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Digital television

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A map depictingdigital terrestrial televisionstandards
List ofdigital televisionbroadcast standards
DVBstandards(countries)
ATSCstandards(countries)
ISDBstandards(countries)
DTMBstandards(countries)
  • DTMB(terrestrial/mobile)
    • DTMB-A
  • CMMB(handheld)
  • ABS-S (satellite)
DMBstandard(countries)
Codecs
TerrestrialFrequency bands
SatelliteFrequency bands

Digital television(DTV) is the transmission oftelevisionsignals usingdigitalencoding, in contrast to the earlieranalog televisiontechnology which usedanalog signals.At the time of its development it was considered an innovative advancement and represented the first significant evolution in television technology sincecolor televisionin the 1950s.[1]Modern digital television is transmitted inhigh-definition television(HDTV) with greater resolution than analog TV. It typically uses awidescreenaspect ratio (commonly16:9) in contrast to the narrower format (4:3) of analog TV. It makes more economical use of scarceradio spectrumspace; it can transmit up to seven channels in the samebandwidthas a single analog channel,[2]and provides many new features that analog television cannot. Atransition from analog to digital broadcastingbegan around 2000. Different digital television broadcasting standards have been adopted in different parts of the world; below are the more widely used standards:

History

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Background

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Digital television's roots are tied to the availability of inexpensive, high performancecomputers.It was not until the 1990s that digital TV became a real possibility.[7]Digital television was previously not practically feasible due to the impractically highbandwidthrequirements ofuncompressed video,[8][9]requiring around 200Mbit/sfor astandard-definition television(SDTV) signal,[8]and over 1Gbit/sforhigh-definition television(HDTV).[9]

Development

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In the mid-1980s,Toshibareleased atelevision setwith digital capabilities, usingintegrated circuitchips such as amicroprocessorto convert analog television broadcast signals todigital videosignals, enabling features such as freezing pictures andshowing two channels at once.In 1986,SonyandNEC Home Electronicsannounced their own similar TV sets with digital video capabilities. However, they still relied on analog TV broadcast signals, with true digital TV broadcasts not yet being available at the time.[10][11]

A digital TV broadcast service was proposed in 1986 byNippon Telegraph and Telephone(NTT) and theMinistry of Posts and Telecommunication(MPT) in Japan, where there were plans to develop an "Integrated Network System" service. However, it was not possible to practically implement such a digital TV service until the adoption ofmotion-compensated DCT video compression formatssuch asMPEGmade it possible in the early 1990s.[8]

In the mid-1980s, as Japanese consumer electronics firms forged ahead with the development ofHDTVtechnology, and as theMUSEanalog format was proposed by Japan's public broadcasterNHKas a worldwide standard. Japanese advancements were seen as pacesetters that threatened to eclipse US electronics companies. Until June 1990, the Japanese MUSE standard—based on an analog system—was the front-runner among the more than 23 different technical concepts under consideration.

Between 1988 and 1991, several European organizations were working onDCT-based digitalvideo coding standardsfor both SDTV and HDTV. The EU 256 project by the CMTT andETSI,along with research by Italian broadcasterRAI,developed a DCTvideo codecthat broadcast SDTV at 34Mbit/s and near-studio-quality HDTV at about 70–140Mbit/s. RAI demonstrated this with a1990 FIFA World Cupbroadcast in March 1990.[9][12]An American company,General Instrument,also demonstrated the feasibility of a digital television signal in 1990. This led to the FCC being persuaded to delay its decision on an advanced television (ATV) standard until a digitally based standard could be developed.

When it became evident that a digital standard might be achieved in March 1990, the FCC took several important actions. First, the Commission declared that the new TV standard must be more than an enhancedanalog signal,but be able to provide a genuine HDTV signal with at least twice the resolution of existing television images. Then, to ensure that viewers who did not wish to buy a new digital television set could continue to receive conventional television broadcasts, it dictated that the new ATV standard must be capable of beingsimulcaston different channels. The new ATV standard also allowed the new DTV signal to be based on entirely new design principles. Although incompatible with the existing NTSC standard, the new DTV standard would be able to incorporate many improvements.[7]

