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MPEG glossary
371
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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 365) 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 366).
E
ach 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.
This factor is also a direct measure of the sought image quality of the MPEG
data stream, since the "Q" in "Q" factor stands for quantization and quality.