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APPENDIX
341
Another point to consider is that while devices promote a specific data
transfer rate, very few can maintain that rate over an extended period. As
such, if your video comes in just under 300 KB per second, it doesn't
necessarily mean that most double speed CD-ROMs can play it smoothly.
In fact, most CD-ROM developers target their video files at between 50 -
70% of the specified data rate to make sure that no frames are dropped.
This comes out to transferring around 150 -190 KB per second for smooth
playback on a double speed CD-ROM. There are four factors that affect
the data transfer rate for video files: frame rate, frame size, data type,
and compression.
Understanding the frame rate
The frame rate determines how many images get displayed during every
second of a video. The higher the frame rate, the smoother your playback
can be because motion can be shown in smaller increments and at a faster
rate. But higher frame rates require more memory as a greater number of
images must be stored and shown each second. So, if you are producing
fast motion video, such as recording a sporting event, you probably need
to retain as high a frame rate as possible. But if you are working on a
project where there is little or slow movement, you might be able to save
some space by using a slower frame rate.
Understanding the frame size
The frame size determines how big the video image appears on screen.
By using a larger frame size, your image can be bigger and assuming your
source material is of sufficient quality, you can include more detail in your
video. But larger frame sizes carry two distinct problems. First, each
frame requires more data to store the information. For example, an
uncompressed frame with a size of 320 x 240 might require about 250 KB
of memory. A 640 x 480 frame (twice the size) would require about 1 MB,
(four times as much memory!). The frame size also affects how well the
video plays back for another reason. Larger frames require more informa-
tion to be redrawn. Some displays cannot refresh themselves fast enough
to show each change smoothly.
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