Digital television (DTV) is the transmission of television signals using digital encoding, in contrast to the earlier analog television technology which used analog signals. In the 2000s it was represented as the first major evolution in television technology since color television in the 1950s. Modern digital television is transmitted in high-definition television (HDTV) formats with greater resolution than analog TV. It typically uses a widescreen aspect ratio (commonly 16:9) in contrast to the narrower format (4:3) of analog TV. It makes more economical use of scarce radio spectrum space; it can transmit numerous digital channels in the same bandwidth as a single analog channel, and provides many new features that analog television cannot. While digital satellite and cable TV deployments began in the 1990s, using standard definition television resolutions (digital SDTV), over-the-air digital TV began transitioning from analog to digital broadcasting in the late 1990s, primarily using high definition television formats (digital HDTV). Different digital television broadcasting standards have been adopted in different parts of the world; below are the more widely used standards: Digital Video Broadcasting (DVB) uses coded orthogonal frequency-division multiplexing (OFDM) modulation and supports hierarchical transmission. This standard has been adopted in Europe, Africa, Asia and Australasia, for a total of approximately 60 countries. 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, the Bahamas, Jamaica, the Dominican Republic, Haiti and Suriname. 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 uses MPEG-2 video and Advanced Audio Coding. This standard has been adopted in Japan and the Philippines. ISDB-T International is an adaptation of this standard using H.264/MPEG-4 AVC, which has been adopted in most of South America as well as Botswana and Angola. 1seg (1-segment) is a special form of ISDB. Each channel is further divided into 13 segments. Twelve are allocated for HDTV and the other for narrow-band receivers such as mobile televisions and cell 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.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 Digital Multimedia Broadcasting (DMB) is a digital radio transmission technology developed and adopted in South Korea as part of the national information technology project for sending multimedia such as TV, radio and datacasting to mobile devices such as mobile phones, laptops and GPS navigation systems.
With the advent of the broadband Internet in the late 1990s, facilitated by high-speed cable modems and Digital Subscriber Line (DSL) modems, it became possible to stream entertainment-quality video to broadband Internet subscribers. By the late 2000s, broadband Internet speeds were sufficiently high to enable HD-quality video to consumers, allowing Internet Protocol (IP) video to become more competitive with traditional digital TV services. Many Internet content companies emerged, providing expanding libraries of on-demand content. Typically, these Internet TV streaming apps are received directly by "smart" HDTV sets through a program guide and Wi-Fi, or through Wi-Fi-enabled streaming boxes or streaming USB sticks connected to the HDTV set. Many of the same streaming apps are available to other IP-capable devices inside and outside the home, such as smartphones, tablets, and laptops.
History Background Digital television's roots in the 1990s were tied to the availability of high-performance computers containing video coding algorithms that could compress video. Digital television was previously impractical due to high bandwidth requirements of uncompressed video, requiring around for a standard-definition television (SDTV) signal, and over for high-definition television (HDTV).
Development In the mid-1980s, Toshiba commercially released one of the first television sets with digital capabilities, using integrated circuit chips such as a microprocessor to convert analog television broadcast signals to digital video signals, enabling features such as freezing pictures and showing two channels at once. Following in 1986, Sony and NEC 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.
A digital TV broadcast service was proposed in 1986 by Nippon Telegraph and Telephone (NTT) and the Ministry of Posts and Telecommunication (MPT) in Japan, where there were plans to develop an "Integrated Network System" service. However, practical digital TV service implementation was not available until the adoption of motion-compensated DCT video compression formats such as MPEG made it possible in the early 1990s.
In the mid-1980s, as Japanese consumer electronics firms forged ahead with the development of HDTV technology, and the MUSE analog format was proposed by Japan's public broadcaster NHK 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.
In Europe, through the leadership of French President François Mitterrand and German Chancellor Helmut Kohl, government bodies were subsidizing the development of HD-MAC, Europe's analog HDTV response to Japan's MUSE technology.
Simultaneously, between 1988 and 1991, European organizations: CMMT, ETSI, etc. were working on DCT-based digital video coding standards for both SDTV and HDTV. The EU 256 project by the CMTT and ETSI, along with research by Italian broadcaster RAI, developed a DCT video codec that broadcast SDTV at and near-studio-quality HDTV at about . RAI demonstrated this with a 1990 FIFA World Cup broadcast in June 1990.
In 1987, the FCC had begun considering potential new systems for HDTV. After the feasibility of a digital system became clear in 1990, the FCC began to take action. First, the FCC declared that the new TV standard must be more than an enhanced analog 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 being simulcast with NTSC 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. Third, various companies already involved in the FCC's advanced television standard activity began changing their approaches from analog HDTV to digital HDTV.
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 consumer electronics 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 or progressive—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 sequence, from top to bottom. The computer industry argued that progressive scanning is superior because it does not flicker 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.
Inaugural launches DirecTV in the US launched the first commercial digital satellite platform in May 1994, using the Digital Satellite System (DSS) standard. Digital cable broadcasts were tested and launched in the US in 1996 by TCI and Time Warner. The first digital terrestrial platform was launched in November 1998 as ONdigital in the UK, using the DVB-T standard.
Technical information
Formats and bandwidth
Digital television supports many different picture formats defined by the broadcast television systems, which are a combination of size and aspect ratio (width to height ratio).
With digital terrestrial television (DTT) broadcasting, the range of formats can be broadly divided into two categories: high-definition television (HDTV) for the transmission of high-definition video and standard-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: pixels in progressive scan mode (abbreviated 720p) or pixels in interlaced video mode (1080i). Each of these uses a 16:9 aspect ratio. Uncompressed HDTV cannot be transmitted over analog television channels because of channel 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 a resolution in 4:3 and in 16:9, while PAL can give in 4:3 and in 16:9. However, broadcasters may choose to reduce these resolutions to reduce bit rate (e.g., many DVB-T channels in the UK use a horizontal resolution of 544 or 704 pixels per line).Latest snapshots - Freeview/DTT bitrates (Mendip transmitter, UK)
Each commercial broadcasting terrestrial television DTV channel in North America is allocated enough bandwidth to broadcast up to 19 megabits per second using 8VSB modulation. However, the broadcaster does not need to use this entire bandwidth for just one broadcast channel. Instead, the broadcast can use Program and System Information Protocol and subdivide across several video subchannels (a.k.a. feeds) of varying quality and compression rates, including non-video datacasting services.