Image Processing Reference
In-Depth Information
Physical Raster Area
The physical raster is often described as if the image filled all the available lines. When a 525-
or 625-line system is mentioned, this is a reference to the physical raster. You will not actually
see 525 or even 625 lines on your TV receiver. The vertical blanking time required for the CRT
electron beam to fly back to the top of the screen and the horizontal fly-back time accounts
for about 8% of the raster. The raster is reduced from 525 to 480 in USA broadcasts and from
625 to 576 lines of usable picture area in Europe. The value of 480 has been argued about and
is considered to be a nominal value. Other values are quoted in some literature. Anything
bigger than 482 lines of visible picture may be cropped somewhere in the distribution process
even when operating in Europe. This is because video equipment and processing software
may be operated with settings that allow for worldwide distribution of the content.
In fact you lose some of the horizontal time because the imaging is only scanned
from left to right, and about 15% of the horizontal time is lost to blanking while the beam
goes back to start another line.
This is wholly an analog TV issue; digital TV systems don't require fly back or blank-
ing time. However, they must make sure the image areas they work in are usable on ana-
log systems.
This is probably best represented by drawing the physical raster and indicating
where to place the blanking to see how much usable raster area remains.
Visible Raster Area
Within the usable raster area is a somewhat smaller picture area. The viewer normally sees
an even smaller area than that. This is because TV sets are designed to stretch the image
past the edges of the screen. This is called over-scanning. Studio TV monitors allow for this
to be switched in and out so that you can see the entire raster. Computer monitors do not
over-scan, and you should expect to see the whole addressable picture area on the screen.
Vertical blanking period
Visible raster area
Figure 5-9 Physical and visible raster areas.
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