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For other uses, see Radio (disambiguation).

Radio is the wireless transmission of signals, by modulation of electromagnetic waves with frequencies below those of light.

Contents 1 Radio waves 2 History and invention 2.1 Origin of the word 2.2 Invention 2.3 Discovery and development 2.4 Wireless age 2.5 "Wireless" factories and vacuum tubes 2.6 20th century 3 Uses of radio 3.1 Audio 3.2 Telephony 3.3 Video 3.4 Navigation 3.5 Radar 3.6 Emergency services 3.7 Data (digital radio) 3.8 Heating 3.9 Mechanical force 3.10 Other 4 See also 5 References 6 External links

Radio waves

Radio waves are a form of electromagnetic radiation, created whenever a charged object (e.g. an electron) accelerates with a frequency that lies in the radio frequency (RF) portion of the electromagnetic spectrum. In radio, this acceleration is caused by an alternating current in an antenna. Radio frequencies occupy the range from a few tens of hertz to a few hundred gigahertz.

ELF - SLF - ULF/VF - VLF - LF/LW - MW - HF/SW - VHF - UHF - SHF - EHF Electromagnetic radio spectrum

Other types of electromagnetic radiation, with frequencies above the RF range are infrared, visible light, ultraviolet, X-rays and gamma rays. Since the energy of an individual photon of radio frequency is too low to remove an electron from an atom, radio waves are classified as non-ionizing radiation.

Electromagnetic radiation travels (propagates) by means of oscillating electromagnetic fields that pass through the air and the vacuum of space equally well, and does not require a medium of transport (such as the aether). When radio waves pass an electrical conductor, the oscillating electric or magnetic field (depending on the shape of the conductor) induces an alternating current and voltage in the conductor. This can be transformed into audio or other signals that carry information. Although the word 'radio' is used to describe this phenomenon, the transmissions which we know as television, radio, radar, and cell phone are all classed as radio frequency emissions.

History and invention

Main article: History of radio

Origin of the word

Originally, radio technology was called 'wireless telegraphy', which was shortened to 'wireless'. The prefix radio- in the sense of wireless transmission is first recorded in the word radioconductor, coined by the French physicist Edouard Branly in 1897 and based on the verb to radiate. 'Radio' had appeared as a noun by 1907 (when it was used in an article by Lee de Forest), was adopted by the United States Navy in 1912 and became common by the time of the first commercial broadcasts in the United States in the 1920s. (The noun 'broadcasting' itself came from an agricultural term, meaning 'scattering seeds'.) The American term was then adopted by other languages in Europe and Asia, although Britain retained the term 'wireless' until the mid-20th century.

Invention

The identity of the original inventor of radio, at the time called wireless telegraphy, is contentious. The controversy over who invented the radio, with the benefit of hindsight, can be broken down as follows:

Q1: Who invented 'wireless transmission of data' (spark-gap radio)?

A1: Nikola Tesla holds the US patent for the invention of the radio.

Q2: Who invented amplitude-modulated (AM) radio, so that more than one station can send signals (as opposed to spark-gap radio, where one transmitter covers the entire bandwidth of the spectrum)?

A2: Reginald Fessenden _[1] and Lee de Forest.

Q3: Who invented frequency-modulated (FM) radio, so that an audio signal can avoid "static," that is, interference from electrical equipment and atmospherics?

A3: Edwin H. Armstrong and Lee de Forest.

Early radios ran the entire power of the transmitter through a carbon microphone. While some early radios used some type of amplification through electric current or battery, through the mid 1920s the most common type of receiver was the crystal set. In the 1920s, amplifying vacuum tubes revolutionized both radio receivers and transmitters.

Discovery and development

The theoretical basis of the propagation of electromagnetic waves was first described in 1873 by James Clerk Maxwell in his paper to the Royal Society A dynamical theory of the electromagnetic field, which followed his work between 1861 and 1865. In 1878 David E. Hughes was the first to transmit and receive radio waves when he noticed that his induction balance caused noise in the receiver of his homemade telephone. He demonstrated his discovery to the Royal Society in 1880 but was told it was merely induction. It was Heinrich Rudolf Hertz who, between 1886 and 1888, first validated Maxwell's theory through experiment, demonstrating that radio radiation had all the properties of waves (now called Hertzian waves), and discovering that the electromagnetic equations could be reformulated into a partial differential equation called the wave equation.

