Broadcast television systems
From Wikipedia, the free encyclopedia
This article is about television technology. For the Canadian term used for a type of minor television network, see
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 but one analogue television system began life in monochrome. Each country, faced with local political, technical, and economic issues, adopted a color system which was effectively grafted onto 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,
PAL, or
SECAM.
[edit] Frames
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 analogue television systems are
interlaced; 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. It is related to the
frequency at which the
electric power grid operates, to avoid flicker resulting from the
beat between the television screen deflection system and nearby mains generated magnetic fields. All digital, or "fixed pixel", displays have
progressive scanning and must
deinterlace an interlaced source. Use of inexpensive deinterlacing hardware is a typical difference between lower- vs. higher-priced flat panel displays (
PDP,
LCD, etc.).
All
movies and other filmed material shot at 24 frames per second must be transferred to video
frame rates in order to prevent severe motion jitter effects. Typically, for 25 frame/s formats (countries with 50 Hz mains supply), the content is
sped up, while a techniques known as "
3:2 pulldown" is used for 30 frame/s formats (countries with 60 Hz mains supply) to match the film frames to the video frames without speeding up the play back. (See
Telecine.)
[edit] Viewing technology
Analog television signal standards are designed to be displayed on a
cathode ray tube (CRT), and so the physics of these devices necessarily controls the format of the video signal. 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
frequency 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.
[edit] Hidden signalling
Broadcasters later developed mechanisms to transmit digital information on the phantom lines, used mostly for
teletext and
closed captioning:
[edit] Overscan
Television images are unique in that they must incorporate regions of the picture with reasonable-quality content, that will never be seen by some viewers.
For more information, see
overscan in television.
[edit] Interlacing
In a purely analogue system, frame order is merely a matter of convention. For digitally recorded material it becomes necessary to rearrange the subframe order when conversion takes place from one standard to another.
[edit] Image polarity
Another parameter of analogue television systems, minor by comparison, is the choice of whether vision modulation is positive or negative. Some of the earliest electronic television systems such as the British 405 line (system A) used positive modulation. It was also used in the two Belgian systems (system C, 625 lines and System F, 819 lines) and the two French systems (system E, 819 lines and system L, 625 lines). In positive modulation systems, the maximum luminance value is represented by the maximum carrier power; in negative
modulation, the maximum luminance value is represented by zero carrier power. Most newer analogue video systems were defined to use negative modulation.
Impulsive noise, especially from older automotive ignition systems, caused white spots to appear on the screens of television receivers using positive modulation but they could use simple synchronisation circuits. Impulsive noise in negative modulation systems appears as dark spots that are less visible, but picture synchronisation was seriously degraded when using simple synchronisation. The synchronisation problem was overcome with the invention of phase-locked synchronisation circuits. When these first appeared in Britain in the early 1950s one name used to describe them was "flywheel synchronisation".
Older televisions for positive modulation systems were sometimes equipped with a peak video signal inverter that would turn the white interference spots dark. This was usually user-adjustable with a control on the rear of the television labelled "White Spot Limiter" in Britain or "Antiparasite" in France. If adjusted incorrectly it would turn bright white picture content dark. Most of the positive modulation television systems ceased operation by the mid 1980s. The French System L continued on up to the transition to digital broadcasting. Positive modulation was one of several unique technical features that originally protected the French electronics and broadcasting industry from foreign competition and rendered French TV sets incapable of receiving broadcasts from neighboring countries.
Another advantage of negative modulation is, that since the synchronising pulses represent maximum carrier power, it is relatively easy to arrange the receiver
Automatic Gain Control to only operate during sync pulses and thus get a constant amplitude video signal to drive the rest of the TV set. This was not possible for many years with positive modulation as the peak carrier power varied depending on picture content. Modern digital processing circuits have achieved a similar effect but using the front porch of the video signal.
[edit] Modulation
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 one sideband is partially removed. This reduces the bandwidth of the transmitted signal, enabling narrower channels to be used.
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 (known under a variety of names, notably
Zweikanalton, A2 Stereo, West German Stereo, German Stereo or IGR Stereo), in which case each audio channel is separately modulated in FM and added to the broadcast signal; and
BTSC (also known as
MTS), which multiplexes additional audio channels into the FM audio carrier. All three systems are compatible with monaural FM audio, but only
NICAM may be used with the French AM audio systems.its uses FM instead of AM so as to reduce noise and mixing with other sounds
[edit] Evolution
For historical reasons, some 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 (although was never officially broadcast with color encoding). 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.
The French
System E was a post-war effort to advance
France's standing in television technology. Its 819 scan lines were almost high definition even by today's standards. Like the British system A, it was VHF only and remained black & white until its shutdown in 1984 in France and 1985 in Monaco. It was tested with SECAM in the early stages, but later the decision was made to adopt color in 625 lines. Thus France adopted system L on UHF only and abandoned system E.
In some urban areas of
Germany, notably in and around
Berlin and some other major cities, all analogue TV broadcasting has been shut down in 2003–2005 in favor of reallocating the frequencies to digital broadcasting in the
DVB-T standard. See
http://www.ueberallfernsehen.de/ for a map of coverage areas and near-future switchovers. Analogue signals are still on air in the non-colored areas of the map. The rest of the country is scheduled to follow suit by 2010. Many other countries are planning a shutdown of analogue broadcasting, and as of 2007 a few smaller countries have already done so. (See
Digital television transition-article for further details.)
[edit] List of analogue television systems
[edit] Pre–World War II systems
A number of experimental and broadcast pre WW2 systems were tested. The first ones were mechanically based and of very low resolution, sometimes with no sound. Later TV systems were electronic.
- The UK 405 line system was the first to have an allocated ITU System Letter Designation.
[edit] ITU identification scheme
On an international conference in
Stockholm in 1961, the
International Telecommunications Union has defined an identification scheme for broadcast television systems. Each monochrome system is assigned a letter designation (A-M); in combination with a color system (NTSC, PAL, SECAM), this completely specifies all of the monaural analogue television systems in the world (for example, PAL-B, NTSC-M, etc).
The following table gives the principal characteristics of each system. Defunct TV systems are shown in grey text, previous ones never designated by ITU are not yet shown. Except for
lines and
frame rates, other units are
megahertz (MHz).
