In radio communications, a radio receiver known as a receiver, wireless or radio is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna; the antenna intercepts radio waves and converts them to tiny alternating currents which are applied to the receiver, the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, recovers the desired information through demodulation; the information produced by the receiver may be in the form of sound, moving data. A radio receiver may be a separate piece of electronic equipment, or an electronic circuit within another device. Radio receivers are widely used in modern technology, as components of communications, remote control, wireless networking systems. In consumer electronics, the terms radio and radio receiver are used for receivers designed to reproduce sound transmitted by radio broadcasting stations the first mass-market commercial radio application.
The most familiar form of radio receiver is a broadcast receiver just called a radio, which receives audio programs intended for public reception transmitted by local radio stations. The sound is reproduced either by a loudspeaker in the radio or an earphone which plugs into a jack on the radio; the radio requires electric power, provided either by batteries inside the radio or a power cord which plugs into an electric outlet. All radios have a volume control to adjust the loudness of the audio, some type of "tuning" control to select the radio station to be received. Modulation is the process of adding information to a radio carrier wave. Two types of modulation are used in analog radio broadcasting systems. In amplitude modulation the strength of the radio signal is varied by the audio signal. AM broadcasting is allowed in the AM broadcast bands which are between 148 and 283 kHz in the longwave range, between 526 and 1706 kHz in the medium frequency range of the radio spectrum. AM broadcasting is permitted in shortwave bands, between about 2.3 and 26 MHz, which are used for long distance international broadcasting.
In frequency modulation the frequency of the radio signal is varied by the audio signal. FM broadcasting is permitted in the FM broadcast bands between about 65 and 108 MHz in the high frequency range; the exact frequency ranges vary somewhat in different countries. FM stereo radio stations broadcast in stereophonic sound, transmitting two sound channels representing left and right microphones. A stereo receiver contains the additional circuits and parallel signal paths to reproduce the two separate channels. A monaural receiver, in contrast, only receives a single audio channel, a combination of the left and right channels. While AM stereo transmitters and receivers exist, they have not achieved the popularity of FM stereo. Most modern radios are "AM/FM" radios, are able to receive both AM and FM radio stations, have a switch to select which band to receive. Digital audio broadcasting is an advanced radio technology which debuted in some countries in 1998 that transmits audio from terrestrial radio stations as a digital signal rather than an analog signal as AM and FM do.
Its advantages are that DAB has the potential to provide higher quality sound than FM, has greater immunity to radio noise and interference, makes better use of scarce radio spectrum bandwidth, provides advanced user features such as electronic program guide, sports commentaries, image slideshows. Its disadvantage is that it is incompatible with previous radios so that a new DAB receiver must be purchased; as of 2017, 38 countries offer DAB, with 2,100 stations serving listening areas containing 420 million people. Most countries plan an eventual switchover from FM to DAB; the United States and Canada have chosen not to implement DAB. DAB radio stations work differently from AM or FM stations: a single DAB station transmits a wide 1,500 kHz bandwidth signal that carries from 9 to 12 channels from which the listener can choose. Broadcasters can transmit a channel at a range of different bit rates, so different channels can have different audio quality. In different countries DAB stations broadcast in either Band L band.
The signal strength of radio waves decreases the farther they travel from the transmitter, so a radio station can only be received within a limited range of its transmitter. The range depends on the power of the transmitter, the sensitivity of the receiver and internal noise, as well as any geographical obstructions such as hills between transmitter and receiver. AM broadcast band radio waves travel as ground waves which follow the contour of the Earth, so AM radio stations can be reliably received at hundreds of miles distance. Due to their higher frequency, FM band radio signals cannot travel far beyond the visual horizon; however FM radio has higher fidelity. So in many countries serious music is only broadcast by FM stations, AM stations specialize in radio news, talk radio, sports. Like FM, DAB signals travel by line of sight so reception distances are
A wireless microphone, or cordless microphone, is a microphone without a physical cable connecting it directly to the sound recording or amplifying equipment with which it is associated. Known as a radio microphone, it has a small, battery-powered radio transmitter in the microphone body, which transmits the audio signal from the microphone by radio waves to a nearby receiver unit, which recovers the audio; the other audio equipment is connected to the receiver unit by cable. In one type the transmitter is contained within the handheld microphone body. In another type, called a "lavalier microphone" or "lav", a small microphone clipped to the user's lapel is connected by wire to a transmitter unit concealed under his clothes. In a third type the transmitter is a headset, with a microphone on a boom extending in front of the user's mouth. Wireless microphones are used in the entertainment industry, television broadcasting, public speaking to allow public speakers, interviewers and entertainers to move about while using a microphone without requiring a cable attached to the microphone.
