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
DBZ stands for decibel relative to Z. It is a logarithmic dimensionless technical unit used in radar in weather radar, to compare the equivalent reflectivity factor of a radar signal reflected off a remote object to the return of a droplet of rain with a diameter of 1 mm, it is proportional to the number of drops per unit volume and the sixth power of drops' diameter and is thus used to estimate the rain or snow intensity. With other variables analyzed from the radar returns it helps to determine the type of precipitation. Both the radar reflectivity factor and its logarithmic version are referred to as reflectivity when the context is clear; the radar reflectivity factor of precipitation is dependent on the number and size of reflectors, which includes rain, snow and hail. Sensitive radars can measure the reflectivity of cloud drops and ice. For an exponential distribution of reflectors, Z is expressed by: Z = ∫ 0 D m a x N 0 e − Λ D D 6 d D As rain droplets have a diameter of the order of 1 millimetre, Z is in mm6m−3.
By dividing Z with the equivalent return of a 1 mm drop in a volume of a meter cube and using the logarithm of the result, one obtains the logarithmic reflectivity LZ, in dBZ: L Z = 10 log 10 Z Z 0 dBZdBZ values can be converted to rainfall rates in millimetres per hour using the Marshall-Palmer formula: R m m / h = 5 8 The definition of Z above shows that a large number of small hydrometeors will reflect as one large hydrometeor. The signal returned to the radar will be equivalent in both situations, so a group of small hydrometeors is indistinguishable from one large hydrometeor on the resulting radar image; the reflectivity image is just one type of image produced by a radar. Using it alone a meteorologist could not tell with certainty the type of precipitation and distinguish any artifacts affecting the radar return. In combination with other information gathered by the radar during the same scan, meteorologists can distinguish between hail, snow and other atmospheric phenomena. ≈
A logarithmic scale is a nonlinear scale used when there is a large range of quantities. Common uses include earthquake strength, sound loudness, light intensity, pH of solutions, it is based on orders of magnitude, rather than a standard linear scale, so the value represented by each equidistant mark on the scale is the value at the previous mark multiplied by a constant. Logarithmic scales are used in slide rules for multiplying or dividing numbers by adding or subtracting lengths on the scales; the following are examples of used logarithmic scales, where a larger quantity results in a higher value: Richter magnitude scale and moment magnitude scale for strength of earthquakes and movement in the earth sound level, with units bel and decibel neper for amplitude and power quantities frequency level, with units cent, minor second, major second, octave for the relative pitch of notes in music logit for odds in statistics Palermo Technical Impact Hazard Scale logarithmic timeline counting f-stops for ratios of photographic exposure the rule of'nines' used for rating low probabilities entropy in thermodynamics information in information theory particle-size-distribution curves of soilThe following are examples of used logarithmic scales, where a larger quantity results in a lower value: pH for acidity stellar magnitude scale for brightness of stars Krumbein scale for particle size in geology absorbance of light by transparent samplesSome of our senses operate in a logarithmic fashion, which makes logarithmic scales for these input quantities appropriate.
In particular our sense of hearing perceives equal ratios of frequencies as equal differences in pitch. In addition, studies of young children in an isolated tribe have shown logarithmic scales to be the most natural display of numbers in some cultures, it can be used for geographical purposes like for measuring the speed of earthquakes. The top left graph is linear in the X and Y axis, the Y-axis ranges from 0 to 10. A base-10 log scale is used for the Y axis of the bottom left graph, the Y axis ranges from 0.1 to 1,000. The top right graph uses a log-10 scale for just the X axis, the bottom right graph uses a log-10 scale for both the X axis and the Y axis. Presentation of data on a logarithmic scale can be helpful when the data: covers a large range of values, since the use of the logarithms of the values rather than the actual values reduces a wide range to a more manageable size. A slide rule has logarithmic scales, nomograms employ logarithmic scales; the geometric mean of two numbers is midway between the numbers.
Before the advent of computer graphics, logarithmic graph paper was a used scientific tool. If both the vertical and horizontal axes of a plot are scaled logarithmically, the plot is referred to as a log–log plot. If only the ordinate or abscissa is scaled logarithmically, the plot is referred to as a semi-logarithmic plot. A logarithmic unit is a unit that can be used to express a quantity on a logarithmic scale, that is, as being proportional to the value of a logarithm function applied to the ratio of the quantity and a reference quantity of the same type; the choice of unit indicates the type of quantity and the base of the logarithm. Examples of logarithmic units include units of data storage capacity, of information and information entropy, signal level. Logarithmic frequency quantities are used in electronics and for music pitch intervals. Other logarithmic scale units include the Richter magnitude scale point. Bit, byte hartley nat shannon bel, decibel neper decade, savart octave, semitone, cent The motivation behind the concept of logarithmic units is that defining a quantity on a logarithmic scale in terms of a logarithm to a specific base amounts to making a choice of a unit of measurement for that quantity, one that corresponds to the specific logarithm base, selected.
