Broadcast television systems
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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.
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[edit] 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 analogue 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. Digital, or "fixed pixel", displays are all progressive scan 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
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.
[edit] Hidden signalling
Broadcasters later developed mechanisms to transmit digital information on the phantom lines, used mostly for teletext and closed captioning:
- PAL-Plus uses a hidden signalling scheme to indicate if it exists, and if so what operational mode it is in.
- NTSC has an anti-ghosting signal that is inserted on a non-visible scan line.
- Teletext uses hidden signalling to transmit information pages.
- NTSC Closed Captioning signalling uses signalling that is nearly identical to teletext signalling.
[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. This concept is analogous to producing widescreen content that will be cropped for some viewers who do not have widescreen.
[edit] Interlacing
In the PAL standard the odd (upper) field is drawn first and the even (lower) field second. In the NTSC standard, the even (lower) field is drawn first and the odd (upper) field second opposite to PAL.
[edit] 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 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.
[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.
[edit] 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 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.
[edit] Evolution
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.
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 "Analogue switch-off" in the digital television article for more information.)
[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 mechanical based and of very low resolution, some times with no sound. Latter 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. Most values are measured in MHz.
- Also see: television channel frequencies
| System | Lines | Frame rate | Channel bandwidth (in MHz) | Visual bandwidth (in MHz) | Sound offset | Vestigial sideband | Vision mod. | Sound mod. |
|---|---|---|---|---|---|---|---|---|
| (A) | 405 | 25 | 5 | 3 | −3.5 | 0.75 | Pos. | AM |
| B | 625 | 25 | 7 | 5 | +5.5 | 0.75 | Neg. | FM |
| (C) | 625 | 25 | 7 | 5 | +5.5 | 0.75 | Pos. | AM |
| D | 625 | 25 | 8 | 6 | +6.5 | 0.75 | Neg. | FM |
| (E) | 819 | 25 | 14 | 10 | ±11.15 | 2.00 | Pos. | AM |
| (F) | 819 | 25 | 7 | 5 | +5.5 | 0.75 | Pos. | AM |
| G | 625 | 25 | 8 | 5 | +5.5 | 0.75 | Neg. | FM |
| H | 625 | 25 | 8 | 5 | +5.5 | 1.25 | Neg. | FM |
| I | 625 | 25 | 8 | 5.5 | +5.9996 | 1.25 | Neg. | FM |
| J | 525 | 29.97 | 6 | 4.2 | +4.5 | 0.75 | Neg. | FM |
| K | 625 | 25 | 8 | 6 | +6.5 | 0.75 | Neg. | FM |
| K' | 625 | 25 | 8 | 6 | +6.5 | 1.25 | Neg. | FM |
| L | 625 | 25 | 8 | 6 | +6.5 | 1.25 | Pos. | AM |
| M | 525 | 29.97 | 6 | 4.2 | +4.5 | 0.75 | Neg. | FM |
| N | 625 | 25 | 6 | 4.2 | +4.5 | 0.75 | Neg. | FM |
Obsolete TV systems are shown in grey text, with the ITU designation in brackets.
Notes by system:
- A
- Old United Kingdom and Ireland VHF system (BW only). First electronic TV system, introduced in 1936. Vestigal sideband filtering introduced in 1949. Discontinued 23 Nov 1982 (Ireland) and 2 Jan 1985 (UK). [1] [2]
- B
- VHF only in most countries (combined with system G and H on UHF); VHF and UHF in Australia.
- C
- Old VHF system; used only in Belgium, as a compromise between Systems B and L. Discontinued in 1977. [3]
- D
- Used on VHF only in most countries (combined with system K on UHF). Used in the People's Republic of China on both VHF and UHF.
- E
- Old French VHF system; very good (near HDTV) picture quality but uneconomical use of bandwidth. Sound carrier separation +11.15 MHz on odd numbered channels, -11.15 MHz on even numbered channels. Discontinued in 1984 (France) and 1985 (Monaco). [4]
- F
- Old VHF system used only in Belgium and Luxembourg; allowed French 819-line programming to be broadcast on the 7 MHz VHF channels used in those countries, at a substantial cost in horizontal resolution. Discontinued in 1969. [5]
- G
- UHF only; used in countries with system B on VHF, except Australia.
- H
- UHF only; used in Belgium, Luxembourg and the former Yugoslavia. Similar to System G with an 1.25 MHz vestigal sideband.
- I
- UK, Ireland, South Africa, Macau, Hong Kong and Falkland Islands.
- J
- VHF and UHF in Japan (see system M below). Identical to system M except that a different black level of 0 IRE (unit) is used instead of 7.5 IRE.
- K
- UHF only; used in countries with system D on VHF, and identical to it in most respects.
- K'
- Used only in French overseas departments and territories.
- L
- Used only in France. On VHF Band 1 only, the audio is at −6.5 MHz.
- M
- Used in most of the Americas and Caribbean, Philippines, South Korea, Taiwan (all NTSC-M) and Brazil (PAL-M).
- N
- Used in Argentina, Paraguay and Uruguay (all PAL-N). Allows 625-line, 50-frame/s video to be broadcast in a 6 MHz channel, at some cost in horizontal resolution.
[edit] 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.
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 converter 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 100 evenly-spaced lines from every frame, and then duplicate some of those frames to make up the difference in frame rate. More complex systems include inter-field interpolation, adaptive interpolation, and phase correlation.
[edit] 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.
[edit] ATSC
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.
[edit] DMB-T/H
DMB-T/H is the digital television broadcasting standard of the People's Republic of China (including Hong Kong).
[edit] DVB-T
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.)
[edit] DVB-S
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 ExpressVu in Canada. The transport stream delivered by DVB-S is mandated as MPEG-2.
[edit] DVB-C
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.
[edit] ISDB
The ISDB system 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 is used mainly in Japan.
[edit] See also
Transmission technology standards
- Lists of television channels for lists by country and language.
- Broadcast safe
- Television channel frequencies
- Display resolution
- Amateur television
- North American cable television frequencies
Defunct analogue systems
- 405 lines
- 819 lines
- MUSE an Analogue high-definition television system still in use in Japan until 2007.
Analogue television systems
- NTSC (525/60)
- PAL-M (television)
- PAL (color encoding usually used with 625/50 systems)
- PALplus
- SECAM
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
History
[edit] References
- Characteristics of television systems. International Telecommunication Union, ITU-R Recommendation BT.470-2.
[edit] External links
- World Analogue Television Standards and Waveforms by Alan Pemberton
- Analogue TV Broadcast Systems by Paul Schlyter
- European Television Stations in 1932 a scan from a1932 French magazine
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