MPEG-2 Video Coding - Digital Video and Audio Broadcasting Technology - page 130

The data rate of the elementary video stream can be constant or can vary
depending on the current picture content. The data rate is controlled by
changing the quantization factors in dependence on the level of the output
buffer of the MPEG encoder (Fig.7.25.).
The macroblocks of an I, P or B picture can be coded in various ways.
The most numerous variants occur especially in the case of the B picture
where a macroblock can be coded in the following ways:

• Intraframe coded (completely new)

• Forward coded

•

Forward and backward coded

• Skipped (not coded at all)

Frame encoding

DCT1 DCT1 DCT2 DCT2

DCT4 DCT4

DCT3 DCT3

Field encoding

DCT1 DCT1 DCT2 DCT2

DCT3 DCT3 DCT4 DCT4

Fig. 7.27. Frame/field coding of macroblocks

The type of coding is decided by the encoder (Fig.7.25.) with reference
to the current picture content and the available channel capacity (data rate).
In contrast to analog television, no fields are transmitted but only
frames. The fields are then recreated at the receiving end by reading out
the frame buffer in a particular way.