Greenbelt is a city in Prince George's County, United States, a suburb of Washington, D. C.. Greenbelt is notable for being the first and the largest of the three experimental and controversial New Deal Greenbelt Towns and built by the Federal government of the United States; the cooperative community was conceived in 1935, by Undersecretary of Agriculture Rexford Guy Tugwell, perceived by some of his contemporaries as having held a collectivist ideology and was utilized as a source of opposition to the Greenbelt Towns project throughout its short duration. The project came into legal existence in the spring of 1935. On April 8, 1935, the United States Congress passed the Emergency Relief Appropriation Act of 1935. Under the authority granted to him from this legislation, President Franklin D. Roosevelt issued an executive order, on May 1, 1935, establishing the United States Resettlement Administration. Referred to as Maryland Special Project No. 1, the project was given the name Greenbelt when the Division of Suburban Resettlement of the Resettlement Administration began construction on January 13, 1936 8 miles north of Washington, D.
C. The complete Greenbelt plans were reviewed at the White House by President Roosevelt and First Lady Eleanor Roosevelt on April 30, 1936; the first tenants, after being selected through a stringent and restrictive application process, moved into the town, which consisted of structures built in the Art Deco, Streamline Moderne, Bauhaus architectural styles, on September 30, 1937. Greenbelt is credited as a historic milestone in urban development, which includes its role as having served as the initial model for the constructed suburban Washington, D. C. planned cities of Reston and Columbia, Maryland. Known locally as Old Greenbelt, the original, federally-built core of the city was recognized as the Greenbelt Historic District by the Maryland Historical Trust, is listed on the National Register of Historic Places as a National Historic Landmark District. Greenbelt's population, which includes residents of private sector dwellings that were constructed over several decades subsequent to the federal government's ownership of the city, was recorded as 23,068 in the 2010 U.
S. Census. Greenbelt is located at 38°59′41″N 76°53′07″W. According to United States Census Bureau data, as of January 1, 2018, the city has a total area of 6.23 square miles, of which, 6.18 square miles is land, 0.06 square miles is water. Greenbelt's ZIP Codes are 20770, 20771, 20768; the ZIP Code 20770 contains all residential and business addresses that correspond to actual physical locations inside the geographic boundaries of the City of Greenbelt. The 20771 ZIP Code is assigned to post-office box addresses, while 20768 is the designated ZIP Code for Goddard Space Flight Center, situated on federal government owned land, contiguous with a portion of Greenbelt's eastern border. NASA's Goddard Space Flight Center, located directly adjacent to Greenbelt's eastern boundary, utilizes a Greenbelt postal address, as well, it is within the former Goddard census-designated place. Greenbelt Park, a unit of the National Park System, is located within the City of Greenbelt's boundaries, at its southernmost portion.
Two major highways pass through and have interchanges in Greenbelt: the Capital Beltway and the National Park Service's owned and maintained portion of the Baltimore–Washington Parkway. The Greenbelt portion of the Baltimore–Washington Parkway is part of the parkway's 19-mile section, listed on the National Register of Historic Places in 1991. Additionally, Greenbelt Road is part of state highway MD 193, which connects several suburban communities in both Prince George's and Montgomery counties. Kenilworth Avenue traverses Greenbelt in a north-south direction, running parallel to the B–W Parkway, providing an alternate travel route into Washington, D. C. from Greenbelt. The southernmost Maryland portion of Kenilworth Avenue forms a major interchange with both the B–W Parkway and US 50 near the Maryland–D. C. Line, continues into Washington, as the Kenilworth Avenue Freeway. Washington Metro′s rapid transit rail system serves Washington, D. C. and neighboring communities in Maryland and Northern Virginia, by operating 91 Metro stations, which includes the Greenbelt station, the northern terminus of Metro′s Green Line.
Commuter rail service to the station is provided by MARC Train′s Camden Line, which connects the District of Columbia′s Washington Union Station with Camden Station in Baltimore. The Camden Line provides service by utilizing the original 1835 Baltimore and Ohio Railroad track route between Washington and Baltimore, now part of the CSX System. Available at the station, is a weekday express Metrobus service, the Greenbelt–BWI Thurgood Marshall Airport Express Line, designated route B30, to Baltimore/Washington International Thurgood Marshall Airport, a mode of transportation to and from the airport for airline passengers, in addition to allowing for connections to Baltimore′s regional transit services. Metrobus, Prince George′s County′s THE BUS, the University of Maryland′s Shuttle-UM each have bus routes which serve the city of Greenbelt. Through a city–university partnership that began in 2017, Greenbelt residents are permitted to unlimited travel on Shuttle UM, with the purchase of a $10 annual pass.
