In communication systems, signal processing, electrical engineering, a signal is a function that "conveys information about the behavior or attributes of some phenomenon". In its most common usage, in electronics and telecommunication, this is a time varying voltage, current or electromagnetic wave used to carry information. A signal may be defined as an "observable change in a quantifiable entity". In the physical world, any quantity exhibiting variation in time or variation in space is a signal that might provide information on the status of a physical system, or convey a message between observers, among other possibilities; the IEEE Transactions on Signal Processing states that the term "signal" includes audio, speech, communication, sonar, radar and musical signals. In a effort of redefining a signal, anything, only a function of space, such as an image, is excluded from the category of signals, it is stated that a signal may or may not contain any information. In nature, signals can take the form of any action by one organism able to be perceived by other organisms, ranging from the release of chemicals by plants to alert nearby plants of the same type of a predator, to sounds or motions made by animals to alert other animals of the presence of danger or of food.
Signaling occurs in organisms all the way down to the cellular level, with cell signaling. Signaling theory, in evolutionary biology, proposes that a substantial driver for evolution is the ability for animals to communicate with each other by developing ways of signaling. In human engineering, signals are provided by a sensor, the original form of a signal is converted to another form of energy using a transducer. For example, a microphone converts an acoustic signal to a voltage waveform, a speaker does the reverse; the formal study of the information content of signals is the field of information theory. The information in a signal is accompanied by noise; the term noise means an undesirable random disturbance, but is extended to include unwanted signals conflicting with the desired signal. The prevention of noise is covered in part under the heading of signal integrity; the separation of desired signals from a background is the field of signal recovery, one branch of, estimation theory, a probabilistic approach to suppressing random disturbances.
Engineering disciplines such as electrical engineering have led the way in the design and implementation of systems involving transmission and manipulation of information. In the latter half of the 20th century, electrical engineering itself separated into several disciplines, specialising in the design and analysis of systems that manipulate physical signals. Definitions specific to sub-fields are common. For example, in information theory, a signal is a codified message, that is, the sequence of states in a communication channel that encodes a message. In the context of signal processing, signals are analog and digital representations of analog physical quantities. In terms of their spatial distributions, signals may be categorized as point source signals and distributed source signals. In a communication system, a transmitter encodes a message to create a signal, carried to a receiver by the communications channel. For example, the words "Mary had a little lamb" might be the message spoken into a telephone.
The telephone transmitter converts the sounds into an electrical signal. The signal is transmitted to the receiving telephone by wires. In telephone networks, for example common-channel signaling, refers to phone number and other digital control information rather than the actual voice signal. Signals can be categorized in various ways; the most common distinction is between discrete and continuous spaces that the functions are defined over, for example discrete and continuous time domains. Discrete-time signals are referred to as time series in other fields. Continuous-time signals are referred to as continuous signals. A second important distinction is between continuous-valued. In digital signal processing, a digital signal may be defined as a sequence of discrete values associated with an underlying continuous-valued physical process. In digital electronics, digital signals are the continuous-time waveform signals in a digital system, representing a bit-stream. Another important property of a signal is its information content.
Two main types of signals encountered in practice are digital. The figure shows a digital signal that results from approximating an analog signal by its values at particular time instants. Digital signals are quantized. An analog signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e. analogous to another time varying signal. For example, in an analog audio signal, the instantaneous voltage of the signal varies continuously with the pressure of the sound waves, it differs from a digital signal, in which the continuous quantity is a representation of a sequence of discrete values which can only take on one of a finite number of values. The term analog signal refers to electrical signals. An analog signal uses some property of the medium to convey the signal's information. For ex
Magnetic tape is a medium for magnetic recording, made of a thin, magnetizable coating on a long, narrow strip of plastic film. It was developed in Germany based on magnetic wire recording. Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders respectively. A device that stores computer data on magnetic tape is known as a tape drive. Magnetic tape revolutionized reproduction and broadcasting, it allowed radio, which had always been broadcast live, to be recorded for or repeated airing. It allowed gramophone records to be recorded in multiple parts, which were mixed and edited with tolerable loss in quality, it was a key technology in early computer development, allowing unparalleled amounts of data to be mechanically created, stored for long periods, accessed. In recent decades, other technologies have been developed that can perform the functions of magnetic tape. In many cases, these technologies have replaced tape. Despite this, innovation in the technology continues, Sony and IBM continue to produce new magnetic tape drives.