Wireless microphones use the VHF or UHF frequency bands since they allow the transmitter to use a small unobtrusive antenna. Cheap units use a fixed frequency but most units allow a choice of several frequency channels, in case of interference on a channel or to allow the use of multiple microphones at the same time. FM modulation is used, although some models use digital modulation to prevent unauthorized reception by scanner radio receivers; some models transmit on two frequency channels for diversity reception to prevent nulls from interrupting transmission as the performer moves around. A few low cost models use infrared light, although these require a direct line of sight between microphone and receiver. Various individuals and organizations claim to be the inventors of the wireless microphone. From about 1945 there were schematics and hobbyist kits offered in Popular Science and Popular Mechanics for making a wireless microphone that would transmit the voice to a nearby radio. Figure skater and Royal Air Force flight engineer Reg Moores developed a radio microphone in 1947 that he first used in the Tom Arnold production "Aladdin on Ice" at Brighton's sports stadium from September 1949 through the Christmas season.
Moores affixed the wireless transmitter to the costume of the character Abanazar, it worked perfectly. Moores did not patent his idea; the producers of the ice show decided. In 1972 Moores donated his 1947 prototype to the Science Museum in London. Herbert "Mac" McClelland, founder of McClelland Sound in Wichita, fabricated a wireless microphone to be worn by baseball umpires at major league games broadcast by NBC from Lawrence–Dumont Stadium in 1951; the transmitter was strapped to the umpire's back. Mac's brother was Harold M. McClelland, the chief communications architect of the U. S. Air Force. Shure Brothers claims that its "Vagabond" system from 1953 was the first "wireless microphone system for performers." Its field of coverage was a circle of "approximately 700 square feet," which corresponds to a line-of-sight distance of only 15 feet from the receiver. In 1957, the German audio equipment manufacturer Sennheiser, at that time called Lab W, working with the German broadcaster Norddeutscher Rundfunk, exhibited a wireless microphone system.
From 1958 the system was marketed through Telefunken under the name of Mikroport. The pocket-sized Mikroport incorporated a dynamic moving-coil cartridge microphone with a cardioid pickup pattern, it transmitted at 37 MHz with a specified range of 300 feet. The first recorded patent for a wireless microphone was filed by Raymond A. Litke, an American electrical engineer with Educational Media Resources and San Jose State College, who invented a wireless microphone in 1957 to meet the multimedia needs for television and classroom instruction, his U. S. patent number 3134074 was granted in May 1964. Two microphone types were made available for purchase in 1959: lavalier; the main transmitter module was a cigar-sized device. Vega Electronics Corporation manufactured the design in 1959, producing it as a product called the Vega-Mike; the device was first used by the broadcast media at the 1960 Democratic and Republican National Conventions. It allowed television reporters to roam the floor of the convention to interview participants, including presidential candidates John F. Kennedy and Richard Nixon.
Introduced in 1958, the Sony CR-4 wireless microphone was being recommended as early as 1960 for theatre performances and nightclub acts. Animal trainers at Marineland of the Pacific in California were wearing the $250 device for performances in 1961; the 27.12 MHz solid-state FM transmitter was capable of fitting into a shirt pocket. Said to be effective out to 100 feet, it mounted a flexible dangling antenna and a detachable dynamic microphone; the tube-based receiver incorporated a carrying drawer for the transmitter and a small monitor loudspeaker with volume control. Another German equipment manufacturer, claims that the first wireless microphone was invented by Hung C. Lin. Called the "transistophone," it went into production in 1962; the first time that a wireless microphone was used to record sound during filming of a motion picture was on Rex Harrison in the 1964 film My Fair Lady, through the efforts of Academy Award-winning H
The Universal Mobile Telecommunications System is a third generation mobile cellular system for networks based on the GSM standard. Developed and maintained by the 3GPP, UMTS is a component of the International Telecommunications Union IMT-2000 standard set and compares with the CDMA2000 standard set for networks based on the competing cdmaOne technology. UMTS uses wideband code division multiple access radio access technology to offer greater spectral efficiency and bandwidth to mobile network operators. UMTS specifies a complete network system, which includes the radio access network, the core network and the authentication of users via SIM cards; the technology described in UMTS is sometimes referred to as Freedom of Mobile Multimedia Access or 3GSM. Unlike EDGE and CDMA2000, UMTS requires new base stations and new frequency allocations. UMTS supports maximum theoretical data transfer rates of 42 Mbit/s when Evolved HSPA is implemented in the network. Users in deployed networks can expect a transfer rate of up to 384 kbit/s for Release'99 handsets, 7.2 Mbit/s for High-Speed Downlink Packet Access handsets in the downlink connection.
