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HDTV/High Definition Television:
High-definition television (HDTV) is a digital television broadcasting system with higher 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.
In 1958, the U.S.S.R. created transformator, "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 commercialized.
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. 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.
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, 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, 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. 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. 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.
Current HDTV broadcast standards include ATSC (US and Canada) 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.
HDTV Sources:
The rise in popularity of large screens and projectors has made the limitations of conventional Standard Definition TV (SDTV) increasingly evident. A HDTV compatible television set will not improve the quality of SDTV channels. To display a superior picture, high definition televisions require a High Definition (HD) signal. Typical sources of HD signals are as follows:
1. 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 high definition televisions have an HD tuner built in. For HDTV televisions without an built in HD tuner, a separate set-top HD tuner box can be rented from a cable or satellite company or purchased.
2. Cable television companies often offer HDTV broadcasts as part of their digital 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.
3. Satellite-based TV companies, such as DirecTV and Dish Network (both in North America), Sky Digital (in the UK and Ireland), Bell ExpressVu and Star Choice (both in Canada) and NTV Plus (in Russia), offer HDTV to customers as an upgrade. New satellite receiver boxes and a new satellite dish are often required to receive HD content.
4. Video game systems, such as the Xbox, PlayStation 3, and Xbox 360, and digital set-top boxes that rely on an Internet connection, such as the Apple TV, can output a HD signal. The Xbox Live Marketplace, iTunes Music Store, and PlayStation Network services offer HD movies, TV shows, movie trailers, and clips for download, but generally at lower bitrates than Blu-ray Disc. Most newer computer graphics cards have either HDMI or DVI interfaces, which can be used to output images or video to a HDTV.
5. The optical disc standard Blu-ray Disc (25GB-50GB) can provide enough digital storage to store hours of HD video content.
Notation
HDTV broadcast systems are defined threefold, by:
1. The number of lines in the vertical display resolution.
2. The scanning system: progressive scanning (p) or interlaced scanning (i). Progressive scanning simply draws a complete image frame (all the lines) per image refresh, whereas interlaced scanning draws a partial image field (every second line) during a first pass, then fills-in the remaining lines during a second pass, per image refresh. Interlaced scanning requires significantly lower signal/data bandwidth, but an interlaced signal loses half of the vertical resolution and suffers "combing" artifacts when showing a moving subject on a progressive display (although the worst effects can be mitigated by suitable image post-processing known as 'deinterlacing'). As some compensation, however, interlaced mode provides finer time-sampling, giving two (half-resolution) image samples in the same time interval as one (full-resolution) image sample in progressive mode.
3. The number of frames per second or fields per second. The 720p60 format is 1280 × 720 pixels progressive scanning with 60 fields per second (60 Hz). The 1080i50 format is 1920 × 1080 pixels (ie 2 MP) interlaced scanning with 50 fields per second. Sometimes interlaced fields are called half-frames, but they are not, because two fields of one frame are temporally shifted. Frame pulldown and segmented frames are special techniques that allow transmitting full frames via an interlaced video stream.
For commercial naming of the product, either the frame rate or the field rate is often dropped, e.g. a "1080i television set" label indicates only the image resolution. Often, the rate is inferred from the context, usually assumed to be either 50 or 60 Hz, except for 1080p, which denotes 1080p24, 1080p25, and 1080p30, but may include 1080p50 and 1080p60 in the future.
A frame or field rate can also be specified without a resolution. For example 24p 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.
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