Encoded Output Delivered as a Bit Stream - A Practical Guide to Video and Audio Compression - page 253

You have a variety of parameters in the compression engine that control the quality
of the output. Chapters 31 to 38 discuss the optimum values for these as part of the step-
by-step walk-through of the compression process. Whether CBR or VBR is selected in the
parameter settings of your compression software controls the bit-rate usage.
A very useful bit-rate calculator is available at the DVD-HQ Web site. Also, Pioneer
provides some DVD technical guidelines at their Web site.

14.3.1 Bursting

Bursting happens when the content being compressed momentarily becomes more com-
plex than usual. This occurs when two scenes are lap dissolved from one to the other.
A consequence of this kind of scene transition is that every pixel changes value on every
frame, and any motion compensation is also compromised since subjects are partially
transparent and may be moving in completely different directions in the two scenes.
This all becomes much harder to compress because there is far less chance of finding
similar macroblocks in previous or later frames on which to base the delta information. As
a consequence, virtually the whole frame must be recompressed as if it were an I-frame.
Few of the gains associated with B-frames are available.
You should edit sympathetically in order to reduce compression artifacts like this.
Editing sympathetically is when you avoid things like cutting abruptly and frequently
(every couple of frames) or using transitions that create a lot of frame-to-frame difference
information.
Straightforward cut edits are less attractive visually but compress much better. If you
want to enhance the scene transitions, a wipe, possibly with a soft edge, would compress
far more easily than a dissolve and is not unattractive. Figure 14-2 shows the two differ-
ent transition effects.

14.3.2 Variable Bit Rate

With a variable bit rate (VBR), the video quality remains the same but the bit rate is
allowed to burst occasionally in order to accommodate a more complex scene.
Some encoders provide a hybrid VBR that has a maximum bit-rate setting. The video
will be compressed at a low bit rate most of the time. When it bursts, the bit rate is allowed
to rise to a preset maximum value. This gives an all-around better performance if it is con-
figured correctly. The quality will only degenerate when an extremely complex piece of
video is presented.
Figure 14-3 also shows how a VBR signal uses more or less capacity but must remain
below some arbitrarily defined threshold to avoid multiplexing problems.
Variable bit-rate streams have a major advantage in the area of video quality. Because
they are variable, additional bit rate can be made available when a sequence of frames

DVD HQ bit-rate calculator: http://dvd-hq.info/Calculator.html
Pioneer DVD specifications: http://www.pioneer.co.jp/crdl/tech/dvd/4-3-e.html