Science Fair Project Encyclopedia
Broadcast television system
There are several broadcast television systems in use in the world today. An analogue television system includes several components: a set of technical parameters for the broadcast signal, a system for encoding color, and possibly a system for encoding multi-channel audio. In digital television, all of these elements are combined in a single digital transmission system.
Analogue television systems
All analogue television systems began life in monochrome. Each country, faced with local political, technical, and economic issues, adopted a color system which was effectively grafted on to an existing monochrome system, using gaps in the video spectrum (explained below) to allow the color information to fit in the channels allotted. In theory, any color system could be used with any monochrome video system, but in practice some of the original monochrome systems proved impractical to adapt to color and were abandoned when the switch to color broadcasting was made. All countries use one of three color systems: NTSC, SECAM, or PAL.
Ignoring color, all television systems work in essentially the same manner. The monochrome image seen by a camera (now, the luminance component of a color image) is divided into horizontal scan lines, some number of which make up a single image or frame. A monochrome image is theoretically continuous, and thus unlimited in horizontal resolution, but to make television practical a limit had to be placed on the bandwidth of the television signal, which puts an ultimate limit on the horizontal resolution possible. (When color was introduced, this limit of necessity became fixed.) All current television systems are interlaced; that is to say, alternate rows of the frame are transmitted in sequence, followed by the remaining rows in their sequence. Each half of the frame is called a field, and the rate at which fields are transmitted is one of the fundamental parameters of a video system. Usually it is closely related to the frequency at which the electric power grid operates, to avoid the appearance of a flicker resulting from the beat between the television screen and nearby electric lights.
In systems that use a 50 field / 25 frame rate, movies and other filmed material shot at 24 frames per second must be transferred to video at 25 frame/s in order to prevent severe motion judder effects. The resulting increase in speed is usually not noticeable to the eye, but there is also a distinct increase in the pitch of the soundtrack, although nowadays this is sometimes corrected using digital technology.
Since television was originally implemented using cathode-ray tubes, the physics of these devices necessarily intrudes on the format of the video they can be used to display. The image on a CRT is painted by a moving beam of electrons which hits a phosphor coating on the front of the tube. This electron beam is steered by a magnetic field generated by powerful electromagnets close to the source of the electron beam. In order to reorient this magnetic steering mechanism, a certain amount of time is required due to the inductance of the magnets; the greater the change, the greater the time it takes for the electron beam to settle in the new spot. For this reason, it is necessary to shut off the electron beam (corresponding to a video signal of zero luminance) during the time it takes to reorient the beam from the end of one line to the beginning of the next (horizontal retrace) and from the bottom of the screen to the top (vertical retrace or vertical blanking interval). The horizontal retrace is accounted for in the time allotted to each scan line, but the vertical retrace is accounted for as phantom lines which are never displayed but which are included in the number of lines per frame defined for each video system. Since the electron beam must be turned off in any case, the result is gaps in the television signal, which can be used to transmit other information, such as test signals or color identification signals. The temporal gaps translate into a comb-like spectrum for the signal, where the teeth are spaced at line frequency and concentrate most of the energy; the space between the teeth can be used to insert a color subcarrier. Broadcasters later developed mechanisms to transmit digital information on the phantom lines, used mostly for teletext and closed captioning.
Another parameter of analogue television systems, minor by comparison, is the choice of whether vision modulation is positive or negative. In positive modulation, the maximum luminance value is represented by the maximum electrical signal; in negative modulation, the maximum luminance value is represented by a zero electrical signal. Most video systems were defined to use negative modulation to reduce the appearance of noise, on the theory that dark spots in the image would be less noticeable than bright white spots in the image, given a particularly common sort of noise.
Given all of these parameters, the result is a mostly-continuous analogue signal which can be modulated onto a radio-frequency carrier and transmitted through an antenna. All analogue television systems use vestigial sideband modulation, a form of amplitude modulation in which the lower sideband is incompletely suppressed. This provides a small guard band between the actual video carrier and the bottom frequency in the channel, which helps to reduce interference between transmitters on adjoining channels at a receiver which receives strong signals from both. At the time television was developed, the vestigial sideband was easier to accomplish than true single-sideband modulation; with today's technology, there is no reason for it except to be compatible with existing technology.
In analogue television, the sound portion of a broadcast is invariably modulated separately from the video. Most commonly, the audio and video are combined at the transmitter before being presented to the antenna, but in some cases separate aural and visual antennas can be used. In almost all cases, standard wideband frequency modulation is used for the standard monaural audio; the exception is systems used by France, which are AM. Stereo, or more generally multi-channel, audio is encoded using a number of schemes which (except in the French systems) are independent of the video system. The principal systems are NICAM, which uses a digital audio encoding; double-FM, in which case each audio channel is separately modulated in FM and added to the broadcast signal; and BTSC, which multiplexes additional audio channels on the existing FM audio carrier. All three systems are compatible with monaural FM audio, but only NICAM may be used with the French AM audio systems.
