Video - A Practical Guide to Video and Audio Compression - page 93

Table 5-4 Common Aspect Ratios

Description Aspect ratio Normalized value

Standard TV 4:3 1.33:1

Widescreen TV 16:9 1.76:1

Compromised broadcast 14:9 1.56:1

4:3 image padded each side to fill 16:9

Visible region on 16:9 screen

Figure 5-15 Ratio of 4:3 on a wide-screen raster (center cutout).

5.9.3 Wide-Screen Aspect Ratio

Wide-screen presentation is based on the width to height ratio being 16:9 and uses the
full visible raster area. There is no additional picture detail being provided to add pixels to
either side of the 4:3 aspect ratio picture. It is just the standard 720-pixel width image with
the pixels being stretched horizontally even more than they were for normal presentation.
Fortunately, the human optical mechanisms are very tolerant of a loss of detail in the
horizontal axis when watching TV.

5.9.4 Stretched Display Mode

Manufacturers of wide-screen TV sets added a “justify” display mode where the horizontal
scanning is purposely nonlinear. This is another way to work the compromise between wide-
screen and normal aspect ratio. The middle section of the screen is not stretched as much as
the sides. A circle placed in the middle of the screen still looks roughly circular. Objects
placed nearer the edges are stretched more and more as they move away from the center of
the screen. Fortunately, you can turn this stretching feature on and off and select the viewing
mode you prefer. The original linear geometry is shown in Figure 5-16 for reference.
The effects of the “justify” distortion are visible in Figure 5-17. This is fine most of the
time but shows up when text crawls horizontally across the screen. It moves faster at
the sides and slows down in the middle. Because you are able to see the whole screen,