Television allows video images to be sent over long distances. There are basically four ways of sending such pictures to the consumer; terrestrial transmission, satellite transmission, cable distribution and Internet streaming. Older methods of transmission are based on analogue television standards whilst more recent systems use digital television (DTV) technology.
The transmission of pictures over an electrical circuit or radio link posed a tremendous challenge to inventors. In theory, it wasn’t difficult to make a camera using a lens and an array of photocells. These could then be connected via numerous wires to a display device containing an array of lamps.
Unfortunately, conventional communications systems only allows the use of one wire or radio channel. Today, with modern electronics, a multiplexer could be used at the camera, with a demultiplexer at the receiver. Indeed, such techniques are the basis of modern digital video systems. Sadly, early inventors didn’t have this technology and had to resort to more basic methods.
The pioneers of television had to devise a system for converting the shades of light in an image into variations of electrical voltage over a period of time. The technique that made this possible is known as scanning. In 1883, Paul Nipkow devised the Nipkow scanning disk. This metal disk, perforated with holes along the line of a spiral, was placed in front of a picture and rotated. This effectively scanned the image in front of it, the variations in light being converted to an electrical signal by a photocell. This varying voltage could then be sent via normal telegraph wires to another location. And, by rapidly changing the picture, it was possible to convey an illusion of movement.
In 1925, the inventor Charles Francis Jenkins (1867-1934) designed an experimental system based on the Nipkow disk. By 1928, in Washington DC, he began broadcasting cinema films in the form of radiomovies. John Logie Baird (1888-1946), generally attributed as the inventor of television, began experiments in 1922, finally transmitting his 30-line image of a human face in 1926. Two years later he demonstrated colour television and sent his pictures via a transatlantic telegraph cable. Finally, he developed the Phonodisc, an early videodisc system that was sold for several years at Selfridge’s in London. Although the British Broadcasting Corporation (BBC) adopted Baird’s mechanical system in 1929, it wasn’t entirely reliable and was soon overtaken by alternative systems.
Whilst Baird was working on his disks, both Vladimir Kosma Zworykin at Westinghouse and Philo T Farnsworth were developing cathode-ray tube (CRT) technology for television receivers and transmitters. Such tubes, based on a device created in 1897 by Karl Ferdinand Braun, avoided cumbersome mechanical equipment, providing what was known as electronic television. In 1929, David Sarnoff invested in Zworykin’s work, eventually demonstrating a working system in 1939 at the New York World’s Fair, with president Franklin D Roosevelt appearing on the screen.
This technology was developed by Marconi in the United Kingdom, resulting in their 405-line system. Using this format in 1936, the BBC in London sent out the world’s first regular television programmes. Meanwhile, in the USA, black-and-white transmissions based on the 525-line system devised by the Radio Corporation of America (RCA) and developed by the National Television System Committee (NTSC) were authorised by the Federal Communications Commission (FCC).The FCC set aside 12 very high frequency (VHF) channels and 70 ultra high frequency (UHF) channels, although regular transmissions didn’t begin until after World War II.
The earliest type of camera tube, as developed by Zworykin, was known as the Iconoscope. This was superseded by the Orthicon and Vidicon camera tubes, used for black-and-white television, followed by the Plumbicon and Saticon for colour material. All of these devices converted energy from incident light into free electrons by means of a process known as photoemission. The electrical charge so created on the camera’s mosaic was detected by a moving electron beam within the tube.
Following the war, most European countries adopted a common 625-line system that was incompatible with the systems in the UK and USA. Despite this, most of the Western Hemisphere and much of the Far East followed the American lead and employed the 525-line format.
In 1953, the FCC approved the 525-line NTSC colour system. Unlike alternative systems, this was compatible with existing receivers and transmission equipment. It worked by inserting a colour subcarrier into the video signal. This contained a chrominance signal, conveying information about hue (the shade of colour) and colour saturation (the amount of colour to be added). In order to maintain compatibility, the subcarrier had to be transmitted within the spectrum originally assigned to black-and-white picture information, now known as luminance (brightness). This meant that some degree of interference patterning occurred on both black-and-white and colour receivers, although this could be minimised by filtering and by avoiding certain programme content.
