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High-definition television (HDTV) is a digital television broadcasting system with a significantly higher resolution than traditional formats (NTSC, SECAM, PAL). While some early analog HDTV formats were broadcast in Europe and Japan, HDTV is usually broadcast digitally, because digital television (DTV) broadcasting requires much less bandwidth. HDTV technology was first introduced in the US during the 1990s by a group of electronics companies cal led the Digital HDTV Grand Alliance.



History

High-Definition television was first developed by Nippon Hs and was unveiled in 1969. However, the system did not become mainstream until the late 1990s.In the early 2000s, a number of high-definition television standards were competing for the still-developing niche markets.


Three HDTV standards are currently defined by the International Telecommunication Union (ITU-R BT.709). They include 1080i (1,080 actively interlaced lines), 10 80p (1,080 progressively scanned lines), and 720p (720 progressively scanned lines). All standards use a 16:9 aspect ratio, leading many consumers to the incorrect conclusion of equating widescreen television with HDTV. All current HDTV broadcasting standards are encompassed within the DVB specification. Projection screen in a home theater, displaying a high-definition television image. Projection screen in a home theater, displaying a high-definition television image.


HDTV is also capable of "theater-quality" audio because it uses the Dolby Digital (AC-3) format to support "5.1" surround sound. It should be noted that while H DTV is more like a theater in quality than conventional television, 35 mm and 70 mm film projectors used in theaters still have the highest resolution and best viewing quality on very large screens. Many HDTV programs are produced from movies on film as well as content shot in HD video.

The term "high-definition" can refer to the resolution specifications themselves , or more loosely to media capable of similar sharpness, such as photographic film and digital video. As of July 2007, HDTV saturation in the US has reached 30% in other words, three out of every ten American households own at least one HDTV. However, only 44% of those that do own an HDTV are actually receiving HDTV programming, as many consumers are not aware that they must obtain special receivers to receive HDTV from cable or satellite, or use ATSC tuners to receive over -the-air broadcasts; others may not even know what HDTV is.[4]

HDTV Sources

The rise in popularity of large screens and projectors has made the limitations of conventional Standard Definition TV (SDTV) increasingly evident. An HDTV compatible television set will not improve the quality of SDTV channels. To get a better picture HDTV televisions require a High Definition (HD) signal.


Typical sources of HD signals are as follows:
  • Over the air with an antenna. Most cities in the US with major network affiliates broadcast over the air in HD. To receive this signal an HD tuner is required. Most newer HDTV televisions have a HD tuner built in. For HDTV televisions without a built in HD tuner, a separate set-top HD tuner box can be purchased.
  • Cable television companies often offer HDTV broadcasts as part of their dig ital broadcast service. This is usually done with a set-top box or CableCARD issued by the cable company. Alternatively one can usually get the network HDTV channels for free with basic cable by using a QAM tuner built into their HDTV or set-top box. Some cable carriers also offer HDTV on-demand playback of movies and commonly viewed shows.
  • Satellite-based TV companies, such as Optimum, DirecTV, Sky Digital, Virgin Media (in the UK and Ireland) and Dish Network, offer HDTV to customers as an upgrade. New satellite receiver boxes and a new satellite dish are often required to receive HD content.
  • Video game systems, such as the Xbox (NTSC only), Xbox 360, and Playstation 3, can output an HD signal.
  • Two optical disc standards, Blu-ray and HD DVD, can provide enough digital storage to store hours HD video content.

    Notation

    In the context of HDTV, the formats of the broadcasts are referred to using a notation describing:
  • The number of lines in the vertical display resolution.
  • Whether progressive scan (p) or interlaced scan (i) are used. Progressive scan redraws all the lines (a frame) of a picture in each refresh. Interlaced scan redraws every second line (a field) in one refresh and the remaining lines in a second refresh. Interlaced scan increases picture resolution while saving bandwidth but at the expense of some flicker or other artifacts.
  • The number of frames or fields per second.

    The format 720p60 is 1280 720 pixels, progressive encoding with 60 frames per second (60 Hz). The format 1080i50 is 1920 by 1080 pixels, interlaced encoding with 50 fields (25 frames) per second. Often the frame or field rate is left out, indicating only the resolution and type of the frames or fields, and leading to confusion.[5] Sometimes the rate is to be inferred from the context, in which case it can usually be assumed to be either 50 or 60, except for 1080p which is of ten used to denote either 1080p24, 1080p25 or 1080p30 at present but will also denote 1080p50 and 1080p60 in the future.


    A frame or field rate can also be specified without a resolution. For example 24 p means 24 progressive scan frames per second and 50i means 25 interlaced frames per second, consisting of 50 interlaced fields per second. Most HDTV systems support some standard resolutions and frame or field rates. The most common are noted below.



