X4

MPEG glossary 395
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 (view page 397)).
Interlace
For historical reasons, pictures in a movie are always recorded and transmitted
in the form of two fields; first the lines with even numbers and then those with
odd numbers. These fields are alternatively displayed with double the frame
rate. The (lazy) eye of the viewer or the processing of the TV tube puts the two
frames together to form one.
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The output image First field Second field
You normally don’t have to worry about field processing. The video material
goes through the entire processing chain as fields and is exported again as
fields or burned onto DVD or shown on TV when played back on a DVD as a
full picture. Only in certain rare conditions is it necessary to go deeper into this
process. Two problems can occur:
1. Interlace artifacts
To be displayed on a computer monitor (during recording, in your TV/VCR, and
in the arranger during editing), the two fields must be combined to form a full
screen.