Apple DVD Studio Pro 4 - Manual (Page 67)

Note: 24p assets have their GOP structure based on their display rate, not the encoded
rate. A 24p asset encoded to play in an NTSC project has a display rate of 29.97 interlaced
frames (with an 18 frames per GOP maximum) or 59.94 progressive frames (with a
36 frames per GOP maximum).
The integrated MPEG encoder uses a GOP length of 15 frames for NTSC and 12 frames
for PAL HD assets.
Open or Closed GOPs
An open GOP allows the B-frames from one GOP to look at an I- or P-frame from an
adjacent GOP. Open GOPs are more efficient, but cannot be used with multi-angle or
mixed-angle titles. A closed GOP keeps all references within itself, standing alone with
respect to other GOPs. DVD Studio Pro works with either type in single-angle titles and
requires closed GOPs with multi-angle and mixed-angle titles.
It is important to know that the same pattern can provide different results when used
with an open or closed GOP. For example, the integrated MPEG encoder uses a closed
GOP type, and it starts its IBBP pattern with an I-frame. Other encoders using an open
GOP may start with a B-frame. In this case, starting with a B-frame is a little more efficient;
if you start with an I-frame, an extra P-frame must be added to the end (a GOP cannot
end with a B-frame).
General Quality Issues
The quality of the MPEG encoding determines the quality of the title. It is well worth
putting some effort into getting the best possible MPEG encoding environment. For
example, start with the best quality source material you can. An uncompressed digital
source is the best, followed by a DV source, and then an analog Component source. Of
the other analog formats, S-Video (Y/C) is much preferable to Composite.
Noisy Video
Because a large portion of MPEG’s compression power depends on its ability to locate
areas of the image that do not change between successive frames, “noisy” video can
reduce the efficiency of MPEG encoding. While the actual video may be identical between
two frames, if there are noise artifact differences between the two frames, they are
detected as a change in video and require more bits to encode. A lot of noise causes poor
encodes, because the encoding resources are tied up trying to faithfully reproduce the
noise.
Transcoding Artifacts
Every time video is transcoded from one format to another—for example, from analog
to digital, or DV to MPEG-2—artifacts are introduced into it. The visibility of these artifacts
depends on the quality of the transcoding device and the content. While transcoding
cannot be avoided, its effects can be minimized. Use the highest quality equipment and
software you can, and choose products that work well together and support each other’s
formats.
67 Chapter 4 Preparing Video Assets