Encoded Output Delivered as a Bit Stream - A Practical Guide to Video and Audio Compression - page 255

14.3.3 Constant Bit Rate

It is possible to even out any bit-rate bursting by capping the available bit rate. The prob-
lem here is that bit-rate requirements may periodically and momentarily exceed the cap-
ping level. Those same dissolve and color-change effects will look quite different if the bit
rate is capped at the average level. Figure 14-4 shows what happens if the same bit-rate
pattern is delivered within a much lower threshold and that available bit rate is capped.
The encoder might eke out the dips in the constant bit rate (CBR) if it is smart enough to
move bits around and alter the buffering.
You will notice a severe blocking artifact at the points where the bit rate is capped
because the compressor has to throw information away. The best solution is to reduce the
information in the coded macroblocks.
A CBR setting forces the encoder to constrain the output so that even if the video
compression becomes more complex, the maximum bit rate is not exceeded under any cir-
cumstances. During times of low complexity, the video quality will be better than when
the encoder has to work hard. Because the bit rate remains constant, the trade-off has to
be made with the video quality. In order to remain within the budgeted bit rate, more
information must be discarded at times and this will lower the output quality of the result-
ing compressed video.

14.3.4 Statistical Multiplexing

A combination of variable and constant bit rate is available when statistical multiplexing
is used. This is very useful in digital TV applications and is covered in Chapter 27 where
digital TV services are discussed in detail.
The concept is essentially very simple. When two streams are transmitted together,
it is statistically unlikely that they will both be bursting to their maximum bit rate at the
same time. It is far more likely that one will trough while the other peaks.
The encoders are able to take advantage of this and create a multiplexed bit stream
that maximizes both channels. In fact, the concept is extended to as many as five or six
channels and still works very well.

CBR capped here

Maximum
bit rate
available

Constant bit rate

Figure 14-4 CBR bit stream.