出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2016/08/24 11:41:46」(JST)
| この項目では、高精細度な映像を用いるテレビ放送について説明しています。高精細度映像全般については「高精細度ビデオ」を、NHKが開発した高精細度テレビジョン放送のアナログ伝送方式「MUSE」や日本における高精細度テレビジョン放送については「ハイビジョン」をご覧ください。 |
高精細度テレビジョン放送（こうせいさいどテレビジョンほうそう、HDTV : High-definition television、ハイ・デフィニション・テレビジョン）とは走査線数を増やし、かつワイドアスペクト比 (16:9) を採用することにより鮮明な映像を実現したテレビジョン放送である[1]。
HDTVは概ね従来の標準放送（NTSC、PAL、SECAMなどの方式）の2倍程度の走査線を持つものをこう呼ぶ。日本の放送では電波法施行規則において
を「高精細度テレビジヨン放送」、その他のテレビ放送を「標準テレビジヨン放送」と規定している（第2条第1項 28の3による）。
国際電気通信連合では国際標準規格（2000年）で、走査線1125本（有効走査線1080本、画面の縦横比（アスペクト比）16:9）のシステムのみを「HDTV」として定義している。
HDTVは日本、アメリカ、ヨーロッパがそれぞれ独自に研究・開発している。世界に先駆けて日本のNHK放送技術研究所が「高品位テレビ」として1964年より研究を始め、1970年代後半にHDTVのアナログ伝送方式「MUSE」を開発し、「ハイビジョン」という愛称で欧米に先駆けてHDTV放送を開始した。NHKは自ら開発したMUSE方式を世界統一のHDTV放送規格にすることを目指し、「高品位テレビ」の英訳として「High Definition Television」という言葉を使って1974年にCCIR（国際電気通信連合の前組織）で提唱し、欧米で精力的な標準化活動を続けたが、デジタル伝送技術の実用化、政治的その他さまざまな理由から日米欧はそれぞれ異なる方式でデジタル伝送技術を使ったHDTV放送を行うことになった。「High Definition Television」という言葉が日本に逆輸入されたときに、「高品位テレビ」でなく直訳の「高精細度テレビジョン (HDTV)」という言葉が作られた。
日本では2000年12月1日よりBSデジタル放送、2003年12月1日より地上デジタルテレビ放送の本放送がそれぞれ開始され、デジタル伝送方式によるHDTV放送が行われている。
アメリカでも地上デジタル放送の規格として高精細度放送の規格が決定され、関連の政策が整備されたことで高精細度放送を受信できるテレビ受像機が出回るようになり、また高精細度放送用の番組制作が行われるようになりつつある。
HDTVはSDTVより最低でも2倍以上の解像度を持つのでアナログテレビや一般のDVDと比較して、より細密な映像の描写が可能である。また、HDTV放送の技術標準ではレターボックス無しに縦横比が16:9の映像を処理することができるので映画のような映像を視聴する場合には解像度をより效率的に調整することができる。
SDTVより4倍高いHDTVの解像度
SDTVの解像度
HDTVを言う時に放送信号の形式は次のような値を利用して表記する。
例えば、720/60pという形式は1280pixel×720lineの有効画素が順次走査 (progressive scan) 方式で60frame/secで伝送されるベースバンド信号を示す。1080/50iという形式は1920pixel×1080lineの有効画素が飛び越し走査 (interlace scan) で、50field/sec (=25frame/sec) で伝送されるベースバンド信号を指す。サフィックスのフレームあるいはフィールド数/secが省略されている場合は、当該地域の標準テレビシステムによって50あるいは60と想定される。例えば、”1080p”とのみ表記されている場合は、一般に1080/60p、1080/50pを指す（映画ソース等では、1080/24pのベースバンド信号も存在する）。フレームレート（frame/secまたはfield/sec）も有効line/frameとは独立に明記される。例えば60pはprogressive方式の60frame/secを意味し、60iは秒当たり30frame/secのinterlace方式 (=60field/sec) を意味する。
標準解像度（従来の解像度）との比較を以下に示す。
（1ピクセルを正方形で同一サイズと仮定した場合の画面サイズ比較イメージ図。詳しくは脚注参照）
アメリカでは従来のテレビジョン放送向けのデジタル伝送方式としてMPEG-2、HDTV向けにMPEG-3が策定される予定であったがMPEG-2とMPEG-3の基本技術が同じであったため最終的にはMPEG-2がMPEG-3を吸収・統合することが決定しMPEG-3規格は欠番となっている。現在では、HDTVを含めた映像のデジタル伝送方式としてMPEG-2が幅広く利用されている。
MPEG-2では最大4:2:2YUVサンプリング (chroma subsampling) と10ビット量子化をサポートしているが、一般的にHDTV放送では帯域幅を減らすために4:2:0方式と8ビット量子化を利用する。韓国のDMBとドイツのDVB-S2ではH.264 (MPEG-4 AVC) を利用している。今後、あらゆるヨーロッパのHDTV放送でもH.264を使う予定でアイルランドなどまだデジタル放送が始まらない所ではHDTV放送は勿論のことSDTVデジタル放送でもH.264の利用が検討されている。日本では2008年10月からスカパー!でのHDTV放送用（スカパー!プレミアムサービス《旧スカパー!HD》）として、2009年12月から国際放送・NHKワールドTVのHDTV放送用としてそれぞれH.264を利用している。
HDTVは5.1chサラウンドサウンドをサポートするAAC（エーエーシー、Advanced Audio Coding）とドルビーデジタル (AC-3) 形式のサウンドを使うので、劇場（映画館）水準のオーディオを鑑賞することができる。
HD信号のピクセルアスペクト比は1.0である（ピクセルの幅と高さとが等しい。スクエアピクセルと呼ばれる）。新しいHD圧縮/録画方式ではもっと効率的な圧縮のために長方形のピクセルを利用することもある（例えば、日本の地上デジタルテレビ放送では、水平画素数1440ドットで送信されることが多く、受信機で1920ドットに拡大している）。
テレビ放送局や地域内の分配を引き受けている業者、あるいはプロダクション業者ではベースバンド（非圧縮）HDTV信号を送るため、HD-SDI (SMPTE-292M : YPbPr/4:2:2/10bit/1.485Gbps) を使用する。
ベースバンドHDTV信号は空中波やケーブルで送るには非常に広い帯域幅を占めるので公衆放送では圧縮された映像信号を送信し、テレビ受信機でこれをデコードする。また民生用テレビ受像機ではHDMIインターフェースを経由することにより、BDプレーヤー等から出力される暗号化されたベースバンドHDTV信号を受像することができる。
System-M/NTSC互換のためのフレームレート : 29.97フレーム/秒 (30×1000/1001)、59.94 フレーム/秒(60×1000/1001) はそれぞれ30、60と丸めて表現される場合が多い。
放送のための最適の形式は映像を記録する保存メディアと該当の映像の特性とによって変わる。解像度とフレームレートは映像形式に依存する。つまり、非常に高い解像度の映像を損失なしに（できるだけ綺麗に）送るためにはより高い帯域幅が必要になる。現在、すべてのデジタルHDTV放送と多くの保存システムでは損失のある圧縮方式が使われるので原本映像と比較して若干の映像劣化が見られる。
劇場用フィルムは一般的に高い解像度を持っており、秒当たり24フレームを記録する。この映像を送信するための最適な形式は720p24や1080p24となる。この映像をPAL方式のテレビで視聴するためには秒当たり25フレームに変換されなければならない。また、NTSC方式の場合には2:3pulldownと言われる技術が使われる。これは一つのフィルムフレームは3フィールドに割り当てられ（1/20秒）その次のフレームは2フィールドに割り当てられる（1/30秒）形式で、これを繰り返せば2フレームが1/12秒間で送信される。
HDTV以前のベータカムSPのような媒体に記録された映像の場合には483i60または576i50の形式になっている。これらはより高い解像度である720iに変換されることができるが飛び越し走査された映像を一般的な形式である720pに変換する過程では映像の劣化が起きることがあり、デインターレイシングという過程が必要になる。
映画ではないHDTV映像は720pや1080i形式として記録される。放送形式はテレビ放送会社で決めるが形式を決めるには多くの要素が影響を与える。一般的に720pは順次走査方式であり早い動きが多い映像に適当であり、1080iは飛び越し走査方式であり早い動きを表現すると画質が落ちることがある。また720pはHD映像をインターネットを通じて配布する時よく使われる。一般にコンピューターは順次走査方式を使っており、飛び越し走査方式の映像の表示は不得手である。720p映像は1080iや1080p映像に比べて少ない容量と復号演算しか必要としない。720pは一般的な1280×1024解像度のLCDでスケーリングせず表示可能である。
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High-definition television (HDTV) is a television system providing an image resolution that is substantially higher than that of standard-definition television.
HDTV may be transmitted in various formats:
The letter "p" here stands for progressive scan, while "i" indicates interlaced.
When transmitted at two megapixels per frame, HDTV provides about five times as many pixels as SD (standard-definition television).
The term high definition once described a series of television systems originating from August 1936; however, these systems were only high definition when compared to earlier systems that were based on mechanical systems with as few as 30 lines of resolution. The ongoing competition between companies and nations to create true "HDTV" spanned the entire 20th century, as each new system became more HD than the last.In the beginning of the 21st century, this race has continued with 4k, 5k and current 8K systems.
The British high-definition TV service started trials in August 1936 and a regular service on 2 November 1936 using both the (mechanical) Baird 240 line sequential scan (later to be inaccurately rechristened 'progressive') and the (electronic) Marconi-EMI 405 line interlaced systems. The Baird system was discontinued in February 1937.[1] In 1938 France followed with their own 441-line system, variants of which were also used by a number of other countries. The US NTSC 525-line system joined in 1941. In 1949 France introduced an even higher-resolution standard at 819 lines, a system that should have been high definition even by today's standards, but was monochrome only and the technical limitations of the time prevented it from achieving the definition of which it should have been capable. All of these systems used interlacing and a 4:3 aspect ratio except the 240-line system which was progressive (actually described at the time by the technically correct term "sequential") and the 405-line system which started as 5:4 and later changed to 4:3. The 405-line system adopted the (at that time) revolutionary idea of interlaced scanning to overcome the flicker problem of the 240-line with its 25 Hz frame rate. The 240-line system could have doubled its frame rate but this would have meant that the transmitted signal would have doubled in bandwidth, an unacceptable option as the video baseband bandwidth was required to be not more than 3 MHz.
Colour broadcasts started at similarly higher resolutions, first with the US NTSC color system in 1953, which was compatible with the earlier monochrome systems and therefore had the same 525 lines of resolution. European standards did not follow until the 1960s, when the PAL and SECAM color systems were added to the monochrome 625 line broadcasts.
The Nippon Hōsō Kyōkai (NHK, the Japan Broadcasting Corporation) began conducting research to "unlock the fundamental mechanism of video and sound interactions with the five human senses" in 1964, after the Tokyo Olympics. NHK set out to create an HDTV system that ended up scoring much higher in subjective tests than NTSC's previously dubbed "HDTV". This new system, NHK Color, created in 1972, included 1125 lines, a 5:3 aspect ratio and 60 Hz refresh rate. The Society of Motion Picture and Television Engineers (SMPTE), headed by Charles Ginsburg, became the testing and study authority for HDTV technology in the international theater. SMPTE would test HDTV systems from different companies from every conceivable perspective, but the problem of combining the different formats plagued the technology for many years.
There were four major HDTV systems tested by SMPTE in the late 1970s, and in 1979 an SMPTE study group released A Study of High Definition Television Systems:
Since the formal adoption of digital video broadcasting's (DVB) widescreen HDTV transmission modes in the mid to late 2000s; the 525-line NTSC (and PAL-M) systems, as well as the European 625-line PAL and SECAM systems, are now regarded as standard definition television systems.
Early HDTV broadcasting used analog technology, but today it is transmitted digitally and uses video compression.
In 1949, France started its transmissions with an 819 lines system (with 737 active lines). The system was monochrome only, and was used only on VHF for the first French TV channel. It was discontinued in 1983.
In 1958, the Soviet Union developed Тransformator (Russian: Трансформатор, meaning Transformer), the first high-resolution (definition) television system capable of producing an image composed of 1,125 lines of resolution aimed at providing teleconferencing for military command. It was a research project and the system was never deployed by either the military or consumer broadcasting.[3]
In 1979, the Japanese state broadcaster NHK first developed consumer high-definition television with a 5:3 display aspect ratio.[4] The system, known as Hi-Vision or MUSE after its Multiple sub-Nyquist sampling encoding for encoding the signal, required about twice the bandwidth of the existing NTSC system but provided about four times the resolution (1080i/1125 lines). Satellite test broadcasts started in 1989, with regular testing starting in 1991 and regular broadcasting of BS-9ch commencing on November 25, 1994, which featured commercial and NHK programming.
In 1981, the MUSE system was demonstrated for the first time in the United States, using the same 5:3 aspect ratio as the Japanese system.[5] Upon visiting a demonstration of MUSE in Washington, US President Ronald Reagan was impressed and officially declared it "a matter of national interest" to introduce HDTV to the US.[6]
Several systems were proposed as the new standard for the US, including the Japanese MUSE system, but all were rejected by the FCC because of their higher bandwidth requirements. At this time, the number of television channels was growing rapidly and bandwidth was already a problem. A new standard had to be more efficient, needing less bandwidth for HDTV than the existing NTSC.
The limited standardization of analog HDTV in the 1990s did not lead to global HDTV adoption as technical and economic constraints at the time did not permit HDTV to use bandwidths greater than normal television.
Early HDTV commercial experiments, such as NHK's MUSE, required over four times the bandwidth of a standard-definition broadcast. Despite efforts made to reduce analog HDTV to about twice the bandwidth of SDTV, these television formats were still distributable only by satellite.
In addition, recording and reproducing an HDTV signal was a significant technical challenge in the early years of HDTV (Sony HDVS). Japan remained the only country with successful public broadcasting of analog HDTV, with seven broadcasters sharing a single channel.
Since 1972, International Telecommunication Union's radio telecommunications sector (ITU-R) had been working on creating a global recommendation for Analog HDTV. These recommendations, however, did not fit in the broadcasting bands which could reach home users. The standardization of MPEG-1 in 1993 also led to the acceptance of recommendations ITU-R BT.709.[7] In anticipation of these standards the Digital Video Broadcasting (DVB) organisation was formed, an alliance of broadcasters, consumer electronics manufacturers and regulatory bodies. The DVB develops and agrees upon specifications which are formally standardised by ETSI.[8]
DVB created first the standard for DVB-S digital satellite TV, DVB-C digital cable TV and DVB-T digital terrestrial TV. These broadcasting systems can be used for both SDTV and HDTV. In the US the Grand Alliance proposed ATSC as the new standard for SDTV and HDTV. Both ATSC and DVB were based on the MPEG-2 standard, although DVB systems may also be used to transmit video using the newer and more efficient H.264/MPEG-4 AVC compression standards. Common for all DVB standards is the use of highly efficient modulation techniques for further reducing bandwidth, and foremost for reducing receiver-hardware and antenna requirements.
In 1983, the International Telecommunication Union's radio telecommunications 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, the world already having split into two camps, 25/50 Hz and 30/60 Hz, largely due to the differences in mains frequency. The IWP11/6 working party considered many views and throughout 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 upon at the first meeting of the IWP11/6 working party at the BBC's Research and Development 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). The British Freeview HD trials used MBAFF, which contains both progressive and interlaced content in the same encoding.
It also includes the alternative 1440×1152 HDMAC scan format. (According to some reports, a mooted 750-line (720p) format (720 progressively scanned lines) was viewed by some at the ITU as an enhanced television format rather than a true HDTV format,[9] and so was not included, although 1920×1080i and 1280×720p systems for a range of frame and field rates were defined by several US SMPTE standards.)
HDTV technology was introduced in the United States in the late 1980s and made official in 1993 by the Digital HDTV Grand Alliance, a group of television, electronic equipment, communications companies consisting of AT&T Bell Labs, General Instrument, Philips, Sarnoff, Thomson, Zenith and the Massachusetts Institute of Technology. Field testing of HDTV at 199 sites in the United States was completed August 14, 1994.[10] The first public HDTV broadcast in the United States occurred on July 23, 1996 when the Raleigh, North Carolina television station WRAL-HD began broadcasting from the existing tower of WRAL-TV southeast of Raleigh, winning a race to be first with the HD Model Station in Washington, D.C., which began broadcasting July 31, 1996 with the callsign WHD-TV, based out of the facilities of NBC owned and operated station WRC-TV.[11][12][13] The American Advanced Television Systems Committee (ATSC) HDTV system had its public launch on October 29, 1998, during the live coverage of astronaut John Glenn's return mission to space on board the Space Shuttle Discovery.[14] The signal was transmitted coast-to-coast, and was seen by the public in science centers, and other public theaters specially equipped to receive and display the broadcast.[14][15]
The first HDTV transmissions in Europe, albeit not direct-to-home, began in 1990, when the Italian broadcaster RAI used the HD-MAC and MUSE HDTV technologies to broadcast the 1990 FIFA World Cup. The matches were shown in 8 cinemas in Italy and 2 in Spain. The connection with Spain was made via the Olympus satellite link from Rome to Barcelona and then with a fiber optic connection from Barcelona to Madrid.[16] After some HDTV transmissions in Europe the standard was abandoned in the mid-1990s.
The first regular broadcasts started on January 1, 2004 when the Belgian company Euro1080 launched the HD1 channel with the traditional Vienna New Year's Concert. Test transmissions had been active since the IBC exhibition in September 2003, but the New Year's Day broadcast marked the official launch of the HD1 channel, and the official start of direct-to-home HDTV in Europe.[17]
Euro1080, a division of the former and now bankrupt Belgian TV services company Alfacam, broadcast HDTV channels to break the pan-European stalemate of "no HD broadcasts mean no HD TVs bought means no HD broadcasts ..." and kick-start HDTV interest in Europe.[18] The HD1 channel was initially free-to-air and mainly comprised sporting, dramatic, musical and other cultural events broadcast with a multi-lingual soundtrack on a rolling schedule of 4 or 5 hours per day.
These first European HDTV broadcasts used the 1080i format with MPEG-2 compression on a DVB-S signal from SES's Astra 1H satellite. Euro1080 transmissions later changed to MPEG-4/AVC compression on a DVB-S2 signal in line with subsequent broadcast channels in Europe.
Despite delays in some countries,[19] the number of European HD channels and viewers has risen steadily since the first HDTV broadcasts, with SES's annual Satellite Monitor market survey for 2010 reporting more than 200 commercial channels broadcasting in HD from Astra satellites, 185 million HD capable TVs sold in Europe (£60 million in 2010 alone), and 20 million households (27% of all European digital satellite TV homes) watching HD satellite broadcasts (16 million via Astra satellites).[20]
In December 2009 the United Kingdom became the first European country to deploy high definition content using the new DVB-T2 transmission standard, as specified in the Digital TV Group (DTG) D-book, on digital terrestrial television.
The Freeview HD service currently contains 13 HD channels (as of April 2016[update]) and was rolled out region by region across the UK in accordance with the digital switchover process, finally being completed in October 2012. However, Freeview HD is not the first HDTV service over digital terrestrial television in Europe; Italy's Rai HD channel started broadcasting in 1080i on April 24, 2008 using the DVB-T transmission standard.
In October 2008 France deployed five high definition channels using DVB-T transmission standard on digital terrestrial distribution.
HDTV broadcast systems are identified with three major parameters:
If all three parameters are used, they are specified in the following form: [frame size][scanning system][frame or field rate] or [frame size]/[frame or field rate][scanning system].[citation needed] Often, frame size or frame rate can be dropped if its value is implied from context. In this case, the remaining numeric parameter is specified first, followed by the scanning system.
For example, 1920×1080p25 identifies progressive scanning format with 25 frames per second, each frame being 1,920 pixels wide and 1,080 pixels high. The 1080i25 or 1080i50 notation identifies interlaced scanning format with 25 frames (50 fields) per second, each frame being 1,920 pixels wide and 1,080 pixels high. The 1080i30 or 1080i60 notation identifies interlaced scanning format with 30 frames (60 fields) per second, each frame being 1,920 pixels wide and 1,080 pixels high. The 720p60 notation identifies progressive scanning format with 60 frames per second, each frame being 720 pixels high; 1,280 pixels horizontally are implied.
50 Hz systems support three scanning rates: 50i, 25p and 50p. 60 Hz systems support a much wider set of frame rates: 59.94i, 60i, 23.976p, 24p, 29.97p, 30p, 59.94p and 60p. In the days of standard definition television, the fractional rates were often rounded up to whole numbers, e.g. 23.976p was often called 24p, or 59.94i was often called 60i. 60 Hz high definition television supports both fractional and slightly different integer rates, therefore strict usage of notation is required to avoid ambiguity. Nevertheless, 29.97i/59.94i is almost universally called 60i, likewise 23.976p is called 24p.
For the commercial naming of a product, the frame rate is often dropped and is implied from context (e.g., a 1080i television set). A frame 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.[21]
There is no single standard for HDTV color support. Colors are typically broadcast using a (10-bits per channel) YUV color space but, depending on the underlying image generating technologies of the receiver, are then subsequently converted to a RGB color space using standardized algorithms. When transmitted directly through the Internet, the colors are typically pre-converted to 8-bit RGB channels for additional storage savings with the assumption that it will only be viewed only on a (sRGB) computer screen. As an added benefit to the original broadcasters, the losses of the pre-conversion essentially make these files unsuitable for professional TV re-broadcasting.
Most HDTV systems support resolutions and frame rates defined either in the ATSC table 3, or in EBU specification. The most common are noted below.
| Video format supported [image resolution] | Native resolution [inherent resolution] (W×H) | Pixels | Aspect ratio (W:H) | Description | ||
|---|---|---|---|---|---|---|
| Actual | Advertised (Megapixels) | Image | Pixel | |||
| 720p 1280×720 |
1024×768 XGA |
786,432 | 0.8 | 4:3 | 1:1 | Typically a PC resolution (XGA); also a native resolution on many entry-level plasma displays with non-square pixels. |
| 1280×720 |
921,600 | 0.9 | 16:9 | 1:1 | Standard HDTV resolution and a typical PC resolution (WXGA), frequently used by high-end video projectors; also used for 750-line video, as defined in SMPTE 296M, ATSC A/53, ITU-R BT.1543. | |
| 1366×768 WXGA |
1,049,088 | 1.0 | 683:384 (approx. 16:9) |
1:1 | A typical PC resolution (WXGA); also used by many HD ready TV displays based on LCD technology. | |
| 1080p/i 1920×1080 |
1920×1080 |
2,073,600 | 2.1 | 16:9 | 1:1 | Standard HDTV resolution, used by Full HD and HD ready 1080p TV displays such as high-end LCD, plasma and rear projection TVs, and a typical PC resolution (lower than WUXGA); also used for 1125-line video, as defined in SMPTE 274M, ATSC A/53, ITU-R BT.709; |
| Video format supported | Screen resolution (W×H) | Pixels | Aspect ratio (W:H) | Description | ||
|---|---|---|---|---|---|---|
| Actual | Advertised (Megapixels) | Image | Pixel | |||
| 720p 1280×720 |
1248×702 Clean Aperture |
876,096 | 0.9 | 16:9 | 1:1 | Used for 750-line video with faster artifact/overscan compensation, as defined in SMPTE 296M. |
| 1080p 1920×1080 |
1888×1062 Clean aperture |
2,005,056 | 2.0 | 16:9 | 1:1 | Used for 1124-line video with faster artifact/overscan compensation, as defined in SMPTE 274M. |
| 1080i 1920×1080 |
1440×1080 HDCAM/HDV |
1,555,200 | 1.6 | 16:9 | 4:3 | Used for anamorphic 1125-line video in the HDCAM and HDV formats introduced by Sony and defined (also as a luminance subsampling matrix) in SMPTE D11. |
At a minimum, HDTV has twice the linear resolution of standard-definition television (SDTV), thus showing greater detail than either analog television or regular DVD. The technical standards for broadcasting HDTV also handle the 16:9 aspect ratio images without using letterboxing or anamorphic stretching, thus increasing the effective image resolution.
A very high resolution source may require more bandwidth than available in order to be transmitted without loss of fidelity. The lossy compression that is used in all digital HDTV storage and transmission systems will distort the received picture, when compared to the uncompressed source.
ATSC and DVB define the following frame rates for use with the various broadcast standards:[22][23]
The optimum format for a broadcast depends upon the type of videographic recording medium used and the image's characteristics. For best fidelity to the source the transmitted field ratio, lines, and frame rate should match those of the source.
PAL, SECAM and NTSC frame rates technically apply only to analogue standard definition television, not to digital or high definition broadcasts. However, with the roll out of digital broadcasting, and later HDTV broadcasting, countries retained their heritage systems. HDTV in former PAL and SECAM countries operates at a frame rate of 25/50 Hz, while HDTV in former NTSC countries operates at 30/60 Hz.[24]
Standard 35mm photographic film used for cinema projection has a much higher image resolution than HDTV systems, and is exposed and projected at a rate of 24 frames per second (frame/s). To be shown on standard television, in PAL-system countries, cinema film is scanned at the TV rate of 25 frame/s, causing a speedup of 4.1 percent, which is generally considered acceptable. In NTSC-system countries, the TV scan rate of 30 frame/s would cause a perceptible speedup if the same were attempted, and the necessary correction is performed by a technique called 3:2 Pulldown: Over each successive pair of film frames, one is held for three video fields (1/20 of a second) and the next is held for two video fields (1/30 of a second), giving a total time for the two frames of 1/12 of a second and thus achieving the correct average film frame rate.