Over time, magnetic tape made in the 1970s and 1980s can suffer from a type of deterioration called sticky-shed syndrome. It can render the tape unusable; the oxide side of a tape is the surface. This is the side that stores the information, the opposite side is a substrate to give the tape strength and flexibility; the name originates from the fact that the magnetic side of most tapes is made of iron oxide, though chromium is used for some tapes. An adhesive binder between the oxide and the substrate holds the two sides together. In all tape formats, a tape drive uses motors to wind the tape from one reel to another, passing over tape heads to read, write or erase as it moves. Magnetic tape was invented for recording sound by Fritz Pfleumer in 1928 in Germany, based on the invention of magnetic wire recording by Oberlin Smith in 1888 and Valdemar Poulsen in 1898. Pfleumer's invention used a ferric oxide powder coating on a long strip of paper; this invention was further developed by the German electronics company AEG, which manufactured the recording machines and BASF, which manufactured the tape.
In 1933, working for AEG, Eduard Schuller developed the ring-shaped tape head. Previous head designs were tended to shred the tape. Another important discovery made in this period was the technique of AC biasing, which improved the fidelity of the recorded audio signal by increasing the effective linearity of the recording medium. Due to the escalating political tensions, the outbreak of World War II, these developments in Germany were kept secret. Although the Allies knew from their monitoring of Nazi radio broadcasts that the Germans had some new form of recording technology, its nature was not discovered until the Allies acquired captured German recording equipment as they invaded Europe at the end of the war, it was only after the war that Americans Jack Mullin, John Herbert Orr, Richard H. Ranger, were able to bring this technology out of Germany and develop it into commercially viable formats. A wide variety of recorders and formats have been developed since, most reel-to-reel and Compact Cassette.
The practice of recording and editing audio using magnetic tape established itself as an obvious improvement over previous methods. Many saw the potential of making the same improvements in recording the video signals used by television. Video signals use more bandwidth than audio signals. Existing audio tape recorders could not capture a video signal. Many set to work on resolving this problem. Jack Mullin and the BBC both created crude working systems that involved moving the tape across a fixed tape head at high speeds. Neither system saw much use, it was the team at Ampex, led by Charles Ginsburg, that made the breakthrough of using a spinning recording head and normal tape speeds to achieve a high head-to-tape speed that could record and reproduce the high bandwidth signals of video. The Ampex system was called Quadruplex and used 2-inch-wide tape, mounted on reels like audio tape, which wrote the signal in what is now called transverse scan. Improvements by other companies Sony, led to the development of helical scan and the enclosure of the tape reels in an easy-to-handle videocassette cartridge.
Nearly all modern videotape systems use helical cartridges. Videocassette recorders used to be common in homes and television production facilities, but many functions of the VCR have been replaced with more modern technology. Since the advent of digital video and computerized video processing, optical disc media and digital video recorders can now perform the same role as videotape; these devices offer improvements like random access to any scene in the recording and the ability to pause a live program and have replaced videotape in many situations. Magnetic tape was first used to record computer data in 1951 on the Eckert-Mauchly UNIVAC I; the system's UNISERVO I tape drive used a thin strip of one half inch wide metal, consisting of nickel-plated bronze. Recording density was 100 characters per inch on eight tracks. Early IBM 7 track tape drives were floor-standing and used vacuum columns to mechanically buffer long U-shaped loops of tape; the two tape reels visibly fed tape through the columns, intermittently spinning the reels in rapid, unsynchronized bursts, resulting in visually striking action.
Stock shots of such vacuum-column tape drives in motion were used to represent "the computer" in movies and televis
A communications satellite is an artificial satellite that relays and amplifies radio telecommunications signals via a transponder. Communications satellites are used for television, radio and military applications. There are 2,134 communications satellites in Earth’s orbit, used by both private and government organizations. Many are in geostationary orbit 22,200 miles above the equator, so that the satellite appears stationary at the same point in the sky, so the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track it; the high frequency radio waves used for telecommunications links travel by line of sight and so are obstructed by the curve of the Earth. The purpose of communications satellites is to relay the signal around the curve of the Earth allowing communication between separated geographical points. Communications satellites use a wide range of microwave frequencies. To avoid signal interference, international organizations have regulations for which frequency ranges or "bands" certain organizations are allowed to use.
