Twisted pair cabling is a type of wiring in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted balanced pair, a twisted pair reduces electromagnetic radiation from the pair and crosstalk between neighboring pairs and improves rejection of external electromagnetic interference, it was invented by Alexander Graham Bell. In a balanced line, the two wires carry equal and opposite signals, the destination detects the difference between the two; this is known as differential signaling. Noise sources introduce signals into the wires by coupling of electric or magnetic fields and tend to couple to both wires equally; the noise thus produces a common-mode signal which can be canceled at the receiver when the difference signal is taken. Differential signaling starts to fail; this problem is apparent in telecommunication cables where pairs in the same cable lie next to each other for many miles.
Twisting the pairs counters this effect as on each half twist the wire nearest to the noise-source is exchanged. Provided the interfering source remains uniform, or nearly so, over the distance of a single twist, the induced noise will remain common-mode; the twist rate makes up part of the specification for a given type of cable. When nearby pairs have equal twist rates, the same conductors of the different pairs may lie next to each other undoing the benefits of differential mode. For this reason it is specified that, at least for cables containing small numbers of pairs, the twist rates must differ. In contrast to shielded or foiled twisted pair, UTP cable is not surrounded by any shielding. UTP is the primary wire type for telephone usage and is common for computer networking; the earliest telephones used open-wire single-wire earth return circuits. In the 1880s electric trams were installed in many cities. Lawsuits being unavailing, the telephone companies converted to balanced circuits, which had the incidental benefit of reducing attenuation, hence increasing range.
As electrical power distribution became more commonplace, this measure proved inadequate. Two wires, strung on either side of cross bars on utility poles, shared the route with electrical power lines. Within a few years, the growing use of electricity again brought an increase of interference, so engineers devised a method called wire transposition, to cancel out the interference. In wire transposition, the wires exchange position once every several poles. In this way, the two wires would receive similar EMI from power lines; this represented an early implementation of twisting, with a twist rate of about four twists per kilometre, or six per mile. Such open-wire balanced lines with periodic transpositions still survive today in some rural areas. Twisted-pair cabling was invented by Alexander Graham Bell in 1881. By 1900, the entire American telephone line network was either twisted pair or open wire with transposition to guard against interference. Today, most of the millions of kilometres of twisted pairs in the world are outdoor landlines, owned by telephone companies, used for voice service, only handled or seen by telephone workers.
Unshielded twisted pair cables are found in many Ethernet networks and telephone systems. For indoor telephone applications, UTP is grouped into sets of 25 pairs according to a standard 25-pair color code developed by AT&T Corporation. A typical subset of these colors shows up in most UTP cables; the cables are made with copper wires measured at 22 or 24 American Wire Gauge, with the colored insulation made from an insulator such as polyethylene or FEP and the total package covered in a polyethylene jacket. For urban outdoor telephone cables containing hundreds or thousands of pairs, the cable is divided into small but identical bundles; each bundle consists of twisted pairs. The bundles are in turn twisted together to make up the cable. Pairs having the same twist rate within the cable can still experience some degree of crosstalk. Wire pairs are selected to minimize crosstalk within a large cable. UTP cable is the most common cable used in computer networking. Modern Ethernet, the most common data networking standard, can use UTP cables.
Twisted pair cabling is used in data networks for short and medium length connections because of its lower costs compared to optical fiber and coaxial cable. UTP is finding increasing use in video applications in security cameras. Many cameras include a UTP output with screw terminals; as UTP is a balanced transmission line, a balun is needed to connect to unbalanced equipment, for example any using BNC connectors and designed for coaxial cable. Twisted pair cables incorporate shielding in an attempt to prevent electromagnetic interference. Shielding provides an electrically conductive barrier to attenuate electromagnetic waves external to the shield; such shielding can be applied to individual quads. Individual pairs are foil shielded, while an overall cable may use any of braided screen or foi
Western Electric Company was an American electrical engineering and manufacturing company that served as the primary supplier to AT&T from 1881 to 1996, to the local Bell Operating Companies until 1984. The company was responsible for many technological innovations and seminal developments in industrial management, it served as the purchasing agent for the member companies of the Bell System. In 1856, George Shawk purchased an electrical engineering business in Ohio. On December 31, 1869, he became partners with Enos M. Barton and the same year, sold his share to inventor Elisha Gray. In 1872 Barton, Gray moved the business to Clinton Street, Chicago and incorporated it as the Western Electric Manufacturing Company, they manufactured a variety of electrical products including typewriters and lighting and had a close relationship with telegraph company Western Union, to whom they supplied relays and other equipment. In 1875, Gray sold his interests to Western Union, including the caveat that he had filed against Alexander Graham Bell's patent application for the telephone.