These speeds are faster than the 9.6 kbit/s of a single GSM error-corrected circuit switched data channel, multiple 9.6 kbit/s channels in High-Speed Circuit-Switched Data and 14.4 kbit/s for CDMAOne channels. Since 2006, UMTS networks in many countries have been or are in the process of being upgraded with High-Speed Downlink Packet Access, sometimes known as 3.5G. HSDPA enables downlink transfer speeds of up to 21 Mbit/s. Work is progressing on improving the uplink transfer speed with the High-Speed Uplink Packet Access. Longer term, the 3GPP Long Term Evolution project plans to move UMTS to 4G speeds of 100 Mbit/s down and 50 Mbit/s up, using a next generation air interface technology based upon orthogonal frequency-division multiplexing; the first national consumer UMTS networks launched in 2002 with a heavy emphasis on telco-provided mobile applications such as mobile TV and video calling. The high data speeds of UMTS are now most utilised for Internet access: experience in Japan and elsewhere has shown that user demand for video calls is not high, telco-provided audio/video content has declined in popularity in favour of high-speed access to the World Wide Web—either directly on a handset or connected to a computer via Wi-Fi, Bluetooth or USB.
UMTS combines three different terrestrial air interfaces, GSM's Mobile Application Part core, the GSM family of speech codecs. The air interfaces are called UMTS Terrestrial Radio Access. All air interface options are part of ITU's IMT-2000. In the most popular variant for cellular mobile telephones, W-CDMA is used, it is called "Uu interface", as it links User Equipment to the UMTS Terrestrial Radio Access Network Please note that the terms W-CDMA, TD-CDMA and TD-SCDMA are misleading. While they suggest covering just a channel access method, they are the common names for the whole air interface standards. W-CDMA or WCDMA, along with UMTS-FDD, UTRA-FDD, or IMT-2000 CDMA Direct Spread is an air interface standard found in 3G mobile telecommunications networks, it supports conventional cellular voice, text and MMS services, but can carry data at high speeds, allowing mobile operators to deliver higher bandwidth applications including streaming and broadband Internet access. W-CDMA uses the DS-CDMA channel access method with a pair of 5 MHz wide channels.
In contrast, the competing CDMA2000 system uses one or more available 1.25 MHz channels for each direction of communication. W-CDMA systems are criticized for their large spectrum usage, which delayed deployment in countries that acted slowly in allocating new frequencies for 3G services; the specific frequency bands defined by the UMTS standard are 1885–2025 MHz for the mobile-to-base and 2110–2200 MHz for the base-to-mobile. In the US, 1710–1755 MHz and 2110–2155 MHz are used instead, as the 1900 MHz band was used. While UMTS2100 is the most deployed UMTS band, some countries' UMTS operators use the 850 MHz and/or 1900 MHz bands, notably in the US by AT&T Mobility, New Zealand by Telecom New Zealand on the XT Mobile Network and in Australia by Telstra on the Next G network; some carriers such as T-Mobile use band numbers to identify the UMTS frequencies. For example, Band I, Band IV, Band V. UMTS-FDD is an acronym for Universal Mobile Telecommunications System - frequency-division duplexing and a 3GPP standardized version of UMTS networks that makes use of frequency-division duplexing for duplexing over an UMTS Terrestrial Radio Access air interface.
