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720p
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720p is the shorthand name for a category of HDTV video modes. The number 720 stands for 720 lines of vertical display 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 (sharpness) resolution is much closer to 1080i than the number of scan lines would suggest.
Contents
- 1 Specifications
- 2 Compatibility
- 3 History
- 4 720p versus 1080i
- 5 See also
- 6 References
- 7 External links
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Specifications
720p assumes a widescreen aspect ratio of 16:9, and a horizontal resolution of 1280 pixels for a total of about 0.92 million pixels. The frame rate (in this case equal to the field 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 fps). 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.
Compatibility
720p is directly compatible with newer flat panel technology such as
plasma and
LCD which are inherently progressive and must perform
deinterlacing to display
1080i source material. 720p must be scan converted for display on most
CRT-based consumer televisions which are generally interlaced-only display devices.
[1] However, CRTs intended for use as computer monitors are progressive-only devices that can be run at 1280×720p60 either natively or through a refresh rate tweaking utility.
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 (in Murray Hill), along with Barry Haskell (in Holmdel) and other image processing experts at Bell Labs, and William Schreiber at MIT, quickly devised a digital standard using DCT block coding. About 50 engineers were hired and a prototype was assembed in Murray Hill using Xilinx programable 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. 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 United States broadcasters use 720p60 as their primary high-definition format; others use the 1080i standard. While 720p presents a complete 720 line frame to the viewer between 24 to 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 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 - in CRT type display. 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 has to weave together both 540-line segments to form the full-resolution frame. It does so by holding the first field in its memory, receiving the next field, then electronically knitting the two fields together. The combined fields are displayed at once as a complete 1080p frame. The main tradeoff between the two is that 1080i may show more detail than 720p for a stationary shot of a subject at the expense of a lower effective refresh rate and the introduction of interlace artifacts during motion. 720p is used by ABC and ESPN because the smoother image is desirable for fast-action sports telecasts. Fox Broadcasting Company uses the tagline "the nation's finest high-definition standard" in advertising its 720p programming.
The
European Broadcasting Union (EBU) recommends to its members to use 720p50 for emission with the possibility of 1080i50 on a programme-by-programme choice and
1080p50 as a future option.
[2][3] 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 broadcasts in 720p50. All commercial European HDTV services so far use 1080i50(except Polish
Cyfra+).
Digital video resolutions
| Designation
| Usage examples
| Definition (lines)
| Rate (Hz)
|
| Interlaced (fields)
| Progressive (frames)
|
| Low; MP@LL
| LDTV, VCD
| 240; 288 (SIF) | | 24, 30; 25
|
| Standard; MP@ML
| SDTV, SVCD, DVD, DV
| 480 (NTSC, PAL-M) | 60 | 24, 30
|
| 576 (PAL, SECAM) | 50 | 25
|
| Enhanced
| EDTV
| 480; 576 | | 60; 50
|
| High; MP@HL
| HDTV, HD DVD, Blu-ray Disc, HDV
| 720 | | 24, 30, 60; 25, 50
|
| 1080 | 50, 60 | 24, 30; 25
|
| Image:Common Video Resolutions.svg
|
| This table illustrates total horizontal and vertical pixel resolution via box size. It does not accurately reflect the screen shape (aspect ratio) of these formats, which is either 4:3 or 16:9.
|
See also
References
- ^ http://www.avsforum.com/avs-vb/archive/index.php/t-489948.html
- ^ EBU Technical Recommendation R112 - 2004
- ^ http://www.ebu.ch/en/technical/trev/trev_home.html
