Saturday, March 15, 2008

High-definition television

High-definition television (HDTV) is a digital television broadcasting system with greater resolution than traditional television systems (NTSC, SECAM, PAL). HDTV is digitally broadcast because digital television (DTV) requires less bandwidth if sufficient video compression is used.

Projection screen in a home theater, displaying a high-definition television image.

Projection screen in a home theater, displaying a high-definition television image.

History of high-definition television

Further information: Analog high-definition television system

The term high definition described the television systems of the 1930s and 1940s beginning with the British 405-line black-and-white system, introduced in 1936; however, it, and the American 525-line NTSC system established in 1941, were only high definition in comparison with previous mechanical and electronic television systems. Today, the American 525-line NTSC system and the European 625-line PAL and SECAM systems are only regarded as standard definition. The post–WWII French 819-line black-and-white system was high definition in the contemporary sense, but was discontinued in 1986, a year after the final British 405-line broadcast.

In 1958, the U.S.S.R. created Трансформатор (Transformer), the first high-resolution (definition) television system capable of producing an image composed of 1,125 lines of resolution for the purpose of television conferences among military commands; as it was a military product, it was not commercialised.[1]

In 1969, the Japanese state broadcaster NHK first developed consumer high-definition television with a 5:3 aspect ratio, a slightly wider screen format than the usual 4:3 standard.[2] However, the system was not launched publicly until late in the 1990s.

In 1981, the first HDTV demonstration in the United States was held. It had the same 5:3 aspect ratio as the Japanese system.[3]

In 1983, the International Telecommunication Union's radiotelecommunications sector (ITU-R) set up a working party (IWP11/6) with the aim of setting a single international HDTV standard. One of the thornier issues concerned a suitable frame/field refresh rate, with the world already strongly demarcated into two camps, 25/50Hz and 30/60Hz, related by reasons of picture stability to the frequency of their mains electrical supplies. The WP considered many views and through the 1980s served to encourage development in a number of video digital processing areas, not least conversion between the two main frame/field rates using motion vectors, which led to further developments in other areas. While a comprehensive HDTV standard was not in the end established, at least agreement on the aspect ratio was achieved. Initially the existing 5:3 aspect ratio had been the main candidate, but due to the influence of widescreen cinema, the aspect ratio 16:9 (1.78) eventually emerged as being a reasonable compromise between 5:3 (1.67) and the common 1.85 widescreen cinema format. An aspect ratio of 16:9 was duly agreed at the first meeting of the WP at the BBC's R & D establishment in Kingswood Warren.

The resulting ITU-R Recommendation ITU-R BT.709-2 ("Rec. 709") includes the 16:9 aspect ratio, a specified colorimetry, and the scan modes 1080i (1,080 actively-interlaced lines of resolution) and 1080p (1,080 progressively-scanned lines). It also includes the alternative 1440 x 1152 HDMAC scan format. (According to some reports, a mooted 720p format (720 progressively-scanned lines) was viewed by some at the ITU as an "enhanced" television format rather than a true HDTV format[4], and so was not included, although 1920x1080 and 1280x720p systems for a range of frame and field rates were defined by several US SMPTE standards.)

However, even that limited standardization of HDTV did not lead to its adoption, principally for technical and economic reasons. Early HDTV commercial experiments such as NHK's MUSE required over four times the bandwidth of a standard-definition (SDTV) broadcast, and despite efforts made to shrink the required bandwidth down to about 2 times that of SDTV, it was still only distributable by satellite. In addition, recording and reproducing a HDTV signal was a significant technical challenge in the early years of HDTV.

HDTV technology was introduced in the United States in the 1990s by the Digital HDTV Grand Alliance, a group of television companies and MIT.[5][6] On 6th April 1997, CBS went on the air with WCBS-HD from the top of the Empire State Building, New York, doing demos and evaluations.[7] The first HDTV sets went on sale in the United States in 1998.

In Europe, analog 1,125-line HD-MAC test broadcasts were performed in the early 1990s, but did not lead to any established public broadcast service.

Japan remains the only country with successful public broadcast analog HDTV, known as "Hi-vision", featuring a 5:3 aspect ratio screen with 1,125 interlaced lines (1,035 active lines) at the rate of 60 fields per second.

It was not until the early 2000s that technology had progressed enough to deliver sufficient storage capacity and processing power to support compression algorithms powerful enough to make HDTV affordable for consumers and profitable for broadcasters and other programme makers. The main enabling factor was the transition from analog to digital TV standards. Digital compression methods such as MPEG-2 and MPEG-4 allow the bandwidth of a single analogue TV channel (6 MHz in the US) to carry up to 5 standard-definition or up to 2 high-definition digital TV channels instead. Most developed nations have plans in place for a transition to digital television, but not necessarily or exclusively HDTV; for example, on 17th February 2009, the US intends to terminate all full-power terrestrial analog broadcasting (although some smaller local stations have later deadlines), with both standard definition TV (SDTV) and HDTV being allowed. [8]

Current HDTV broadcast standards include ATSC (US) and DVB (Europe, and most of the rest of the world). HDTV can also provide 5.1-channel surround sound audio using e.g. the Dolby Digital (AC-3) format.

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