William Henry Ward was issued U.S. Patent 126356 on April 30, 1872. Mahlon Loomis was issued U.S. Patent 129971 on July 30, 1872. Landell de Moura, a Brazilian priest and scientist, conducted experiments after 1893 (but at least by 1894). He did not publicize his achievement until 1900. Claims have been made that Nathan Stubblefield invented radio before either Tesla or Marconi, but his device seems to have worked by induction transmission rather than radio transmission.

Wireless age

In 1893 in St. Louis, Missouri, Tesla made devices for his experiments with the electricity. Addressing the Franklin Institute in Philadelphia and the National Electric Light Association, he described and demonstrated in detail the principles of their work. [2] They contained all the elements that were later incorporated into radio systems before the development of the vacuum tube. He initially experimented with magnetic receivers, unlike the coherers used by Marconi and other early experimenters. [3]. Tesla is usually considered the first to apply the mechanism of electrical conduction to wireless practices.

On 19 August 1894, British physicist Sir Oliver Lodge demonstrated the reception of Morse code signalling using radio waves using a detecting device called a coherer, a tube filled with iron filings which had been invented by Temistocle Calzecchi-Onesti at Fermo in Italy in 1884. Edouard Branly of France and Popov of Russia later produced improved versions of the coherer.

Alexander Popov, who was the first to develop a practical communication system based on the coherer, is usually considered to have been the inventor of radio. In 1894 he built a coherer and presented it to the Russian Physical and Chemical Society on May 7, 1895 [4]. In March 1896, he effected transmission of radio waves between different campus buildings in Saint Petersburg, but didn't care to apply for a patent.

The Indian physicist, Jagdish Chandra Bose, during the years 1894-1900, performed pioneering research on radio waves and created waves as short as 5 mm. [5] In November 1894 J.C. Bose ignited gunpowder and rang a bell at a distance using electromagnetic waves, confirming that communication signals can be sent without using wires. But he was not interested in patenting his work too.

In 1896 Marconi was awarded what is sometimes recognised as the world's first patent for radio with British Patent 12039, Improvements in transmitting electrical impulses and signals and in apparatus there-for. In 1897 he established the world's first radio station on the Isle of Wight, England. The same year in the U.S., some key developments in radio's early history were created and patented by Tesla. The U.S. Patent Office reversed its decision in 1904, awarding Marconi a patent for the invention of radio, possibly influenced by Marconi's financial backers in the States, who included Thomas Edison and Andrew Carnegie. Some believe this was made for financial reasons, allowing the U.S. government to avoid having to pay the royalties that were being claimed by Tesla for use of his patents.

In 1909, Marconi, with Karl Ferdinand Braun, was also awarded the Nobel Prize in Physics for "contributions to the development of wireless telegraphy". However, Tesla's patent (number 645576) was reinstated in 1943 by the U.S. Supreme Court, shortly after his death. This decision was based on the fact that prior art existed before the establishment of Marconi's patent. Some believe the decision was also made for financial reasons, to allow the U.S. government to avoid having to pay damages that were being claimed by the Marconi Company for use of its patents during World War I.

"Wireless" factories and vacuum tubes

Marconi opened the world's first "wireless" factory in Hall Street, Chelmsford, England in 1898, employing around 50 people. Around 1900, Tesla opened the Wardenclyffe Tower facility and advertised services. By 1903, the tower structure neared completion. Various theories exist on how Tesla intended to achieve the goals of this wireless system (reportedly, a 200 kW system). Tesla claimed that Wardenclyffe, as part of a World System of transmitters, would have allowed secure multichannel transceiving of information, universal navigation, time synchronization, and a global location system.

The next great invention was the vacuum tube detector, invented by a team of Westinghouse engineers. On Christmas Eve, 1906, Reginald Fessenden (using his heterodyne principle) transmitted the first radio audio broadcast in history from Brant Rock, Massachusetts. Ships at sea heard a broadcast that included Fessenden playing O Holy Night on the violin and reading a passage from the Bible. The world's first radio news program was broadcast August 31, 1920 by station 8MK in Detroit, Michigan. The world's first regular wireless broadcasts for entertainment commenced in 1922 from the Marconi Research Centre at Writtle near Chelmsford, England.

20th century

Developments in the early 20th century (1900-1959):

• Aircraft used commercial AM radio stations for navigation. This continued through the early 1960s when VOR systems finally became widespread (though AM stations are still marked on U.S. aviation charts).
• In the early 1930s, single sideband and frequency modulation were invented by amateur radio operators. By the end of the decade, they were established commercial modes.
• Radio was used to transmit pictures visible as television as early as the 1920s. Standard analog transmissions started in North America and Europe in the 1940s.
• In 1954, Regency introduced a pocket transistor radio, the TR-1, powered by a "standard 22.5V Battery".