This allocation of bands minimizes the risk of signal interference. The concept of the geostationary communications satellite was first proposed by Arthur C. Clarke, along with Vahid K. Sanadi building on work by Konstantin Tsiolkovsky. In October 1945 Clarke published an article titled "Extraterrestrial Relays" in the British magazine Wireless World; the article described the fundamentals behind the deployment of artificial satellites in geostationary orbits for the purpose of relaying radio signals. Thus, Arthur C. Clarke is quoted as being the inventor of the communications satellite and the term'Clarke Belt' employed as a description of the orbit. Decades a project named Communication Moon Relay was a telecommunication project carried out by the United States Navy, its objective was to develop a secure and reliable method of wireless communication by using the Moon as a passive reflector and a natural communications satellite. The first artificial Earth satellite was Sputnik 1. Put into orbit by the Soviet Union on October 4, 1957, it was equipped with an on-board radio-transmitter that worked on two frequencies: 20.005 and 40.002 MHz.
Sputnik 1 was launched as a major step in the exploration of rocket development. However, it was not placed in orbit for the purpose of sending data from one point on earth to another; the first satellite to relay communications was an intended lunar probe. Though the spacecraft only made it about halfway to the moon, it flew high enough to carry out the proof of concept relay of telemetry across the world, first from Cape Canaveral to Manchester, England; the first satellite purpose-built to relay communications was NASA's Project SCORE in 1958, which used a tape recorder to store and forward voice messages. It was used to send a Christmas greeting to the world from U. S. President Dwight D. Eisenhower. Courier 1B, built by Philco, launched in 1960, was the world's first active repeater satellite; the first artificial satellite used to further advances in global communications was a balloon named Echo 1. Echo 1 was the world's first artificial communications satellite capable of relaying signals to other points on Earth.
It soared 1,600 kilometres above the planet after its Aug. 12, 1960 launch, yet relied on humanity's oldest flight technology — ballooning. Launched by NASA, Echo 1 was a 30-metre aluminized PET film balloon that served as a passive reflector for radio communications; the world's first inflatable satellite — or "satelloon", as they were informally known — helped lay the foundation of today's satellite communications. The idea behind a communications satellite is simple: Send data up into space and beam it back down to another spot on the globe. Echo 1 accomplished this by serving as an enormous mirror, 10 stories tall, that could be used to reflect communications signals. There are two major classes of communications satellites and active. Passive satellites only reflect the signal coming from the source, toward the direction of the receiver. With passive satellites, the reflected signal is not amplified at the satellite, only a small amount of the transmitted energy reaches the receiver. Since the satellite is so far above Earth, the radio signal is attenuated due to free-space path loss, so the signal received on Earth is very weak.
Active satellites, on the other hand, amplify the received signal before retransmitting it to the receiver on the ground. Passive satellites are little used now. Telstar was the second direct relay communications satellite. Belonging to AT&T as part of a multi-national agreement between AT&T, Bell Telephone Laboratories, NASA, the British General Post Office, the French National PTT to develop satellite communications, it was launched by NASA from Cape Canaveral on July 10, 1962, in the first sponsored space launch. Relay 1 was launched on December 13, 1962, it became the first satellite to transmit across the Pacific Ocean on November 22, 1963. An immediate antecedent of the geostationary satellites was the Hughes Aircraft Company's Syncom 2, launched on July 26, 1963. Syncom 2 was the first communications satellite in a geosynchronous orbit, it revolved around the earth once per day at constant speed, but because it still had north-south motion, special equipment was needed to track it. Its successor, Syncom 3 was the first geostationary communications satellite.
Syncom 3 obt
Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure is the pressure relative to the ambient pressure. Various units are used to express pressure; some of these derive from a unit of force divided by a unit of area. Pressure may be expressed in terms of standard atmospheric pressure. Manometric units such as the centimetre of water, millimetre of mercury, inch of mercury are used to express pressures in terms of the height of column of a particular fluid in a manometer. Pressure is the amount of force applied at right angles to the surface of an object per unit area; the symbol for it is p or P. The IUPAC recommendation for pressure is a lower-case p. However, upper-case P is used; the usage of P vs p depends upon the field in which one is working, on the nearby presence of other symbols for quantities such as power and momentum, on writing style. Mathematically: p = F A, where: p is the pressure, F is the magnitude of the normal force, A is the area of the surface on contact.