The ensuing legal battle between Western Union and the Bell Telephone Company over patent rights ended in 1879 with Western Union withdrawing from the telephone market and Bell acquiring Western Electric in 1881. Western Electric was the first company to join in a Japanese joint venture with foreign capital. In 1899, it invested in a 54 % share of Ltd.. Western Electric's representative in Japan was Walter Tenney Carleton. In 1901, Western Electric secretly purchased a controlling interest in a principal competitor, the Kellogg Switchboard & Supply Company, but in 1909 was forced by a lawsuit to sell back to Milo Kellogg. On July 24, 1915, employees of the Hawthorne Works boarded the SS Eastland in downtown Chicago for a company picnic; the ship rolled over at the dock and over 800 people died. In 1920, Alice Heacock Seidel was the first of Western Electric's female employees to be given permission to stay on after she had married; this set a precedent in the company, which had not allowed married women in their employ.
Miss Heacock had worked for Western Electric for sixteen years before her marriage, was at the time the highest-paid secretary in the company. In her memoirs, she wrote that the decision to allow her to stay on "required a meeting of the top executives to decide whether I might remain with the Company, for it established a precedent and a new policy for the Company - that of married women in their employ. If the women at the top were permitted to remain after marriage all women would expect the same privilege. How far and how fast the policy was expanded is shown by the fact that a few years women were given maternity leaves with no loss of time on their service records."In 1925, ITT purchased the Bell Telephone Manufacturing Company of Brussels and other worldwide subsidiaries from AT&T, to avoid an antitrust action. The company manufactured rotary system switching equipment under the Western Electric brand. Early on, Western Electric managed an electrical equipment distribution business, furnishing its customers with non-telephone products made by other manufacturers.
This electrical distribution business was spun off from Western Electric in 1925 and organized into a separate company, Graybar Electric Company, in honor of the company's founders, Elisha Gray and Enos Barton. Bell Telephone Laboratories was half-owned by Western Electric, the other half belonging to AT&T. Western Electric used various logos during its existence. Starting in 1914 it used an image of AT&T's statue Spirit of Communication. In 1915, the assets of Western Electric Manufacturing were transferred to a newly incorporated company in New York, New York named Western Electric Company, Inc, a wholly owned subsidiary of AT&T; the sole reason for the transfer was to provide for the issuance of a non-voting preferred class of capital stock, disallowed under the statutes of the state of Illinois. All telephones in areas where AT&T subsidiaries provided local service, all components of the public switched telephone network, all devices connected to the network were made by Western Electric and no other devices were allowed to be connected to AT&T's network.
AT&T and Bell System companies were rumored to employ small armies of inspectors to check household line impedance levels to determine if non-leased phones were in use by consumers. Western Electric telephones were owned not by end customers but by the local Bell System telephone companies—all of which were subsidiaries of AT&T, which owned Western Electric; each phone was leased from the phone company on a monthly basis by customers who paid for their phone as part of the recurring lease fees. This system had the effect of subsidizing basic telephone service, keeping local phone service inexpensive, under $10 per month, including the leased phone. After divestiture, basic service prices increased, customers were now responsible for inside building wiring and telephone equipment; the Bell System had an extensive policy and infrastructure to recycle or refurbish phones taken out of service, replacing all defective, weak, or otherwise unusable parts for new installations. This resulted in an extraordinary longevity of Western Electric telephone models and limited the variety of new designs introduced into the market place.