Beltsville Agricultural Research Center Berwyn Heights College Park Goddard Lanham New Carrollton Glenn D
CKLW is a 50,000-watt, Class B, AM radio station broadcasting on the Mexican clear-channel frequency of 800 kHz and located in Windsor, serving Windsor and Detroit. CKLW uses a five-tower directional antenna with differing patterns day and night, to protect Class-A clear-channel station XEROK-AM in Ciudad Juárez and other neighboring stations on the same frequency; the transmitter is located in southern Essex County near Harrow only a few miles from the Lake Erie shoreline. CKLW was an internationally known Top 40 station in the 1970s. During this era, CKLW used a tight Top 40 format known as Boss Radio, devised by radio programmer Bill Drake. However, CKLW never used the handle "boss" on the air, just the style. Rather than a Boss 30, CKLW's weekly music survey was known as a Big 30, and instead of calling itself Boss Radio, CKLW called itself The Big 8. During this period it was the top-rated radio station not only in Windsor, but across the river in Detroit, in cities as far away as Toledo and Cleveland in Ohio.
In its current incarnation, CKLW is a news/talk radio station serving listeners in Windsor, Essex County and Chatham-Kent in Ontario, with a mix of local and syndicated programs, including some programs from the United States. CKLW first came on the air on June 2, 1932 as CKOK on 540 kilocycles, with 5,000 watts of power, was owned by a group of Windsor-area businessmen led by Malcolm Campbell, operating as "Essex Broadcasters, Ltd." CKOK became CKLW in 1933, when Essex Broadcasters, Ltd. merged with the London Free Press and its station CJGC, became "Western Ontario Broadcasting", co-owned by Essex Broadcasters, the London Free Press. The "LW" in the callsign is said to have stood for "London, Windsor", considered to be the two chief cities in the station's listening area; when the station's power increased to 50,000 watts, its listening area increased accordingly. In 1934, when London Free Press's station CJGC pulled out of the agreement, the station's ownership became wholly owned by Western Ontario Broadcasters.
CJGC evolved into today's CFPL 980, while CKLW moved from 840 to 1030 kc. in 1934, before settling on its present frequency of 800 kHz in 1941, thanks to a shuffle of frequency allocations. CKLW for most of its history had a distinct American accent to its programming, for a number of years served as the Detroit affiliate of the Mutual Broadcasting System, an affiliation that began with its switch from CBS to Mutual September 29, 1935, which would last from until its purchase by RKO General in 1963; when Mutual was restructured as a cooperative in 1940, CKLW was one of the major shareholders in the network. Alongside its affiliation with Mutual, it gained a dual affiliation with the Dominion Network of the CBC in 1935, replacing its CBS Radio affiliation with that of Mutual/Dominion, its affiliation with Dominion would last until 1950. The Mutual System's owner, General Tire and Rubber Company, purchased a controlling interest in CKLW and its owner at the time, Western Ontario Broadcasting in 1956, along with RKO General.
RKO would increase their stake to 100% in 1963. In the late 1930s and early 1940s, CKLW was home to Happy Joe's Early Morning Frolic with Joe Gentile and Toby David, one of the first popular comedy-oriented radio morning shows in Detroit; the show continued strong after David left CKLW for Washington, D. C. in 1940, was replaced by Ralph Binge. The duo kept listeners entertained with an endless stream of comedic situations; the show's sponsors got in on the fun as well, as Gentile and Binge's trademark was their ability to turn a standard 60-second commercial announcement into a comedy sketch which could run for three minutes or longer. A typical three-and-a-half-hour Gentile and Binge show might feature such comedic commercials for as many as fifty legitimate products, some imaginary ones as well. Sometimes listeners didn't get the joke. For example, according to popular legend, after promoting a miracle weight-loss aid called "Dr. Quack's Slim Jim Reducing Pills" with the story of an obese woman who got stuck in a telephone booth and Binge received over $3,000 from listeners requesting a $1 trial of the pills as advertised, the station had to hire a clerk to return the money.
