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Broadcast television system
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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.
but one analogue television system began life in monochrome.
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Encyclopedia
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.
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.)
Viewing technology
Since television was originally implemented using cathode ray tubes (CRT), 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 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.
Hidden signalling
Broadcasters later developed mechanisms to transmit digital information on the phantom lines, used mostly for teletext and closed captioning:
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.
Interlacing
In a purely analogue system, frame order is merely a matter of convention. For digitally recorded material it becomes necessary to rearrange the sub frame order when conversion takes place from one standard to another.
Image polarity
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 carrier power; in negative modulation, the maximum luminance value is represented by a zero carrier power. Most current video systems were defined to use negative modulation since this system has a far greater immunity to noise. The original 405 line system (System A) used positive modulation and suffered consideably from even modest amounts of interference as it was interpreted as synchronising information. The French (older) system C and the current system L transmissions continue to be so plagued. Positive modulation was chosen for no reason other than the render French TV sets incapable of receiving 'unsuitable' 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.
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.
Audio
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 on the video carrier. All three systems are compatible with monaural FM audio, but only NICAM may be used with the French AM audio systems.
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 the 1980s. 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.)
List of analogue television systems
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. Latter TV systems were electronic.
- The UK 405 line system was the first to have an allocated ITU System Letter Designation.
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).
World television systems System | Lines | Frame rate | Channel bandwidth | Video bandwidth | Audio offset | Vestigial sideband | Video mod. | Sound mod. | Usual color | A | 405 | 25 | 5 | 3 | -3.5 | 0.75 | pos. | AM | none | B | 625 | 25 | 7 | 5 | +5.5 | 0.75 | neg. | FM | PAL | C | 625 | 25 | 7 | 5 | +5.5 | 0.75 | pos. | AM | none | D | 625 | 25 | 8 | 6 | +6.5 | 0.75 | neg. | FM | SECAM | E | 819 | 25 | 14 | 10 | ±11.15 | 2.00 | pos. | AM | none | F | 819 | 25 | 7 | 5 | +5.5 | 0.75 | pos. | AM | none | G | 625 | 25 | 8 | 5 | +5.5 | 0.75 | neg. | FM | PAL | H | 625 | 25 | 8 | 5 | +5.5 | 1.25 | neg. | FM | PAL | I | 625 | 25 | 8 | 5.5 | +5.9996 | 1.25 | neg. | FM | PAL | J | 525 | 30 | 6 | 4.2 | +4.5 | 0.75 | neg. | FM | NTSC | K | 625 | 25 | 8 | 6 | +6.5 | 0.75 | neg. | FM | SECAM | K' | 625 | 25 | 8 | 6 | +6.5 | 1.25 | neg. | FM | SECAM | L | 625 | 25 | 8 | 6 | +6.5 | 1.25 | pos. | AM | SECAM | M | 525 | 30 | 6 | 4.2 | +4.5 | 0.75 | neg. | FM | NTSC** | N | 625 | 25 | 6 | 4.2 | +4.5 | 0.75 | neg. | FM | PAL |
Why that number of lines?
Because an interlaced system requires accurate positioning of scanning lines it is important to make sure that the horizontal and vertical timebase are in a precise ratio. This is accomplished by passing the one through a series of electronic divider circuits to produce the other. Each division is by a prime number.
Therefore there has to be a straightforward mathematical relationship between the line and field frequencies, the latter being derived by dividing down from the former. Technology constraints of the 1930s meant that this division process could only be done using small integers, preferably no greater than 7, for good stability. The number of lines was odd because of 2:1 interlace. The 405 line system used a vertical frequency of 50 Hz (Standard AC mains supply frequency in Britain) and a horizontal one of 10,125 Hz (50 × 405 ÷ 2)
- 2 × 3 × 3 × 5 Gives 90 (Non Interlaced)
- 2 × 2 × 2 × 2 × 2 × 3 Gives 96 (Non Interlaced)
- 2 × 2 × 3 × 3 × 5 Gives 180 (Non Interlaced)
- 2 × 2 × 2 × 2 × 3 × 5 Gives 240 (Used for the experimental Baird transmissions in Britain [See Note 1])
- 3 × 3 × 3 × 3 × 3 Gives 243
- 7 × 7 × 7 Gives 343 (Early North American system also used in Poland before WW2)
- 3 × 5 × 5 × 5 Gives 375
- 3 × 3 × 3 × 3 × 5 Gives 405 (Used in Britain, Ireland and Hong Kong before 1985)
- 2 × 2 × 2 × 5 × 11 Gives 440 (Non Interlaced)
- 3 × 3 × 7 × 7 Gives 441 (Used by RCA in North America before the 525 NTSC standard was adopted and widely used before WW2 in Continental Europe with different frame rates)
- 2 × 3 × 3 × 5 × 5 Gives 450 (Non Interlaced)
- 5 × 7 × 13 Gives 455 (Used in France before WW2)
- 3 × 5 × 5 × 7 Gives 525 (A compromise between the RCA and Philco systems. Still used today in most of the Americas and parts of Asia)
- 3 × 3 × 3 × 3 × 7 produces 567 (used for a while after WW2 in the Netherlands)
- 5 × 11 × 11 Gives 605 (Proposed by Philco in North America before the 525 standard was adopted)
- 5 × 5 × 5 × 5 Gives 625 (Still used today in most parts of the world)
- 2 × 3 × 5 × 5 × 5 Gives 750 @ 50 fields (Used for 720p/50 [See Note 2])
- 2 x 2 x 2 x 2 x 3 x 3 x 5 Gives 750 @ 60 fields (Used for 720p/60 [See Note 2])
- 3 × 3 × 7 × 13 Gives 819 (Used in France in the 1950’s)
- 3 × 7 × 7 × 7 Gives 1029 (Proposed but never adopted around 1948 in France)
- 3 × 3 × 5 × 5 x 5 Gives 1125 @ 25 fields (Used for 1080i/25 and 1080p/25 [See Note 2])
- 2 × 3 × 3 × 3 × 5 × 5 Gives 1125 @ 30 fields (Used for 1080i/30 and 1080p/30 [See Note 2])
Notes
- The division of the 240 line system is academic as the scan ratio was determined entirely by the construction of the mechanical scanning system used with the cameras used with this transmission system.
