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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.

1 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 fps 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 electromagnetAn electromagnet is a type of magnet in which the magnetic field is induced by a flow of electric current. The magnetic field disappears when the current ceases. right-hand rule. The simplest type of electromagnet is a coil of wire. A coil forming the shas 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 inductanceInductance is a physical characteristic of an inductor, which is an electrical device that produces at any time a voltage proportional to the instantaneous rate of change in current flowing through it. The symbol L is used for inductance in honour of the 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 spectrumThe noun spectrum (plural: spectra has a variety of meanings. Ghosts Originally a spectrum was what is now called a spectre, i. a phantom or apparition. Spectral evidence is testimony about what was done by spectres of persons not present physically, or h 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 teletextTeletext is an information retrieval service provided by television broadcast companies. Teletext pages can be viewed on television sets with suitable decoders. They offer a range of text-based information, usually including national, international and sp and closed captioningClosed captioning allows deaf, hard of hearing / hearing-impaired, and other people to read, through captions, a transcript of the audio portion of a video that they cannot hear. As the video plays, captions showing what is being said and by who, as well.

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 modulationAmplitude modulation (AM is a method used to modulate a signal, typically using radio. In the case of an analog signal to be sent, the amplitude of the radio wave is modulated to be directly proportional to the value of the analog signal at the time. 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 modulationSingle-sideband modulation SSB is a refinement of the technique of amplitude modulation designed to be more efficient in its use of electrical power and bandwidth. It is closely related to vestigial sideband modulation (VSB) (see below). Amplitude modulat; 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 until 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.

Systems A, C, E, and F are no longer in use.

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