W-CDMA is the basis of Japan's NTT DoCoMo's FOMA service and the most-commonly used member of the Universal Mobile Telecommunications System family and sometimes used as a synonym for UMTS. It uses the DS-CDMA channel access method and the FDD duplexing method to achieve higher speeds and support more users compared to most used time division multiple access and time division duplex schemes. While not an evolutionary upgrade on the airside, it uses the same core network as the 2G GSM networks deployed worldwide, allowing dual mode mobile operation al
In electronics and telecommunications, a transmitter or radio transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, applied to the antenna; when excited by this alternating current, the antenna radiates radio waves. Transmitters are necessary component parts of all electronic devices that communicate by radio, such as radio and television broadcasting stations, cell phones, walkie-talkies, wireless computer networks, Bluetooth enabled devices, garage door openers, two-way radios in aircraft, spacecraft, radar sets and navigational beacons; the term transmitter is limited to equipment that generates radio waves for communication purposes. Generators of radio waves for heating or industrial purposes, such as microwave ovens or diathermy equipment, are not called transmitters though they have similar circuits; the term is popularly used more to refer to a broadcast transmitter, a transmitter used in broadcasting, as in FM radio transmitter or television transmitter.
This usage includes both the transmitter proper, the antenna, the building it is housed in. A transmitter can be a separate piece of electronic equipment, or an electrical circuit within another electronic device. A transmitter and a receiver combined in one unit is called a transceiver; the term transmitter is abbreviated "XMTR" or "TX" in technical documents. The purpose of most transmitters is radio communication of information over a distance; the information is provided to the transmitter in the form of an electronic signal, such as an audio signal from a microphone, a video signal from a video camera, or in wireless networking devices, a digital signal from a computer. The transmitter combines the information signal to be carried with the radio frequency signal which generates the radio waves, called the carrier signal; this process is called modulation. The information can be added to the carrier in several different ways, in different types of transmitters. In an amplitude modulation transmitter, the information is added to the radio signal by varying its amplitude.
In a frequency modulation transmitter, it is added by varying the radio signal's frequency slightly. Many other types of modulation are used; the radio signal from the transmitter is applied to the antenna, which radiates the energy as radio waves. The antenna may be enclosed inside the case or attached to the outside of the transmitter, as in portable devices such as cell phones, walkie-talkies, garage door openers. In more powerful transmitters, the antenna may be located on top of a building or on a separate tower, connected to the transmitter by a feed line, a transmission line. Electromagnetic waves are radiated by electric charges undergoing acceleration. Radio waves, electromagnetic waves of radio frequency, are generated by time-varying electric currents, consisting of electrons flowing through a metal conductor called an antenna which are changing their velocity or direction and thus accelerating. An alternating current flowing back and forth in an antenna will create an oscillating magnetic field around the conductor.
The alternating voltage will charge the ends of the conductor alternately positive and negative, creating an oscillating electric field around the conductor. If the frequency of the oscillations is high enough, in the radio frequency range above about 20 kHz, the oscillating coupled electric and magnetic fields will radiate away from the antenna into space as an electromagnetic wave, a radio wave. A radio transmitter is an electronic circuit which transforms electric power from a power source into a radio frequency alternating current to apply to the antenna, the antenna radiates the energy from this current as radio waves; the transmitter impresses information such as an audio or video signal onto the radio frequency current to be carried by the radio waves. When they strike the antenna of a radio receiver, the waves excite similar radio frequency currents in it; the radio receiver extracts the information from the received waves. A practical radio transmitter consists of these parts: A power supply circuit to transform the input electrical power to the higher voltages needed to produce the required power output.
An electronic oscillator circuit to generate the radio frequency signal. This generates a sine wave of constant amplitude called the carrier wave, because it serves to "carry" the information through space. In most modern transmitters, this is a crystal oscillator in which the frequency is controlled by the vibrations of a quartz crystal; the frequency of the carrier wave is considered the frequency of the transmitter. A modulator circuit to add the information to be transmitted to the carrier wave produced by the oscillator; this is done by varying some aspect of the carrier wave. The information is provided to the transmitter either in the form of an audio signal, which represents sound, a video signal which represents moving images, or for data in the form of a binary digital signal which represents a sequence of bits, a bitstream. Different types of transmitters use different modulation methods to transmit information: In an AM transmitter the amplitude of the carrier wave is varied in proportion to the modulation signal.