A universal standard for scanning formats, aspect ratios, or lines of resolution was not produced by the FCC's final standard. This outcome resulted from a dispute between the consumerelectronics industry(joined by some broadcasters) and the computer industry (joined by the film industry and some public interest groups) over which of the two scanning processes—interlacedorprogressive—is superior. Interlaced scanning, which is used in televisions worldwide, scans even-numbered lines first, then odd-numbered ones. Progressive scanning, which is the format used in computers, scans lines in sequences, from top to bottom. The computer industry argued that progressive scanning is superior because it does notflickerin the manner of interlaced scanning. It also argued that progressive scanning enables easier connections with the Internet and is more cheaply converted to interlaced formats than vice versa. The film industry also supported progressive scanning because it offers a more efficient means of converting filmed programming into digital formats. For their part, the consumer electronics industry and broadcasters argued that interlaced scanning was the only technology that could transmit the highest quality pictures then (and currently) feasible, i.e., 1,080 lines per picture and 1,920 pixels per line. Broadcasters also favored interlaced scanning because their vast archive of interlaced programming is not readily compatible with a progressive format.[7]

Inaugural launches

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DirecTVin the US launched the first commercial digitalsatelliteplatform in May 1994, using theDigital Satellite System(DSS) standard.[13][14]Digital cablebroadcasts were tested and launched in the US in 1996 byTCIandTime Warner.[15][16]The firstdigital terrestrialplatform was launched in November 1998 asONdigitalin the UK, using theDVB-Tstandard.[17]

Technical information

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Formats and bandwidth

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Comparison of image quality betweenISDB-T(1080i broadcast, top) andNTSC(480i transmission, bottom)

Digital television supports many different picture formats defined by thebroadcast television systemswhich are a combination of size andaspect ratio(width to height ratio).

Withdigital terrestrial television(DTT) broadcasting, the range of formats can be broadly divided into two categories:high-definition television(HDTV) for the transmission ofhigh-definition videoandstandard-definition television(SDTV). These terms by themselves are not very precise and many subtle intermediate cases exist.

One of several different HDTV formats that can be transmitted over DTV is:1280 × 720pixelsinprogressive scanmode (abbreviated720p) or1920 × 1080pixels in interlaced video mode (1080i). Each of these uses a16:9aspect ratio. HDTV cannot be transmitted over analogtelevision channelsbecause ofchannel capacityissues.

SDTV, by comparison, may use one of several different formats taking the form of various aspect ratios depending on the technology used in the country of broadcast.NTSCcan deliver a640 × 480resolution in 4:3 and854 × 480in16:9,whilePALcan give768 × 576in4:3and1024 × 576in16:9.However, broadcasters may choose to reduce these resolutions to reducebit rate(e.g., many DVB-T channels in the UK use a horizontal resolution of 544 or 704 pixels per line).[18]

Eachcommercial broadcastingterrestrial televisionDTV channel in North America is allocated enough bandwidth to broadcast up to 19megabitsper second. However, the broadcaster does not need to use this entire bandwidth for just one broadcast channel. Instead, the broadcast can useProgram and System Information Protocoland subdivide across several videosubchannels(a.k.a. feeds) of varying quality and compression rates, including non-videodatacastingservices.

A broadcaster may opt to use a standard-definition (SDTV) digital signal instead of anHDTVsignal, because current convention allows the bandwidth of a DTV channel (or "multiplex") to be subdivided into multipledigital subchannels,(similar to what most FMradio stationsoffer withHD Radio), providing multiple feeds of entirely differenttelevision programmingon the same channel. This ability to provide either a single HDTV feed or multiple lower-resolution feeds is often referred to as distributing one's bit budget or multicasting. This can sometimes be arranged automatically, using astatistical multiplexer.With some implementations, image resolution may be less directly limited by bandwidth; for example inDVB-T,broadcasters can choose from several different modulation schemes, giving them the option to reduce the transmissionbit rateand make reception easier for more distant or mobile viewers.

Reception

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There are several different ways to receive digital television. One of the oldest means of receiving DTV (and TV in general) is from terrestrial transmitters using anantenna(known as anaerialin some countries). This delivery method is known asdigital terrestrial television(DTT). With DTT, viewers are limited to channels that have a terrestrial transmitter in range of their antenna.