AT&T strictly enforced policies against using telephone equipment by other manufacturers on their network. A customer who insisted on using a telephone not supplied by the Bell System had to first transfer the phone to the local Bell operating company, who leased the phone back to the customer for a monthly charge in addition to a re-wiring fee. In the 1970s when consumers incr
A courtesy telephone is a telephone located in airport terminals, large train stations, hotel lobbies, other places where many travellers are expected, used to relay messages to a specific person. It is used in connection with a public address system announcement of the style "Jane Doe, please pick up the nearest white courtesy telephone." Courtesy telephones may have a distinctive color, traditionally white in US airports, most have no dialing capabilities but rather are simple ringdown stations to reach an operator or other fixed number. Some double as emergency telephones, having buttons by which a user can distinguish between emergency use and inquiry. Customers can use a courtesy phone to seek information, such as where to find further transport or a person trying to meet them; some courtesy phones provide a direct line to a number of advertised businesses, such as motels or taxis. They may be located near baggage claim, ticketing areas, security checkpoints. Other telephones in public or semi-public places which may be used to make outside calls are sometimes called "courtesy telephones".
Courtesy telephones are featured prominently as a running gag in the cult comedy film Airplane!. Notes
In telecommunications, frequency-division multiplexing is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency bands, each of, used to carry a separate signal. This allows a single transmission medium such as a cable or optical fiber to be shared by multiple independent signals. Another use is to carry separate serial segments of a higher rate signal in parallel; the most natural example of frequency-division multiplexing is radio and television broadcasting, in which multiple radio signals at different frequencies pass through the air at the same time. Another example is cable television, in which many television channels are carried on a single cable. FDM is used by telephone systems to transmit multiple telephone calls through high capacity trunklines, communications satellites to transmit multiple channels of data on uplink and downlink radio beams, broadband DSL modems to transmit large amounts of computer data through twisted pair telephone lines, among many other uses.
An analogous technique called wavelength division multiplexing is used in fiber-optic communication, in which multiple channels of data are transmitted over a single optical fiber using different wavelengths of light. The multiple separate information signals that are sent over an FDM system, such as the video signals of the television channels that are sent over a cable TV system, are called baseband signals. At the source end, for each frequency channel, an electronic oscillator generates a carrier signal, a steady oscillating waveform at a single frequency that serves to "carry" information; the carrier is much higher in frequency than the baseband signal. The carrier signal and the baseband signal are combined in a modulator circuit; the modulator alters some aspect of the carrier signal, such as its amplitude, frequency, or phase, with the baseband signal, "piggybacking" the data onto the carrier. The result of modulating the carrier with the baseband signal is to generate sub-frequencies near the carrier frequency, at the sum and difference of the frequencies.
The information from the modulated signal is carried in sidebands on each side of the carrier frequency. Therefore, all the information carried by the channel is in a narrow band of frequencies clustered around the carrier frequency, this is called the passband of the channel. Additional baseband signals are used to modulate carriers at other frequencies, creating other channels of information; the carriers are spaced far enough apart in frequency that the band of frequencies occupied by each channel, the passbands of the separate channels, do not overlap. All the channels are sent through the transmission medium, such as a coaxial cable, optical fiber, or through the air using a radio transmitter; as long as the channel frequencies are spaced far enough apart that none of the passbands overlap, the separate channels will not interfere with each other. Thus the available bandwidth is divided into "slots" or channels, each of which can carry a separate modulated signal. For example, the coaxial cable used by cable television systems has a bandwidth of about 1000 MHz, but the passband of each television channel is only 6 MHz wide, so there is room for many channels on the cable.
At the destination end of the cable or fiber, or the radio receiver, for each channel a local oscillator produces a signal at the carrier frequency of that channel, mixed with the incoming modulated signal. The frequencies subtract; this is called demodulation. The resulting baseband signal is filtered out of the other frequencies and output to the user. For long distance telephone connections, 20th century telephone companies used L-carrier and similar coaxial cable systems carrying thousands of voice circuits multiplexed in multiple stages by channel banks. For shorter distances, cheaper balanced pair cables were used for various systems including Bell System K- and N-Carrier; those cables didn't allow such large bandwidths, so only 12 voice channels and 24 were multiplexed into four wires, one pair for each direction with repeaters every several miles 10 km. See 12-channel carrier system. By the end of the 20th Century, FDM voice circuits had become rare. Modern telephone systems employ digital transmission, in which time-division multiplexing is used instead of FDM.
