Television transmitter
A television transmitter is a transmitter, used for terrestrial television broadcasting. It is an electronic device that radiates radio waves that carry a video signal representing moving images, along with a synchronized audio channel, received by television receivers belonging to a public audience, which display the image on a screen. A television transmitter, together with the broadcast studio which originates the content, is called a television station. Television transmitters must be licensed by governments, are restricted to a certain frequency channel and power level, they transmit on frequency channels in the UHF bands. Television transmitters use one of two different technologies: analog, in which the picture and sound are transmitted by analog signals modulated onto the radio carrier wave, digital in which the picture and sound are transmitted by digital signals; the original television technology, analog television, began to be replaced in a transition beginning in 2006 in many countries with digital television systems.
These transmit pictures in a new format called HDTV which has higher resolution and a wider screen aspect ratio than analog. DTV makes more efficient use of scarce radio spectrum bandwidth, as several DTV channels can be transmitted in the same bandwidth as a single analog channel. In both analog and digital television, different countries use several incompatible modulation standards to add the video and audio signals to the radio carrier wave, The principles of analog systems are summarized as they are more complex than digital transmitters due to the multiplexing of VSB and FM modulation stages. There are many types of transmitters depending on The system standard Output power Back up facility the Modulator and Power Amplifier Stereophonic facility, for analogue TV systems Aural and visual power combining principal, for analogue TV systems Active circuit element in the final amplifier stage An international plan by ITU on broadcast standards, known as Stockholm plan defines standards used in broadcasting.
In this plan, most important figures for transmitters are radio frequency, frequency separation between aural and visual carriers and band width. The audio input is a signal with 15 kHz maximum bandwidth and 0 dBm maximum level. Preemphasis time constant is 50 µs; the signal after passing buffer stages is applied to a modulator, where it modulates an intermediate frequency carrier. The modulation technique is frequency modulation with a typical maximum deviation of 50 kHz; the video input is a composite video signal of maximum 1 volt on 75 Ω impedance. After buffer and 1 V clipping circuits, the signal is applied to the modulator where it modulates an intermediate frequency signal The modulator is an amplitude modulator which modulates the IF signal in a manner where 1 V VF corresponds to low level IF and 0 volt VF corresponds to high level IF. AM modulator produces two symmetrical side bands in the modulated signals. Thus, IF band width is two times the video band width. However, the modulator is followed by a special filter known as Vestigal sideband filter.
This filter is used to suppress a portion of one side band, thus bandwidth is reduced. Although the suppression causes phase delay problems the VSB stage includes correction circuits to equalise the phase; the modulated signal is applied to a mixer. Another input to the mixer, produced in a crystal oven oscillator is known as subcarrier; the two outputs of the mixer are the difference of two signals. Unwanted signal is filtered out and the remaining signal is the radio frequency signal; the signal is applied to the amplifier stages. The number of series amplifiers depends on the required output power; the final stage is an amplifier consisting of many parallel power transistors. But in older transmitters tetrodes or klystrons are utilized. In modern solid-state VHF and UHF transmitters, LDMOS power transistors are the device of choice for the output stage, with the latest products employing 50V LDMOS devices for higher efficiency and power density. Higher energy efficiency is possible using Envelope Tracking, which in the broadcast industry is referred to as'drain modulation'.