Due to the identity log b a = log c a log c b, the logarithms of any given number a to two different bases differ only by the constant factor logc b. This constant factor can be considered to represent the conversion factor for converting a numerical representation of the pure logarithmic quantity Log from one arbitrary unit of measurement to another, since Log = =. For example, Boltzmann's standard definition of entropy S = k ln W can be written more as just S = Log, where "Log" here denotes the indefinite logarithm, we let k =; this identity works because ln W = log e W = Log ( W
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
A-weighting is the most used of a family of curves defined in the International standard IEC 61672:2003 and various national standards relating to the measurement of sound pressure level. A-weighting is applied to instrument-measured sound levels in an effort to account for the relative loudness perceived by the human ear, as the ear is less sensitive to low audio frequencies, it is employed by arithmetically adding a table of values, listed by octave or third-octave bands, to the measured sound pressure levels in dB. The resulting octave band measurements are added to provide a single A-weighted value describing the sound. Other weighting sets of values – B, C, D and now Z – are discussed below; the curves were defined for use at different average sound levels, but A-weighting, though intended only for the measurement of low-level sounds, is now used for the measurement of environmental noise and industrial noise, as well as when assessing potential hearing damage and other noise health effects at all sound levels.
It is used when measuring low-level noise in audio equipment in the United States. In Britain and many other parts of the world and audio engineers more use the ITU-R 468 noise weighting, developed in the 1960s based on research by the BBC and other organizations; this research showed that our ears respond differently to random noise, the equal-loudness curves on which the A, B and C weightings were based are only valid for pure single tones. A-weighting began with work by Fletcher and Munson which resulted in their publication, in 1933, of a set of equal-loudness contours. Three years these curves were used in the first American standard for sound level meters; this ANSI standard revised as ANSI S1.4-1981, incorporated B-weighting as well as the A-weighting curve, recognising the unsuitability of the latter for anything other than low-level measurements. But B-weighting has since fallen into disuse. Work, first by Zwicker and by Schomer, attempted to overcome the difficulty posed by different levels, work by the BBC resulted in the CCIR-468 weighting maintained as ITU-R 468 noise weighting, which gives more representative readings on noise as opposed to pure tones.
A-weighting is valid to represent the sensitivity of the human ear as a function of the frequency of pure tones, but only for quiet levels of sound. In effect, the A-weighting is based on the 40-phon Fletcher–Munson curves which represented an early determination of the equal-loudness contour for human hearing. However, because decades of field experience have shown a good correlation between the A scale and occupational deafness in the frequency range of human speech, this scale is employed in many jurisdictions to evaluate the risks of occupational deafness and other auditory problems related to signals or speech intelligibility in noisy environnements; because of perceived discrepancies between early and more recent determinations, the International Organization for Standardization revised its standard curves as defined in ISO 226, in response to the recommendations of a study coordinated by the Research Institute of Electrical Communication, Tohoku University, Japan. The study produced new curves by combining the results of several studies, by researchers in Japan, Denmark, UK, USA.
This has resulted in the recent acceptance of a new set of curves standardized as ISO 226:2003. The report comments on the large differences, the fact that the original Fletcher–Munson contours are in better agreement with recent results than the Robinson-Dadson, which appear to differ by as much as 10–15 dB in the low-frequency region, for reasons that are not explained. Fortuitously, the 40-phon Fletcher–Munson curve is close to the modern ISO 226:2003 standard, it will be noted that A-weighting would be a better match to the loudness curve if it fell much more steeply above 10 kHz, it is that this compromise came about because steep filters were difficult to construct in the early days of electronics. Nowadays, no such limitation need exist. If A-weighting is used without further band-limiting it is possible to obtain different readings on different instruments when ultrasonic, or near ultrasonic noise is present. Accurate measurements therefore require a 20 kHz low-pass filter to be combined with the A-weighting curve in modern instruments.