Other delivery methods includedigital cableanddigital satellite.In some countries where transmissions of TV signals are normally achieved bymicrowaves,digitalmultichannel multipoint distribution serviceis used. Other standards, such asdigital multimedia broadcasting(DMB) anddigital video broadcasting - handheld(DVB-H), have been devised to allow handheld devices such asmobile phonesto receive TV signals. Another way isInternet Protocol television(IPTV), which is the delivery of TV over a computer network. Finally, an alternative way is to receive digital TV signals via the open Internet (Internet television), whether from a central streaming service or a P2P (peer-to-peer) system.

Some signals are protected byencryptionand backed up with the force of law under theWIPO Copyright Treatyand national legislation implementing it, such as the USDigital Millennium Copyright Act.Access to encrypted channels can be controlled by a removable card, for example via theCommon InterfaceorCableCard.

Protection parameters

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Digital television signals must not interfere with each other and they must also coexist with analog television until it is phased out. The following table gives allowable signal-to-noise and signal-to-interference ratios for various interference scenarios. This table is a crucial regulatory tool for controlling the placement and power levels of stations. Digital TV is more tolerant of interference than analog TV.[19]

System Parameters
(protection ratios) 		Canada US EBU
ITU-mode M3 		Japan & Brazil[A]
C/N for AWGN Channel +19.5 dB
(16.5 dB[B]) 		+15.19 dB +19.3 dB +19.2 dB
Co-Channel DTV into Analog TV +33.8 dB +34.44 dB +34 ≈37 dB +38 dB
Co-Channel Analog TV into DTV +7.2 dB +1.81 dB +4 dB +4 dB
Co-Channel DTV into DTV +19.5 dB
(16.5 dB[B]) 		+15.27 dB +19 dB +19 dB
Lower Adjacent Channel DTV into Analog TV −16 dB −17.43 dB −5 ~ −11 dB[C] −6 dB
Upper Adjacent Channel DTV into Analog TV −12 dB −11.95 dB −1 ~ −10[C] −5 dB
Lower Adjacent Channel Analog TV into DTV −48 dB −47.33 dB −34 ~ −37 dB[C] −35 dB
Upper Adjacent Channel Analog TV into DTV −49 dB −48.71 dB −38 ~ −36 dB[C] −37 dB
Lower Adjacent Channel DTV into DTV −27 dB −28 dB −30 dB −28 dB
Upper Adjacent Channel DTV into DTV −27 dB −26 dB −30 dB −29 dB
  1. ^ISDB-T (6 MHz, 64QAM, R=2/3), Analog TV (M/NTSC).
  2. ^abThe Canadian parameter, C/(N+I) of noise plus co-channel DTV interface should be 16.5 dB.
  3. ^abcdDepending on analog TV systems used.

Interaction

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People can interact with a DTV system in various ways. One can, for example, browse theelectronic program guide.Modern DTV systems sometimes use a return path providing feedback from the end user to the broadcaster. This is possible over cable TV or through an Internet connection but is not possible with a standard antenna alone.

Some of these systems supportvideo on demandusing acommunication channellocalized to a neighborhood rather than a city (terrestrial) or an even larger area (satellite).

1seg

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1seg(1-segment) is a special form ofISDB.Each channel is further divided into 13 segments. Twelve are allocated forHDTVand the other for narrow-band receivers such as mobile televisions andcell phones.

Comparison to analog

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DTV has several advantages overanalog television,the most significant being that digital channels take up less bandwidth and the bandwidth allocations are flexible depending on the level of compression and resolution of the transmitted image. This means that digital broadcasters can provide more digital channels in the same space, providehigh-definition televisionservice, or provide other non-television services such as multimedia or interactivity. DTV also permits special services such as multiplexing (more than one program on the same channel), electronic program guides and additional languages (spoken or subtitled). The sale of non-television services may provide an additional revenue source to broadcasters.

Digital and analog signals react to interference differently. For example, common problems with analog television includeghostingof images, noise from weak signals and other problems that degrade the quality of the image and sound, although the program material may still be watchable. With digital television, because of thecliff effect,reception of the digital signal must be very nearly complete; otherwise, neither audio nor video will be usable.

