Resolution in computer and home entertainment system terminology describes two different specifications. One tells the amount of information in a signal sent to a monitor and the other tells the number of pixels or dots on the screen that a monitor can illuminate. VGA describes a signal format with various resolutions, and 720p describes either a high-definition signal format or an HDTV set specification. Making a relationship between the two terms may cause confusion.
Released in 1987, visual graphics array sent high-quality analog video information to a computer monitor at 640 by 480 pixels of resolution. Over the years, upgrades increased resolution up to 3,840 by 2,400 with various letter designations ending in "GA" including XGA at 1,024 by 768 and SXGA at 1,280 by 1,024 for standard screens, and 1,280 by 800, 1,360 by 768 and 1,366 by 768 for widescreens. However widescreens are not necessarily high-definition. A high-definition television set that supports a VGA input can display any resolution that a VGA signal produces.
Standard television sets have an aspect ratio of 4:3 and a resolution of 640 by 480 pixels with interlaced scanning. High-definition television sets have an aspect ratio of 16:9 with a resolution of either 1,280 by 720 or 1,920 by 1,080 with either interlaced or progressive scanning. To simplify resolution specifications, statistics drop the horizontal figure and use only the vertical, calling signals and HDTV sets 720i, 720p, 1080i or 1080p. HDTV sets are designed with only one of the four resolutions. Specifications for VGA signal resolutions don't carry the "i" or "p" scanning designations.
An electronic beam scans a television screen from left to right and top to bottom illuminating pixels as it goes. With interlaced scanning, the set illuminates the odd vertical lines on one sweep and the even ones on the next. With progressive scanning, the set illuminates all lines in sequence. Standard TV sets use interlaced scanning, but HDTV sets offer an option, with progressive scanning producing the better picture. The resolution of the signal sent to an HDTV set can improve the quality of the picture, but cannot increase the set's designed resolution or the way it scans.
Up- and Down-converting
Whatever signal an HDTV receives, it up- or down-converts it to the resolution and scanning of the set's design. On a set with 720p resolution, any VGA signal produces a 720p image. So technically speaking, no VGA signal equals 720p but all can result in 720p on the screen. The difference in quality results from the amount of information the set must convert. At lower resolutions, it must create more missing pixels than with signals at higher resolutions. If the signal has higher resolution than the set's design, it down-converts it to the resolution of the screen.
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