    Changes in notation

    The terminology described above was invented for digital systems in the 1990s. A digital signal encodes the color of each pixel, or dot on the screen as a series of numbers. Before that, analog TV signals encoded values for one monochrome, or 3 color signals as they scanned a screen continuously from line to line. By comparison, radio encodes an analog signal of the sound to be sent to an amplified speaker, typically up to 20 kHz, but video signals are in the MHz range, which is why they are much higher in the broadcast spectrum than audio radio. Analog video signals have no true "pixels" to measure horizontal resolution. The vertical scan-line count included off-screen scan lines with no picture information while the CRT beam returned to the top of the screen to begin another field. Thus NTSC was considered to have "525 lines" even though only 486 of them had a picture (625/576 for PAL). Similarly the Japanese MUSE system was called "1125 line", but is only 1035i by today's measuring standards. This change was made because digital systems have no need of blank retrace lines unless the signal was converted to analog to drive a CRT.



    Standard resolutions
    Visual comparison of common TV display resolutions.

    When resolution is considered, both the resolution of the transmitted signal and the (native) displayed resolution of a TV set are taken into account. Digital N TSC- and PAL/SECAM-like signals (480i60 and 576i50 respectively) are transmitted at a horizontal resolution of 720 or 704 "pixels". However these transmitted DT V "pixels" are not square, and have to be stretched for correct viewing. PAL TV sets with an aspect ratio of 4:3 use a fixed pixel grid of 768 by 576 or 720 by 540; with an aspect ratio of 16:9 they use 1440 x 768, 1024 by 576 or 960 by 540 ; NTSC ones use 640 by 480 and 852 by 480 or, seldom, 720 by 540. High Definition usually refers to one million pixels or more.


    In Australia, the 576p50 format is also considered a HDTV format, as it has doubled temporal resolution though the use of progressive scanning. Thus, a number o f Australian networks broadcast a 576p signal as their High-definition DVB-T signal, while others use the more conventional 720p and 1080i formats. Technically, however, the 576p format is defined as Enhanced-definition television.


    Standard frame or field rates:
    • 23.977p (allow easy conversion to NTSC)
    • 24p (cinematic film)
    • 25p (PAL, SECAM DTV progressive material)
    • 30p (NTSC DTV progressive material)
    • 50p (PAL, SECAM DTV progressive material)
    • 60p (NTSC DTV progressive material)
    • 50i (PAL & SECAM)
    • 60i (NTSC, PAL-M)

    HDTV has at least twice the linear resolution of standard-definition television (SDTV), thus allowing much more detail to be shown compared with analog television or regular DVD. In addition, the technical standards for broadcasting HDTV are also able to handle 16:9 aspect ratio pictures without using letterboxing or anamorphic stretching, thus further increasing the effective resolution for such content.


    Format considerations

    The optimum formats for a broadcast depends on the type of media used for the re cording and the characteristics of the content. The field and frame rate should match the source, as should the resolution. On the other hand, a very high resolution source may require more bandwidth than is available in order to be transmitted without loss of fidelity. The lossy compression that is used in all digital HDTV storage/transmission systems will then cause the received picture to appear distorted when compared to the uncompressed source.


    Photographic film destined for the theater typically has a high resolution and i s photographed at 24 frames per second. Depending on the available bandwidth and the amount of detail and movement in the picture, the optimum format for video transfer is thus either 720p24 or 1080p24. When shown on television in countries using PAL, film must be converted to 25 frames per second by speeding it up by 4.1%. In countries using the NTSC standard, 30 frames per second, a technique called 3:2 pull down is used. One film frame is held for three video fields, (1/20 of a second) and then the next is held for two video fields (1/30 of a second) and then the process repeats, thus achieving the correct film rate with two film frames shown in 1/12 of a second.


    Older (pre-HDTV) recordings on video tape such as Betacam SP are often either in the form 480i60 or 576i50. These may be upconverted to a higher resolution format (720i), but removing the interlace to match the common 720p format may distort the picture or require filtering which actually reduces the resolution of the final output.


    Non-cinematic HDTV video recordings are recorded in either 720p or 1080i format. The format used depends on the broadcast company (if destined for television broadcast); however, in other scenarios the format choice will vary depending on a variety of factors. In general, 720p is more appropriate for fast action as it uses progressive scan frames, as opposed to 1080i which uses interlaced fields and thus can have a degradation of image quality with fast motion.


    In addition, 720p is used more often with Internet distribution of HD video, as all computer monitors are progressive, and most graphics cards do a poor job of de-interlacing video in real time. 720p Video also has lower storage and decoding requirements than 1080i or 1080p.


