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Overview Analog television systems use interlace scanning with two sequential scans (50 or 60 fields per second), one with the odd numbered lines, the other with the even numbered lines to give a complete picture (25 or 30 frames per second). This is done to save transmission bandwidth but a consequence is that in picture tube (CRT) displays, the full vertical resolution cannot be realized. For example, the maximum detail in the vertical direction would be for adjacent lines to be alternately black then white. This is not a problem in a progressive display but an interlace display will have an unacceptable flicker or twitter at the slower frame rate. This is why interlace is unacceptable for fine detail such as computer word processing or spreadsheets. For television it means that if the picture is intended for interlace displays the picture must be vertically filtered to remove this objectionable flicker with a reduction of vertical resolution to about 70%. So a 576 line PAL interlace display only has about 400 lines vertical resolution and 350 in the case of a 480 line NTSC interlace display. Similarly, 1080i HD interlaced video would need to be filtered to about 700 lines for an interlaced display. Any interlaced broadcast television pictures and for that matter DVDs are filtered to that vertical resolution to reduce the interline twitter on fine detail. Fixed pixel array displays such as LCDs, Plasmas, DLPs, LCoS, etc. need a "scaling" processor with frame memory, which depending on the processing system, effectively converts an incoming interlaced picture into progressive. A similar process occurs in a PC and its display with interlaced video (eg. from a TV tuner card). The downside is that interlace motion artifacts are almost impossible to remove resulting in horizontal "toothed" edges on moving objects. Also in analog connected picture displays such as CRT TV sets, the horizontal scanlines are not divided into pixels, and therefore the horizontal resolution is related to the bandwidth of the luminance and chroma signals. For television, the analog bandwidth for luminance in standard definition should be flat to 5 MHz and in high definition, about 30/31 MHz. Current standards in resolution Currently 1024×768 (XGA/XVGA, eXtended), 1280×1024 (SXGA Super eXtended Graphics Array), and 1600×1200 resolution (UXGA, Ultra-eXtended) are the most common display resolutions. Many computer users, including CAD users, many graphic artists and video game players, run their computers at 1600×1200 resolution (UXGA, Ultra-eXtended) or higher if they have the necessary equipment. When a computer display resolution is set higher than the physical screen resolution, some systems make the virtual screen scrollable over the physical screen, however, most CRT monitors will accept higher resolutions than their native resolution without scrolling. The true maximum resolution is calculated from the dot pitch. Few CRT manufacturers will quote the true native resolution in their documentation, but most LCD manufacturers do. With digital television and HDTV, vertical resolutions of 720 or 1080 scan lines are typical. Overscan Most television display manufacturers "overscan" the pictures on their displays (CRTs and PDPs, LCDs etc.), so that the effective on-screen picture may be reduced from 720×576(480) to 680×550(450), for example. The size of the invisible area somewhat depends on the display device. Computer displays including projectors generally do not overscan although many models (particularly CRT displays) allow it. Evolution of resolution standards Many personal computers introduced in the late 1970s and the 1980s were designed to use television sets as their display devices, making the resolutions dependent on the television standards in use, including PAL and NTSC. Picture sizes were usually limited in order to ensure the visibility of all the pixels in the major television standards and the broad range of television sets with varying amounts of overscan. The actual drawable picture area was therefore somewhat smaller than the whole screen, and was usually surrounded by a static-colored border. Also, the interlace scanning was usually omitted in order to provide more stability to the picture, effectively halving the vertical resolution in progress. 320×200 and 640×200 on NTSC and 320×256 and 640×256 on PAL were relatively common resolutions in the era. In the PC world, these resolutions came to be used by the Color Graphics Adapter. The 640×480 resolution, introduced with the IBM PS/2 VGA and MCGA (multi-color) on-board graphics chips, was the standard resolution in the IBM PC compatibles from 1990 to around 1996, partly due to its 4:3 ratio. 800×600 was the standard resolution until around 2000. Since then, 1024×768 has been the standard resolution. Many web sites and multimedia products are designed for this resolution. Most of today's computer games released during the "128-bit video game era", such as SimCity 4, do not support 640×480 at all. Microsoft Windows XP is designed to run at 800×600 minimum although it is possible to select 640×480 in the Advanced Settings Window, and an application is also able to switch to any desired mode. GNU/Linux, FreeBSD, and most Unix variants use the X Window System and can run at any desired resolution as long as the display and video card support it. The Apple's Mac OS and Mac OS X operating systems are able to run with most available display resolutions, although 800×600 is the minimum. Common display resolutions Computer graphics Note 1: WXGA defines a range of resolutions with widths of 1280 to 1366 pixels and heights of 720 to 800 pixels. Television/movies ~ = horizontal resolution is an approximation See also | ||||||||||
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