Developments in the latter half of the 20th century (1960-1999):

• In 1960, Sony introduced their first transistorized radio, small enough to fit in a vest pocket, and able to be powered by a small battery. It was durable, because there were no tubes to burn out. Over the next twenty years, transistors displaced tubes almost completely except for very high power, or very high frequency, uses.
• In 1963 color television was commercially transmitted, and the first (radio) communication satellite, TELSTAR, was launched.
• In the late 1960s, the U.S. long-distance telephone network began to convert to a digital network, employing digital radios for many of its links.
• In the 1970s, LORAN became the premier radio navigation system. Soon, the U.S. Navy experimented with satellite navigation, culminating in the invention and launch of the GPS constellation in 1987.
• In the early 1990s, amateur radio experimenters began to use personal computers with audio cards to process radio signals. In 1994, the U.S. Army and DARPA launched an aggressive, successful project to construct a software radio that could become a different radio on the fly by changing software.
• Digital transmissions began to be applied to broadcasting in the late 1990s.

Uses of radio

Many of radio's early uses were maritime, for sending telegraphic messages using Morse code between ships and land. One of the earliest users included the Japanese Navy scouting the Russian fleet during the Battle of Tsushima in 1905. One of the most memorable uses of marine telegraphy was during the sinking of the RMS Titanic in 1912, including communications between operators on the sinking ship and nearby vessels, and communications to shore stations listing the survivors.

Radio was used to pass on orders and communications between armies and navies on both sides in World War I; Germany used radio communications for diplomatic messages once its submarine cables were cut by the British. The United States passed on President Woodrow Wilson's Fourteen Points to Germany via radio during the war.

Broadcasting began to become feasible in the 1920s, with the widespread introduction of radio receivers, particularly in Europe and the United States. Besides broadcasting, point-to-point broadcasting, including telephone messages and relays of radio programs, became widespread in the 1920s and 1930s.

Another use of radio in the pre-war years was the development of detecting and locating aircraft and ships by the use of radar (RAdio Detection And Ranging).

Today, radio takes many forms, including wireless networks, mobile communications of all types, as well as radio broadcasting. Read more about radio's history.

Before the advent of television, commercial radio broadcasts included not only news and music, but dramas, comedies, variety shows, and many other forms of entertainment. Radio was unique among dramatic presentation that it used only sound. For more, see radio programming.

There are a number of uses of radio:

Audio

• AM broadcast radio sends music and voice in the Medium Frequency (MF—0.300 MHz to 3 MHz) radio spectrum. AM radio uses amplitude modulation, in which louder sounds at the microphone causes wider fluctuations in the transmitter power while the transmitter frequency remains unchanged. Transmissions are affected by static because lightning and other sources of radio add their radio waves to the ones from the transmitter.
• FM broadcast radio sends music and voice, with higher fidelity than AM radio. In frequency modulation, louder sounds at the microphone cause the transmitter frequency to fluctuate farther, the transmitter power stays constant. FM is transmitted in the Very High Frequency (VHF—30 MHz to 300 MHz) radio spectrum. FM requires more radio frequency space than AM and there are more frequencies available at higher frequencies, so there can be more stations, each sending more information. Another effect is that shorter VHF radio waves act more like light, travelling in straight lines, hence the reception range is generally limited to about 50-100 miles. During unusual upper atmospheric conditions, FM signals are occasionally reflected back towards the Earth by the ionosphere, resulting in Long distance FM reception. FM receivers are subject to the capture effect, which causes the radio to only receive the strongest signal when multiple signals appear on the same frequency. FM receivers are relatively immune to lightning and spark interference.
• FM Subcarrier services are secondary signals transmitted "piggyback" along with the main program. Special receivers are required to utilize these services. Analog channels may contain alternative programming, such as reading services for the blind, background music or stereo sound signals. In some extremely crowded metropolitan areas, the subchannel program might be an alternate foreign language radio program for various ethnic groups. Subcarriers can also transmit digital data, such as station identification, the current song's name, web addresses, or stock quotes. In some countries, FM radios automatically retune themselves to the same channel in a different district by using sub-bands.
• Aviation voice radios use VHF AM. AM is used so that multiple stations on the same channel can be received. (Use of FM would result in stronger stations blocking out reception of weaker stations due to FM's capture effect). Aircraft fly high enough that their transmitters can be received hundreds of miles (kilometres) away, even though they are using VHF.
• Marine voice radios can use AM in the shortwave High Frequency (HF—3 MHz to 30 MHz) radio spectrum for very long ranges or narrowband FM in the VHF spectrum for much shorter ranges.
• Government, police, fire and commercial voice services use narrowband FM on special frequencies. Fidelity is sacrificed to use a smaller range of radio frequencies, usually five kilohertz of deviation (5 thousand cycles per second), rather than the 75 used by FM broadcasts and 25 used by TV sound.
• Civil and military HF (high frequency) voice services use shortwave radio to contact ships at sea, aircraft and isolated settlements. Most use single sideband voice (SSB), which uses less bandwidth than AM. SSB sounds like ducks quacking on an AM radio. Viewed as a graph of frequency versus power, an AM signal shows power where the frequencies of the voice add and subtract with the main radio frequency. SSB cuts the bandwidth in half by suppressing the carrier and (usually) lower sideband. This also makes the transmitter about three times more powerful, because it doesn't need to transmit the unused carrier and sideband.
• TETRA, Terrestrial Trunked Radio is a digital cell phone system for military, police and ambulances.
• Commercial services such as XM and Sirius offer digital Satellite radio.