    In North America, Fox, My Network TV (also owned by Fox), ABC, and ESPN (ABC and ESPN are both owned by Disney) currently broadcast 720p content. NBC, Universal HD (both owned by General Electric), CBS, PBS, The CW, HBO, Showtime, Starz!, I NHD, HDNet ,TNT, and Discovery HD Theater currently broadcast 1080i content.


    In UK on Sky Digital there's BBC HD, Sky One HD, Sky Arts HD, Sky Movies HD1 & 2, Sky Sports HD1,2 & X, Discovery HD, National Geographic Channel HD, The History Channel HD & Sky Box Office HD1 & 2. With MTV HD, FX HD, Living HD Rush HD, Ultra HD & Eurosport HD to come in the near future. BBC HD is also available on Virgin Media



    Technical details

    MPEG-2 is most commonly used as the compression codec for digital HDTV broadcast s. Although MPEG-2 supports up to 4:2:2 YCbCr chroma subsampling and 10-bit quantization, HD broadcasts use 4:2:0 and 8-bit quantization to save bandwidth. Some broadcasters also plan to use MPEG-4 AVC, such as the BBC which is trailing such a system via satellite broadcast, which will save considerable bandwidth compared to MPEG-2 systems. Some German broadcasters already use MPEG-4 AVC together with DVB-S2 (Pro 7, Sat.1 and Premiere). Although MPEG-2 is more widely used at present, it seems likely that in the future all European HDTV may be MPEG-4 AVC, and Ireland and Norway, which have not yet begun any digital television broadcasts, are considering MPEG-4 AVC for SD Digital as well as HDTV on terrestrial broadcasts.


    HDTV is capable of "theater-quality" audio because it uses the Dolby Digital (AC -3) format to support "5.1" surround sound. The pixel aspect ratio of native HD signals is a "square" 1.0, in which each pixel's height equals its width. New HD compression and recording formats such as HDV use rectangular pixels to save bandwidth and to open HDTV acquisition for the consumer market. For more technical details see the articles on HDV, ATSC, DVB, and ISDB.



    Television studios as well as production and distribution facilities, use HD-SDI SMPTE 292M interconnect standard (a nominally 1.485 Gbit/s, 75-ohm serial digital interface) to route uncompressed HDTV signals. The native bitrate of HDTV for mats cannot be supported by 6-8 MHz standard-definition television channels for over-the-air broadcast and consumer distribution media, hence the widespread use of compression in consumer applications. SMPTE 292M interconnects are generally unavailable in consumer equipment, partially due to the expense involved in sup porting this format, and partially because consumer electronics manufacturers are required (typically by licensing agreements) to provide encrypted digital outputs on consumer video equipment, for fear that this would aggravate the issue of video piracy.


    Newer dual-link HD-SDI signals are needed for the latest 4:4:4 camera systems (Sony HDC-F950 & Thomson Viper), where one link/coax cable contains the 4:2:2 YCbCr info and the other link/coax cable contains the additional 0:2:2 CbCr information.



    Advantages of HDTV expressed in non-engineering terms


    High-definition television (HDTV) potentially offers a much better picture quality than standard television. HD's greater clarity means the picture on screen ca n be less blurred and less fuzzy. HD also brings other benefits such as smoother motion, richer and more natural colors, surround sound, and the ability to allow a variety of input devices to work together. However, there are a variety of reasons why the best HD quality is not usually achieved. The main problem is a la ck of HD input. Many cable and satellite channels and even some "high definition " channels are not broadcast in true HD. Also, image quality may be lost if the television is not properly connected to the input device or not properly configured for the input's optimal performance.


    Almost all commercially available HD is digital, so the system cannot produce a snowy or washed out image from a weak signal, effects from signal interference, such as herringbone patterns, or vertical rolling. HD digital signals will either deliver an excellent picture, a picture with noticeable pixilation, a series o f still pictures, or no picture at all. Any interference will render the signal unwatchable. As opposed to a lower-quality signal one gets from interference in an analogue television broadcast, interference in a digital television broadcast will freeze, skip, or display "garbage" information.


    With HDTV the lack of imperfections in the television screen often seen on traditional television is another reason why many prefer high definition to analog. A s mentioned, problems such as snow caused from a weak signal, double images from ghosting or multi-path and picture sparkles from impulse noise are a thing of t he past. These problems often seen on a conventional television broadcast just d o not occur on HDTV.


