Display resolution

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Image:Video Standards.PNG The display resolution of a digital television or computer display is the number of pixels (or maximal image resolution) that can be displayed on the screen, usually given as a product of the number of columns (horizontal, "X"), which is always stated first and the number or rows (vertical, "Y") to make up the aspect ratio. There are two types of resolution, the number of discrete pixels or scanlines and the number of resolvable lines, the latter determined by the number of alternating black and white stripes that can be displayed across the picture without them merging together. The relationship between the two is known as Kell factor.

The way the lines or rows are displayed will affect the perceived resolution in the vertical direction. This relates to the differences produced by interlace or progressive scanning and now by the use of memory to hold each pixel's displayed brightness values between picture refreshes.

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 realised. 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 objectional 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 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 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

It should be noted that 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). Computer displays including projectors generally do not overscan.

Evolution of Resolution Standards

The 640×480 resolution, introduced with the IBM PS/2 VGA and MCGA (multi-color) on-board graphics chips, was the standard resolution 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 able to switch to any other 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 a reasonable minimum.

Common Display Resolutions

Computer Graphics

Television/Movies

See also

• Computer display standardscs:Rozlišení

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