In an FM transmitter the frequency of the carrier is varied by the modulation signal. In an FSK transmitter, which transmits digital data, the frequency of the carrier is shifted between two frequencies which represent the two binary digits, 0 and 1. Many oth
Asymmetry is the absence of, or a violation of, symmetry. Symmetry is an important property of both physical and abstract systems and it may be displayed in precise terms or in more aesthetic terms; the absence of or violation of symmetry that are either expected or desired can have important consequences for a system. Due to how cells divide in organisms, asymmetry in organisms is usual in at least one dimension, with biological symmetry being common in at least one dimension. Louis Pasteur proposed that biological molecules are asymmetric because the cosmic forces that preside over their formation are themselves asymmetric. While at his time, now, the symmetry of physical processes are highlighted, it is known that there are fundamental physical asymmetries, starting with time. Asymmetry is an important and widespread trait, having evolved numerous times in many organisms and at many levels of organisation. Benefits of asymmetry sometimes have to do with improved spatial arrangements, such as the left human lung being smaller, having one fewer lobes than the right lung to make room for the asymmetrical heart.
In other examples, division of function between the right and left half may have been beneficial and has driven the asymmetry to become stronger. Such an explanation is given for mammal hand or paw preference, an asymmetry in skill development in mammals. Training the neural pathways in a skill with one hand may take less effort than doing the same with both hands. Nature provides several examples of handedness in traits that are symmetric; the following are examples of animals with obvious left-right asymmetries: Most snails, because of torsion during development, show remarkable asymmetry in the shell and in the internal organs. Fiddler crabs have one small claw; the narwhal's tusk is a left incisor which can grow up to 10 feet in length and forms a left-handed helix. Flatfish have evolved to swim with one side upward, as a result have both eyes on one side of their heads. Several species of owls exhibit asymmetries in the size and positioning of their ears, thought to help locate prey. Many animals have asymmetric male genitalia.
The evolutionary cause behind this is, in most cases, still a mystery. Certain disturbances during the development of the organism, resulting in birth defects. Injuries after cell division that cannot be biologically repaired, such as a lost limb from an accident. Since birth defects and injuries are to indicate poor health of the organism, defects resulting in asymmetry put an animal at a disadvantage when it comes to finding a mate. In particular, a degree of facial symmetry is associated with physical attractiveness, but complete symmetry is both impossible and unattractive. Pre-modern architectural styles tended to place an emphasis on symmetry, except where extreme site conditions or historical developments lead away from this classical ideal. To the contrary and postmodern architects became much more free to use asymmetry as a design element. While most bridges employ a symmetrical form due to intrinsic simplicities of design and fabrication and economical use of materials, a number of modern bridges have deliberately departed from this, either in response to site-specific considerations or to create a dramatic design statement.
Some asymmetrical structures In fire-resistance rated wall assemblies, used in passive fire protection, but not limited to, high-voltage transformer fire barriers, asymmetry is a crucial aspect of design. When designing a facility, it is not always certain, that in the event of fire, which side a fire may come from. Therefore, many building codes and fire test standards outline, that a symmetrical assembly, need only be tested from one side, because both sides are the same. However, as soon as an assembly is asymmetrical, both sides must be tested and the test report is required to state the results for each side. In practical use, the lowest result achieved is the one. Neither the test sponsor, nor the laboratory can go by an opinion or deduction as to which side was in more peril as a result of contemplated testing and test only one side. Both must be tested in order to be compliant with test standards and building codes There are no a and b such that a < b and b < a. This form of asymmetry is an asymmetrical relation.
