Databases Reference
In-Depth Information
A
5
+
B
h
1
=
+
0
.
2
C
5
+
D
h
2
=
+
0
.
2
A
5
+
C
v
1
=
+
0
.
2
B
5
+
D
v
2
=
+
0
.
(1)
2
The pixel
c
is obtained as the average of the four neighboring pixels from the coded original:
A
5
+
B
+
C
+
D
c
=
+
0
.
4
We have described motion compensation in very general terms in this section. The various
schemes in this chapter use specific motion compensation schemes that differ from each other.
The differences generally involve the region of search for the matching block and the search
procedure. We will look at the details with the study of the compression schemes. But
before we begin our study of compression schemes, we briefly discuss how video signals are
represented in the next section.
19.4 Video Signal Representation
The development of different representations of video signals has depended a great deal on
past history. We will also take a historical view, starting with black-and-white television
and proceeding up to digital video formats. The history of the development of analog video
signal formats is different for the United States than for Europe. Although we will show the
development using the formats used in the United States, the basic ideas are the same for all
formats.
A black-and-white analog television picture is generated by exciting the phosphor on the
television screen using an electron beam whose intensity is modulated to generate the image
we see. The path that the modulated electron beam traces is shown in Figure
19.4
. The line
created by the horizontal traversal of the electron beam is called a line of the image. In order
to trace a second line, the electron beam has to be deflected back to the left of the screen.
During this period, the gun is turned off in order to prevent the retrace from becoming visible.
The image generated by the traversal of the electron gun has to be updated rapidly enough for
persistence of vision to make the image appear stable. However, higher rates of information
transfer require higher bandwidths, which translate to higher costs.
In order to keep the cost of bandwidth low it was decided to send 525 lines 30 times a
second. These 525 lines are said to constitute a
frame
. However, a thirtieth of a second between
frames is long enough for the image to appear to flicker. To avoid the flicker, it was decided
to divide the image into two interlaced fields. A field is sent once every sixtieth of a second.
