Sony DTV-01, Manual

Page: 13 / 132

16
The Bit Rate Fluctuates
For DVD, variable bit rate is a tremendous advantage. If the bit rate were
fixed, it could not accommodate the changing needs of video scenes.
Consider the fast-paced action of a football player, running for a touch-
down as the camera pans past the crowd. Full of motion, this is an ex-
tremely demanding scene, one that requires the bit rate to be very high.
Now picture the same football player after the game, sitting in a restau-
rant, talking to his girlfriend. Almost nothing in the scene moves, so the bit
rate can be quite low.
If DVD used a fixed bit rate, the system might fall short on the football
scene. And it would definitely be wasting bits on the restaurant scene.
DVD-Video accommodates both scenes by varying the bit rate. In fact,
the maximum bit rate is 9.8 megabits per second which is nearly three
times as high as the “average” rate.
If DVD used a fixed rate, it would have to be at least 7 Mbps to maintain
picture quality! At that rate, total recording time would be cut in half. So
the goal of capturing a full-length movie on a 3-3/4-inch disc could not be
realized. Variable bit rate is one of the key technologies that makes DVD
possible.
By the way, Video CD uses MPEG-1 to yield a fixed bit rate of 1.15 Mbps.
The fixed rate and low number translate into the vast quality difference
between DVD-Video and Video CD.