Digital television (DTV) is the transmission oftelevision signals usingdigital encoding, in contrast to the earlieranalog television technology which usedanalog signals. In the 2000s[1] it was represented as the first significant evolution in television technology sincecolor television in the 1950s.[2] Modern digital television is transmitted inhigh-definition television (HDTV) with greater resolution than analog TV. It typically uses awidescreen aspect ratio (commonly16:9) in contrast to the narrower format (4:3) of analog TV. It makes more economical use of scarceradio spectrum space; it can transmit up to seven channels in the samebandwidth as a single analog channel,[3] and provides many new features that analog television cannot. Atransition from analog to digital broadcasting began around 2000. Different digital television broadcasting standards have been adopted in different parts of the world; below are the more widely used standards:
Advanced Television System Committee (ATSC) standard uses eight-level vestigial sideband (8VSB) for terrestrial broadcasting. This standard has been adopted by 9 countries: the United States, Canada, Mexico, South Korea, Bahamas, Jamaica, the Dominican Republic, Haiti and Suriname.[citation needed]
Integrated Services Digital Broadcasting (ISDB) is a system designed to provide good reception to fixed receivers and also portable or mobile receivers utilizing OFDM and two-dimensional interleaving. It supports hierarchical transmission of up to three layers and usesMPEG-2 video andAdvanced Audio Coding. This standard has been adopted in Japan and the Philippines.ISDB-T International is an adaptation of this standard usingH.264/MPEG-4 AVC, which has been adopted in most ofSouth America as well asBotswana andAngola.1seg (1-segment) is a special form ofISDB. Each channel is further divided into 13 segments. Twelve are allocated forHDTV and the other for narrow-band receivers such as mobile televisions andcell phones.
Digital Terrestrial Multimedia Broadcast (DTMB) adopts time-domain synchronous (TDS) OFDM technology with a pseudo-random signal frame to serve as the guard interval (GI) of the OFDM block and the training symbol. The DTMB standard has been adopted in China, including Hong Kong and Macau.[4]
In the mid-1980s,Toshiba commercially released one of the firsttelevision sets with digital capabilities, usingintegrated circuit chips such as amicroprocessor to convert analog television broadcast signals todigital video signals, enabling features such as freezing pictures andshowing two channels at once. Following in 1986,Sony andNEC Home Electronics announced their own similar TV sets with digital video capabilities. However, these television sets still relied on analog TV broadcast signals, with true digital TV broadcasts not yet being available at the time.[11][12]
In the mid-1980s, as Japanese consumer electronics firms forged ahead with the development ofHDTV technology, and theMUSE analog format was proposed by Japan's public broadcasterNHK as a worldwide standard. Until June 1990, the Japanese MUSE standard—based on an analog system—was the front-runner, set to eclipse US electronics company solutions, among the more than 23 different technical concepts under consideration.
Simultaneously, between 1988 and 1991, European organizations: CMMT, ETSI, etc. were working onDCT-based digitalvideo coding standards for both SDTV and HDTV. The EU 256 project by the CMTT andETSI, along with research by Italian broadcasterRAI, developed a DCTvideo codec that broadcast SDTV at34 Mbit/s and near-studio-quality HDTV at about70–140 Mbit/s. RAI demonstrated this with a1990 FIFA World Cup broadcast in March 1990.[10][13]
Simultaneously, in March 1990, American companyGeneral Instrument demonstrated the feasibility of a digital television signal; persuading the FCC to delay its decision on an advanced television (ATV) standard until a digitally based standard could be developed; resulting in several actions. First, the FCC declared that the new TV standard must be more than an enhancedanalog signal, capable of providing a genuine HDTV signal with at least twice the resolution of existing television images. Second, 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 beingsimulcast withNTSC on different channels. The new ATV standard also allowed the new DTV signal to be based on entirely new design principles incorporating many improvements over existing analog television.[8]
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—interlaced orprogressive—is superior. Interlaced scanning, which is used by the electronics industry 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 notflicker in 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. 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.[8]
Digital television supports many different picture formats defined by thebroadcast television systems which are a combination of size andaspect ratio (width to height ratio).
