X3

368 MPEG glossary
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 369)).
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.
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.
These two fields are not the same, since two fields are created during the
recording between which a 1/50 of a second gap is evident. Moving objects
can therefore produce artifacts on vertical edges.
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