DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Pixel
72 ppi
0.35 mm
CRT spot
0.63 mm
CRT triad
0.31 mm
Figure 5.9 Pixel/spot/triad.
Triad refers to the smallest
complete set of red-producing,
green-producing, and blueproducing
elements of a
display. CRT triads have no
direct relationship to pixels;
what is usually called dot pitch
is properly called triad pitch.
A direct-view color CRT display has several hundred
thousand, or perhaps a million or more, triads of red,
green, and blue phosphor dots deposited onto the back
of the display panel. (A Sony Trinitron CRT has
a thousand or more vertical stripes of red, green, and
blue phosphor.) Triad pitch is the shortest distance
between like-colored triads (or stripes), ordinarily
expressed in millimeters. There is not a one-to-one relationship
between pixels and triads (or stripes). A typical
CRT has a Gaussian spot whose width exceeds both the
distance between pixels and the distance between
triads. Ideally, there are many more triads (or stripes)
across the image width than there are pixels – 1.2 times
as many, or more.
You saw at the beginning of this chapter that in order
to avoid visible pixel structure in image display some
overlap is necessary in the distributions of light
produced by neighboring display elements. Such
overlap reduces sharpness, but by how much? How
much overlap is necessary? I will discuss these issues in
the Chapter Resolution, on page 65. First, though,
I introduce the fundamentals of raster scanning.
50 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES