High-definition television

High-definition television (or HDTV, or just HD) refers to video having resolution substantially higher than traditional television systems (standard-definition TV, or SDTV, or SD). HD has one or two million pixels per frame, roughly five times that of SD. Early HDTV broadcasting used analog techniques, but today HDTV is digitally broadcast using video compression. Some personal video recorders (PVRs) with hard disk storage but without high-definition tuners are legitimately described as "HD", for "Hard Disk", which can be a cause of confusion.

History of high-definition television

Further information: Analog high-definition television system

The term high definition once described a series of television systems originating from the late 1930s; however, these systems were only high definition when compared to earlier systems that were based on mechanical systems as few as 30 lines of resolution.

The British high definition TV service started trials in August 1936 and a regular service in November 1936 using both the (mechanical) Baird 240 line and (electronic) Marconi-EMI 405 line (377i) systems. The Baird system was discontinued in February 1937. 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 system joined in 1941. In 1949 France introduced an even higher resolution standard at 819 lines (768i), a system that would be high definition even by today's standards, but it was monochrome only. 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.

Color broadcasts started at similarly higher resolutions, first with the US NTSC color system in 1953, which was compatible with the earlier B&W systems and therefore had the same 525 lines (480i) of resolution. European standards did not follow until the 1960s, when the PAL and SECAM colour systems were added to the monochrome 625 line (576i) broadcasts.

Since the formal adoption of Digital Video Broadcasting's (DVB) widescreen HDTV transmission modes in the early 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. In Australia, the 625-line digital progressive system (with 576 active lines) is officially recognized as high definition.^[1]

Analog systems

In 1949, France started its transmissions with an 819 lines system (768i). It was monochrome only, it was used only on VHF for the first French TV channel, and it was discontinued in 1985.

In 1958, the Soviet Union developed Тransformator (Russian: Трансформатор, 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 in the military or broadcasting.^[2]

In 1979, the Japanese state broadcaster NHK first developed consumer high-definition television with a 5:3 display aspect ratio.^[3] 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 commenced on 25 November 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.^[4] Upon visiting a demonstration of MUSE in Washington, US President Ronald Reagan was most impressed and officially declared it "a matter of national interest" to introduce HDTV to the USA.^[5]

Several systems were proposed as the new standard for the USA, 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.

Demise of analog HD systems

The limited standardization of analogue HDTV in the 1990s did not lead to global HDTV adoption as technical and economic reasons 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 -- and HD-MAC was not much better. Despite efforts made to reduce analog HDTV to about 2x the bandwidth of SDTV these television formats were still only distributable by satellite.

In addition, recording and reproducing an HDTV signal was a significant technical challenge in the early years of HDTV. Japan remained the only country with successful public broadcast analog HDTV, with seven broadcasters sharing a single channel. Digital HDTV broadcasting started in 2000 in Japan, and the analog service ended in the early hours of 1 October 2007.

Rise of digital compression

Since 1972, International Telecommunication Union's radio telecommunications sector (ITU-R) has been working on creating a global recommendation for Analogue 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.^[6] 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 on specifications which are formally standardised by ETSI.^[7]

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 USA the Grand Alliance proposed ATSC as the new standard for SDTV and HDTV. Both ATSC and DVB were based on the MPEG-2 standard. The DVB-S2 standard is based on 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, related by reasons of picture stability to the frequency of their main electrical supplies.

The IWP11/6 working party 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. (Bob Morris explained that the 16:9 ratio was chosen as being the geometric mean of 4:3, Academy ratio, and 2.4:1, the widest cinema format in common use, in order to minimize wasted screen space when displaying content with a variety of aspect ratios.^[8])

An aspect ratio of 16:9 was duly agreed 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 current Freeview HD trials use 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-ine (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.)

Inaugural HDTV broadcast in the United States

HDTV technology was introduced in the United States in the 1990s by the Digital HDTV Grand Alliance, a group of television companies and MIT.^[10][11] Field testing of HDTV at 199 sites in the United States was completed August 14, 1994.^[12] 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 south-east 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.^[13][14][15] 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.^[16] 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.^[16][17]

European HDTV broadcasts

Although HDTV broadcasts had been demonstrated in Europe since the early 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 start of the HD1 channel, and the start of HDTV in Europe.^[18]

Euro1080, a division of the 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.^[19] 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 Astra's 1H satellite. Euro1080 transmissions later changed to MPEG-4/AVC compression on a DVB-S2 signal in line with subsequent broadcast channels in Europe.

The first Russian HDTV broadcast commenced in 2007 by NTV Plus, followed by Platform HD in 2008. Both companies broadcast via satellite using MPEG-4/AVC video compression.

In December 2009 the UK became the first European country to deploy high definition content on digital terrestrial television (branded as Freeview) using the new DVB-T2 transmission standard as specified in the Digital TV Group (DTG) D-Book.

