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High-definition television (HDTV) is a television broadcasting system with a significantly higher resolution than traditional formats (NTSC, SECAM, PAL) allow. Except for early analog formats in Europe and Japan, HDTV is broadcasted digitally, and therefore its introduction sometimes coincides with the introduction of digital television (DTV): this technology was first introduced in the USA during the 1990s, by the Digital HDTV Grand Alliance (grouping together AT&T, General Instrument, MIT, Philips, Sarnoff, Thomson, and Zenith)<ref name="spectrum">Carlo Basile et al. (1995). "The U.S. HDTV standard: the Grand Alliance". IEEE Spectrum (4): 36–45.</ref>.

While a number of high-definition television standards have been proposed or implemented on a limited basis, the current HDTV standards are defined in ITU-R BT.709 as 1080 active interlaced or progressive lines, or 720 progressive lines, using a 16:9 aspect ratio. The term "high-definition" can refer to the resolution specifications themselves, or more loosely to media capable of similar sharpness, such as photographic film.

Contents 1 Notation 1.1 Changes in notation 1.2 Standard resolutions 1.3 Standard frame or field rates 2 Comparison to SDTV 3 Format considerations 4 Technical details 5 Advantages of HDTV expressed in non-engineering terms 6 Early systems 7 Contemporary systems 8 Recording and compression 9 Table of terrestrial HDTV transmission systems 10 References 11 See also 12 External links

[edit] 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 frames (p) or interlaced fields (i) are used.
• The number of frames or fields per second.

For example, the format 720p60 is 1280 × 720 pixels, progressive encoding with 60 frames per second (60 Hz). The format 1080i50 is 1920 × 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. Sometimes the rate is then 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 often 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 24p means 24 progressive 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.

[edit] Changes in notation

It should be noted that the terminology described above was invented for digital systems in the 1990s. Before that, analog TV had no true "pixels" to measure horizontal resolution, and 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 480 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.

[edit] Standard resolutions

Image:Standard video res.svg

We distinguish (a) the resolution of the transmitted signal and (b) the (native) displayed resolution of a TV set. Digital NTSC- and PAL/SECAM-like signals (480i60 and 576i50 respectively) are transmitted at a horizontal resolutions of 720 or 704 "pixels". However these transmitted DTV "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 × 576 or 720 x 540; with an aspect ratio of 16:9 they use 1024 × 576 or 960 × 540; NTSC ones use 640 × 480 and 852 × 480 or, seldomly, 720 × 540.

[edit] 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, PAL-M & SECAM)
• 60i (NTSC)

[edit] Comparison to SDTV

Close-up view
Image:Raster graphic fish 40X46squares hdtv-example.png	Image:Raster graphic fish 20x23squares sdtv-example.png
HDTV at four times the spatial resolution of SDTV	SDTV resolution

HDTV has at least twice the lineal resolution of standard-definition television (SDTV), thus allowing much more detail to be shown compared to 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.

[edit] Format considerations

The optimum formats for a broadcast depends on the type of media used for the recording 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 is photographed at 24 frame/s. 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 pulldown 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.

See also: Telecine

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.

See also: Deinterlacing

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 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, and few people possess displays capable of displaying the 1920x1080 resolution without scaling.

In North America, 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, HBO-HD, INHD, HDNet ,TNT, and Discovery HD Theater currently broadcast 1080i content.

[edit] Technical details

MPEG-2 is most commonly used as the compression codec for digital HDTV broadcasts. 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 trialling such a system via satellite broadcast, which save considerable bandwidth compared to MPEG-2 systems. Some German broadcasters already use MPEG-4 together with DVB-S2 (ProSieben, Sat1 and Three Premiere Channels). Although MPEG-2 is more widely used at present, it seems likely that in the future all European HDTV may be MPEG-4, and Ireland and Norway, which have not yet begun any digital television broadcasts, are considering MPEG4 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, or 1 pixel length = 1 pixel width. New HD compression and recording formats such as HDV use rectangular pixels for more efficient compression and to open HDTV acquisition for the consumer market.

For more technical details see the articles on HDV, ATSC, DVB, and ISDB.

Within television studios and other production and distribution facilities, HD-SDI SMPTE 292M interconnect standard (a nominally 1.485 Gbit/s, 75-ohm serial digital interface) is used to route uncompressed HDTV signals. The native bitrate of HDTV formats cannot be supported by 6-8MHz 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 supporting 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.