There are two methods: Split sound system: Actually there are two parallel transmitters one for aural and one for visual signal. The two signals are combined at the output via a high power combiner. In addition to a combiner, this system requires separate mixer and amplifiers for aural and visual signals; this is the system used in most high power applications. Intercarrier system: There are two input stages one for AF and one for VF, but the two signals are combined in low power IF circuits The mixer and the amplifiers are common to both signals and the system needs no high power combiners. So both the price of the transmitter and the power consumption is lower than that of split sound system of the same power level, but two signals passing through amplifiers produce some intermodulation products. So intercarrier system is not suitable for high p
Electromagnetic radiation
In physics, electromagnetic radiation refers to the waves of the electromagnetic field, propagating through space, carrying electromagnetic radiant energy. It includes radio waves, infrared, ultraviolet, X-rays, gamma rays. Classically, electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields that propagate at the speed of light, which, in a vacuum, is denoted c. In homogeneous, isotropic media, the oscillations of the two fields are perpendicular to each other and perpendicular to the direction of energy and wave propagation, forming a transverse wave; the wavefront of electromagnetic waves emitted from a point source is a sphere. The position of an electromagnetic wave within the electromagnetic spectrum can be characterized by either its frequency of oscillation or its wavelength. Electromagnetic waves of different frequency are called by different names since they have different sources and effects on matter. In order of increasing frequency and decreasing wavelength these are: radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.
Electromagnetic waves are emitted by electrically charged particles undergoing acceleration, these waves can subsequently interact with other charged particles, exerting force on them. EM waves carry energy and angular momentum away from their source particle and can impart those quantities to matter with which they interact. Electromagnetic radiation is associated with those EM waves that are free to propagate themselves without the continuing influence of the moving charges that produced them, because they have achieved sufficient distance from those charges. Thus, EMR is sometimes referred to as the far field. In this language, the near field refers to EM fields near the charges and current that directly produced them electromagnetic induction and electrostatic induction phenomena. In quantum mechanics, an alternate way of viewing EMR is that it consists of photons, uncharged elementary particles with zero rest mass which are the quanta of the electromagnetic force, responsible for all electromagnetic interactions.
Quantum electrodynamics is the theory of. Quantum effects provide additional sources of EMR, such as the transition of electrons to lower energy levels in an atom and black-body radiation; the energy of an individual photon is greater for photons of higher frequency. This relationship is given by Planck's equation E = hν, where E is the energy per photon, ν is the frequency of the photon, h is Planck's constant. A single gamma ray photon, for example, might carry ~100,000 times the energy of a single photon of visible light; the effects of EMR upon chemical compounds and biological organisms depend both upon the radiation's power and its frequency. EMR of visible or lower frequencies is called non-ionizing radiation, because its photons do not individually have enough energy to ionize atoms or molecules or break chemical bonds; the effects of these radiations on chemical systems and living tissue are caused by heating effects from the combined energy transfer of many photons. In contrast, high frequency ultraviolet, X-rays and gamma rays are called ionizing radiation, since individual photons of such high frequency have enough energy to ionize molecules or break chemical bonds.
These radiations have the ability to cause chemical reactions and damage living cells beyond that resulting from simple heating, can be a health hazard. James Clerk Maxwell derived a wave form of the electric and magnetic equations, thus uncovering the wave-like nature of electric and magnetic fields and their symmetry; because the speed of EM waves predicted by the wave equation coincided with the measured speed of light, Maxwell concluded that light itself is an EM wave. Maxwell's equations were confirmed by Heinrich Hertz through experiments with radio waves. According to Maxwell's equations, a spatially varying electric field is always associated with a magnetic field that changes over time. A spatially varying magnetic field is associated with specific changes over time in the electric field. In an electromagnetic wave, the changes in the electric field are always accompanied by a wave in the magnetic field in one direction, vice versa; this relationship between the two occurs without either type of field causing the other.
