ADS Technologies USBAV-708 INSTANT VIDEOMPX, User Manual

Page: 24 / 116

The
Guide
24
Fo
rw
a
rd
Ge
ne
ral
c
on
ce
p
ts,
Fr
a
m
es
,
Ma
cr
ob
lo
ck
s a
nd
m
ot
io
n
Forward
In order to understand all of the encoder features that will be covered in the next
section it is first necessary to cover the core concepts behind the MPEG 4 Video
Standard upon which DivX 5.1 is based. Lets begin with the way videos are
composed.
Frames
At its most basic a
video is a series of
pictures shown one
after the other in
quick succession.
When the pictures
are played fast
enough the image appears to move. Each picture in a video is called a
frame and the speed at which they are shown is the frame rate, given in
frames per second , or fps .
If we were to slow a video down to such an extent that it were possible
to see individual frames it would become evident there is generally little
change in the picture between adjacent frames. Frames in sequence
would appear very similar to each other, objects in each consecutive
frame moving position only slightly. DivX takes advantage of this
behavior as a means to compress the data required to store a video.
Macroblocks and motion
The encoder divides a frame into a grid of blocks, called macroblocks ,
and attempts to track the motion of each macroblock in the current
frame back to a matching area of picture in the previous frame. This
process is known as the
motion search .
General concepts
Frame 1 Frame 2 Frame 3
Frame 1 Frame 2 Frame 3