Pressure is a scalar quantity. It relates the vector surface element with the normal force acting on it; the pressure is the scalar proportionality constant that relates the two normal vectors: d F n = − p d A = − p n d A. The minus sign comes from the fact that the force is considered towards the surface element, while the normal vector points outward; the equation has meaning in that, for any surface S in contact with the fluid, the total force exerted by the fluid on that surface is the surface integral over S of the right-hand side of the above equation. It is incorrect to say "the pressure is directed in such or such direction"; the pressure, as a scalar, has no direction. The force given by the previous relationship to the quantity has a direction, but the pressure does not. If we change the orientation of the surface element, the direction of the normal force changes accordingly, but the pressure remains the same. Pressure is distributed to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point.
It is a fundamental parameter in thermodynamics, it is conjugate to volume. The SI unit for pressure is the pascal, equal to one newton per square metre; this name for the unit was added in 1971. Other units of pressure, such as pounds per square inch and bar, are in common use; the CGS unit of pressure is 0.1 Pa.. Pressure is sometimes expressed in grams-force or kilograms-force per square centimetre and the like without properly identifying the force units, but using the names kilogram, kilogram-force, or gram-force as units of force is expressly forbidden in SI. The technical atmosphere is 1 kgf/cm2. Since a system under pressure has the potential to perform work on its surroundings, pressure is a measure of potential energy stored per unit volume, it is therefore related to energy density and may be expressed in units such as joules per cubic metre. Mathematically: p =; some meteorologists prefer the hectopascal for atmospheric air pressure, equivalent to the older unit millibar. Similar pressures are given in kilopascals in most other fields, where the hecto- prefix is used.
The inch of mercury is still used in the United States. Oceanographers measure underwater pressure in decibars because pressure in the ocean increases by one decibar per metre depth; the standard atmosphere is an established constant. It is equal to typical air pressure at Earth mean sea level and is defined as 101325 Pa; because pressure is measured by its ability to displace a column of liquid in a manometer, pressures are expressed as a depth of a particular fluid. The most common choices are water; the pressure exerted by a column of liquid of height h and density ρ is given by the hydrostatic pressure equation p = ρgh, where g is the gravitational acceleration. Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of a fluid column
Videotape is magnetic tape used for storing video and sound in addition. Information stored can be in the form of either digital signal. Videotape is used in both video tape recorders or, more videocassette recorders and camcorders. Videotapes are used for storing scientific or medical data, such as the data produced by an electrocardiogram; because video signals have a high bandwidth, stationary heads would require high tape speeds, in most cases, a helical-scan video head rotates against the moving tape to record the data in two dimensions. Tape is a linear method of storing information and thus imposes delays to access a portion of the tape, not under the heads; the early 2000s saw the introduction and rise to prominence of high quality random-access video recording media such as hard disks and flash memory. Since videotape has been relegated to archival and similar uses; the electronics division of entertainer Bing Crosby's production company, Bing Crosby Enterprises, gave the world's first demonstration of a videotape recording in Los Angeles on November 11, 1951.
Developed by John T. Mullin and Wayne R. Johnson since 1950, the device gave what were described as "blurred and indistinct" images using a modified Ampex 200 tape recorder and standard quarter-inch audio tape moving at 360 inches per second. A year an improved version using one-inch magnetic tape was shown to the press, who expressed amazement at the quality of the images although they had a "persistent grainy quality that looked like a worn motion picture". Overall the picture quality was still considered inferior to the best kinescope recordings on film. Bing Crosby Enterprises hoped to have a commercial version available in 1954 but none came forth; the BBC experimented from 1952 to 1958 with a high-speed linear videotape system called VERA, but this was unfeasible. It used half-inch tape on 20-inch reels traveling at 200 inches per second. RCA demonstrated the magnetic tape recording of both black-and-white and color television programs at its Princeton laboratories on December 1, 1953.
The high-speed longitudinal tape system, called Simplex, in development since 1951, could record and play back only a few minutes of a television program. The color system used half-inch tape on 10-1/2 inch reels to record five tracks, one each for red, green and audio; the black-and-white system used quarter-inch tape on 10-1/2 inch reels with two tracks, one for video and one for audio. Both systems ran at 360 inches per second with 2,500 feet on a reel. RCA-owned NBC first used it on The Jonathan Winters Show on October 23, 1956 when a prerecorded song sequence by Dorothy Collins in color was included in the otherwise live television program. In 1953, Dr. Norikazu Sawazaki developed a prototype helical scan video tape recorder. BCE demonstrated a color system in February 1955 using a longitudinal recording on half-inch tape. CBS, RCA's competitor, was about to order BCE machines when Ampex introduced the superior Quadruplex system. BCE was acquired by 3M Company in 1956. In 1959, Toshiba released the first commercial helical scan video tape recorder.
