8.0

GOP
Group of P
ictures: The sequence of I frames and the P and B frames that belong to them.
e.g. I B B P B B P B B I ...
(This GOP has a length of 9, with 2 P frames and 2 B frames)
I frames contain the entire image information of a frame, while P and B have part of the information.
So-called prediction
and movement approximation are methods used for reduction.
The combination P B B is called a subgroup.
I frames must appear in regular intervals in the data stream for image and sound to be synchronized.
Between the I frames only a limited count of P and B frames is allowed. This explains a few things: Since
P and B frames contain only differential information, these differences will be larger with time, since more
and more changes takes place from frame to frame. A large count does not make much sense, since
GOP has a maximum length of 15 (4P, 2B) in PAL and 18 (5P, 2B) in NTSC. (More than 2 B frames
between P frames is not allowed).
In a closed GOP
, B frames of the last subgroup may contain only backward predictions or references to the preceding P
frame, but no references to the following I frame, since it belongs to the next GOP.
I frames
Intra-frames: In these pictures, the entire image information of a frame is saved and only information from
this frame is used ("intra-frame encoded"). In contrast to the I frame, P and B frames save only the
differences between the current frame, and preceding and/or following frame are also found in MPEG
video (P frame = "predicted frame", B frame = "bidirectional frame", see Prediction
).
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