For historical reasons, many countries use a different video system on UHF than they do on the VHF bands. In a few countries, most notably the United Kingdom, television broadcasting on VHF has been entirely shut down. (Note that the British system A, unlike all the other systems, suppressed the upper sideband rather than the lower — befitting its status as the oldest operating television system to survive into the color era. System A was tested with all three color systems, and production equipment was designed and ready to be built; system A might have survived, as NTSC-A, had the British government not decided to harmonize with the rest of Europe on a 625-line video standard, implemented in Britain as PAL-I on UHF only.)
In some urban areas of Germany, notably in and around Berlin, all analogue TV broadcasting has been shut down in 2004 in favour of reallocating the frequencies to digital broadcasting. The rest of the country is expected to follow suit in 2010.
The International Telecommunications Union has defined an identification scheme for broadcast television systems. Each monochrome system is assigned a letter designation; in combination with a color system, this completely specifies all of the monaural analogue television systems in the world. The following table gives the principal characteristics of each system. Most values are measured in MHz.
|A||405||25||5||3||−3.5||0.75||Pos.||AM||Old UK VHF system (B/W only)|
|B||625||25||7||5||+5.5||0.75||Neg.||FM||VHF only in most countries. VHF & UHF in Australia (see systems G and H)|
|C||625||25||7||5||+5.5||0.75||Pos.||AM||Old VHF system used only in Belgium|
|D||625||25||8||6||+6.5||0.75||Neg.||FM||VHF only (see system K)|
|E||819||25||14||10||±11.15||2||Pos.||AM||Old French VHF system|
|F||819||25||7||5||+5.5||0.75||Pos.||AM||Old VHF system used only in Belgium and Luxembourg|
|G||625||25||8||5||+5.5||0.75||Neg.||FM||UHF only (see system B)|
|H||625||25||8||5||+5.5||1.25||Neg.||FM||UHF only (see system B)|
|I||625||25||8||5.5||+5.996||1.25||Neg.||FM||UK, Ireland, South Africa & Hong Kong|
|K||625||25||8||6||+6.5||0.75||Neg.||FM||UHF only (see system D)|
|L||625||25||8||6||+6.5||1.25||Pos.||AM||France: audio −6.5 MHz on VHF Band 1 only|
|M||525||29.97||6||4.2||+4.5||0.75||Neg.||FM||Americas, Japan, Philippines, South Korea (all NTSC-M), and Brazil (PAL-M)|
|N||625||25||6||4.2||+4.5||0.75||Neg.||FM||Argentina, Bolivia, Paraguay, Uruguay|
Systems A, C, E, and F are no longer in use.
Digital television systems
The situation with worldwide digital television is much simpler by comparison. All current digital television systems are based on the MPEG-2 multiplexed data stream standard, and use the MPEG-2 video codec. They differ significantly in the details of how the MPEG stream is converted into a broadcast signal, in the video format prior to encoding (or alternately, after decoding), and in the audio format. This has not prevented the creation of an international standard that includes both major systems, even though they are incompatible in almost every respect.
The two principal digital broadcasting systems are ATSC, developed by the Advanced Television Systems Committee and adopted as a standard in the United States and Canada, and DVB-T, the Digital Video Broadcast -- Terrestrial system used in most of the rest of the world. DVB-T was designed for format compatibility with existing direct broadcast satellite services in Europe (which use the DVB-S standard), and there is also a DVB-C version for cable television. While the ATSC standard also includes support for satellite and cable television systems, operators of those systems have chosen other technologies (principally DVB-S for satellite and 64-QAM for cable). Japan uses a third system, closely related to DVB-T, called ISDB-T.
The ATSC system uses a Zenith-developed modulation called 8-VSB; as the name implies, it is a vestigial sideband technique. Essentially, analogue VSB is to regular amplitude modulation as 8-VSB is to eight-way quadrature amplitude modulation. This system was chosen specifically to provide for maximum spectral compatibility between existing analogue TV and new digital stations in the United States' already-crowded television allocations system. After demodulation and error-correction, the 8-VSB modulation supports a digital data stream of about 19.2 Mbit/s, enough for one high-definition video stream or several "standard-definition" services.
DVB-T uses coded orthogonal frequency division multiplexing (COFDM), which uses as many as 256 independent carriers, each transmitting data at a comparatively low rate. This system was designed to provide superior immunity from multipath interference , although at the cost of supporting a lower data rate than 8-VSB. One U.S. broadcaster, Sinclair Communications , petitioned the Federal Communications Commission to permit the use of COFDM instead of 8-VSB, on the theory that this would improve prospects for digital TV reception by households without outside antennas (a majority in the U.S.), but this request was denied. (However, one U.S. digital station, WNYE-DT in New York, was temporarily converted to COFDM modulation on an emergency basis for datacasting information to emergency services personnel in lower Manhattan in the aftermath of the September 11th terrorist attacks.)
- "Table of Voltage, Frequency, TV Broadcasting system, Radio Broadcasting, by Country"
- "World Analogue Television Standards and Waveforms"
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