    HD programming and films will be presented in 16:9 widescreen format (although films created in even wider ratios will still display "letterbox" bars on the top and bottom of even 16:9 sets.) Older films and programming that retain their 4:3 ratio display will be presented in a version of letterbox commonly called "pillar box," displaying bars on the right and left of 16:9 sets (rendering the term "fullscreen" a misnomer). While this is an advantage when it comes to playing 16:9 movies, it creates the same disadvantage when playing 4:3 television shows that standard televisions have playing 16:9 movies. A way to address this is to zoom the 4:3 image to fill the screen or reframe its material to 14:9 aspect ratio, either during preproduction or manually in the TV set.


    The colors will generally look more realistic, due to their greater bandwidth. The visual information is about 2-5 times more detailed overall. The gaps between scanning lines are smaller or invisible. Legacy TV content that was shot and preserved on 35 mm film can now be viewed at nearly the same resolution as that at which it was originally photographed. A good analogy for television quality is looking through a window. HDTV offers a degree of clarity that is much closer to this.


    The "i" in these numbers stands for "interlaced" while the "p" stands for "progressive". With interlaced scan, the 1,080 lines are split into two, the first 540 being "painted" on a frame, followed by the second 540 painted on another frame . This method reduces the bandwidth and raises the frame rate to 50-60 per second. A progressive scan displays all 1,080 lines at the same time at 60 frames per second, using more bandwidth.


    Dolby Digital 5.1 surround sound is broadcast along with standard HDTV video signals, allowing full surround sound capabilities. (Standard broadcast television signals usually only include monophonic or stereophonic audio. Stereo broadcasts can be encoded with Dolby Surround, an early home video surround format.) Both designs make more efficient use of electricity than SDTV designs of equivalent size, which can mean lower operating costs. LCD is a leader in energy conservation.



    Early systems


    The term "high definition" was used to describe the electronic television system s of the late 1930s and 1940s beginning with the former British 405-line black-and-white system, introduced in 1936; however, this and the subsequent 525-line U .S. NTSC system, established in 1941, were high definition only in comparison with previous mechanical and electronic television systems, and NTSC, along with t he later European 625-line PAL and SECAMs, is described as standard definition today. On the other hand, the 819-line French black-and-white television system introduced after World War II arguably was high definition in the modern sense, as it had a line count and theoretical maximum resolution considerably higher than those of the 625-line systems introduced across most of postwar Europe. However, it required far more bandwidth than other systems, and was switched off in 1986, a year after the final British 405-line broadcasts.


    Japan was the only country where commercial analog HDTV was launched and had some success. In other places, such as Europe, analog (HD-MAC) HDTV failed. Finally , although the United States experimented with analog HDTV (there were about 10 proposed formats), it soon moved towards a digital approach.


    Contemporary systems Components of a typical satellite HDTV system:1. HDTV Monitor 2. HD satellite receiver 3. Standard satellite dish 4. HDMI or DVI-D cable Components of a typical satellite HDTV system: 1. HDTV Monitor 2. HD satellite receiver 3. Standard satellite dish 4. HDMI or DVI-D cable

    In addition to an HD Ready television, other equipment is often needed for the home user to view High Definition Television. Cable Ready TV sets can display HD content without the use of an external box. They provide a card slot that accept s a CableCARD. There are several sources of high definition content and the equipment used to control each of these must be HD compatible.


    High Definition picture sources include terrestrial broadcast, direct broadcast satellite, digital cable, high definition discs (BD and HD DVD), internet downloads and the latest generation of games consoles. The availability of television pictures broadcast in the HD format varies by region and country.


    HDTV can be recorded to D-VHS (Data-VHS), W-VHS (analog only), to a HDTV-capable digital video recorder (for example DirecTV's high-definition Digital video recorder, Sky HD's set-top box, or TiVo's Series 3 recorder), or an HDTV-ready HTPC . Cable boxes from Comcast are capable of receiving or recording two broadcasts at a time in HDTV format, and HDTV programming, some free, some for a fee, can b e played back with the on-demand feature. The massive amount of data storage required to archive uncompressed streams make it unlikely that an uncompressed storage option will appear in the consumer market soon. Realtime MPEG-2 compression of an uncompressed digital HDTV signal is also prohibitively expensive for the consumer market at this time, but should become inexpensive within several years (although this is more relevant for consumer HD camcorders than recording HDTV). Analog tape recorders with bandwidth capable of recording analog HD signals such as W-VHS recorders are no longer produced for the consumer market and are both expensive and scarce in the secondary market.


    In the United States, as part of the FCC's "plug and play" agreement, cable companies are required to provide customers that rent HD set-top boxes with a set-to p box with "functional" Firewire (IEEE 1394) upon request. None of the direct broadcast satellite providers have offered this feature on any of their supported boxes, but some cable TV companies have. As of July 2004, boxes are not included in the FCC mandate. This content is protected by encryption known as 5C.[7] This encryption can prevent duplication of content or simply limit the number of co pies permitted.



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