Since the late 20th century digital subscriber lines have used a Discrete multitone system to divide their spectrum into frequency channels. The concept corresponding to frequency-division multiplexing in the optical domain is known as wavelength-division multiplexing. A once commonplace FDM system, used for example in L-carrier, uses crystal filters which operate at the 8 MHz range to form a Channel Group of 12 channels, 48 kHz bandwidth in the range 8140 to 8188 kHz by selecting carriers in the range 8140 to 8184 kHz selecting upper sideband this group can be translated to the standard range 60 to 108 kHz by a carrier of 8248 kHz; such systems are used in DTL and DFSG. 132 voice channels can be formed using DTL plane the modulation and frequency plan are given in FIG1 and FIG2 use of DTL technique allows the formation of a maximum of 132 voice channels that can be placed direct to line. DTL eliminates group and super group equipment. DFSG can take similar steps where a direct formation of a number of super groups can be obtained in the 8 kHz the DFSG eliminates group
The volt is the derived unit for electric potential, electric potential difference, electromotive force. It is named after the Italian physicist Alessandro Volta. One volt is defined as the difference in electric potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those points, it is equal to the potential difference between two parallel, infinite planes spaced 1 meter apart that create an electric field of 1 newton per coulomb. Additionally, it is the potential difference between two points that will impart one joule of energy per coulomb of charge that passes through it, it can be expressed in terms of SI base units as V = potential energy charge = J C = kg ⋅ m 2 A ⋅ s 3. It can be expressed as amperes times ohms, watts per ampere, or joules per coulomb, equivalent to electronvolts per elementary charge: V = A ⋅ Ω = W A = J C = eV e; the "conventional" volt, V90, defined in 1987 by the 18th General Conference on Weights and Measures and in use from 1990, is implemented using the Josephson effect for exact frequency-to-voltage conversion, combined with the caesium frequency standard.
For the Josephson constant, KJ = 2e/h, the "conventional" value KJ-90 is used: K J-90 = 0.4835979 GHz μ V. This standard is realized using a series-connected array of several thousand or tens of thousands of junctions, excited by microwave signals between 10 and 80 GHz. Empirically, several experiments have shown that the method is independent of device design, measurement setup, etc. and no correction terms are required in a practical implementation. In the water-flow analogy, sometimes used to explain electric circuits by comparing them with water-filled pipes, voltage is likened to difference in water pressure. Current is proportional to the amount of water flowing at that pressure. A resistor would be a reduced diameter somewhere in the piping and a capacitor/inductor could be likened to a "U" shaped pipe where a higher water level on one side could store energy temporarily; the relationship between voltage and current is defined by Ohm's law. Ohm's Law is analogous to the Hagen–Poiseuille equation, as both are linear models relating flux and potential in their respective systems.
The voltage produced by each electrochemical cell in a battery is determined by the chemistry of that cell. See Galvanic cell § Cell voltage. Cells can be combined in series for multiples of that voltage, or additional circuitry added to adjust the voltage to a different level. Mechanical generators can be constructed to any voltage in a range of feasibility. Nominal voltages of familiar sources: Nerve cell resting potential: ~75 mV Single-cell, rechargeable NiMH or NiCd battery: 1.2 V Single-cell, non-rechargeable: alkaline battery: 1.5 V. Some antique vehicles use 6.3 volts. Electric vehicle battery: 400 V when charged Household mains electricity AC: 100 V in Japan 120 V in North America, 230 V in Europe, Asia and Australia Rapid transit third rail: 600–750 V High-speed train overhead power lines: 25 kV at 50 Hz, but see the List of railway electrification systems and 25 kV at 60 Hz for exceptions. High-voltage electric power transmission lines: 110 kV and up Lightning: Varies often around 100 MV.
