DVD Demystified

130
Figure 3.22
Display sizes at equal
heights
Chapter 3
horizontal and vertical lines). Therefore, too wide a tube is not desirable.
The 16:9 aspect ratio was chosen in part because it is an exact multiple of
4:3. That is, 4/3 � 4/3 = 16/9. The clean mathematical relationship between
4:3 and 16:9 makes it easy to convert between the two. Going from 4:3 to 16:9
merely entails adding one horizontal pixel for every three (3➝4), and going
from 16:9 to 4:3 requires simply removing one pixel from every four (4➝3). 23
This makes the scaling circuitry for letterbox and pan and scan functions
much simpler and cheaper. It also makes the resulting picture cleaner.
The 16:9 aspect ratio is also a reasonable compromise between television
and movies. It is very close to 1.85, and it is close to the mean of 1.33 and
2.35. That is, 4/3 � 4/3 � 4/3 � 2.35. Choosing a wider display aspect ratio,
such as 2:1, would have made 2.35 movies look wonderful but would have
required huge mattes on the side when showing 4:3 video (as in Figure
3.17f, but even wider).
Admittedly, the extra space could be used for picture outside picture (POP,
the converse of PIP), but it would be very expensive extra space. To make a
2:1 display the same height as a 35-inch television (21 inches) requires a
width of 42 inches, giving a diagonal measure of 47 inches. In other words,
to keep the equivalent 4:3 image size of 35-inch television, you must get a
47-inch 2:1 television. Figure 3.22 shows additional widescreen display sizes
required to maintain the same height of common television sizes.
23 In each case, a weighted scaling function generally is used. For example, when going from 4 to
3 pixels, 3/4 of the first pixel is combined with 1/4 of the second to make the new first, 1/2 of the
second is combined with 1/2 of the third to make the new second, and 1/4 of the third is combined
with 3/4 of the fourth to make the new third (see Figure 6.25). This kind of scaling causes the picture
to become slightly softer but is generally preferable to the cheap alternative of simply throwing
away every fourth line. Similar weighted averages can be used when going from 3 to 4 (see
Figure 6.27).