Job Segmenting and Two-Pass or Multi-Pass Encoding
If you choose the two-pass or the multi-pass mode, and you have 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 number 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 throughout 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.
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Chapter 8
Finalizing Jobs and Submitting Batches
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 two 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 deselecting 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.
Note: The Allow Job Segmenting checkbox affects only the segmenting of individual
jobs (source files). If you are submitting batches with multiple jobs, the distributed
processing system will continue to speed up processing by distributing (unsegmented)
jobs, even with job segmenting turned off.
For more information on variable bit-rate encoding (VBR), see
Quality Tab
. For more
information on the Apple Qmaster distributed processing system, see
Apple Qmaster and
Distributed Processing
.