In 1800, as the result of a professional disagreement over the galvanic response advocated by Luigi Galvani, Alessandro Volta developed the so-called voltaic pile, a forerunner of the battery, which produced a steady electric current. Volta had determined that the most effective pair of dissimilar metals to produce electricity was zinc and silver. In 1861, Latimer Clark and Sir Charles Bright coined the name "volt" for the unit of resistance. By 1873, the British Association for the Advancement of Science had defined the volt and farad. In 1881, the International Electrical Congress, now the International Electrotechnical Commission, approved the volt as the unit for electromotive force, they made the volt equal to 108 cgs units of voltage
The XLR connector is a style of electrical connector found on professional audio and stage lighting equipment. The connectors have between three and seven pins, they are most associated with balanced audio interconnection, including AES3 digital audio, but are used for lighting control, low-voltage power supplies, other applications. XLR connectors are available from a number of manufacturers and are covered by an international standard for dimensions, IEC 61076-2-103, they are superficially similar to the smaller DIN connector range, but are not physically compatible with them. A smaller version, the Mini XLR Connector, is used on smaller equipment; the XLR connector was invented by James H. Cannon, founder of Cannon Electric in Los Angeles and for this reason it was sometimes colloquially known as a Cannon plug or Cannon connector. Manufactured as the Cannon X series, by 1950 a latching mechanism was added and by 1955 a version surrounding the female contacts with a synthetic rubber polychloroprene insulation using the part number prefix XLR.
There was an XLP series which used a hard plastic insulation, but was otherwise the same. ITT Cannon manufactured XLR connectors in two locations Kanagawa and Melbourne, Australia; the Australian operation was sold to Alcatel Components in 1992 and acquired by Amphenol in 1998. ITT Cannon continues to manufacture XLR connectors in Japan; the Switchcraft corporation started manufacturing compatible connectors, followed by Neutrik. Neutrik made a number of improvements to the connector and its second-generation design had just four parts for the cable connector and eliminated the small screws used by both Cannon and Switchcraft, which were prone to working loose, falling out and becoming lost. XLR connectors are available in male and female versions in both cable and chassis mounting designs, a total of four styles; this is unusual as many other connector designs omit one of the styles. The female XLR connectors are designed to first connect pin 1, before the other pins make contact, when a male XLR connector is inserted.
With the ground connection established before the signal lines are connected, the insertion of XLR connectors in live equipment is possible without picking up external signals. The number of pins varies; as of 2016, XLR connectors are available with up to 10 pins, mini XLR connectors with up to eight. XLR connectors from different manufacturers will intermate, with the exception of six-pin models, which are available in two incompatible designs; the older Switchcraft 6 pin design adds a center pin to the standard 5 pin design, whereas the newer Neutrik design is a different pattern. The Switchcraft 6 pin female will accept a standard 5 pin male plug whereas the Neutrik 6 pin design will not. Neutrik offers connectors in both 6 pin designs; the terminology for labelling the corresponding members of a pair of mating connectors follows the usual rules for the gender of connectors: a'male' connector is the one with pins on the smallest element,'female' has corresponding receptacles. A'plug' connector enters the'socket' connector, judged by the largest element.
For most XLR, plugs are male and sockets are female. XLR are unusual as, at least in audio applications, all four combinations of male and female and sockets are common. A common misnomer is that'plugs' are free connectors and'sockets' are panel-mounted, but XLR uses many free female sockets and panel-mounted male plugs. There is a loose convention for audio work that signals are generated by equipment with male pins and transmitted to that with female receptacles. Three-pin XLR connectors are by far the most common style, are an industry standard for balanced audio signals; the great majority of professional microphones use the XLR connector. In previous years, they were used for loudspeaker connections, for instance by Trace Elliot in its bass enclosures; the XLR could accept 14 AWG wire with a current-carrying capacity of 15 amps, suitable for most loudspeakers, but they have been superseded by the Speakon connector for professional loudspeakers. The Speakon connector accepts larger wire and carries more current, it provides a better shield for the contacts, which may carry dangerous voltages when connected to an amplifier.
Three-pin XLR connectors are used to interconnect powered speakers with line-level signals. This use is seen in PA system applications and seems to be growing more common. Rechargeable devices exist; these can be found on electric powered mobility scooters. The connectors carry from 2 to 10 amps at 24 volts. An obsolete use for three-pin XLR connectors was for MIDI data on some Octave-Plateau synthesizers including the Voyetra-8; the three-pin XLR connector is used for DMX512, on lighting and related control equipment. At the budget / DJ end of the market. However, using three-pin XLR connectors for DMX512 is prohibited by section 7.1.2 of the DMX512 standard. Use of the three-pin XLR in this context firstly presents a risk of damage to the lighting equipment should an audio cable carrying 48 volt phantom power be accidentally connected, secondly encourages the use of cable following analogue audio specifications for DMX, which can lead to signal degradation and unreliable operation of the DMX network.