This is defined in IEC 61012 as AU weighting and while desirable, is fitted to commercial sound level meters. A-frequency-weighting is mandated by the international standard IEC 61672 to be fitted to all sound level meters; the old B- and D-frequency-weightings have fallen into disuse, but many sound level meters provide for C frequency-weighting and its fitting is mandated — at least for testing purposes — to precision sound level meters. D-frequency-weighting was designed for use when measuring high level aircraft noise in accordance with the IEC 537 measurement standard; the large peak in the D-weighting curve is not a feature of the equal-loudness contours, but reflects the fact that humans hear random noise differently from pure tones, an effect, pronounced around 6 kHz. This is because individual neurons from different regions of the cochlea in the inner ear respond to narrow ba
The decibel is a unit of measurement used to express the ratio of one value of a power or field quantity to another on a logarithmic scale, the logarithmic quantity being called the power level or field level, respectively. It can be used to express a change in an absolute value. In the latter case, it expresses the ratio of a value to a fixed reference value. For example, if the reference value is 1 volt the suffix is "V", if the reference value is one milliwatt the suffix is "m". Two different scales are used when expressing a ratio in decibels, depending on the nature of the quantities: power and field; when expressing a power ratio, the number of decibels is ten times its logarithm to base 10. That is, a change in power by a factor of 10 corresponds to a 10 dB change in level; when expressing field quantities, a change in amplitude by a factor of 10 corresponds to a 20 dB change in level. The decibel scales differ by a factor of two so that the related power and field levels change by the same number of decibels in, for example, resistive loads.
The definition of the decibel is based on the measurement of power in telephony of the early 20th century in the Bell System in the United States. One decibel is one tenth of one bel, named in honor of Alexander Graham Bell. Today, the decibel is used for a wide variety of measurements in science and engineering, most prominently in acoustics and control theory. In electronics, the gains of amplifiers, attenuation of signals, signal-to-noise ratios are expressed in decibels. In the International System of Quantities, the decibel is defined as a unit of measurement for quantities of type level or level difference, which are defined as the logarithm of the ratio of power- or field-type quantities; the decibel originates from methods used to quantify signal loss in telegraph and telephone circuits. The unit for loss was Miles of Standard Cable. 1 MSC corresponded to the loss of power over a 1 mile length of standard telephone cable at a frequency of 5000 radians per second, matched the smallest attenuation detectable to the average listener.
The standard telephone cable implied was "a cable having uniformly distributed resistance of 88 Ohms per loop-mile and uniformly distributed shunt capacitance of 0.054 microfarads per mile". In 1924, Bell Telephone Laboratories received favorable response to a new unit definition among members of the International Advisory Committee on Long Distance Telephony in Europe and replaced the MSC with the Transmission Unit. 1 TU was defined such that the number of TUs was ten times the base-10 logarithm of the ratio of measured power to a reference power. The definition was conveniently chosen such that 1 TU approximated 1 MSC. In 1928, the Bell system renamed the TU into the decibel, being one tenth of a newly defined unit for the base-10 logarithm of the power ratio, it was named the bel, in honor of the telecommunications pioneer Alexander Graham Bell. The bel is used, as the decibel was the proposed working unit; the naming and early definition of the decibel is described in the NBS Standard's Yearbook of 1931: Since the earliest days of the telephone, the need for a unit in which to measure the transmission efficiency of telephone facilities has been recognized.
The introduction of cable in 1896 afforded a stable basis for a convenient unit and the "mile of standard" cable came into general use shortly thereafter. This unit was employed up to 1923 when a new unit was adopted as being more suitable for modern telephone work; the new transmission unit is used among the foreign telephone organizations and it was termed the "decibel" at the suggestion of the International Advisory Committee on Long Distance Telephony. The decibel may be defined by the statement that two amounts of power differ by 1 decibel when they are in the ratio of 100.1 and any two amounts of power differ by N decibels when they are in the ratio of 10N. The number of transmission units expressing the ratio of any two powers is therefore ten times the common logarithm of that ratio; this method of designating the gain or loss of power in telephone circuits permits direct addition or subtraction of the units expressing the efficiency of different parts of the circuit... In 1954, J. W. Horton argued that the use of the decibel as a unit for quantities other than transmission loss led to confusion, suggested the name'logit' for "standard magnitudes which combine by addition".
