Digital television


Digital television 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 significant evolution in television technology since color television in the 1950s. Modern digital television is transmitted in high-definition television with greater resolution than analog TV. It typically uses a widescreen aspect ratio in contrast to the narrower format of analog TV. It makes more economical use of scarce radio spectrum space; it can transmit up to seven channels in the same bandwidth as a single analog channel, and provides many new features that analog television cannot. A transition 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:

Background

Digital television's roots in the 1990s are tied to the availability of inexpensive, high-performance computers that can compress video. Digital television was previously impractical due to high bandwidth requirements of uncompressed video, requiring around for a standard-definition television signal, and over for high-definition television.

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 and the Ministry of Posts and Telecommunication 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.
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 March 1990.
Simultaneously, in March 1990, American company General Instrument demonstrated the feasibility of a digital television signal, persuading the FCC to delay its decision on an advanced television 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 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.
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 and the computer industry 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 sequences, 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 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

in the US launched the first commercial digital satellite platform in May 1994, using the Digital Satellite System 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.
With digital terrestrial television broadcasting, the range of formats can be broadly divided into two categories: high-definition television for the transmission of high-definition video and standard-definition television. 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 or pixels in interlaced video mode. 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.
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 of varying quality and compression rates, including non-video datacasting services.
A broadcaster may opt to use a standard-definition digital signal instead of an HDTV signal, because current convention allows the bandwidth of a DTV channel to be subdivided into multiple digital subchannels,, providing multiple feeds of entirely different television 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 a statistical multiplexer. With some implementations, image resolution may be less directly limited by bandwidth; for example in DVB-T, broadcasters can choose from several different modulation schemes, giving them the option to reduce the transmission bit rate and possibly improve reception for more distant or mobile viewers.