DVD Demystified

Essentials of DVD Production
Balancing the Bit Budget
515
Bit budgeting is a critical step before you encode the audio and video. You
must determine the data rate for each segment of the disc. If you underestimate
the bit budget, your assets won’t fit and you will have to re-encoded
and re-author. If you overestimate the bit budget, you will waste space on
your disc that could have been used for better quality encoding.
The idea is to list all the audio and video assets and determine how many
of the total bits available on the disc can be allocated to each. Part of the
early bit budgeting process involves balancing program length and video
quality. Two axes of control exist: data rate and capacity. Within the resulting
two-dimensional space, you must balance title length, picture quality,
number of audio tracks, quality of audio tracks, number of camera angles,
amount of additional footage for seamless branching, and other details. The
data rate (in megabits per second) multiplied by the playing time (in seconds)
gives the size (in megabits). At a desired level of quality, a range of
data rates will determine the size of the program. If the size is too big,
reduce the amount of video, reduce the data rate (and thus the quality), or
move up to a bigger capacity disc. Once you have determined the disc size
and the total playing time, maximize the data rate to fill the disc, which will
provide the best quality within the other constraints. See Figure 12.5 to get
a general idea of needed disc sizes. A DVD-5 holds 1 to 9 hours of video or 2
to 160 hours of audio. A DVD-9 holds 2 to 16 hours video or 3 to 300 hours
of audio. Of course the video quality at high-end playing times is much
lower than what is expected from DVD.
The easiest way to make a bit budget is to use a spreadsheet. One is
included on the sample disc that comes with this book. For simple projects
with only one video segment, use the DVDCalc spreadsheet on the sample
disc. Some authoring programs will calculate bit budgets for you when you
use their layout features.
To simplify calculations, keep track of sizes in megabits, rather than
megabytes (see Table 12.3). Allow an overhead of 3 to 4 percent for control
data and backup files which are added during formatting and multiplexing.
This also allows a bit of breathing room to make sure everything fits
on the disc.
In addition to fitting the assets into the total space on the disc, also make
sure that the combined data rate of the streams in a video program do not
exceed the maximum data rate (or instantaneous bit rate) of 10.08 Mbps.