The first commercial professional broadcast quality videotape machines capable of replacing kinescopes were the two-inch quadruplex videotape machines introduced by Ampex on April 14, 1956 at the National Association of Broadcasters convention in Chicago. Quad employed a transverse four-head system on a two-inch tape, stationary heads for the sound track. CBS Television first used the Ampex VRX-1000 Mark IV at its Television City studios in Hollywood on November 30, 1956 to play a delayed broadcast of Douglas Edwards and the News from New York City to the Pacific Time Zone. On January 22, 1957, the NBC Television game show Truth or Consequences, produced in Hollywood, became the first program to be broadcast in all time zones from a prerecorded videotape. Ampex introduced a color videotape recorder in 1958 in a cross-licensing agreement with RCA, whose engineers had developed it from an Ampex black-and-white recorder. NBC's special, An Evening With Fred Astaire, is the oldest surviving television network color videotape, has been restored by the UCLA Film and Television Archive.
On December 7, 1963, instant replay was used for the first time during the live transmission of the Army–Navy Game by its inventor, director Tony Verna. Although Quad became the industry standard for thirty years, it has drawbacks such as an inability to freeze pictures, no picture search. In early machines, a tape could reliably be played back using only the same set of hand-made tape heads, which wore out quickly. Despite these problems, Quad is capable of producing excellent images. Subsequent videotape systems have used helical scan, where the video heads record diagonal tracks onto the tape. Many early videotape recordings were not preserved. While much less expensive and more convenient than kinescope, the high cost of 3M Scotch 179 and other early videotapes meant that most broadcasters erased and reused them, regarded videotape as a better and more cost-effective means of time-delaying broadcasts than kinescopes, it was the four time zones of the continental United States which had made the system desirable in the first place.
However, some classic television programs recorded on studio videotape still exist, are available on DVD – among them NBC's Peter Pan with Mary Martin as Peter, several episodes o
Frequency is the number of occurrences of a repeating event per unit of time. It is referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency; the period is the duration of time of one cycle in a repeating event, so the period is the reciprocal of the frequency. For example: if a newborn baby's heart beats at a frequency of 120 times a minute, its period—the time interval between beats—is half a second. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio signals, radio waves, light. For cyclical processes, such as rotation, oscillations, or waves, frequency is defined as a number of cycles per unit time. In physics and engineering disciplines, such as optics and radio, frequency is denoted by a Latin letter f or by the Greek letter ν or ν; the relation between the frequency and the period T of a repeating event or oscillation is given by f = 1 T.
The SI derived unit of frequency is the hertz, named after the German physicist Heinrich Hertz. One hertz means. If a TV has a refresh rate of 1 hertz the TV's screen will change its picture once a second. A previous name for this unit was cycles per second; the SI unit for period is the second. A traditional unit of measure used with rotating mechanical devices is revolutions per minute, abbreviated r/min or rpm. 60 rpm equals one hertz. As a matter of convenience and slower waves, such as ocean surface waves, tend to be described by wave period rather than frequency. Short and fast waves, like audio and radio, are described by their frequency instead of period; these used conversions are listed below: Angular frequency denoted by the Greek letter ω, is defined as the rate of change of angular displacement, θ, or the rate of change of the phase of a sinusoidal waveform, or as the rate of change of the argument to the sine function: y = sin = sin = sin d θ d t = ω = 2 π f Angular frequency is measured in radians per second but, for discrete-time signals, can be expressed as radians per sampling interval, a dimensionless quantity.
Angular frequency is larger than regular frequency by a factor of 2π. Spatial frequency is analogous to temporal frequency, but the time axis is replaced by one or more spatial displacement axes. E.g.: y = sin = sin d θ d x = k Wavenumber, k, is the spatial frequency analogue of angular temporal frequency and is measured in radians per meter. In the case of more than one spatial dimension, wavenumber is a vector quantity. For periodic waves in nondispersive media, frequency has an inverse relationship to the wavelength, λ. In dispersive media, the frequency f of a sinusoidal wave is equal to the phase velocity v of the wave divided by the wavelength λ of the wave: f = v λ. In the special case of electromagnetic waves moving through a vacuum v = c, where c is the speed of light in a vacuum, this expression becomes: f = c λ; when waves from a monochrome source travel from one medium to another, their frequency remains the same—only their wavelength and speed change. Measurement of frequency can done in the following ways, Calculating the frequency of a repeating event is accomplished by counting the number of times that event occurs within a specific time period dividing the count by the length of the time period.