One of several different HDTV formats that can be transmitted over DTV is:1280 × 720pixels inprogressive scan mode (abbreviated720p) or1920 × 1080 pixels in interlaced video mode (1080i). Each of these uses a16:9 aspect ratio. Uncompressed HDTV cannot be transmitted over analogtelevision channels because ofchannel capacity issues.
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.NTSC can deliver a640 × 480 resolution in 4:3 and854 × 480 in16:9, whilePAL can give768 × 576 in4:3 and1024 × 576 in16:9. 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).[19]
Eachcommercial broadcastingterrestrial television DTV channel in North America is allocated enough bandwidth to broadcast up to 19megabits per second using 8VSB modulation.[20] However, the broadcaster does not need to use this entire bandwidth for just one broadcast channel. Instead, the broadcast can useProgram and System Information Protocol and subdivide across several videosubchannels (a.k.a. feeds) of varying quality and compression rates, including non-videodatacasting services.
A broadcaster may opt to use a standard-definition (SDTV) digital signal instead of anHDTV signal, because current convention allows the bandwidth of a DTV channel (or "multiplex") to be subdivided into multipledigital subchannels, (similar to what most FMradio stations offer withHD Radio), providing multiple feeds of entirely differenttelevision programming on 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 rate and possibly improve reception for more distant or mobile viewers.
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 anaerial in some countries). This delivery method is known asdigital terrestrial television (DTT). With DTT, viewers are limited to channels that have a terrestrial transmitter within range of their antenna.
Digital television signals should not interfere with each other and many times 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 digital television stations. Digital TV is more tolerant of interference than analog TV.[22]
Viewers can interact and provide data back to broadcasters with DTV systems in various ways shown in the list below. Return path to the broadcaster is possible in post 2020s DTV systems typically via the viewers internet connection. Some DTV systems supportvideo on demand using acommunication channel localized to a neighborhood rather than a city (terrestrial) or an even larger area (satellite)
More efficient bandwidth usage provide more digital channels in the same space and/or providehigh-definition television service
Digital TV signals require less transmission power than analog TV signals to be broadcast and received satisfactorily.[25]
Flexible bandwidth allocations are flexible depending on the level of compression and resolution of the transmitted image.
More sound channels. Analog TV began with monophonic sound and later developedmultichannel television sound with two independent audio signal channels. DTV allows up to 5 audio signal channels plus asubwoofer bass channel, producing broadcasts similar in quality to movie theaters and DVDs.[26]
Can provide for sale of non-television services such as multimedia on demand or interactive purchasing.
Permits special services such as multiplexing (more than one program on the same channel), electronic program guides and additional languages (spoken or subtitled).
Digital and analog signals react to interference differently. For example, common problems with analog television includeghosting of 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.
Compression artifacts, picture quality monitoring and allocated bandwidth
DTV images have some picture defects 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.[27]
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 artifacts during 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 theNetflixVMAF video quality monitoring system.
Quantising effects can create contours—rather than smooth gradations—on areas with small graduations in amplitude. Typically, a veryflat scene, such as a cloudless sky, will exhibit visiblesteps across 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 ascattering effect as the digital processing dithers and is unable to consistently allocate a value of either absolute black or the next step up the greyscale.
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.[28] This phenomenon is known as the digital cliff effect.[29]
Block errors may 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.
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 boxes to tune in the digital signals. In the United States, agovernment-sponsored coupon was available to offset the cost of an external converter box.
Thedigital television transition began around the late 1990s and has been completed on a country-by-country basis in most parts of the world.
Prior to the conversion to digital TV, analog television broadcast audio for TV channels on a separate FMcarrier signal from 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 radio is not the same thing.
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."[30] 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.[31] The digital television transition, migration tohigh-definition television receivers 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 aslead as well as lesser amounts of materials such asbarium,cadmium andchromium.[32][33]
^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