Telephony

• Cell phones transmit to a local cell transmitter/receiver site, which connects to the public service telephone network through an optic fiber or microwave radio. When the phone leaves the cell radio's area, the central computer switches the phone to a new cell. Cell phones originally used FM, but now most use various digital encodings.
• Satellite phones come in two types: INMARSAT and Iridium. Both types provide world-wide coverage. INMARSAT uses geosynchronous satellites, with aimed high-gain antennas on the vehicles. Iridium provides cell phones, except the cells are satellites in orbit.

Video

• Television sends the picture as AM, and the sound as FM, on the same radio signal.
• Digital television encodes three bits as eight strengths of AM signal. The bits are sent out-of-order to reduce the effect of bursts of radio noise. A Reed-Solomon error correction code lets the receiver detect and correct errors in the data. Although any data could be sent, the standard is to use MPEG-2 for video, and five CD-quality (44.1 kHz) audio channels (center, left, right, left-back and right back). With all this, it takes only half the bandwidth of an analog TV signal because the video data is compressed.

Navigation

• All satellite navigation systems use satellites with precision clocks. The satellite transmits its position, and the time of the transmission. The receiver listens to four satellites, and can figure its position as being on a line that is tangent to a spherical shell around each satellite, determined by the time-of-flight of the radio signals from the satellite. A computer in the receiver does the math.
• Loran systems also used time-of-flight radio signals, but from radio stations on the ground.
• VOR systems (used by aircraft), have a antenna array that transmits two signals simultaneously. A directional signal rotates like a lighthouse at a fixed rate. When the directional signal is facing north, an omnidirectional signal pulses. By measuring the difference in phase of these two signals, an aircraft can determine its bearing from the station. An aircraft can get readings from two VORs, and locate its position at the intersection of the two beams.
• Radio direction-finding is the oldest form of radio navigation. Before 1960 navigators used movable loop antennas to locate commercial AM stations near cities. In some cases they used marine radiolocation beacons, which share a range of frequencies just above AM radio with amateur radio operators.

Radar

• Radar detects things at a distance by bouncing radio waves off them. The delay caused by the echo measures the distance. The direction of the beam determines the direction of the reflection. The polarization and frequency of the return can sense the type of surface.
• Navigational radars scan a wide area two to four times per minute. They use very short waves that reflect from earth and stone. They are common on commercial ships and long-distance commercial aircraft
• General purpose radars generally use navigational radar frequencies, but modulate and polarize the pulse so the receiver can determine the type of surface of the reflector. The best general-purpose radars distinguish the rain of heavy storms, as well as land and vehicles. Some can superimpose sonar data and map data from GPS position.
• Search radars scan a wide area with pulses of short radio waves. They usually scan the area two to four times a minute. Sometimes search radars use the doppler effect to separate moving vehicles from clutter.
• Targeting radars use the same principle as search radar but scan a much smaller area far more often, usually several times a second or more.
• Weather radars resemble search radars, but use radio waves with circular polarization and a wavelength to reflect from water droplets. Some weather radar use the doppler to measure wind speeds.

