Apple Compressor 2 - Manual (Page 112)

112 Chapter 8 Creating H.264 DVD Output Files
 Frame Sync: Use the slider to choose the Frame Sync rate for the output video or
enter a value directly. Also known as the key frame interval, this value represents how
often a keyframe is inserted in the H.264 stream. More frequent keyframes will
increase the quality and the bit rate; less frequent keyframes will reduce the quality
and bit rate. The default setting is 2 seconds.
 Multi-pass: Use this checkbox to turn on multi-pass encoding. Similar to two pass
MPEG-2 encoding, multi-pass offers the best possible quality. For faster (single-pass)
encodes, turn this feature off by deselecting the checkbox.
 Include PCM Audio: Use this checkbox to include a 16-bit, 48kHz stereo PCM audio
track in the output QuickTime movie. Most DVD producers will use a Dolby Digital
Professional (AC-3) setting for the audio with its much higher efficiency. In that case,
you can deselect this checkbox and make sure that a Dolby 2.0 setting is applied to
the job in the Batch window. If you select the checkbox, encode the H.264 movie,
and then import the movie into DVD Studio Pro, the video track and the audio track
will appear as two separate items in the Assets tab, as if you had imported one video
asset and one audio asset.
Job Segmenting and Multi-pass
If you choose 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 Multi-pass (and two-pass VBR
MPEG-2), 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.