Apple Compressor 2 - Manual (Page 135)

Chapter 10 Creating MPEG-2 Output Files 135
 “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 10–29Mbps average, 12–29Mbps
maximum. Compared to one pass VBR, one pass VBR Best provides higher quality,
especially for difficult material at low bit rates.
Note: 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. Like 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.