Unfortunately, the NTSC system had a significant shortcoming. When colour signals were sent over long distances, the phase of the subcarrier would get shifted, causing a significant change in the displayed colour spectrum. Although this could be corrected, it often wasn’t, leading some to label NTSC as ‘Never Twice the Same Colour’. With the benefit of this knowledge, the Europeans, including the UK with its new 625-line UHF bands, adopted the Phase Alternate Line (PAL) system. This introduced a phase reversal of the subcarrier on every alternate line, ensuring that the average colour remained constant, irrespective of any fixed shift in phase. Meanwhile, France, being different as usual, decided to adopt the 625-line SECAM system, as used in the USSR.
Although colour technology was introduced quite early, it took a long time to realise. For example, in the USA, prime-time television didn’t appear in colour until 1964.
In theory, broadcasting from a satellite should be cheaper than using numerous terrestrial transmitters, despite the initial high cost of putting a satellite into orbit. RCA launched its first communications satellite for relaying television pictures in 1975. Although intended purely for the use of North American cable-TV companies, numerous individuals in rural locations used small Earth station dishes. By 1986, such material was frequently scrambled, requiring the use of a special decoder.
Direct Broadcast Satellite (DBS) systems, employing a 460 mm to 610 mm dish, appeared in Europe and Asia during the late eighties, arriving in the USA in the early nineties. These systems frequently employ digital signals, giving high quality, although such signals are often converted into conventional analogue form via a set-top decoder.
A Digital Satellite System (DSS) provides both digital television and access to the Internet. Older systems of this type are often unidirectional, requiring a conventional modem for Internet data that you send out, otherwise known as your upstream connection. More recent systems are bidirectional, using the satellite link for Internet data in both directions.
Terrestrial is the oldest television system, requiring an appropriate number of transmitters, usually with an omnidirectional broadcast pattern, so as to cover all of the areas of population.
Although there can be problems with analogue reception in the proximity of buildings and other metallic objects, this kind of broadcasting is ideal for urban communities. Unfortunately, it’s often uneconomic in isolated rural areas, where satellite systems are often preferred.
In common with Japan and several other countries, the USA employs the NTSC 525-line system for analogue television transmissions. The alternative PAL 625-line system is used in Europe, Australia, Asia, Africa and some parts of South America.
In the North American system each channel occupies a 6 MHz wide single-sideband signal. The video information resides in an area between 1.25 and 5.45 MHz, giving a video bandwidth of 4.2 MHz. The frequency of 1.25 MHz above the main carrier is known as the video carrier.
Colour information is conveyed by means of a colour subcarrier that’s inserted into the video spectrum at 3.58 MHz. Although this is within the visible range, careful filtering at the receiver can minimise the colour patterning that can occur. Even so, certain programme content, such as a weatherman’s striped tie or jacket, can result in odd effects.
Finally, a sound carrier is provided. In the USA this is placed 4.5 MHz above the video carrier, whilst in Europe the spacing is 6 MHz and in Britain it’s 5.5 MHz. Although older television systems outside the USA use amplitude modulation (AM), this suffers from interference from other electrical equipment and has largely been replaced by frequency modulation (FM).
All broadcasting frequencies are set by the International Telecommunications Union (ITU), whilst in the USA they’re also determined by the FCC. In North America, the following blocks or bands of frequencies are employed, each divided into several channels:-
|Very ||54-72 ||2-4|
|Ultra ||470-806 ||14-69|
|Super ||0.9-40 ||-|
Similar VHF bands were originally used in the United Kingdom for 405-line television transmissions, with the frequencies split into Bands I, II and III. As in North America, Band II, occupying frequencies from 88 to 103 MHz, was set aside for FM radio transmissions. The entire VHF band is internationally recognised as being in the range of 45.0 MHz to 229.75 MHz.