- The division ratio though relevant to CRT based systems is largely academic today because modern LCD and plasma displays are not constrained to having the scanning in precise ratios.
Converting from one TV system to another
Converting between different numbers of lines and different frequencies of fields/frames in video pictures is not an easy task.
Perhaps the most technically challenging conversion to make is from any of the 625-line, 25-frame/s systems to system M, which has 525 lines at 29.97 frames per second. Historically this required a frame store to hold those parts of the picture not actually being output (since the scanning of any point was not time coincident). In more recent times, conversion of standards is relatively easy task for a computer.
Aside from the line count being different, it's easy to see that generating 60 fields every second from a format that has only 50 fields might pose some interesting problems. Every second, an additional 10 fields must be generated seemingly from nothing. The conversion has to create new frames (from the existing input) in real time.
There are several methods used to do this, depending on the desired cost and conversion quality. The simplest possible converters simply drop every 5th line from every frame (when converting from 625 to 525) or duplicate every 4th line (when converting from 525 to 625), and then duplicate or drop some of those frames to make up the difference in frame rate. More complex systems include inter-field interpolation, adaptive interpolation, and phase correlation.
Digital television systems
The situation with worldwide digital television is much simpler by comparison. Most 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 also sees some use in direct-to-home satellite dish providers in North America), 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 OpenCable for cable). Japan uses a third system, closely related to DVB-T, called ISDB-T, which is compatible with Brazil's SBTVD. The People's Republic of China has developed a fourth system, named DMB-T/H.
ATSC
The terrestrial ATSC system (unofficially ATSC-T) uses a proprietary 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 8VSB 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, although it is inferior to the other digital systems in dealing with multipath interference; however, it is better at dealing with impulse noise which is especially present on the VHF bands that other countries have discontinued from TV use, but are still used in the U.S. There is also no hierarchical modulation. 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.
On cable, ATSC usually uses 256QAM, although some use 16VSB. Both of these double the throughput to 38.4Mb/s within the same 6MHz bandwidth. ATSC is also used over satellite. While these are logically called ATSC-C and ATSC-S, these terms were never officially defined. ATSC was never designed for mobile use, but the ATSC group is currently considering how this can be done through its ATSC-M/H.
DMB-T/H
DMB-T/H is the digital television broadcasting standard of the People's Republic of China (including Hong Kong). This is a hybrid system, part of which is ATDB, in turn very similar to ATSC-T.
DVB
DVB-T uses coded orthogonal frequency division multiplexing (COFDM), which uses as many as 8000 independent carriers, each transmitting data at a comparatively low rate. This system was designed to provide superior immunity from multipath interference, and has a choice of system variants which allow data rates from 4 MBit/s up to 24 MBit/s. One U.S. broadcaster, Sinclair Broadcasting, 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 11 terrorist attacks.) In Spain is also known as TDT (Television Digital Terreste).
DVB-S is the original Digital Video Broadcasting forward error coding and modulation standard for satellite television and dates from 1995. It is used via satellites serving every continent of the world, this is even true in North America. DVB-S is used in both MCPC and SCPC modes for broadcast network feeds, as well as for direct broadcast satellite services like Sky Digital (UK & Ireland) via Astra in Europe, Dish Network in the U.S., and Bell TV in Canada. The transport stream delivered by DVB-S is mandated as MPEG-2.
DVB-C stands for Digital Video Broadcasting - Cable and it is the DVB European consortium standard for the broadcast transmission of digital television over cable. This system transmits an MPEG-2 family digital audio/video stream, using a QAM modulation with channel coding.
ISDB
ISDB is very similar to DVB, however it is broken into 13 subchannels. Twelve are used for TV, while the last serves either as a guard band, or for the 1seg (ISDB-H) service. Like the other DTV systems, the ISDB types differ mainly in the modulations used, due to the requirements of different frequency bands. The 12 GHz band ISDB-S uses PSK modulation, 2.6 GHz band digital sound broadcasting uses CDM and ISDB-T (in VHF and/or UHF band) uses COFDM with PSK/QAM. It was developed in Japan with MPEG-2, and is now used in Brazil with MPEG-4.
See also
Transmission technology standards
Defunct analogue systems
- 405 lines
- 819 lines
- MUSE an analogue high-definition television system still in use in Japan until 2007.
Analogue television systems
Analogue television system audio
- NICAM (digital, analogue pre-emphasis curve)
- BTSC
- Zweiton
- The defunct MUSE system had a very unusual digital audio subsystem completely unrelated to NICAM.
Digital television systems
- HDTV systems all use MPEG transmission technology
History
External links
- by Alan Pemberton
- by Paul Schlyter
- a scan from a1932 French magazine
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