Emergency services

• emergency position-indicating rescue beacons (EPIRBs), emergency locating transmitters or personal locator beacons are small radio transmitters that satellites can use to locate a person or vehicle needing rescue. Their purpose is to help rescue people in the first day, when survival is most likely. There are several types, with widely-varying performance.

Data (digital radio)

• The oldest form of digital broadcast was spark gap telegraphy, used by pioneers such as Marconi. By pressing the key, the operator could send messages in Morse code by energizing a rotating commutating spark gap. The rotating commutator produced a tone in the receiver, where a simple spark gap would produce a hiss, indistinguishable from static. Spark gap transmitters are now illegal, because their transmissions span several hundred megahertz. This is very wasteful of both radio frequencies and power.
• The next advance was continuous wave telegraphy, or CW, in which a pure radio frequency, produced by a vacuum tube electronic oscillator was switched on and off by a key. A receiver with a local oscillator would "heterodyne" with the pure radio frequency, creating a whistle-like audio tone. CW uses less than 100Hz of bandwidth. CW is still used, these days primarily by amateur radio operators (hams). Strictly, on-off keying of a carrier should be known as "Interrupted Continuous Wave" or ICW.
• Radio teletypes usually operate on short-wave (HF) and are much loved by the military because they create written information without a skilled operator. They send a bit as one of two tones. Groups of five or seven bits become a character printed by a teletype. From about 1925 to 1975, radio teletype was how most commercial messages were sent to less developed countries. These are still used by the military and weather services.
• Aircraft use a 1200 Baud radioteletype service over VHF to send their ID, altitude and position, and get gate and connecting-flight data.
• Microwave dishes on satellites, telephone exchanges and TV stations usually use quadrature amplitude modulation (QAM). QAM sends data by changing both the phase and the amplitude of the radio signal. Engineers like QAM because it packs the most bits into a radio signal. Usually the bits are sent in "frames" that repeat. A special bit pattern is used to locate the beginning of a frame.
• Systems that need reliability, or that share their frequency with other services may use "corrected orthogonal frequency-division multiplexing" or COFDM. COFDM breaks a digital signal into as many as several hundred slower subchannels. The digital signal is often sent as QAM on the subchannels. Modern COFDM systems use a small computer to make and decode the signal with digital signal processing, which is more flexible and far less expensive than older systems that implemented separate electronic channels. COFDM resists fading and ghosting because the narrow-channel QAM signals can be sent slowly. An adaptive system, or one that sends error-correction codes can also resist interference, because most interference can affect only a few of the QAM channels. COFDM is used for WiFi, some cell phones, Digital Radio Mondiale, Eureka 147, and many other local area network, digital TV and radio standards.
• Most new radio systems are digital, see also:Digital TV, Satellite Radio, Digital Audio Broadcasting.

Heating

Radio-frequency energy generated for heating of objects is generally not intended to radiate outside of the generating equipment, to prevent interferance with other radio signals.

• Microwave ovens use intense radio waves to heat food. (Note: It is a common misconception that the radio waves are tuned to the resonant frequency of water molecules. The microwave frequencies used are actually about a factor of 10 below the resonant frequency.)
• Diathermy equipment is used in surgery for sealing of blood vessels.
• Induction furnaces are used for melting metal for casting.

Mechanical force

• Tractor beams: Radio waves exert small electrostatic and magnetic forces. These are enough to perform station-keeping in microgravity environments.
• Conceptually, spacecraft propulsion: Radiation pressure from intense radio waves has been proposed as a propulsion method for an interstellar probe called Starwisp. Since the waves are long, the probe could be a very light metal mesh, and thus achieve higher accelerations than if it were a solar sail.

Other

• Amateur radio is a hobby where enthusiasts who purchase or build their own equipment and use radio for their own enjoyment. They may also provide an emergency and public-service radio service. This has been of great use, saving lives in many instances. Radio amateurs are able to use frequencies in a large number of narrow bands throughout the radio spectrum. Radio amateurs use all forms of encoding, including obsolete and experimental ones. Several forms of radio were pioneered by radio amateurs and later became commercially important, including FM, single-sideband AM, digital packet radio and satellite repeaters.
• Personal radio services such as Citizens' Band Radio, Family Radio Service, Multi-Use Radio Service and others exist in North America to provide simple, (usually) short range communication for individuals and small groups, without the overhead of licensing. Similar services exist in other parts of the world.
• Wireless energy transfer: A number of schemes have been proposed that transmit power using microwaves, and the technique has been demonstrated. (See Microwave power transmission). These schemes include, for example, solar power stations in orbit beaming energy down to terrestrial users.
• Radio remote control: Use of radio waves to transmit control data to a remote object as in some early forms of guided missile, some early TV remotes and a range of model boats, cars and aeroplanes. Large industrial remote-controlled equipment such as cranes and switching locomotives now usually use digital radio techniques to ensure safety and reliability.