Also in the UK, the UHF band is currently used for colour 625-line television broadcasts and is divided into Bands IV and V. The lower of these is normally used for major transmitters, usually broadcasting with horizontal polarisation, whilst the higher band is frequently employed for low-power ‘fill-in’ transmitters, often with vertical polarisation.
The UK’s UHF system was originally designed to provide four channels in each area (BBC 1, BBC 2, ITV 1 and Channel 4), with the frequencies assigned so as to allow a single aerial to be used for all channels. However, Channel 5 transmissions were also squeezed into the system, although universal coverage wasn’t possible, requiring a second aerial for the new channel in some areas. Internationally, the UHF band is defined by frequencies in the range of 471.25 MHz to 860.75 MHz.
Unfortunately, both the VHF and UHF bands suffer from co-channel interference, which is caused by changes in the ionosphere, particularly in summer months. Signals that are normally out of range are then reflected in the atmosphere and received, interfering with local transmissions.
The following table shows some of the uses of the SHF band in the USA:-
|12-14 ||TV |
The original analogue television systems didn’t provide any capacity for stereo sound. This was corrected in the 1980s with the introduction of Near Instantaneous Companded Audio Multiplex (NICAM), as developed by the BBC. This introduces high-quality stereo sound to TV, also enabling broadcasters to include Dolby Surround material in their existing transmissions.
NICAM 728 offers any one of the following: a single channel of stereo digital audio, two entirely separate mono sound channels, a single mono channel plus data at 352 kbit/s or pure data at 704 kbit/s. In the normal stereo mode the sound is sampled to 14-bit resolution at a rate of 32 kHz.
The UK’s version of NICAM 728 introduces an additional carrier at 5.7421875 MHz above the video carrier, whilst retaining the original FM sound carrier at 5.5 MHz. In mainland Europe a higher carrier of 6.552 MHz is used alongside the original carrier located at 6 MHz. In stereo mode the FM carrier continues to carry the mono sum of the left and right-hand channels, whilst the NICAM carrier conveys (rather oddly) only the right-hand channel.
Part or all of a television channel can be used to convey special information. For example, in the USA, subtitles for deaf, also known as closed captioning, are provided via line number
21, an unused line that exists within the vertical blanking interval. Similarly, Teletext uses line lines
18 for pages of extra information. Both these services require special hardware in the receiver.
In the USA, the 6 MHz channels described above are also used for up to eight channels of digitally-compressed standard definition television (SDTV). Alternatively, a single channel of high definition television (HDTV), containing 1050 or 1250 horizontal lines, otherwise requiring a bandwidth of 30 MHz, can be compressed to fit into a standard 6 MHz slot.
In the United Kingdom, the terrestrial channels are also used for a spread-spectrum form of digital transmission known as Digital Video Broadcasting - Terrestrial (DVB-T). Those DVB-T channels that can be viewed without charge are known as free to air services and are marketed under the Freeview banner. These programmes can also be received via the DVB-S satellite service (see below).
Cable is the ideal television medium for television in closely-packed urban areas. Prior to it’s arrival, communal aerial television (CATV) was provided in locations where terrestrial reception wasn’t reliable. Today, the acronym ‘CATV’ usually refers to cable-assisted television, where underground or overhead cables, sometimes also used for telephones, are used for television.
The connections between the subscriber and the source of material, known as the head end, can be in the form of ‘copper’ or fibre-optic circuits. Most cable TV companies obtain their programmes from satellites and receiving dishes. Unfortunately, cable services aren’t available in rural areas.
To use cable television you’ll need a special decoder box. The following description applies to an older type of installation, usually employing analogue signals. The decoder used for this type of system has two TNC coaxial sockets, one for the actual input from the cable system and another for the output. Plugged into these sockets you may find a diplexer box, providing TNC sockets for a UHF output to your own viewing equipment and for connecting a terrestrial television aerial.