Certain molecules are chiral. Chemically identical molecules with different chirality are called enantiomers. Asymmetry arises in physics in a number of different realms; the original non-statistical formulation of thermodynamics was asymmetrical in time: it claimed that the entropy in a closed system can only increase with time. This was using the Clausius' Theorem; the theory of statistical mechanics, however, is symmetric in time. Although it states that a system below maximum entropy is likely to evolve towards higher entropy, it states that such a system is likely to have evolved from higher entropy. Symmetry is one of the most powerful tools in particle physics, because it has become evident that all laws of nature originate in symmetries. Violations of symmetry therefore present theoretical and experimental puzzles that lead to
Closed-circuit television known as video surveillance, is the use of video cameras to transmit a signal to a specific place, on a limited set of monitors. It differs from broadcast television in that the signal is not transmitted, though it may employ point to point, point to multipoint, or mesh wired or wireless links. Though all video cameras fit this definition, the term is most applied to those used for surveillance in areas that may need monitoring such as banks and other areas where security is needed. Though Videotelephony is called'CCTV' one exception is the use of video in distance education, where it is an important tool. Surveillance of the public using CCTV is common in many areas around the world. In recent years, the use of body worn video cameras has been introduced as a new form of surveillance used in law enforcement, with cameras located on a police officer's chest or head. Video surveillance has generated significant debate about balancing its use with individuals' right to privacy when in public.
In industrial plants, CCTV equipment may be used to observe parts of a process from a central control room, for example when the environment is not suitable for humans. CCTV systems may only as required to monitor a particular event. A more advanced form of CCTV, utilizing digital video recorders, provides recording for many years, with a variety of quality and performance options and extra features. More decentralized IP cameras equipped with megapixel sensors, support recording directly to network-attached storage devices, or internal flash for stand-alone operation. There are about 350 million surveillance cameras worldwide as of 2016. About 65% of these cameras are installed in Asia; the growth of CCTV has been slowing in recent years. The first CCTV system was installed by Siemens AG at Test Stand VII in Peenemünde, Nazi Germany in 1942, for observing the launch of V-2 rockets; the noted German engineer Walter Bruch was responsible for the technological design and installation of the system.
In the U. S. the first commercial closed-circuit television system became available in 1949, called Vericon. Little is known about Vericon except it was advertised as not requiring a government permit; the earliest video surveillance systems involved constant monitoring because there was no way to record and store information. The development of reel-to-reel media enabled the recording of surveillance footage; these systems required magnetic tapes to be changed manually, a time consuming and unreliable process, with the operator having to manually thread the tape from the tape reel through the recorder onto an empty take-up reel. Due to these shortcomings, video surveillance was not widespread. VCR technology became available in the 1970s, making it easier to record and erase information, the use of video surveillance became more common. During the 1990s, digital multiplexing was developed, allowing several cameras to record at once, as well as time lapse and motion-only recording; this increased savings of time and money which led to an increase in the use of CCTV.
CCTV technology has been enhanced with a shift toward Internet-based products and systems, other technological developments. Closed-circuit television was used as a form of pay-per-view theatre television for sports such as professional boxing and professional wrestling. Boxing telecasts were broadcast live to a select number of venues theaters, where viewers paid for tickets to watch the fight live; the first fight with a closed-circuit telecast was Joe Louis vs. Joe Walcott in 1948. Closed-circuit telecasts peaked in popularity with Muhammad Ali in the 1960s and 1970s, with "The Rumble in the Jungle" fight drawing 50 million CCTV viewers worldwide in 1974, the "Thrilla in Manila" drawing 100 million CCTV viewers worldwide in 1975. In 1985, the WrestleMania I professional wrestling show was seen by over one million viewers with this scheme; as late as 1996, the Julio César Chávez vs. Oscar De La Hoya boxing fight had 750,000 viewers. Closed-circuit television was replaced by pay-per-view home cable television in the 1980s and 1990s.
In September 1968, New York was the first city in the United States to install video cameras along its main business street in an effort to fight crime. Another early appearance was in 1973 in Times Square in New York City; the NYPD installed it in order to deter crime, occurring in the area. During the 1980s video surveillance began to spread across the country targeting public areas, it was seen as a cheaper way to deter crime compared to increasing the size of the police departments. Some businesses as well those that were prone to theft, began to use video surveillance. From the mid-1990s on, police departments across the country installed an increasing number of cameras in various public spaces including housing projects and public parks departments. CCTV became common in banks and stores to discourage theft, by recording evidence of criminal activity. In 1998, 3,000 CCTV systems were in use in New York City. A study by Nieto in 2008 found many businesses in the United States had invested in video surveillance technology to protect products and promote safe workplace and consumer environments.
A nationwide survey of a wide variety of companies found. In private sector CCTV surveillance technology is operated in a wide variety of establishments such as in industry/manufacturing, financial/insurance/banking and distribution, util
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