See also

• Satellite radio
• Radio propagation and ionosphere
• Radio programming
• Old-time radio
• Music radio
• International broadcasting
• Amateur radio
• Army No. 11 set
• Shortwave
• Mediumwave
• Longwave
• Near Vertical Incidence Skywave
• Transistor radio
• Crystal radio receiver
• Software radio
• Internet radio
• Types of radio emissions
• Dead air
• Radio astronomy
• Tuner (radio)
• Long distance FM reception (FM DX)
• VFO
• Lists
  • Radio network
  • List of radio stations
  • List of Internet stations

References

• A História da Rádio em Datas (1819-1997) (in Portuguese) - notes on etymology
• L. de Forest, article in Electrical World 22 June 1270/1 (1907), early use of word "radio".

External links

• Satellite Radio News.Net Everything you need to know about Satellite Radio.
• Horzepa, Stan, "Surfin': Who Invented Radio?". Arrl.org. 10 October 2003.
• IAteacher: Interactive Explanation of Radio Receiver Construction
• U.S. Supreme Court, "Marconi Wireless Telegraph co. of America v. United States". 320 U.S. 1. Nos. 369, 373. Argued 9 April-12, 1943. Decided 21 June 1943.
• Radio Locator: Find a radio station in your area
• On The Radio.Net: Find phone numbers and websites for commercials you heard on the radio!
• Ottawa Vintage Radio Club of Canada
• XM Satellite Radio
• The Broadcast Archive - Radio History on the Web!
• Radiozone
• George H. Clark Radioana Collection, ca. 1880 - 1950 - Archives Center, National Museum of American History, Smithsonian Institution

• Directories
  • LookSmart - Radio
  • Open Directory Project - Radio
  • Yahoo! - Radio

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"TV" redirects here. For other uses, see TV (disambiguation).

Television is a telecommunication system for broadcasting and receiving moving pictures and sound over a distance. The term has come to refer to all the aspects of television programming and transmission as well.

Contents 1 History 1.1 Electromechanical television 1.2 Electronic television 1.3 Color television 1.4 Broadcast television 2 Technology 2.1 Broadcasting 2.2 Receiving 2.2.1 Television sets 3 Specifications 3.1 Modern displays 3.2 Signal connections 3.3 Aspect ratios 3.4 Aspect ratio incompatibility 3.5 Sound 3.6 Television add-ons 3.7 New developments 4 Geographical usage 5 Content 5.1 Advertising 5.2 Programming 6 Social aspects 6.1 Alleged dangers 6.2 Technology trends 6.3 Suitability for audience 7 Further reading 8 References 9 See also 10 External links

History

The development of television technology can be partitioned along two lines: those developments that depended upon both mechanical and electronic principles, and those which are purely electronic. From the latter descended all modern televisions, but these would not have been possible without discoveries and insights from the mechanical systems.

The word television is a hybrid word, created from both Greek and Latin. Tele- is Greek for "far", while -vision is from the Latin visio, meaning "vision" or "sight". It is often abbreviated as TV or the telly.

Electromechanical television

Main article: Mechanical television

The German student Paul Gottlieb Nipkow proposed and patented the first electromechanical television system in 1885. Nipkow's spinning disk design is credited with being the first television image rasterizer. However, it wasn't until 1907 that developments in amplification tube technology made the design practical. Meanwhile, Constantin Perskyi had coined the word television in a paper read to the International Electricity Congress at the International World Fair in Paris on August 25, 1900. Perskeyi's paper reviewed the existing electromechanical technologies, mentioning the work of Nipkow and others.

In 1911, Boris Rosing and his student Vladimir Kosma Zworykin created a television system that used a mechanical mirror-drum scanner to transmit, in Zworykin's words, "very crude images" over wires to the electronic Braun tube (cathode ray tube) in the receiver. Moving images were not possible because, in the scanner, "the sensitivity was not enough and the selenium cell was very laggy." Zworykin later went to work for RCA to build a purely electronic television, the design of which was eventually found to violate patents by Philo Taylor Farnsworth.

