DVD Demystified

520
Chapter 12
quality conversion is difficult. Unless you will be converting hundreds of
clips, it’s better to find a video production house with the equipment and
expertise to do it correctly. Be aware that the audio must be conformed to
match (see the following section).
You may need to choose between variable bit rate encoding (VBR) or constant
bit rate encoding (CBR). VBR doesn’t directly improve the quality, it
only means that quality can be maintained at lower average bit rates. For
best quality of long-playing video, VBR is the only choice. For short video
segments, CBR set at or near the same data rate of VBR will produce similar
results with less work and lower cost. For VBR encoding, set the average
and the maximum bit rate as determined in the bit budgeting step.
Tips and Tricks The following tips will help you in preparing video assets:
■ The display rate must be 25 frames per second for 625/50
(PAL/SECAM) video or 29.97 frames per second for 525/60 (NTSC)
video, not 30 frames per second. This is especially important when
using NLE systems that allow many different frame rates. Many
authoring tools will reject streams with non-compliant frame rates.
Note that the display frame rate can be different from the source
frame rate. Twenty-four frame-per-second film can be encoded to
display at 29.97 frames per second.
■ Ensure that the encoder can produce DVD-compliant streams, with
proper GOP size, display rates, and so on. Also look for an encoder that
can do segment-based re-encoding, which enables you to re-encode
small segments as needed, rather than reprocessing hours of video to
correct a single, small problem.
■ If you are working with film transferred to video, make sure the
encoder can do inverse telecine.
■ Chapters and programs must start at a sequence header or GOP
header, and the GOP must be closed, not open. Each cell must also start
at a sequence header. If you have very precise points for chapter marks,
make sure they are encoded properly, otherwise the authoring software
may move the chapter point to the nearest I frame.
■ In general, the easiest way to put still video on a disc is to author each
still as a menu.
■ Static video, where the picture doesn’t change over time, such as FBI
warnings, logos, and so on, should be authored as a timed still. Not only
does this look better, with no movement or variation when played back,
it can save space on the disc.