MPEG-2 Video Coding - Digital Video and Audio Broadcasting Technology - page 117

blocks and the horizontal and vertical number of pixels is selected to be
such that it is divisible by 16 and also by 8 (Y: 720 x 576 pixels). At cer-
tain intervals complete reference frames, so-called I (intracoded) frames,
formed without forming the difference, are then repeatedly transmitted and
interspersed between them the delta frames (interframes).
Forming the difference is done at macroblock level, i.e. the respective
macroblock of a following frame is always compared with the macroblock
of the preceding frame. Put more precisely, this macroblock is first exam-
ined to see whether it has shifted in any direction due to movement in the
picture, has not shifted at all or whether the picture information in this
macroblock is completely new. If there is a simple displacement, only a
so-called motion vector is transmitted. In addition to the motion vector, it
is also possible to transmit the difference, if any, with respect to the pre-
ceding macroblock. If the macroblock has neither shifted nor changed in
any way, nothing needs to be transmitted at all. If no correlation with an
adjoining preceding macroblock can be found, the macroblock is com-
pletely recoded. Such pictures produced by simple forward prediction are
called P (predicted) pictures (Fig.7.10.).

GOP

I BBP B I

I = Intra frame encoded picture
P = “predicted" forward encoded picture
B = "bidirectional" encoded picture
GOP = Group of Pictures

Forward
encoding

Backward
encoding

Fig. 7.11. Bidirectionally predicted delta frames

Apart from unidirectionally forward predicted frames there are also
bidirectionally, i.e. forward and backward, predicted delta frames, so-
called B pictures. The reason for this is the much lower data rate in the B
pictures compared with the P pictures or even I pictures, which becomes