Gentile and Binge were a fixture on CKLW until moving to WJBK radio in 1948, attracting audience ratings as high as 80% at their peak. The duo disbanded their partnership in 1956, Gentile returned to CKLW. Toby David eventually returned to AM 800 to host the morning show in the late 1950s and early 1960s. Both Binge and David were stars of early Detroit television kiddie shows: Binge was "Pirate Pete" on WJBK-TV in the mid-1950s, David became CKLW-TV's "Captain Jolly" in the decade; as television's popularity boomed, CKLW, like many other stations, coped with the changes by replacing the dying network radio fare with locally based disc-jockey shows. Throughout most of the 1950s and into the mid-1960s, CKLW was a "variety" radio station which filled in the cracks between full-service features with pop music played by announcers like Bud Davies, Ron Knowles, Joe Van. For a few years in the early 1960s, CKLW featured a country music program in the evenings called Sounds Like Nashville; this ended in 1963.
After RKO General took over the station and
Interstate 495 (Capital Beltway)
Interstate 495 is a 64-mile Interstate Highway that surrounds Washington, D. C. the capital of the United States of America, its inner suburbs in adjacent Maryland and Virginia. I-495 is known as the Capital Beltway, or the Beltway when the context of Washington, D. C. is clear. It is the basis of the phrase "inside the Beltway", used when referring to issues dealing with American government and politics. I-95 utilizes the southern and eastern half of the Capital Beltway to circumnavigate Washington, D. C. and is cosigned with I-495 along that portion. This circumferential roadway is located not only in the states of Virginia and Maryland, but crosses through the District of Columbia, near the western end of the Woodrow Wilson Bridge over the Potomac River; the Beltway passes through Prince George's County and Montgomery County in Maryland, Fairfax County and the independent city of Alexandria in Virginia. The Cabin John Parkway, a short connector between I-495 and the Clara Barton Parkway near the Potomac River along the Maryland–Virginia border, is considered an Interstate spur by the Maryland State Highway Administration.
Except for the westernmost part of Woodrow Wilson Bridge south of downtown Washington, the Capital Beltway encircles Washington, D. C. in adjacent Maryland and Virginia. The two directions of travel and counterclockwise, have become known as the "Inner Loop" and the "Outer Loop"; the route descriptions below follow the direction of the Outer Loop, starting at the Woodrow Wilson Bridge over the Potomac River, south of Washington. Most Beltway interchanges provide access to Washington, with I-95 and I-295 from the south, I-66 from the west, US 50 from both the west and the east among the most used. More scenic routes from the Beltway into the District are offered by the George Washington Memorial Parkway along the Virginia side of the Potomac, the Clara Barton Parkway along the Maryland side of the river, the Baltimore-Washington Parkway, approaching Washington from the northeast; the entire Beltway was I-495, I-95 was planned to serve downtown Washington, D. C. from the south and north, intersecting the Beltway in Maryland.
However, environmental litigation stopped completion of this plan, the built portion of I-95 inside the Beltway from the south northward into downtown Washington was redesignated I-395 in 1977. The small built portion from the north was converted into a park-and-ride lot. I-95 was rerouted along the eastern side of the Beltway, with the I-495 designation left only along the western side. In 1989, the I-495 designation was returned to the eastern portion as well, with the highway co-signed as I-95 and I-495 along this route; the Beltway, four lanes in each direction, travels over the Woodrow Wilson Bridge over the tidal Potomac River between Alexandria and Prince George's County, Maryland. The Federal Highway Administration recognizes 0.11 miles of the bridge as crossing the tip of the southernmost corner of the District of Columbia. However, signage for these boundary crossings is lacking; the Beltway enters Maryland during its Potomac River crossing over the Woodrow Wilson Bridge, west of Forest Heights and National Harbor.
After crossing the Potomac, I-95/I-495 meets the southern terminus of Interstate 295, known as the Anacostia Freeway, a route that serves downtown Washington to the north, connecting in Washington to Interstate 695. The highway next intersects MD 210, a major north–south route from southern D. C. to Indian Head in Charles County, Maryland. Heading northeastward, the Beltway interchanges with various local highways, including MD 5 and MD 4 on either side of Andrews Air Force Base, which the Beltway travels near its northern edge. Past the MD 4 interchange, the Beltway turns north through Glenarden, interchanging with MD 202; the Beltway interchanges with US 50/unsigned I-595, a major highway from downtown Washington eastward to Annapolis and the Chesapeake Bay Bridge to Maryland's Eastern Shore. Both US 50 and MD 450 provide access to the New Carrollton Metro, MARC commuter rail, Amtrak stations, the New Carrollton area. Turning northwest, the Beltway enters Greenbelt Park, intersecting the Baltimore-Washington Parkway in the northeastern edge of the park.