Analog TV began with monophonic sound and later developedmultichannel television soundwith two independent audio signal channels. DTV allows up to 5 audio signal channels plus asubwooferbass channel, producing broadcasts similar in quality to movie theaters and DVDs.[20]

Digital TV signals require less transmission power than analog TV signals to be broadcast and received satisfactorily.[21]

Compression artifacts, picture quality monitoring and allocated bandwidth

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DTV images have some picture defects that are not present on analog television or motion picture cinema, because of present-day limitations of bit rate and compression algorithms such asMPEG-2.This defect is sometimes referred to asmosquito noise.[22]

Because of the way the human visual system works, defects in an image that are localized to particular features of the image or that come and go are more perceptible than defects that are uniform and constant. However, the DTV system is designed to take advantage of other limitations of the human visual system to help mask these flaws, e.g., by allowing morecompression artifactsduring fast motion where the eye cannot track and resolve them as easily and, conversely, minimizing artifacts in still backgrounds that, because time allows, may be closely examined in a scene.

Broadcast, cable, satellite and Internet DTV operators control the picture quality of television signal encoders using sophisticated, neuroscience-based algorithms, such as thestructural similarity index measure(SSIM) video quality measurement tool. Another tool calledvisual information fidelity(VIF), is used in theNetflixVMAFvideo quality monitoring system.

Quantising effects can create contours—rather than smooth gradations—on areas with small graduations in amplitude. Typically, a veryflatscene, such as a cloudless sky, will exhibit visiblestepsacross its expanse, often appearing as concentric circles or ellipses. This is known ascolor banding.Similar effects can be seen in very dark scenes, where true black backgrounds are overlaid by dark gray areas. These transitions may be smooth, or may show ascatteringeffect as the digital processing dithers and is unable to consistently allocate a value of either absolute black or the next step up the greyscale.

Effects of poor reception

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Changes in signal reception from factors such as degrading antenna connections or changing weather conditions may gradually reduce the quality of analog TV. The nature of digital TV results in a perfectly decodable video initially, until the receiving equipment starts picking up interference that overpowers the desired signal or if the signal is too weak to decode. Some equipment will show a garbled picture with significant damage, while other devices may go directly from perfectly decodable video to no video at all or lock up.[23]This phenomenon is known as the digital cliff effect.[24]

Block errorsmay occur when transmission is done with compressed images. A block error in a single frame often results in black boxes in several subsequent frames, making viewing difficult.

For remote locations, distant channels that, as analog signals, were previously usable in a snowy and degraded state may, as digital signals, be perfectly decodable or may become completely unavailable. The use of higher frequencies add to these problems, especially in cases where a clear line-of-sight from the receiving antenna to the transmitter is not available, because usually higher frequency signals can't pass through obstacles as easily.

Effect on old analog technology

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Television sets with only analog tuners cannot decode digital transmissions. When analog broadcasting over the air ceases, users of sets with analog-only tuners may use other sources of programming (e.g., cable, recorded media) or may purchaseset-top converter boxesto tune in the digital signals. In the United States, agovernment-sponsored couponwas available to offset the cost of an external converter box.

Thedigital television transitionbegan around the late 1990s and has been completed on a country-by-country basis in most parts of the world.

Disappearance of TV-audio receivers

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Prior to the conversion to digital TV, analog television broadcast audio for TV channels on a separate FMcarrier signalfrom the video signal. This FM audio signal could be heard using standard radios equipped with the appropriate tuning circuits.

However, after thedigital television transition,no portable radio manufacturer has yet developed an alternative method for portable radios to play just the audio signal of digital TV channels;DTV radiois not the same thing.

Environmental issues

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The adoption of a broadcast standard incompatible with existing analog receivers has created the problem of large numbers of analog receivers being discarded. One superintendent of public works was quoted in 2009 saying; "some of the studies I’ve read in the trade magazines say up to a quarter of American households could be throwing a TV out in the next two years following the regulation change."[25]In Michigan in 2009, one recycler estimated that as many as one household in four would dispose of or recycle a TV set in the following year.[26]The digital television transition, migration tohigh-definition televisionreceivers and the replacement of CRTs with flat screens are all factors in the increasing number of discarded analog CRT-based television receivers. In 2009, an estimated 99 million analog TV receivers were sitting unused in homes in the US alone and, while some obsolete receivers are being retrofitted with converters, many more are simply dumped in landfills where they represent a source of toxic metals such asleadas well as lesser amounts of materials such asbarium,cadmiumandchromium.[27][28]

