720p is the shorthand name for a category of HDTV video modes. The number "720" stands for 720 horizontal scan lines of
display resolution(also known as 720 pixels of vertical resolution), while the letter "p" stands for progressive scanor non- interlaced. When broadcast at 60 frames per second, 720p features the highest temporal (motion) resolution possible under the ATSC standard. Progressive scanning reduces the need to prevent flicker by filtering out fine details, so spatial resolution (sharpness) is much closer to 1080ithan the number of scan lines would suggest.
720p has a
widescreenaspect ratio of 16:9, a vertical resolution of 720 pixels and a horizontal resolution of 1280 pixels for a total of 921,600 pixels. The frame rate(in this case equal to the refresh rate) can be either implied by the context or specified in hertzafter the letter p. The five 720p frame rates in common use are 24, 25, 30, 50 and 60 Hz (or frame/s). In general, traditional PALand SECAMcountries (Europe, Australia, much of Asia, Africa, and parts of South America) are or will be using the 25p and 50p frame or field rates, whereas traditional NTSCcountries (North and Central America, Japan, South Korea, Philippines) are using 24p (for movies), and 60p for high motionprogramming. All variants can be transported by both major digital televisionformats, ATSC and DVB.
720p is directly compatible with newer flat-panel technology such as plasma and
LCD, which are inherently progressive and must perform deinterlacingto display 1080isource material. 720p must be scan-converted for display on most CRT-based consumer televisions, which are generally interlaced-only display devices. [cite web|url=http://archive2.avsforum.com/avs-vb/showthread.php?t=489948|title=archive2.avsforum.com/avs-vb/showthread.php?t=489948 |accessdate=2007-07-06] However, CRTs intended for use as computer monitors are progressive-only devices that are generally able to support 1280×720 at 60 Hz.
720p was designed at AT&T Bell Laboratories in the late 1980s, under the supervision of
Arun Netravali. The project began when Zenith approached AT&T to partner in the design of an analog HDTV format, comparable to the Japanese system. Netravali (in Murray Hill), along with Barry Haskell (in Holmdel) and other image processing experts at Bell Labs, and William Schreiberat MIT, quickly devised a digital standard using DCT block coding. About 50 engineers were hired and a prototype was assembed in Murray Hill using Xilinxprogrammable logic hardware. The leaders of Zenith and AT&T cancelled the analog-HDTV project after the completion of the digital 720p experimental system, and Zenith agreed to design a radio-frequency modem system for broadcasting digital video. The 720p system was tested against competing standards during FCC trials, and was particularly notable for its lack of flicker and shimmer of moving edges.Fact|date=March 2008 The conflict between interlaced formats (supported by the television industry) and progressive scan formats (supported by AT&T, Microsoft and others) was extremely contentious in the early days of format proposals.
720p versus 1080i
Some broadcasters use 720p50/60 as their primary high-definition format; others use the
1080istandard. While 720p presents a complete 720-line frame to the viewer between 24 and 60 times each second (depending on the format), 1080i presents the picture as 50 or 60 partial 540-line "fields" per second (24 complete 1080-line fields, or "24p" is included in the ATSC standardthough) which the human eye or a deinterlacer built into the display device must visually and temporally combine to build a 1080-line picture.
To get all 1080 interlaced lines to appear on the screen at the same time on a progressive high-definition display, the processor within the HD set deinterlaces incoming video by either weaving together two 540-line fields, or by doubling lines in each field, effectively converting fields to frames. The first deinterlacing method is used for static scenes, the second one is used for scenes with motion. Cheaper TVs always use line doubling, while more expensive TVs use complex algorithms to analyze motion between two fields. Because of the deinterlacing, 1080i video with static scenes has more vertical resolution than 720p video, while the resolution in moving scenes is lower because of field doubling.
While 1080i has more scan lines than 720p, they do not translate directly into greater vertical resolution. Interlaced video is usually blurred vertically (filtered) to prevent twitter. Twitter is a flickering of fine horizontal lines in a scene, lines that are so fine that they only occur on a single scan line. Because only half the scan lines are drawn per field, fine horizontal lines may be missing entirely from one of the fields, causing them to flicker. Images are blurred vertically to ensure that no detail is only one scan line in height. Therefore, 1080i material does not deliver 1080 scan lines of vertical resolution. However 1080i provides a 1920-pixel horizontal resolution, greater than 720p's 1280 resolution.
In the USA, 720p is used by ABC,
Fox Broadcasting Companyand ESPNbecause the smoother image is desirable for fast-action sports telecasts, whereas 1080i is used by CBS, NBC, HBO, Showtimeand Discovery HDdue to the crisper picture particularly in non-moving shots.
European Broadcasting Union(EBU) recommends that its members use 720p50 with the possibility of 1080i50 on a programme-by-programme basis and 1080p50 as a future option. [cite web|url=http://www.ebu.ch/CMSimages/en/tec_text_r112-2004_tcm6-16462.pdf?display=EN|title=EBU Technical Recommendation R112 - 2004|accessdate=2007-07-06] [cite web|url=http://www.ebu.ch/en/technical/trev/trev_home.html|title=www.ebu.ch/CMSimages/en/tec_text_r112-2004_tcm6-16462.pdf?display=EN|title=EBU Technical Recommendation R112 - 2004|accessdate=2007-07-06] [cite web|url=http://www.ebu.ch/en/technical/trev/trev_home.html|title=EBU Technical Review|accessdate=2007-07-06] The BBCis one of the EBU members transmitting in HDTV. It has not yet made a final decision on picture scanning format. Sveriges televisionin Sweden, Cyfra+in Poland, SRG SSR idée suissein Switzerland and ORFin Austria broadcast in 720p50. All other commercial European HDTV services so far use 1080i50 . Broadcasters can also use progressive segmented framescaling in their 1080ibroadcasting so that viewers can see a 1080pimage.
1080p, 720i, 576p, 480p
* [http://hometheater.about.com/cs/television/a/aafoxhdtv03a.htm HD standards in use.]
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