Apple Compressor (4.0) - Manual (Page 93)

Chapter 5 Custom settings and output formats 93
Quality tab
The Quality tab allows you to set the bit rate for your video so that it can be transcoded to a
suitable data size and quality.
In addition to the controls, the lower-right corner shows an estimate of how many minutes of
video would fit on a 4.7 GB DVD using the current settings. This time assumes a single AIFF audio
stream to be used with the MPEG-2 stream.
Mode pop-up menu
Time estimate for a 4.7
GB disc with a stereo
AIFF audio stream
Motion Estimation
pop-up menu
•
Mode: Choose one of the following MPEG-2 encoding modes. (HD-quality modes are “One pass
VBR Best” and “Two pass VBR Best.”)
•
One pass CBR: Using this mode, the output MPEG-2 video stream’s bit rate is held approximately
constant from one GOP to the next. This is the fastest Compressor MPEG-2 encoding mode,
and it provides good to very good quality, especially at bit rates between 5 and 9 Mbps.
•
One pass VBR: This VBR (variable bit rate) encoding mode aims to maintain constant quality
(at the expense of constant bit rate) for the transcoded video file. This means that in scenes
with much detail or fast motion, Compressor allows a higher bit rate than it allows in less
detailed or still parts of the stream. In spite of this bit rate variability, Compressor ensures
that your specified average bit rate is achieved accurately and that the maximum bit rate is
not exceeded. Unlike the two-pass modes, “One pass VBR” doesn’t have a “bird’s-eye” view of
the source media prior to encoding, so its bit allocation is less optimized than that of “Two
pass VBR.” Nonetheless, for most standard-definition (SD) media files at bit rates of 3.5 Mbps
and above, this mode provides good to excellent quality and transcodes quickly.
•
One pass VBR Best: This is similar to “One pass VBR,” but Compressor applies greater effort
to its internal decision-making process. 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 high-definition (HD) MPEG-2 can range from 10 to 29
Mbps average, 12 to 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’re 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.