See also

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References

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  1. ^Kruger, Lennard G. (2002).Digital Television: An Overview.New York: Nova Publishers.ISBN1-59033-502-3.
  2. ^"HDTV Set Top Boxes and Digital TV Broadcast Information".Archived fromthe originalon 22 May 2016.Retrieved28 June2014.
  3. ^Ong, C. Y., Song, J., Pan, C., & Li, Y.(2010, May). Technology and Standards of Digital Television Terrestrial Multimedia Broadcasting [Topics in Wireless Communications], IEEE Communications Magazine, 48(5),119–127
  4. ^"Korea's Terrestrial DMB: Germany to begin broadcast this May".ZDNet Korea. 2006-04-06.Retrieved2010-06-17.
  5. ^"picturephoning.com: DMB".Textually.org. Archived fromthe originalon 2010-08-09.Retrieved2010-06-17.
  6. ^"South Korea: Social Media 답변 내용: 악어새 – 리포트월드".Reportworld.co.kr. Archived fromthe originalon 2009-08-17.Retrieved2010-06-17.
  7. ^abc"The Origins and Future Prospects of Digital Television".Benton Foundation.2008-12-23.
  8. ^abcLea, William (1994).Video on demand: Research Paper 94/68.House of Commons Library.Retrieved20 September2019.
  9. ^abcBarbero, M.; Hofmann, H.; Wells, N. D. (14 November 1991)."DCT source coding and current implementations for HDTV".EBU Technical Review(251).European Broadcasting Union:22–33.Retrieved4 November2019.
  10. ^Meigs, James B. (June 1986)."Home Video: Get set for digital".Popular Mechanics.Vol. 163, no. 6.Hearst Magazines.p. 52.ISSN0032-4558.
  11. ^Bateman, Selby (April 1986)."New Technologies: The Converging Digital Universe".Compute!.No. 71. pp. 21-29 (26-8).
  12. ^Barbero, M.; Stroppiana, M. (October 1992)."Data compression for HDTV transmission and distribution".IEE Colloquium on Applications of Video Compression in Broadcasting:10/1–10/5.
  13. ^"History of U.S. Satellite Broadcasting Company, Inc. – FundingUniverse".www.fundinguniverse.com.Retrieved9 August2018.
  14. ^"Business Insider: Digital satellite TV has Indy roots".Retrieved9 August2018.
  15. ^"NextLevel signs cable deal - Dec. 17, 1997".money.cnn.com.Retrieved9 August2018.
  16. ^"TCI faces big challenges - Aug. 15, 1996".money.cnn.com.Retrieved9 August2018.
  17. ^"CANAL+ TECHNOLOGIES and the world's first digital terrestrial television service in the United Kingdom".Retrieved9 August2018.
  18. ^Latest snapshots - Freeview/DTT bitratesArchived2007-11-22 at theWayback Machine(Mendip transmitter, UK)
  19. ^"Frequently Asked Questions -- What Is Digital TV?".ABC News.Retrieved2020-09-30.
  20. ^"Digital TV: A Cringley Crash Course — Digital Vs. Analog".Pbs.org.Retrieved2014-01-13.
  21. ^"Report ITU-R BT.2140-3 (05/2011)"(PDF).Archived(PDF)from the original on 10 June 2020.
  22. ^Le Dinh, Phuc-Tue; Patry, Jacques (February 24, 2006)."Video compression artifacts and MPEG noise reduction".Video Imaging DesignLine.Archived fromthe originalon March 14, 2006.RetrievedApril 30,2010.
  23. ^"Digital TV Info".Antenna Direct.2013-09-25.Retrieved2022-07-22.
  24. ^"Steering clear of the digital cliff".Connected Magazine.2010-03-01.Retrieved2024-01-05.
  25. ^North Tonawanda: council discusses future TV disposalArchived2009-01-31 at theWayback Machine,Neale Gulley, Tonawanda News, January 27, 2009
  26. ^Trashing the tube: Digital conversion may spark glut of toxic waste,Jennifer Chambers,Detroit News,January 23, 2009
  27. ^Old Toxic TVs Cause Problems,USA TODAY, January 27, 2009
  28. ^Unloading that old TV not quite so simple,Lee Bergquist,Milwaukee Journal-Sentinel,January 23, 2009

Further reading

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