720p
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720p is the shorthand name for a category of HDTV video modes. The number 720 stands for the 720 horizontal scan lines of display resolution (also known as 720 pixels of vertical resolution), while the letter p stands for progressive scan or 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 1080i than the number of scan lines would suggest.[1][2]
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[edit] Specifications
720p has a widescreen aspect ratio of 16:9, a vertical resolution of 720 pixels and a horizontal resolution of 1280 pixels, or 1280x720, 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 hertz after 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 PAL and SECAM countries (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 NTSC countries (North and Central America, Japan, South Korea, Philippines) are using 24p (for movies), and 60p for high motion programming. All variants can be transported by both major digital television formats, ATSC and DVB. Newer displays have 720p, but have a native resolution higher or lower than a native 1280×720 resolution. This is because it helps cut manufacturing costs.
[edit] Compatibility
720p is directly compatible with newer flat-panel technology such as plasma and LCD and progressive-scan CRTs. 720p must be scan-converted for display on interlace-only displays. YouTube videos in High Definition use this resolution.
[edit] History
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 (Bell Labs in Murray Hill, New Jersey), along with Barry Haskell (Bell Labs in Holmdel, New Jersey) and other image processing experts at Bell Labs, and William F. Schreiber [3][4] at MIT, quickly devised a digital standard using DCT block coding. About 50 engineers were hired and a prototype was assembled in Murray Hill using Xilinx programmable logic hardware. The leaders of Zenith and AT&T canceled 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.[citation needed] 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.
[edit] 720p versus 1080i
Some broadcasters use 720p50/60 as their primary high-definition format; others use the 1080i standard. 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 "1080p24" is included in the ATSC standard though) 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.
The data rate is also a concern in broadcasting. 1080i has a raw rate of 51.84 million pixels per second at 50Hz, and 62.208 at 60Hz. 720p50 has a rate of 46.08Mpx/s, while 720p60 is 55.296, and the p25 and p30 modes are exactly half, respectively. (For comparison, 480i and 576i modes average around 10Mpx/s.) Transmission of greater total pixel rates from all virtual channels multiplexed on a physical TV channel (whether a TV station or on digital cable) requires greater video data compression. Excessive lossy compression can look much worse than a lower resolution with less compression, which in turn affects the choice of 720p or 1080i, and low or high frame rate.
In the USA, 720p60 is used by ABC, Fox Broadcasting Company and ESPN because the smoother image is desirable for fast-action sports telecasts, where as 1080i is used by CBS, NBC, HBO, Showtime and Discovery HD due to the crisper picture, particularly in non-moving shots. Ion appears to use 720p30 over the air, due to its three other digital subchannels, and lack of sports programming.
The European Broadcasting Union (EBU) recommends that its members use 720p50 with the possibility of 1080i50 on a program-by-program basis and 1080p50 as a future option.[5][6][7] The BBC is one of the EBU members transmitting in HDTV. It has not yet made a final decision on picture scanning format. Sveriges television in Sweden, Cyfra+ in Poland, SRG SSR idée suisse in Switzerland, ORF in Austria and VRT in Belgium broadcast in 720p50. All other commercial European HDTV services so far use 1080i50. Broadcasters can also use progressive segmented frame scaling in their 1080i broadcasting so that viewers can see a 1080p image. Unlike NTSC, the ATSC standards do allow for a true 1080p broadcast leaving this battle almost lost on both sides.
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[edit] See also
- 1080p, 720i, 576p, 480p
- High-definition television (HDTV)
- 720re
- Comparison of video on demand services
[edit] References
- ^ afterdawn.com - 720p
- ^ CNET - Glossary - 720p
- ^ William F. Schreiber, PhD. Archival web site containing personal and professional data and collected writings
- ^ "William F. Schreiber Oral History", 9 January 2009, IEEE
- ^ "EBU Technical Recommendation R112 - 2004". http://www.ebu.ch/CMSimages/en/tec_text_r112-2004_tcm6-16462.pdf?display=EN.
- ^ "EBU Technical Recommendation R112 - 2004". http://www.ebu.ch/en/technical/trev/trev_home.html.
- ^ "EBU Technical Review". http://www.ebu.ch/en/technical/trev/trev_home.html.
[edit] External links
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