For example, if 71 events occur within 15 seconds the frequency is: f = 71 15 s ≈ 4.73 Hz If the number of counts is not large, it is more accurate to measure the time interval for a predetermined number of occurrences, rather than the number of occurrences within a specified time. The latter method introduces a random error into the count of between zero and one count, so on average half a count; this is called gating error and causes an average error in the calculated frequency of Δ f = 1 2 T
Telecommunication is the transmission of signs, messages, writings and sounds or information of any nature by wire, optical or other electromagnetic systems. Telecommunication occurs when the exchange of information between communication participants includes the use of technology, it is transmitted either electrically over physical media, such as cables, or via electromagnetic radiation. Such transmission paths are divided into communication channels which afford the advantages of multiplexing. Since the Latin term communicatio is considered the social process of information exchange, the term telecommunications is used in its plural form because it involves many different technologies. Early means of communicating over a distance included visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, optical heliographs. Other examples of pre-modern long-distance communication included audio messages such as coded drumbeats, lung-blown horns, loud whistles. 20th- and 21st-century technologies for long-distance communication involve electrical and electromagnetic technologies, such as telegraph and teleprinter, radio, microwave transmission, fiber optics, communications satellites.
A revolution in wireless communication began in the first decade of the 20th century with the pioneering developments in radio communications by Guglielmo Marconi, who won the Nobel Prize in Physics in 1909, other notable pioneering inventors and developers in the field of electrical and electronic telecommunications. These included Charles Wheatstone and Samuel Morse, Alexander Graham Bell, Edwin Armstrong and Lee de Forest, as well as Vladimir K. Zworykin, John Logie Baird and Philo Farnsworth; the word telecommunication is a compound of the Greek prefix tele, meaning distant, far off, or afar, the Latin communicare, meaning to share. Its modern use is adapted from the French, because its written use was recorded in 1904 by the French engineer and novelist Édouard Estaunié. Communication was first used as an English word in the late 14th century, it comes from Old French comunicacion, from Latin communicationem, noun of action from past participle stem of communicare "to share, divide out.
Homing pigeons have been used throughout history by different cultures. Pigeon post had Persian roots, was used by the Romans to aid their military. Frontinus said; the Greeks conveyed the names of the victors at the Olympic Games to various cities using homing pigeons. In the early 19th century, the Dutch government used the system in Sumatra, and in 1849, Paul Julius Reuter started a pigeon service to fly stock prices between Aachen and Brussels, a service that operated for a year until the gap in the telegraph link was closed. In the Middle Ages, chains of beacons were used on hilltops as a means of relaying a signal. Beacon chains suffered the drawback that they could only pass a single bit of information, so the meaning of the message such as "the enemy has been sighted" had to be agreed upon in advance. One notable instance of their use was during the Spanish Armada, when a beacon chain relayed a signal from Plymouth to London. In 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system between Lille and Paris.
However semaphore suffered from the need for skilled operators and expensive towers at intervals of ten to thirty kilometres. As a result of competition from the electrical telegraph, the last commercial line was abandoned in 1880. On 25 July 1837 the first commercial electrical telegraph was demonstrated by English inventor Sir William Fothergill Cooke, English scientist Sir Charles Wheatstone. Both inventors viewed their device as "an improvement to the electromagnetic telegraph" not as a new device. Samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837, his code was an important advance over Wheatstone's signaling method. The first transatlantic telegraph cable was completed on 27 July 1866, allowing transatlantic telecommunication for the first time; the conventional telephone was invented independently by Alexander Bell and Elisha Gray in 1876. Antonio Meucci invented the first device that allowed the electrical transmission of voice over a line in 1849.
However Meucci's device was of little practical value because it relied upon the electrophonic effect and thus required users to place the receiver in their mouth to "hear" what was being said. The first commercial telephone services were set-up in 1878 and 1879 on both sides of the Atlantic in the cities of New Haven and London. Starting in 1894, Italian inventor Guglielmo Marconi began developing a wireless communication using the newly discovered phenomenon of radio waves, showing by 1901 that they could be transmitted across the Atlantic Ocean; this was the start of wireless telegraphy by radio. Voice and music had little early success. World War I accelerated the development of radio for military communications. After the war, commercial radio AM broadcasting began in the 1920s and became an important mass medium for entertainment and news. World War II again accelerated development of radio for the wartime purposes of aircraft and land communication, radio navigation and radar. Development of stereo FM broadcasting of radio