Resolution And Aspect Ratio
Dear Gary: I understand that when using a RPTV that you lose resolution if the screen isn’t filled up. Thus the “anamorphic squeeze” option. Seems to me that unless you are receiving hi-def TV you are better off with a 4:3 (1.33:1) screen with squeeze ability than you are with a 16:9 (1.78:1) screen that makes 4:3 pictures lose resolution? Does a front projector work the same way? If you use one of the Texas Instruments widescreen-based projectors like the Sharp, will 4:3 pictures from a cable feed or from VHS tape also lose resolution?
Video Technical Editor Greg Rogers Comments:
The 16:9 (anamorphic) DVD format provides 33 percent more vertical resolution than the 4:3 letterbox format because all of the 480 available scan lines fill a 16:9 (1.78:1) frame rather than a 4:3 (1.33:1) frame. In the latter case, only 360 scan lines occupy a 16:9 image within a 4:3 frame. There are a number of factors to consider if you are debating between a 4:3 and 16:9 RPTV. Some of these issues are technical and some are psychological. A 4:3 TV only needs about 640 TV Lines (TVL) of vertical resolution in its 4:3 mode to display a full vertical resolution (480 TVL) 16:9 standard-definition picture in a raster shrink mode. So that should not be a limitation. Conversely, a 16:9 TV only needs about 960 TVL per picture width of horizontal resolution to fully resolve a 720 x 480 standard-definition DVD picture when it is displayed as a 4:3 image in the center of the 16:9 screen. That is a very reasonable practical resolution for a 16:9 HD-ready TV, so the display’s native resolution itself shouldn’t be a limiting factor in either scenario. However, most 16:9 TVs scale the 4:3 image horizontally using digital signal processing rather than by shrinking the horizontal raster size. In that case, some visible horizontal resolution can be lost in scaling, but it depends on how the scaling is implemented. Fortunately, it is easier to actually test the resolution of the 16:9 display in the 4:3 mode than it is to speculate. The AVIA: Guide To Home Theater DVD resolution patterns will quickly let you know if any visible resolution is lost when displaying 4:3 pictures in the center of a 16:9 display. On most projectors you can compare the resolution pattern in the full screen mode with the same pattern in the 4:3 mode at the touch of a button. I have seen cases where the resolution loss was significant and other cases when it was negligible. You should also bear in mind that the horizontal resolution of cable TV and S-VHS tape is much lower than DVD. The horizontal resolution of NTSC broadcasts is less than 450 TVL per picture width compared to a maximum of 720 TVL per picture width for DVD. Once you have answered the resolution question you should also consider other technical and psychological issues. Do you want the lowest resolution broadcast pictures to be displayed larger than widescreen DVD movies (or HDTV)? Many users sit too close to RPTV screens for the resolution of standard-definition pictures. The maximum resolution of 480i or 480p images can be visually resolved at about 7 picture heights from the screen. That's over 18 feet from a 53-inch diagonal RPTV. Moving closer increases the horizontal viewing angle, which is a positive factor, but it makes the lack of resolution on broadcast images more obvious. Furthermore, do you want the huge letterbox bars that fill a 4:3 screen while watching a 2.35:1 movie? A widescreen movie can appear rather diminutive on a vertically oversized screen. With a 16:9 screen there will be no visible letterbox bars at all for many movies and HDTV broadcasts. Fixed-pixel front projectors (DLP, transmissive LCD, and reflective LCD i.e. D-ILA) also scale 4:3 pictures to fit the center of a 16:9 screen. But in most cases, the scaling, or resampling, is done in such a way that little or no resolution is lost. The new 16:9 1280 x 720 pixel DLP projectors provide a 960 x 720 pixel area to display a 4:3 picture so no resolution is lost due to the native pixel resolution of the projector. As before, a simple check with AVIA will tell you at a glance if any visible resolution is being lost from scaling. In CRT front projectors, the horizontal raster can be shrunk to put a 4:3 image into the center of a 16:9 screen, so visible resolution is seldom lost using that method.
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