X4

398 MPEG glossary
http://pro.magix.com
Something else: Since the decoder is no prophet, the P frames are always
transmitted before the B frames! The GOP explained above will be encoded
and transmitted in the order it is written.
Original
GOP
I
0
B
01
B
02
P
01
B
11
B
12
P
02
B
21
B
22 I1
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
Due to this nested structure, it is easy to see that during direct editing of
MPEG material, 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.
Using Movement prediction (view page 392) P and B frames are likewise
reduced.
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 blocks (view page 393).
Each one of these blocks
is then transformed into an 8 x 8 matrix (a table with
rows and columns) using a DCT (discreet cosinus transformation) mathematical
method. Each of these values is produced using all 64 individual pixels of the
block, but the values in the matrix are ordered in such a way that the image
information is ordered according to its importance.
This matrix will then be multiplied by another matrix, i.e. the quantization
matrix. Exactly how and why this matrix must be created is the biggest secret
of encoder programmers, since this determines the quality of the whole
encoding process. What is known is that the result should contain as many
zeros as possible! These zeros correspond to the "unimportant" image
elements mentioned and will not be transmitted in the data stream.
Depending on the encoder parameters regarding the target bit rate, fewer or
more values of the matrix will be declared unimportant by dividing the
quantization matrix by the quantization scaling factor. Since only whole
numbers are used, a division can produce a zero is the remainder is discarded.