” checkbox at
the top of the Encoder pane). If the distribution of simple and complex areas of the
media is similar in any given segment as it is across the whole source media file, then
you can get the same quality whether segmenting is turned on or not. In that case, it
makes sense to allow segmenting to speed up the processing time.
However, you may have a source media file with an uneven distribution of complex
scenes. For example, suppose you have a 2-hour sports program in which the first hour
is the pregame show with relatively static talking heads, and the second hour is high-
action sports footage. If this source media were evenly split into 2 segments, the bit
rate allocation plan for the first segment would not be able to “donate” some of its bits
to the second segment because the segments would be processed on separate
computers. The quality of the more complex action footage in the second segment
would suffer. In this case, if your goal were ensuring the highest possible quality over
the entire 2-hour program, it would make sense to not allow job segmenting by de-
selecting the checkbox at the top of the Encoder pane. This forces the job (and
therefore, the bit-rate allocation) to be processed on a single computer.
For more information on variable bit rate encoding (VBR), see “
page 134. For more information on the Apple Qmaster distributed processing system,
see the Distributed Processing Setup guide.