Non-cinematic HDTV video recordings intended for broadcast are typically recorded either in 720p or 1080i format as determined by the broadcaster. 720p is commonly used for Internet distribution of high-definition video, because most computer monitors operate in progressive-scan mode. 720p also imposes less strenuous storage and decoding requirements compared to both 1080i and 1080p. 1080p/24, 1080i/30, 1080i/25, and 720p/30 is most often used on Blu-ray Disc.
In the US, residents in the line of sight of television station broadcast antennas can receive free, over the air programming with a television set with an ATSC tuner (most sets sold since 2009 have this). This is achieved with a TV aerial, just as it has been since the 1940s except now the major network signals are broadcast in high definition (ABC, Fox, and Ion Television broadcast at 720p resolution; CBS, My Network TV, NBC, PBS, and The CW at 1080i). As their digital signals more efficiently use the broadcast channel, many broadcasters are adding multiple channels to their signals. Laws about antennas were updated before the change to digital terrestrial broadcasts. These new laws prohibit home owners' associations and city government from banning the installation of antennas.
Additionally, cable-ready TV sets can display HD content without using an external box. They have a QAM tuner built-in and/or a card slot for inserting a CableCARD.[25]
High-definition image sources include terrestrial broadcast, direct broadcast satellite, digital cable, IPTV (including GoogleTV, Roku boxes and AppleTV or built into "Smart Televisions"), Blu-ray video disc (BD), and internet downloads.
Sony's PlayStation 3 has extensive HD compatibility because of its built in Blu-ray disc based player, so does Microsoft's Xbox 360 with the addition of Netflix and Windows Media Center HTPC streaming capabilities, and the Zune marketplace where users can rent or purchase digital HD content.[26] Recently, Nintendo released a next generation high definition gaming platform, The Wii U, which includes TV remote control features in addition to IPTV streaming features like Netflix. The HD capabilities of the consoles has influenced some developers to port games from past consoles onto the PS3, Xbox 360 and Wii U, often with remastered or upscaled graphics.
HDTV can be recorded to D-VHS (Digital-VHS or Data-VHS), W-VHS (analog only), to an HDTV-capable digital video recorder (for example DirecTV's high-definition Digital video recorder, Sky HD's set-top box, Dish Network's VIP 622 or VIP 722 high-definition Digital video recorder receivers, or TiVo's Series 3 or HD recorders), or an HDTV-ready HTPC. Some cable boxes are capable of receiving or recording two or more broadcasts at a time in HDTV format, and HDTV programming, some included in the monthly cable service subscription price, some for an additional fee, can be played back with the cable company's on-demand feature.
The massive amount of data storage required to archive uncompressed streams meant that inexpensive uncompressed storage options were not available to the consumer. In 2008, the Hauppauge 1212 Personal Video Recorder was introduced. This device accepts HD content through component video inputs and stores the content in MPEG-2 format in a .ts file or in a Blu-ray compatible format .m2ts file on the hard drive or DVD burner of a computer connected to the PVR through a USB 2.0 interface. More recent systems are able to record a broadcast high definition program in its 'as broadcast' format or transcode to a format more compatible with Blu-ray.
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 who rent HD set-top boxes with a set-top box with "functional" FireWire (IEEE 1394) on 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[update], boxes are not included in the FCC mandate. This content is protected by encryption known as 5C.[27] This encryption can prevent duplication of content or simply limit the number of copies permitted, thus effectively denying most if not all fair use of the content.
| Wikimedia Commons has media related to High-definition television. |
History
European adoption
Digital video resolutions
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| Enhanced, HMP@HML |
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| High, MP@HL |
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| Ultra-high |
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High-definition (HD)
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| Concepts |
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| Analog broadcast (All defunct) |
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| Digital broadcast |
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| Audio |
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| Filming and storage |
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| HD media and compression |
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| Connectors |
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| Deployments |
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Broadcast video formats
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| 関連記事 | 「definition」「high」 |