In fact, magnetic fields can be viewed as electric fields in another frame of reference, electric fields can be viewed as magnetic fields in another frame of reference, but they have equal significance as physics is the same in all frames of reference, so the close relationship between space and time changes here is more than an analogy. Together, these fields form a propagating electromagnetic wave, which moves out into space and need never again interact with the source; the distant EM field formed in this way by the acceleration of a charge carries energy with it that "radiates" away through space, hence the term. Maxwell's equations established that some charges and currents produce a local type of electromagnetic field near them that does not have the behaviour of EMR. Currents directly produce a magnetic field, but it is of a magnetic dipole type that dies out with distance from the current. In a similar manner, moving charges pushed apart in a conductor by a changing electrical potential produce an electric dipole type electric
Transmission line
In radio-frequency engineering, a transmission line is a specialized cable or other structure designed to conduct alternating current of radio frequency, that is, currents with a frequency high enough that their wave nature must be taken into account. Transmission lines are used for purposes such as connecting radio transmitters and receivers with their antennas, distributing cable television signals, trunklines routing calls between telephone switching centres, computer network connections and high speed computer data buses; this article covers two-conductor transmission line such as parallel line, coaxial cable and microstrip. Some sources refer to waveguide, dielectric waveguide, optical fibre as transmission line, however these lines require different analytical techniques and so are not covered by this article. Ordinary electrical cables suffice to carry low frequency alternating current, such as mains power, which reverses direction 100 to 120 times per second, audio signals. However, they cannot be used to carry currents in the radio frequency range, above about 30 kHz, because the energy tends to radiate off the cable as radio waves, causing power losses.
Radio frequency currents tend to reflect from discontinuities in the cable such as connectors and joints, travel back down the cable toward the source. These reflections act as bottlenecks. Transmission lines use specialized construction, impedance matching, to carry electromagnetic signals with minimal reflections and power losses; the distinguishing feature of most transmission lines is that they have uniform cross sectional dimensions along their length, giving them a uniform impedance, called the characteristic impedance, to prevent reflections. Types of transmission line include parallel line, coaxial cable, planar transmission lines such as stripline and microstrip; the higher the frequency of electromagnetic waves moving through a given cable or medium, the shorter the wavelength of the waves. Transmission lines become necessary when the transmitted frequency's wavelength is sufficiently short that the length of the cable becomes a significant part of a wavelength. At microwave frequencies and above, power losses in transmission lines become excessive, waveguides are used instead, which function as "pipes" to confine and guide the electromagnetic waves.
Some sources define waveguides as a type of transmission line. At higher frequencies, in the terahertz and visible ranges, waveguides in turn become lossy, optical methods, are used to guide electromagnetic waves; the theory of sound wave propagation is similar mathematically to that of electromagnetic waves, so techniques from transmission line theory are used to build structures to conduct acoustic waves. Mathematical analysis of the behaviour of electrical transmission lines grew out of the work of James Clerk Maxwell, Lord Kelvin and Oliver Heaviside. In 1855 Lord Kelvin formulated a diffusion model of the current in a submarine cable; the model predicted the poor performance of the 1858 trans-Atlantic submarine telegraph cable. In 1885 Heaviside published the first papers that described his analysis of propagation in cables and the modern form of the telegrapher's equations. In many electric circuits, the length of the wires connecting the components can for the most part be ignored; that is, the voltage on the wire at a given time can be assumed to be the same at all points.
However, when the voltage changes in a time interval comparable to the time it takes for the signal to travel down the wire, the length becomes important and the wire must be treated as a transmission line. Stated another way, the length of the wire is important when the signal includes frequency components with corresponding wavelengths comparable to or less than the length of the wire. A common rule of thumb is that the cable or wire should be treated as a transmission line if the length is greater than 1/10 of the wavelength. At this length the phase delay and the interference of any reflections on the line become important and can lead to unpredictable behaviour in systems which have not been designed using transmission line theory. For the purposes of analysis, an electrical transmission line can be modelled as a two-port network, as follows: In the simplest case, the network is assumed to be linear, the two ports are assumed to be interchangeable. If the transmission line is uniform along its length its behaviour is described by a single parameter called the characteristic impedance, symbol Z0.