On March 25, 1925, Scottish inventor John Logie Baird gave a demonstration of televised silhouette images at Selfridge's Department Store in London. But if television is defined as the transmission of live, moving, half-tone (grayscale) images, and not silhouette or still images, Baird achieved this privately on October 2, 1925, and gave the world's first public demonstration of a working television system to members of the Royal Institution and a newspaper reporter on January 26, 1926 at his laboratory in London. Unlike later electronic systems with several hundred lines of resolution, Baird's vertically scanned image, using a scanning disc embedded with a double spiral of lenses, had only 30 lines, just enough to reproduce a recognizable human face.

In 1928 Baird's company (Baird Television Development Company / Cinema Television) broadcast the first transatlantic television signal, between London and New York, and the first shore to ship transmission. He also demonstrated an electromechanical colour, infrared (dubbed "Noctovision"), and stereoscopic television, using additional lenses, disks and filters. In parallel he developed a video disk recording system dubbed "Phonovision"; a number of the Phonovision[1] recordings, dating back to 1927, still exist. In 1929 he became involved in the first experimental electromechanical television service in Germany. In 1931 he made the first live transmission, of the Epsom Derby. In 1932 he demonstrated ultra-short wave television. Baird's electromechanical system reached a peak of 240 lines of resolution on BBC television broadcasts in 1936, before being discontinued in favor of a 405 line all-electronic system.

In the U.S., Charles Francis Jenkins was able to demonstrate on June 13, 1925, the transmission of the silhouette image of a toy windmill in motion from a naval radio station to his laboratory in Washington, using a lensed disc scanner with 48 lines per picture, 16 pictures per second. AT&T's Bell Telephone Laboratories transmitted half-tone images of transparencies in May 1925. But Bell Labs gave the most dramatic demonstration of television yet on April 7, 1927, when it field tested reflected-light television systems using small-scale (2 by 2.5 inches) and large-scale (24 by 30 inches) viewing screens over a wire link from Washington to New York City, and over-the-air broadcast from Whippany, New Jersey. The subjects, which included Secretary of Commerce Herbert Hoover, were illuminated by a flying spot beam and scanned by a 50-aperture disc at 16 pictures per second.

Electronic television

Although the discoveries of Nipkow, Rosing, Baird and others were extraordinary, little of their technology is used in modern television. By 1934, all electromechanical television systems were outmoded, although electromechanical broadcasts continued on some stations until 1939.

A.A. Campbell-Swinton wrote a letter to Nature on the 18 June 1908 describing his concept of electronic television using the cathode ray tube, which had been invented in 1897 by the German physicist and Nobel prize winner Karl Ferdinand Braun. He proposed using an electron beam in both the camera and the receiver, which could be steered electronically to produce moving pictures. He lectured on the subject in 1911 and displayed circuit diagrams, but no one, including Swinton, knew how to realize the design. Although his system was never built, the cathode ray tube did come to be used to display images in almost all television sets and computer monitors until the invention of the LCD panel.

A fully electronic system was first achieved by Philo Taylor Farnsworth on September 7, 1927, although the low-resolution, light-insensitive camera tube limited the image to a plate of glass painted black, with a straight line etched across it, rotated in front of a bright carbon arc lamp. Seven years later, on August 25, 1934, at the Franklin Institute in Philadelphia, Farnsworth gave the world's first public demonstration of a working, all-electronic television system, with 220 lines per picture, 30 pictures per second. Over a three week period, vaudeville acts, athletic and sports demonstrations, politicians, and hundreds of ordinary citizens were captured on Farnsworth's cameras in the open air and simultaneously shown on his receiving sets.

Farnsworth, a Mormon farm boy from Rigby, Idaho, first envisioned his system at age 14. He discussed the idea with his high school chemistry teacher, who could think of no reason why it would not work (Farnsworth would later credit this teacher, Justin Tolman, as providing key insights into his invention). He continued to pursue the idea at Brigham Young Academy (now Brigham Young University). At age 21, he demonstrated a working system at his own laboratory in San Francisco. His breakthrough freed television from reliance on spinning discs and other mechanical parts. All modern picture tube televisions descend directly from his design.

