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306 MPEG glossary
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p
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Data stream I
0
P
01
B
01
B
02
P
02
B
11
B
12
B
21
B
22 I1
... for closed GOPs
I
0
P
01
B
01
B
02
P
02
B
11
B
12
I
1
B
21 B22
P
11
... For open GOPs
D
complicated computations have to take place! These are made easier using a frame table.
A frame table contains a list, where the information of every frame in the data stream is
found, identifying the type of frame it is.
U
Quantization scaling
The single pictures in MPEG are saved using a compression method comparable to JPEG
with bitmaps and associated with quality loss. For this single images are divided into 8 x 8
b
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he encoder, and later the decoder, will know that two B frames belong between them.)
his image will also be completely encoded, and afterwards all macroblocks that haven't
hanged in comparison to the I frame will be deleted. They will be replaced by
orresponding references for the decoder that tell it "you already know what should be
hown here, and you can get it from the last I frame".
ow, the 2nd will be completely encoded, and all macroblocks identical to the first I frame
nd the following P frame will be removed. References to previous frames are called
ackward predictions, and references to following frames are called forward
redictions. The third picture will be edited in exactly the same fashion.
he fourth picture we have already explained, and now we need the next P frame, or
icture number 7. Pictures 5 and 6 are B frames again, which are compared to P frames to
oth sides of them (picture 4 and 7); these are followed by the last two B frames. These
ave a special place, since in closed GOPs, they may contain only
backward predictions,
nd no references to the next I frame, because it belongs to the next GOP.
omething else: Since the decoder is no prophet, the P frames are always transmitted
efore the B frames! The GOP explained above will be encoded and transmitted in the
rder it is written.
riginal I
OP
0
B
01
B
02
P
01
B
11
B
12
P
02
B
21
B
22 I1
ue to this nested structure, it is easy to see that during direct editing of MPEG material,
sing
Movement prediction (view page 302) P and B frames are likewise reduced.
locks (view page 302).
ach one of these blocks is then transformed into an 8 x 8 matrix (a table with rows and
olumns) using a
DCT (discreet cosinus transformation) mathematical method. Each of
hese values is produced using all 64 individual pixels of the block, but the values in the
atrix are ordered in such a way that the image information is ordered according to its
mportance.
his matrix will then be multiplied by another matrix, i.e. the
quantization matrix. Exactly
ow and why this matrix must be created is the biggest secret of encoder programmers,
ince this determines the quality of the whole encoding process. What is known is that the