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• One pass VBR Best: This is similar to one-pass VBR, but with this mode, Compressor
applies greater effort to its internal decision-making processes. Although one-pass
VBR Best transcodes slower than one-pass VBR, it provides excellent to outstanding
SD video quality at bit rates of 3 to 3.5 Mbps and above. Bit rates for HD MPEG-2 can
range from 10–29 Mbps average, 12–29 Mbps maximum. Compared to one-pass
VBR, one-pass VBR Best provides higher quality, especially for difficult material at low
bit rates. For short streams of 1 to 2 minutes or less, if you are using one-pass VBR
or one-pass VBR Best, the average bit-rate accuracy may be compromised by as much
as 10 to 30 percent.
• 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 the higher bit rate to complex
scenes and a lower bit rate to easy scenes, so as to average out to the specified
average bit rate while ensuring that the specified maximum bit rate is not exceeded.
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 where the difference
between the most and the least complex scenes is substantial.
• Two pass VBR Best: This mode devotes more effort to its internal decision-making
process than does two-pass VBR. Encoding time for “Two pass VBR Best” is slower
than it is 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 3–3.5 Mbps and above for standard definition (SD) video. See Job
Segmenting and Two-Pass or Multi-Pass Encoding for information about using the
two-pass modes with job segmenting.
• Average Bit Rate slider and field: Choose the average bit rate by dragging the slider or
entering a number into the accompanying field. The range available to you depends
on the Stream Usage setting. See Stream Usage for more information. There is also an
Automatic button you can use 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.
203Chapter 18 Creating MPEG-2 Output Files