This is the ratio of the complex voltage of a given wave to the complex current of the same wave at any point on the line. Typical values of Z0 are 50 or 75 ohms for a coaxial cable, about 100 ohms for a twisted pair of wires, about 300 ohms for a common type of untwisted pair used in radio transmission; when sending power down a transmission line, it is desirable that as much power as possible will be absorbed by the load and as little as possible will be reflected back to the source. This can be ensured by making the load impedance equal to Z0, in which case the transmission line is said to be matched; some of the power, fed into a transmission line is lost because of its resistance. This effect is called resistive loss. At high frequencies, another effect cal
Digital signal
A digital signal is a signal, being used to represent data as a sequence of discrete values. This contrasts with an analog signal. Simple digital signals represent information in discrete bands of analog levels. All levels within a band of values represent the same information state. In most digital circuits, the signal can have two possible values, they are represented by two voltage bands: one near a reference value, the other a value near the supply voltage. These correspond to the two values "zero" and "one" of the Boolean domain, so at any given time a binary signal represents one binary digit; because of this discretization small changes to the analog signal levels do not leave the discrete envelope, as a result are ignored by signal state sensing circuitry. As a result, digital signals have noise immunity. Digital signals having more than two states are used. For example, signals that can assume three possible states are called three-valued logic. In a digital signal, the physical quantity representing the information may be a variable electric current or voltage, the intensity, phase or polarization of an optical or other electromagnetic field, acoustic pressure, the magnetization of a magnetic storage media, etcetera.
Digital signals are used in all digital electronics, notably computing equipment and data transmission. The term digital signal has related definitions in different contexts. In digital electronics a digital signal is a pulse train, i.e. a sequence of fixed-width square-wave electrical pulses or light pulses, each occupying one of a discrete number of levels of amplitude. A special case is a logic signal or a binary signal, which varies between a low and a high signal level. In digital signal processing, a digital signal is a representation of a physical signal, a sampled and quantized. A digital signal is an abstraction, discrete in time and amplitude; the signal's value only exists at regular time intervals, since only the values of the corresponding physical signal at those sampled moments are significant for further digital processing. The digital signal is a sequence of codes drawn from a finite set of values; the digital signal may be stored, processed or transmitted physically as a pulse-code modulation signal.
In digital communications, a digital signal is a continuous-time physical signal, alternating between a discrete number of waveforms, representing a bitstream. The shape of the waveform depends the transmission scheme, which may be either a line coding scheme allowing baseband transmission; such a carrier-modulated sine wave is considered a digital signal in literature on digital communications and data transmission, but considered as a bitstream converted to an analog signal in electronics and computer networking. In communications, sources of interference are present, noise is a significant problem; the effects of interference are minimized by filtering off interfering signals as much as possible and by using data redundancy. The main advantages of digital signals for communications are considered to be the noise immunity to noise capability, the ability, in many cases such as with audio and video data, to use data compression to decrease the bandwidth, required on the communication media.
A waveform that switches representing the two states of a Boolean value is referred to as a digital signal or logic signal or binary signal when it is interpreted in terms of only two possible digits. The two states are represented by some measurement of an electrical property: Voltage is the most common, but current is used in some logic families. A threshold is designed for each logic family; when below that threshold, the signal is low. The clock signal is a special digital signal, used to synchronize many digital circuits; the image shown can be considered the waveform of a clock signal. Logic changes are triggered either by the falling edge; the rising edge is the transition from a low voltage to a high voltage. The falling edge is the transition from a high voltage to a low one. Although in a simplified and idealized model of a digital circuit, we may wish for these transitions to occur instantaneously, no real world circuit is purely resistive and therefore no circuit can change voltage levels.
This means that during a short, finite transition time the output may not properly reflect the input, will not correspond to either a logically high or low voltage. To create a digital signal, an analog signal must be modulated with a control signal to produce it; the simplest modulation, a type of unipolar encoding, is to switch on and off a DC signal, so that high voltages represent a'1' and low voltages are'0'. In digital radio schemes one or more carrier waves are amplitude, frequency or phase modulated by the control signal to produce a digital signal suitable for transmission. Asymmetric Digital Subscriber Line over telephone wires, does not use binary logic.