In April 2003, the International Committee for Weights and Measures considered a recommendation for the inclusion of the decibel in the International System of Units, but decided against the proposal. However, the decibel is recognized by other international bodies such as the International Electrotechnical Commission and International Organization for Standardization; the IEC permits the use of the decibel with field quantities as well as power and this recommendation is followed by many national standards bodies, such as NIST, which justifies the use of the decibel for voltage ratios. The term field quantity is deprecated by ISO 80000-1. In spite of their widespread use, suffixes are not recognized by the IEC or ISO. ISO 80000-3 describes definitions for units of space and time; the decibel for use in acoustics is defined in ISO 80000-8. The major difference from the article below is that for acoustics the decibel has no
In signal processing, white noise is a random signal having equal intensity at different frequencies, giving it a constant power spectral density. The term is used, with this or similar meanings, in many scientific and technical disciplines, including physics, acoustical engineering, telecommunications, statistical forecasting. White noise refers to a statistical model for signals and signal sources, rather than to any specific signal. White noise draws its name from white light, although light that appears white does not have a flat power spectral density over the visible band. In discrete time, white noise is a discrete signal whose samples are regarded as a sequence of serially uncorrelated random variables with zero mean and finite variance. Depending on the context, one may require that the samples be independent and have identical probability distribution. In particular, if each sample has a normal distribution with zero mean, the signal is said to be Additive white Gaussian noise; the samples of a white noise signal may be sequential in time, or arranged along one or more spatial dimensions.
In digital image processing, the pixels of a white noise image are arranged in a rectangular grid, are assumed to be independent random variables with uniform probability distribution over some interval. The concept can be defined for signals spread over more complicated domains, such as a sphere or a torus. An infinite-bandwidth white noise signal is a purely theoretical construction; the bandwidth of white noise is limited in practice by the mechanism of noise generation, by the transmission medium and by finite observation capabilities. Thus, random signals are considered "white noise" if they are observed to have a flat spectrum over the range of frequencies that are relevant to the context. For an audio signal, the relevant range is the band of audible sound frequencies; such a signal is heard by the human ear as a hissing sound, resembling the /sh/ sound in "ash". In music and acoustics, the term "white noise" may be used for any signal that has a similar hissing sound; the term white noise is sometimes used in the context of phylogenetically based statistical methods to refer to a lack of phylogenetic pattern in comparative data.
It is sometimes used analogously in nontechnical contexts to mean "random talk without meaningful contents". Any distribution of values is possible. A binary signal which can only take on the values 1 or –1 will be white if the sequence is statistically uncorrelated. Noise having a continuous distribution, such as a normal distribution, can of course be white, it is incorrectly assumed that Gaussian noise refers to white noise, yet neither property implies the other. Gaussianity refers to the probability distribution with respect to the value, in this context the probability of the signal falling within any particular range of amplitudes, while the term'white' refers to the way the signal power is distributed over time or among frequencies. We can therefore find Gaussian white noise, but Poisson, etc. white noises. Thus, the two words "Gaussian" and "white" are both specified in mathematical models of systems. Gaussian white noise is a good approximation of many real-world situations and generates mathematically tractable models.
These models are used so that the term additive white Gaussian noise has a standard abbreviation: AWGN. White noise is the generalized mean-square derivative of the Wiener Brownian motion. A generalization to random elements on infinite dimensional spaces, such as random fields, is the white noise measure. White noise is used in the production of electronic music either directly or as an input for a filter to create other types of noise signal, it is used extensively in audio synthesis to recreate percussive instruments such as cymbals or snare drums which have high noise content in their frequency domain. A simple example of white noise is a nonexistent radio station. White noise is used to obtain the impulse response of an electrical circuit, in particular of amplifiers and other audio equipment, it is not used for testing loudspeakers as its spectrum contains too great an amount of high frequency content. Pink noise, which differs from white noise in that it has equal energy in each octave, is used for testing transducers such as loudspeakers and microphones.
To set up the equalization for a concert or other performance in a venue, a short burst of white or pink noise is sent through the PA system and monitored from various points in the venue so that the engineer can tell if the acoustics of the building boost or cut any frequencies. The engineer can adjust the overall equalization to ensure a balanced mix. White noise is used as the basis of some random number generators. For example, Random.org uses a system of atmospheric antennae to generate random digit patterns from white noise. White noise is a common synthetic noise source used for sound masking by a tinnitus masker. White noise machines and other white noise sources are sold as privacy enhancers and sleep aids and to mask tinnitus. Alternatively, the use of an FM radio tuned to unused frequencies is a simpler and more cost-effective source of white noise. However, white noise generated from a common commercial radio receiver tuned to an unused frequency is vulnerable to being contaminated with spurious signals, such as adjacent radio stations, harmonics f