Aspect Ratio Switching On CRT Projectors
Dear Gary: First of all, please let me say that your magazine is terrific. I almost never missed an issue. I have a question that I’m compelled to ask... We all know that many LCD prolectors are claimed to support the 16:9/anamorphic video format. In fact, they don’t. When you switch to WIDE mode, they just simply cover the upper and the lower zones of the LCD and this results in the loss of pixels and light output (except for the case of the Sony VPL-VW10HT with native 16:9 LCD panels). Now my question is how does it work with the majority of the CRT projectors out there? When you switch from standard to widescreen, does it also result in a loss of lines and light output, like with the LCDs? I mean, what’s the theory of aspect ratio switching on CRT projectors? I really need to know because I am about to purchase a CRT projector (an old mode—the Sony VPH-V20M) Thank you very much for your admirably wonderful work,
Tuan Anh
Video Technical Editor Greg Rogers Comments:
When a widescreen picture is displayed on an LCD (or other fixed-pixel) projector with 4:3 panels, it is true that some of the pixels won’t be used. The total light output from the projector is lower, but the brightness of the pixels in the image area is the same for a 16:9 picture or a 4:3 picture, when a 4:3 screen is used. The situation is more complex for CRT projectors than for fixed-pixel projectors. CRTs produce light by scanning an electron beam across a phosphor surface deposited on the inside of the CRT face. The electron beam energy is converted to light when it strikes the phosphor. The raster scanned on the phosphor surface determines the size and aspect ratio of the frame projected on the screen. CRT projectors provide the ability to electronically adjust the height of the raster by changing the vertical separation of the scan lines, or the width of the raster by changing the scan line length. Lines are not lost as the aspect ratio is changed unless the vertical height of the raster is reduced too far. Then the scan lines will begin to overlap, and a loss of vertical resolution will occur. A CRT projector’s total light output remains the same when displaying a full 16:9 frame, or a full 4:3 frame in the center of a 16:9 screen. But image brightness is inversely proportional to the image area when the total light output is constant. Since the 4:3 image is 75% smaller, it will appear 33 percent (1/0.75) brighter. When a movie is letterboxed inside a 4:3 frame, the image is displayed by expanding the frame to fill the entire width of the 16:9 screen. Some or all of the black bar area, 25 percent of the 4:3 frame, will extend above and below the 16:9 screen. Since there are 25 percent fewer scan lines in the 16:9 area, and the scan rate has not changed, the image brightness is reduced by 25 percent compared to the 16:9 (anamorphic) frame format.
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