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Glossary
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The I-picture contains all the image information for a single frame or field, and is thus
only moderately compressed. B-pictures and P-pictures contain only information that
is different from the previous or next frame/field, and reference the nearest I-picture
for the remaining information. This means they are much more highly compressed
than I-pictures.
GOP size
The number of frame s or field s in a group of pictures (GOP).
For example, a GOP size of 10 looks like this if the I-, B-, and P-pictures contain
whole frames:
I B B P B B P B B P
and like this if the I-, B-, and P-pictures contain fields:
I P B B B B P P B B B B P P B B B B P P
GOP
structure
The number of B-pictures before each P-picture in a group of pictures. Typical GOP
structures are I B P (one B-picture) and I B B P (two B-pictures).
Hz
Hertz. Cycles per second. A unit of measurement for frequencies.
IFO file
One of the files on a DVD-Video disc. The IFO file contains instructions that tell the
DVD player how to play the
VOB file s on the disc. Because the disc could become
unplayable if the IFO file is damaged, a backup copy with the extension . BUP is
always included in a separate location on the disc.
interlaced
scanning
A method for displaying an analog video signal on a TV screen. This method was
developed because video images flicker when displayed on TVs at 25 or 29.97 frames
per second — each frame fades away before the next one appears.
To prevent flickering, each video frame is divided into two fields. One field contains
all the odd-numbered scan lines in the frame (the “top” field); the other contains all
the even-numbered scan lines (the “bottom” field). The TV displays one field (one set
of lines), followed by the next, at 59.94 fields per second for NTSC or 50 fields per
second for PAL. At these frequencies, human persistence of vision causes the fields to
be perceived as a single image.
Compare with progressive scanning .
I-picture
Intra-picture. An MPEG video picture that contains all the information necessary to
create a single frame or field. Every
GOP in an MPEG video stream starts with an I-
picture, which provides a reference point for the B-picture s and P-picture s in the
GOP.
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