Two-way radio
A two-way radio is a radio that can both transmit and receive a signal, unlike a broadcast receiver which only receives content. It is an audio transceiver designed for bidirectional person-to-person voice communication with other users with similar radios using the same radio frequency. Two-way radios are available in stationary base and hand-held portable configurations. Hand-held two-way radios are called walkie-talkies, handie-talkies or hand-helds. Two-way radio systems operate in a half-duplex mode: the operator can talk, or he can listen, but not at the same time. A push-to-talk or Press To Transmit button activates the transmitter. Other Full-duplex is achieved by the use of two different frequencies or by frequency-sharing methods to carry the two directions of the conversation simultaneously. Methods for mitigating the self interference caused by simultaneous transmission and reception on different but close-spaced frequencies include using two antennas, or dynamic solid-state filters.
Time-division technologies are used for mitigating self interference by simultaneous transmission and reception on the same frequency. Installation of receivers and transmitters at the same fixed location allowed exchange of messages wirelessly; as early as 1907, two-way telegraphy traffic across the Atlantic Ocean was commercially available. By 1912, commercial and military ships carried both transmitters and receivers, allowing two-way communication in close to real-time with a ship, out of sight of land; the first mobile two-way radio was developed in Australia in 1923 by Senior Constable Frederick William Downie of the Victorian Police. The Victoria Police were the first in the world to use wireless communication in cars, putting an end to the inefficient status reports via public telephone boxes, used until that time; the first sets took up the entire back seat of the Lancia patrol cars. As radio equipment became more powerful and easier to use, smaller vehicles had two-way radio communication equipment installed.
Installation of radio equipment in aircraft allowed scouts to report back observations in real-time, not requiring the pilot to drop messages to troops on the ground below or to land and make a personal report. In 1933, the Bayonne, New Jersey police department operated a two-way system between a central fixed station and radio transceivers installed in police cars. During World War II walkie-talkie hand-held radio transceivers were extensively used by air and ground troops, both by the Allies and the Axis. Early two-way schemes allowed only one station to transmit at a time while others listened, since all signals were on the same radio frequency – this was called "simplex" mode. Code and voice operations required a simple communication protocol to allow all stations to cooperate in using the single radio channel, so that one station's transmissions were not obscured by another's. By using receivers and transmitters tuned to different frequencies and solving the problems introduced by operation of a receiver next to a transmitter, simultaneous transmission and reception was possible at each end of a radio link, in so-called "full duplex" mode.
The first radio systems could not transmit voice. This required training of operators in use of Morse code. On a ship, the radio operating officers had no other duties than handling radio messages; when voice transmission became possible, dedicated operators were no longer required and two-way radio use became more common. Today's two-way mobile radio equipment is nearly as simple to use as a household telephone, from the point of view of operating personnel, thereby making two-way communications a useful tool in a wide range of personal and military roles. Two-way radio systems can be classified in several ways depending on their attributes. Conventional radios operate on fixed RF channels. In the case of radios with multiple channels, they operate on one channel at a time; the proper channel is selected by a user. The user operates a channel selector on the radio control panel to pick the appropriate channel. In multi-channel systems, channels are used for separate purposes. A channel may be reserved for a geographic area.
In a functional channel system, one channel may allow City of Springfield road repair crews to talk to the City of Springfield's road maintenance office. A second channel may allow road repair crews to communicate with state highway department crews. In a wide-area or geographic system, a taxi company may use one channel to communicate in the Boston, Massachusetts area and a second channel when taxis are in Providence, Rhode Island; this is referred to as Multisite operation. In this case, the driver or the radio must switch channels to maintain coverage when transitioning between each area. Most modern conventional digital radios and systems are capable of automatic "roaming" where the radio automatically switches channels on a dynamic basis; the radio accomplishes this based on the received signal strength of the radio repeater's recurring "beacon" signal and a "site" or "roam" list that identifies available geographic channels. Some analog conventional systems can be equipped with a feature called "vote-scan" that provides more limited roaming.
