Encoded Output Delivered as a Bit Stream - A Practical Guide to Video and Audio Compression - page 257

Clipping of bit stream
Max bit rate
Figure 14-6 Peaking of content.
up and move ahead of the playback again. Figure 14-6 shows where a bit rate bursts past
the capacity to deliver it, causing a momentary loss of quality.
This is complex to control and will require quite expensive encoders. The traffic
shaping becomes more important when interactive services are packaged with the video.
In that case, the interactivity is delivered opportunistically during the troughs in bit rate.
The overall bit rate is then maintained at a constant level. Some interactive elements need
to be delivered early in the stream so the user is able to interact with them right away.
Figure 14-7 shows how the clipped bit rate is corrected by delivering some of the buffer a
little earlier than originally planned.
This is effectively statistical multiplexing with the additional twist that some of the
bit rate is allocated to non-video use. Interactive assets don't have to be delivered at spe-
cific frame times within the stream. They only have to be delivered before the interactiv-
ity calls them up for use.
Higher frame rates will typically give a smoother motion, but they decrease
the image quality. Lower frame rates result in crisper images but will proba-
bly cause jerky motion.
Some data delivered earlier when bit rate allows
Figure 14-7 Traffic-shaped changes.