Four-pin XLR connectors are used in a variety of applications. They are the standard connector such as systems made by ClearCom and Telex. Two pins are
Outside broadcasting is the electronic field production of television or radio programmes from a mobile remote broadcast television studio. Professional video camera and microphone signals come into the production truck for processing and transmission; the mobile production control room is known as a "production truck", "scanner", "mobile unit", "remote truck", "live truck", "OB van", "OB Truck" or "live eye". In the United States an "OB van" is smaller in size than a production truck and requires two or three people in the field to manage; the BBC's first Outside Broadcast truck MCR 1, short for mobile control room, was built by the joint Marconi-EMI company and delivered to the BBC just in time to televise the Coronation Procession of King George VI in May 1937. MCR 2 was identical to MCR 1 and was delivered in the summer of 1938; the MCRs could handle three cameras. They were standard Emitrons, but were supplemented by Super Emitrons, which performed much better than the standard ones in low light.
The MCRs were built on the chassis of an AEC Regal single decker bus. After the Second World War, the joint company Marconi-EMI ended; the BBC ordered two 3-camera MCRs from EMI. The cameras were equipped with CPS tubes, had a 3 lens turret. MCR 4 was delivered in time to be used on the 1948 Olympics. After developing colour television in the mid 1960s, the BBC began to develop a fleet of colour OB units, known as CMCRs; these trucks were known as Type 2 scanners and were, at the time state of the art. Type 2 scanners first came equipped with Pye PC80 cameras but these were soon superseded by EMI's 2001 colour cameras; these trucks would remain in service into the mid 1980s. Throughout this time, they would see use on some of the BBC's most prestigious programmes, including Royal Events, Dr Who, Wimbledon Tennis, Question Time. Although made from converted HGVs, inside these trucks were cramped as a result of housing an entire mobile television studio; these were made up of three sections: A section to house the camera control units, or CCUs, camera monitoring equipment.
Being so large and complex, these cameras required a team of skilled engineers to keep them functioning. During a production, the camera operator would control the pan and the focus but it was the engineer who controlled the exposure and the colour balance. A section for the production crew, led by the director, who would orchestrate the over all production. A section for the sound crew which housed their mixing desk and other sound equipment. From here the sound crew controlled not only the sound of the programme but all the production communications which allowed the whole crew to communicate to one another. Without which the production would undoubtedly grind to a halt. In the past many outside broadcasting applications have relied on using satellite uplinks in order to broadcast live audio and video back to the studio. While this has its advantages such as the ability to set up anywhere covered by the respective geostationary satellite, satellite uplinking is expensive and the round trip latency is in the range of 240 to 280 milliseconds.
As more venues install Fibre, this is used. For news gathering, contribution over public internet is now used. Modern applications such as hardware and software IP codecs have allowed the use of public 3G/4G networks to broadcast video and audio; the latency of 3G is around 100–500 ms, while 4G is less than 100 ms. A typical modern OB van is divided into five parts. Parts of the television crew are located in the first and largest part, the video production area; the television director, technical director, assistant director, character generator operator and television producers sit in front of a wall of video monitors. The technical director sits in front of the video switcher; the video monitors show all the video feeds from various sources, including computer graphics, professional video cameras, video tape recorder, video servers and slow-motion replay machines. The wall of monitors contains a preview monitor showing what could be the next source on air and a program monitor that shows the feed going to air or being recorded.
The keyed dirty feed is what is transmitted back to the central studio, controlling the outside broadcast. A clean feed could be sent to other trucks for use in their production; the video switcher is operated by one person called the technical director and is responsible for switching the video sources to air as directed. Behind the directors there is a desk with monitors for the editors to operate, it is essential that the directors and editors are in communication with each other during events, so that replays and slow-motion shots can be selected and aired. The "production room" in most sporting events has a graphics operator and sometimes a font coordinator who are in charge of the graphics and the showing of the names of commentators or the players to be shown on air. Most sports have a "box operator" who controls the graphic seen either on the bottom or top of the screen that shows the score as seen at home; these operators can show on-air stats, control the clock, many times are in charge of showing sponsors during play.
The second part of a van is where the audio engineer has an audio mixer (being fed with all the various audio feeds: reporters, commentary, on-field micro