[edit] Advantages of HDTV expressed in non-engineering terms

High-definition television (HDTV) offers a much better picture quality than standard television. HD's greater clarity means the picture on screen is less blurred and less fuzzy. HD also brings other benefits - smoother motion, richer and more natural colours, surround sound and the chance for different equipment to work better together.

1. All commercial HD is digital, so the signal will either deliver an excellent picture, a picture with noticeable pixelation, a series of still pictures, or no picture at all. 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. This is also a disadvantage, because 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.
2. 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). Or, one can usually choose to zoom the image to fill the screen. 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.
3. The colors will generally look more realistic, due to their greater bandwidth.
4. 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.
5. Two new pre-recorded disc formats support HDTV resolutions, namely HD DVD (supporting 720p, 1080i and 1080p) and Blu-ray (supporting up to 1080p). Most players for both systems are backward-compatible with DVDs. However, the two formats are not compatible with each other.
6. The increased clarity and detail make larger screen sizes more comfortable and pleasing to watch.
7. Dolby Digital 5.1 sound is broadcast along with standard HDTV video signals, allowing full surround sound capabilities. (Standard broadcast television signals usually only include stereo audio.)
8. Both designs make more efficient use of electricity than SDTV designs of equivalent size, which can mean lower operating costs.

[edit] Early systems

Main article: Analog high-definition television systems

The term "high definition" was used to describe the electronic television systems 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 the 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.

[edit] Contemporary systems

Main article: List of digital television deployments by country

[edit] Recording and compression

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

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 TiVo or SkyHD's set-top box), a HDTV-ready Windows Media Center, DirecTV's Plus HD DVR H20 or Dish Network's DVR 921, 942 or VIP622.

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 USA, 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-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, boxes are not included in the FCC mandate. This content is protected by encryption known as 5C. <ref>5C Digital Transmission Content Protection White Paper (pdf) (1998-07-14).</ref>. This encryption can prevent someone from recording content at all or simply limit the number of copies.

[edit] Table of terrestrial HDTV transmission systems

Main characteristics of three DTTV systems

Systems	ATSC	DVB-T	ISDB-T
Source coding
Video	Main Profile syntax of ISO/IEC 13818-2 (MPEG-2 – Video)
Audio	ATSC Standard A/52 (Dolby AC-3)	ISO/IEC 13818-2 (MPEG-2 – Layer II Audio) and Dolby AC-3	ISO/IEC 13818-7 (MPEG-2 – AAC Audio)
Transmission system
Channel coding
Outer coding	R-S (207, 187, t = 10)	R-S (204, 188, t = 8)
Outer interleaver	52 R-S block	convolutional (I=12, M=17, J=1)	12 R-S block
Inner coding	rate 2/3 Trellis code	PCC: rate 1/2, 2/3, 3/4, 5/6, 7/8; constraint length = 7, Polynomials (octal) = 171, 133
Inner interleaver	12 to 1 Trellis code	bit-wise, frequency, selectable time
Data randomization	16-bit PRBS
Modulation	8-VSB and 16-VSB	COFDM QPSK, 16QAM and 64QAM Hierarchical modulation: multi-resolution constellation (16QAM and 64QAM) Guard interval: 1/32, 1/16, 1/8 & 1/4 of OFDM symbol Two modes: 2k and 8k FFT	BST-COFDM with 13 frequency segments DQPSK, QPSK, 16QAM and 64QAM Hierarchical modulation: choice of three different modulations on each segment Guard interval: 1/32, 1/16, 1/8 & 1/4 of OFDM symbol Three modes: 2k, 4k and 8k FFT

[edit] References

[edit] See also

• 1080p, 720p, 576p, 480p, 1080i
• Advanced Television Systems Committee (ATSC)
• ATSC tuner
• Integrated Services Digital Broadcasting
• DVB (Digital Video Broadcasting)
• Digital television
• HDTV input and colorspace (YPbPr/YCbCr).
• HD ready
• SDTV (Standard Definition Television)
• Ultra-High Definition Video (UHDV)

[edit] External links

• US Government HDTV and DTV official site
• Canadian Digital Television official website
• ATSC

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