Question About Screen Size
Dear Gary: As I switch between various zooms on my HDTV and see the sacrifice in picture edge visibility that is sometimes inevitable, it occurs to me that video products permit only preset picture size and never a gradual adjustment. Such a control could allow the operator to maximize the display size while minimizing lost information. Why does no one offer this “digitized rheostat” for picture sizing?
Dennis W. Brandt
Video Technical Editor Greg Rogers Comments:
Let me provide some background before I address your question directly. Most direct-view and rear-projection TVs are adjusted at the factory to produce about 5 percent or more overscan on each side and the top and bottom of the picture. That is the basic reason that image edges are not visible. Overscan is used for several reasons. It is difficult to control image geometry along the edges of the raster (active picture area) for CRT-based displays. The vertical and horizontal edges of the raster tend to bow, tilt, and/or become wavy rather than follow the straight lines of a rectangular screen bezel. Without some overscan there would likely be some ugly blank gaps visible between the image raster and the screen bezel. Manufacturers tend to use excessive overscan so they don’t have to take more time to carefully adjust the CRT geometry or design in better performance. In many instances, a professional calibrator will be able to adjust the geometry and reduce the amount of necessary overscan to about 1 to 3 percent. Flat panel displays have perfect geometry, and fixed-pixel projectors should have nearly perfect geometry (there may be some edge bowing from lens distortion). But most manufacturers still include image overscan to hide the visibility of edge problems. Those problems may originate in the video source or the electronics of the display. The video from the source may have ragged sides from passing through multiple video processors, or may be too narrow because processors along the video chain have blanked edge pixels. The video may also be incorrectly framed such that lines are missing at the bottom of the frame, or lines containing non-video data (such as closed captioning information) are visible at the top of the frame. Incorrect framing, horizontally and vertically, can also occur during signal capture and scaling in the electronics of a display. For all of these reasons, fixed-pixel displays also typically include some overscan. That overscan may also be excessive, but it may not be adjustable even by professional calibrators. On most displays the zoom and aspect ratio adjustments are fixed to maintain significant overscan, but there are exceptions. The Yamaha DPX-1100 provides a choice of Standard or Full overscan modes. The Standard mode varies from about 2.5 to 4 percent, and the Full mode varies from 0 to 1 percent, depending on the signal format. But I don’t know of any displays that provide the incrementally variable user size or overscan adjustments that you would like. CRT displays have variable raster size adjustments but they are not user adjustments because they also affect geometry, convergence, and other critical image parameters. One way to provide a user variable size adjustment for a CRT display would be to provide digital scaling to fit the desired portion of the video image within the visible raster. The user would be responsible for setting a scaling factor that pushed any edge defects from the source into the CRT overscan area, but the manufacturer’s preset overscan would eliminate CRT edge geometry problems. A similar variable size adjustment could be provided for fixed-pixel displays by letting the user set a scaling factor that fit the desired image area to the native fixed resolution of the display. I suspect cost and user interface complexity are two reasons we don’t have variable size adjustments on displays. Most of today’s CRT displays don’t have digital circuits capable of arbitrary incremental scaling, but many fixed-pixel projectors do. More importantly, human interface complexity is a significant factor. Some users already find the fixed aspect ratio controls too complex when confronted with multiple aspect ratios from video sources. If you want this capability, you could check out the Lumagen Vision or Vision Pro Video Processors (reviewed in Issue 72 and Issue 80 respectively) as an addition to your CRT or fixed-pixel display. They provide incremental aspect ratio adjustments (0.01 increments), plus horizontal and vertical size and position adjustments to fit the image to the visible area of the display.
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