Encoded Output Delivered as a Bit Stream - A Practical Guide to Video and Audio Compression - page 252

Figure 14-1 Spatial complexity shown by bit-rate usage per macroblock. Source: Image courtesy
of Tektronix.

14.3 Bit-Rate Variability

When video is coded for delivery though a digital transport, the coding application allows
either variable bit rate (VBR) or constant bit rate (CBR) to be selected. Both have advan-
tages and disadvantages.
The video is arriving at the encoder ordered and synchronized to a steady frame rate.
Perhaps it is coming from a file, in which case time is irrelevant at that point, but the video
must be presented to the viewer on a strict time base. This has to be honored in both the
decoder and the player. The player usually decodes on the fly since decoding to an inter-
mediate storage format is not necessary unless you intend to edit the incoming video.
The frames may be delivered with a bit rate that goes up and down according to the
content, or the bit rate may be constant. If it is permissible to deliver a VBR stream, then
rate control is irrelevant unless the available bit rate has an upper limit. You can normally
set that limit in the encoder. For example, Popwire encoders optimally code MPEG-4 video
in a range from 60 Kbps to 2000 Kbps but you have to specify this, taking into account the
bit rate needed for the audio, of course.