MPEG-2 Video Coding - Digital Video and Audio Broadcasting Technology - page 119

ceiving end, however, it is absolutely necessary to have the information of
the preceding I and P pictures and that of the following I or P picture in
each case. But according to MPEG, the GOP structure can be variable. So
that not too much storage space needs to be reserved at the receiving end,
the GOP structure must be altered during the transmission so that the re-
spective backward prediction information is already available before the
actual B pictures. For this reason, the frames are transmitted in an order
which no longer corresponds to the original order.

I 10

B 5

B 4

P 6

B 2

B 1

P 3

B 1

B 2

I 0

Fig. 7.13. Order of picture transmission

Instead of the order I 0 , B 1 , B 2 , P 3 , B 4 , B 5 , P 6 , B 7 , B 8 , P 9 , the pictures are
now transmitted in the following order: I 0 , B -2 , B -1 , P 3 , B 1 , B 2 , P 6 , B 4 , B 5 ,
P 9 , etc. (Fig. 7.12.). That is to say, the P or I pictures following the B pic-
tures are now available at the receiving end before the corresponding B
pictures are received and must be decoded. The storage space to be re-
served at the receiving end is now calculable and limited. To be able to re-
store the original order, the frame numbers must be transmitted coded in
some way. For this purpose, the DTS (decoding time stamp) values con-
tained in the PES header are used, among other things (see Section 3, The
MPEG Data Stream).

7.1.6 Discrete Cosine Transform Followed by Quantization

A very successful method for still-frame compression has been in use since
the end of the eighties: the JPEG method, which today is also being used