Typically, the output of the decoder is wired to the radio frequency (RF) aerial input of a video cassette recorder (VCR), whose output is connected to the UHF aerial input of the television receiver. To receive cable television you must tune one of the channel buttons on the receiver to the signal from the decoder: in the United Kingdom this signal is often on UHF channel
52. Note that the TV doesn’t need a button ‘tuned’ to the VCR if there’s a SCART cable providing a video circuit from the VCR to the television.
Typically, a decoder has its own remote control box, sometimes requiring you to enter actual channel numbers. Free services often use higher channels, such as
21 for BBC 1 or
36 for Channel 5, often corresponding to the UHF channels used for equivalent terrestrial services. Special services appear on non-UHF channel numbers, such as Sky on channel
The television receiver can also receive terrestrial transmissions if a suitable UHF aerial is fitted. In the UK, buttons
5 on the television would normally be assigned to BBC 1 through to Channel 5 whilst the output of the decoder (often channel
52) could be selected using button
Satellite reception is particularly suitable for those rural areas not covered by a terrestrial transmitter or too remote for a cable connection.
Most satellites, launched by private companies and Intelsat (International Telecommunications Satellite Consortium) are in a geosynchronous equatorial orbit, positioned 22,300 miles up. They receive material from a ground station, also known as an uplink, and transmit it back to Earth over a required area. Satellites have several transponders, each of which receives and transmits a signal.
Subscribers must have a suitable receiving dish as well as a matching receiver or decoder. In the USA, you can use a large Earth station dish, identical to that used by television companies, or a smaller Television Receive Only (TVRO) dish. However, Direct Broadcast Satellite (DBS) dishes, with a diameter of a little as 300 mm, are common throughout the world. In the UK, 900 mm dishes were originally used, although most people now employ the compact 450 mm version.
DBS can employ analogue or digital transmission. By using digital compression, it’s possible to provide a hundred or more channels. In North America, part of the SHF band known as Ku Band is used for DBS, occupying frequencies from 12 to 19 GHz.
Traditionally, most types of DBS receiver or decoder convert the incoming SHF signal into a UHF signal that can feeds the aerial socket of a conventional television receiver. The workings of such a decoder are rather similar to those of a cable-TV decoder.
Around 30 different satellites are accessible in the United Kingdom, each employing analogue, Sky digital or European digital transmission systems. To receive such signals you’ll need a suitable receiving dish and one or more decoders of the appropriate type.
The simplest kind of dish, intended for reception from a single satellite, contains a single LNB (the reception device at its centre) and is erected in a fixed position. To receive signals from several satellites you’ll need a motorised dish with two or more LNBs. However, with such equipment in the UK, you can receive free programmes from central Europe, the Middle East and even the USA.
A simple dish and decoder in the UK lets you receive analogue signals from the Astra satellite. At the time of writing, this provides subscription services as well as 43 free analogue channels. The latter are mainly German, although Bloomberg, CNBC, CNN, Eurosport and Fox News are in English. Additional analogue programmes are available from the Hotbird satellite.
The Astra 2/Eurobird satellites are primarily designed for those who subscribe to the Sky digital service in the UK. However, by obtaining a free-to-view card from the BBC, you can receive around 70 free programmes, most of which are in English.
This digital system is also provided via the Astra satellites, accommodating subscription services and another 60 free channels. Most of the latter are in German, apart from AB Moteurs, Bloomberg, Canal Canarias, CNN, CNBC and Fox News. The DVB-Satellite (DVB-S) service in the United Kingdom includes ‘free to air’ services that are marketed under the Freeview banner: these are also transmitted via the DDB-T system.
1997 Grolier Multimedia Encyclopedia, © 1997, Grolier Inc.
BBC website at www.bbc.co.uk
Additional information on satellite television from Phil Harwood
©Ray White 2004.