Radio "simulcast" technology can be used in adjacent areas, where each site is equipped with the same channel. Here, the transmitters must be synchronized, a centralized voter or receiver comparator device is required to select the best quality sign
Radiolocation
Radiolocating is the process of finding the location of something through the use of radio waves. It refers to passive uses radar—as well as detecting buried cables, water mains, other public utilities, it is similar to radionavigation, but radiolocation refers to passively finding a distant object rather than one's own position. Both are types of radiodetermination. Radiolocation is used in real-time locating systems for tracking valuable assets. An object can be located by measuring the characteristics of received radio waves; the radio waves may be transmitted by the object to be located. A stud finder uses radiolocation. One technique measures a distance by using the difference in the power of the received signal strength as compared to the originating signal strength. Another technique uses the time of arrival, when the time of transmission and speed of propagation are known. Combining TOA data from several receivers at different known locations can provide an estimate of position in the absence of knowledge of the time of transmission.
The angle of arrival at a receiving station can be determined by the use of a directional antenna, or by differential time of arrival at an array of antennas with known location. AOA information may be combined with distance estimates from the techniques described to establish the location of a transmitter or backscatterer. Alternatively, the AOA at two receiving stations of known location establishes the position of the transmitter; the use of multiple receivers to locate a transmitter is known as multilateration. Estimates are improved when the transmission characteristics of the medium is factored into the calculations. For RSSI this means electromagnetic permeability. Use of RSSI to locate a transmitter from a single receiver requires that both the transmitted power from the object to be located are known, that the propagation characteristics of the intervening region are known. In empty space, signal strength decreases as the inverse square of the distance for distances large compared to a wavelength and compared to the object to be located, but in most real environments, a number of impairments can occur: absorption, refraction and reflection.
Absorption is negligible for radio propagation in air at frequencies less than about 10 GHz, but becomes important at multi-GHz frequencies where rotational molecular states can be excited. Refraction is important at long ranges due to gradients in moisture content and temperature in the atmosphere. In urban, mountainous, or indoor environments, obstruction by intervening obstacles and reflection from nearby surfaces are common, contribute to multipath distortion: that is, reflected and delayed replicates of the transmitted signal are combined at the receiver. Signals from different paths can add constructively or destructively: such variations in amplitude are known as fading; the dependence of signal strength on position of transmitter and receiver becomes complex and non-monotonic, making single-receiver estimates of position inaccurate and unreliable. Multilateration using many receivers is combined with calibration measurements to improve accuracy. TOA and AOA measurements are subject to multipath errors when the direct path from the transmitter to receiver is blocked by an obstacle.
Time of arrival measurements are most accurate when the signal has distinct time-dependent features on the scale of interest—for example, when it is composed of short pulses of known duration—but Fourier transform theory shows that in order to change amplitude or phase on a short time scale, a signal must use a broad bandwidth. For example, to create a pulse of about 1 ns duration sufficient to identify location to within 0.3 m, a bandwidth of 1 GHz is required. In many regions of the radio spectrum, emission over such a broad bandwidth is not allowed by the relevant regulatory authorities, in order to avoid interference with other narrowband users of the spectrum. In the United States, unlicensed transmission is allowed in several bands, such as the 902-928 MHz and 2.4-2.483 GHz Industrial and Medical ISM bands, but high-power transmission cannot extend outside of these bands. However, several jurisdictions now allow ultrawideband transmission over GHz or multi-GHz bandwidths, with constraints on transmitted power to minimize interference with other spectrum users.
UWB pulses can be narrow in time, provide accurate estimates of TOA in urban or indoor environments. Radiolocation is employed in a wide variety of military activities. Radar systems use a combination of TOA and AOA to determine a backscattering object's position using a single receiver. In Doppler radar, the Doppler shift is taken into account, determining velocity rather than location. Real Time Location Systems RTLS using calibrated RTLS, TDOA, are commercially available; the used Global Positioning System is based on TOA of signals from satellites at known positions. Radiolocation is used in cellular telephony via base stations. Most this is done through trilateration between radio towers; the location of the Caller or handset can be determined several ways: angle of arrival requires at least two towers, locating the caller at the point where the lines along the angles from each tower intersect time difference of arrival resp. time of arrival works using multilaterati
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