User manual

134 MPEG glossary
www.magix.com
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 128) 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 129).
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