Vladimir Kosma Zworykin is also sometimes cited as the father of electronic television because of his invention of the iconoscope in 1923 and his invention of the kinescope in 1929. His design was one of the first to demonstrate a television system with all the features of modern picture tubes. His previous work with Rosing on electromechanical television gave him key insights into how to produce such a system, but his (and RCA's) claim to being its original inventor was largely invalidated by three facts: a) Zworykin's 1923 patent presented an incomplete design, incapable of working in its given form (it was not until 1933 that Zworykin achieved a working implementation), b) the 1923 patent application was not granted until 1938, and not until it had been seriously revised, and c) courts eventually found that RCA was in violation of the television design patented by Philo Taylor Farnsworth, whose lab Zworykin had visited while working on his designs for RCA.

The controversy over whether it was first Farnsworth or Zworykin who invented modern television is still hotly debated today. Some of this debate stems from the fact that while Farnsworth appears to have gotten there first as an inventor, RCA brought television sets to market before Farnsworth, and it was RCA employees who first wrote the history of television. Even though Farnsworth eventually won the legal battle over this issue, he was never able to fully capitalize financially on his invention.

Color television

Most television researchers appreciated the value of color image transmission, with an early patent application in Russia in 1889 for a mechanically-scanned color system showing how early the importance of color was realized. John Logie Baird demonstrated the world's first color transmission on July 3, 1928, using scanning discs at the transmitting and receiving ends with three spirals of apertures, each spiral with filters of a different primary color; and three light sources at the receiving end, with a commutator to alternate their illumination.

Color television in the United States had a protracted history due to conflicting technical systems vying for approval by the Federal Communications Commission for commercial use. Mechanically scanned color television was demonstrated by Bell Laboratories in June 1929 using three complete systems of photoelectric cells, amplifiers, glow-tubes, and color filters, with a series of mirrors to superimpose the red, green, and blue images into one full color image.

In the electronically scanned era, the first color television demonstration was on February 5, 1940, when RCA privately showed to members of the FCC at the RCA plant in Camden, New Jersey, a television receiver producing images in color by a field sequential color system. CBS began non-broadcast color experiments using film as early as August 28, 1940, and live cameras by November 12. The CBS "field sequential" color system was partly mechanical, with a disc made of red, blue, and green filters spinning inside the television camera at 1,200 rpm, and a similar disc spinning in synchronization in front of the cathode ray tube inside the receiver set. RCA's later "dot sequential" color system had no moving parts, using a series of dichroic mirrors to separate and direct red, green, and blue light from the subject through three separate lenses into three scanning tubes, and electronic switching that allowed the tubes to send their signals in rotation, dot by dot. These signals were sorted by a second switching device in the receiver set and sent to red, green, and blue picture tubes, and combined by a second set of dichroic mirrors into a full color image.

The first field test (i.e., broadcast) of color television was by NBC (owned by RCA) on February 20, 1941. CBS began daily color field tests on June 1, 1941. These color systems were not compatible with existing black and white television sets, and as no color television sets were available to the public at this time, viewership of the color field tests was limited to RCA and CBS engineers and the invited press. The War Production Board halted the manufacture of television and radio equipment for civilian use from April 1, 1942 to October 1, 1945, limiting any opportunity to introduce color television to the general public.

The post-war development of color television was dominated by three systems competing for approval by the FCC as the U.S. color broadcasting standard: CBS's field sequential system, which was incompatible with existing black and white sets without an adaptor; RCA's dot sequential system, which in 1949 became compatible with existing black and white sets; and CTI's system (also incompatible with existing black and white sets), which used three camera lenses, behind which were color filters that produced red, green, and blue images side by side on a single scanning tube, and a receiver set that used lenses in front of the picture tube (which had sectors treated with different phosphorescent compounds to glow in red, green, or blue) to project these three side by side images into one combined picture on the viewing screen.

After a series of hearings beginning in September 1949, the FCC found the RCA and CTI systems fraught with technical problems, inaccurate color reproduction, and expensive equipment, and so formally approved the CBS system as the U.S. color broadcasting standard on October 11, 1950. An unsuccessful lawsuit by RCA delayed the world's first network color broadcast until June 25, 1951, when a musical variety special titled simply Premiere was shown over a network of five east coast CBS affiliates. Viewership was again extremely limited: the program could not be seen on black and white sets, and Variety estimated that only thirty prototype color receivers were available in the New York area. Regular color broadcasts began that same week with the daytime series The World Is Yours and Modern Homemakers.

While the CBS color broadcasting schedule gradually expanded to twelve hours per week (but never into prime time), and the color network expanded to eleven affiliates as far west as Chicago, its commercial success was doomed by the lack of color receivers necessary to watch the programs, the refusal of television manufacturers to create adaptor mechanisms for their existing black and white sets, and the unwillingness of advertisers to sponsor