The Freeview HD service currently contains 4 HD channels and is now rolling out region by region across the UK in accordance with the digital switchover process. Some transmitters such as the Crystal Palace and Emley Moor transmitters are broadcasting the Freeview HD service ahead of the digital switchover by means of a temporary, low-power pre-DSO multiplex.

Notation

HDTV broadcast systems are identified with three major parameters:

• Frame size in pixels is defined as number of horizontal pixels × number of vertical pixels, for example 1280 × 720 or 1920 × 1080. Often the number of horizontal pixels is implied from context and is omitted, as in the case of 720p and 1080p.
• Scanning system is identified with the letter p for progressive scanning or i for interlaced scanning.
• Frame rate is identified as number of video frames per second. For interlaced systems an alternative form of specifying number of fields per second is often used.^{[citation needed]}

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.^{[citation needed]} 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.^{[citation needed]} 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: 25i, 25p and 50p. 60 Hz systems support a much wider set of frame rates: 23.976p, 24p, 29.97i/59.94i, 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.^{[citation needed]}

For 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.^{[citation needed]}

There is no standard for HDTV color support. Until recently the color of each pixel was regulated by three 8-bit color values, each representing the level of red, blue, and green which defined a pixel color. Together the 24 total bits defining color yielded just under 17 million possible pixel colors. Recently^{[update][when?]} some manufacturers have produced systems that can employ 10 bits for each color (30 bits total) which provides for a palette of 1 billion colors, saying that this provides a much richer picture, but there is no agreed way to specify that a piece of equipment supports this feature.

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.

Standard frame or field rates

• 23.976 Hz (film-looking frame rate compatible with NTSC clock speed standards)
• 24 Hz (international film and ATSC high definition material)
• 25 Hz (PAL, SECAM film, standard definition, and high definition material)
• 29.97 Hz (NTSC standard definition material)
• 50 Hz (PAL & SECAM high definition material)
• 59.94 Hz (ATSC high definition material)
• 60 Hz (ATSC high definition material)
• 120 Hz (ATSC high definition material)

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.

The minimum form of HDTV that can be obtained is a 720p resolution paired with an HD-Converter DVD player along with component cables instead of an HDMI cable, which would provide a higher band width and include the audio all in one cable. Also a low Refresh Rate such as 60 hertz. This refers to the rate at which the screen refreshes every second. Therefore, 60 hertz would mean the TV screen is refreshed 60 times every second. Compare this to the highest form of HDTV. This would be obtained with 1080p resolution paired with a Blu-ray player or an HD-DVD player. The TV would need to have a high refresh rate such as 600 hertz. Connected with an HDMI cable to obtain optimum clarity.This is considered true HD. ^{[20] [21]}

The optimum format for a broadcast depends upon the type of videographic recording medium used and the image's characteristics. The field and frame rate should match the source and the 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.

There is a wide spread confusion of using terms like PAL or SECAM or NTSC relating to HD material. PAL, SECAM, NTSC are only standard definition standards, not HD. There is no specific technical reason to keep 25 Hz as HD frame rate in a former PAL country (except in case of a need of compatibility with both HD and standard definition television systems).

Types of media

Standard 35mm photographic film used for cinema projection has higher resolution than HDTV systems, and is exposed and projected at a rate of 24 frames per second. To be shown on standard television, in PAL-system countries, cinema film is scanned at the TV rate of 25 frames per second, causing an acceleration of 4.1 percent, which is generally considered acceptable. In NTSC-system countries, the TV scan rate of 30 frames per second would cause a perceptible acceleration 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.

See also: Telecine

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 is usually used for Blu-ray Disc.

Contemporary systems

Main article: Large-screen television technology

Besides an HD-ready television set, other equipment may be needed to view HD television. In the US, 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.^[22]

High-definition image sources include terrestrial broadcast, direct broadcast satellite, digital cable, IPTV, the high definition Blu-ray video disc (BD), internet downloads, the Blu-ray disc compatible Sony PlayStation 3 video game console (PS3), and the Microsoft Xbox 360 video game console.

Recording and compression

Main article: High-definition pre-recorded media and compression

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 free, some for a 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 in the consumer market until recently. In 2008 the Hauppauge 1212 Personal Video Recorder was introduced. This device accepts HD content through component video inputs and stores the content in an uncompressed MPEG transport stream (.ts) file or Blu-ray format .m2ts file on the hard drive or DVD burner of a computer connected to the PVR through a USB 2.0 interface.

Realtime MPEG-2 compression of an uncompressed digital HDTV signal is 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 who rent HD set-top boxes with a set-top 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^[update], boxes are not included in the FCC mandate. This content is protected by encryption known as 5C.^[23] 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.