Just after the B-W Parkway, I-95/I-495 passes an interchange with MD 201, which connects to the southern terminus of the B-W Parkway at US 50 near the D. C. line. Now turned west, the Beltway runs through the northern edge of College Park, interchanging with the access roadway for the Greenbelt Metro and MARC commuter rail stations US 1. Beyond the US 1 interchange, I-95 separates from I-495 at the College Park Interchange. I-495 continues west, alone, on the Capital Beltway, while I-95 turns northeast towards Baltimore, New York City, Boston; the interchange includes access to a Park and Ride lot, paved as part of I-95's route within the Beltway. Continuing west from the College Park Interchange, I-495 crosses into Montgomery County, entering a developed and populated area. Passing underneath MD 212 with no access, the route interchanges with MD 650 near the George Washington Cemetery. After bisecting Northwest Branch Park, the Beltway interchanges with MD 193 and US 29 south of Four Corners.
AM broadcasting is a radio broadcasting technology, which employs amplitude modulation transmissions. It was the first method developed for making audio radio transmissions, is still used worldwide for medium wave transmissions, but on the longwave and shortwave radio bands; the earliest experimental AM transmissions began in the early 1900s. However, widespread AM broadcasting was not established until the 1920s, following the development of vacuum tube receivers and transmitters. AM radio remained the dominant method of broadcasting for the next 30 years, a period called the "Golden Age of Radio", until television broadcasting became widespread in the 1950s and received most of the programming carried by radio. Subsequently, AM radio's audiences have greatly shrunk due to competition from FM radio, Digital Audio Broadcasting, satellite radio, HD radio and Internet streaming. AM transmissions are much more susceptible than FM or digital signals are to interference, have lower audio fidelity.
Thus, AM broadcasters tend to specialise in spoken-word formats, such as talk radio, all news and sports, leaving the broadcasting of music to FM and digital stations. The idea of broadcasting — the unrestricted transmission of signals to a widespread audience — dates back to the founding period of radio development though the earliest radio transmissions known as "Hertzian radiation" and "wireless telegraphy", used spark-gap transmitters that could only transmit the dots-and-dashes of Morse code. In October 1898 a London publication, The Electrician, noted that "there are rare cases where, as Dr. Lodge once expressed it, it might be advantageous to'shout' the message, spreading it broadcast to receivers in all directions". However, it was recognized that this would involve significant financial issues, as that same year The Electrician commented "did not Prof. Lodge forget that no one wants to pay for shouting to the world on a system by which it would be impossible to prevent non-subscribers from benefiting gratuitously?"On January 1, 1902, Nathan Stubblefield gave a short-range "wireless telephone" demonstration, that included broadcasting speech and music to seven locations throughout Murray, Kentucky.
However, this was transmitted using induction rather than radio signals, although Stubblefield predicted that his system would be perfected so that "it will be possible to communicate with hundreds of homes at the same time", "a single message can be sent from a central station to all parts of the United States", he was unable to overcome the inherent distance limitations of this technology. The earliest public radiotelegraph broadcasts were provided as government services, beginning with daily time signals inaugurated on January 1, 1905, by a number of U. S. Navy stations. In Europe, signals transmitted from a station located on the Eiffel tower were received throughout much of Europe. In both the United States and France this led to a small market of receiver lines designed geared for jewelers who needed accurate time to set their clocks, including the Ondophone in France, the De Forest RS-100 Jewelers Time Receiver in the United States The ability to pick up time signal broadcasts, in addition to Morse code weather reports and news summaries attracted the interest of amateur radio enthusiasts.
It was recognized that, much like the telegraph had preceded the invention of the telephone, the ability to make audio radio transmissions would be a significant technical advance. Despite this knowledge, it still took two decades to perfect the technology needed to make quality audio transmissions. In addition, the telephone had been used for distributing entertainment, outside of a few "telephone newspaper" systems, most of which were established in Europe. With this in mind, most early radiotelephone development envisioned that the device would be more profitably developed as a "wireless telephone" for personal communication, or for providing links where regular telephone lines could not be run, rather than for the uncertain finances of broadcasting; the person credited as the primary early developer of AM technology is Canadian-born inventor Reginald Fessenden. The original spark-gap radio transmitters were impractical for transmitting audio, since they produced discontinuous pulses known as "damped waves".
Fessenden realized that what was needed was a new type of radio transmitter that produced steady "undamped" signals, which could be "modulated" to reflect the sounds being transmitted. Fessenden's basic approach was disclosed in U. S. Patent 706,737, which he applied for on May 29, 1901, was issued the next year, it called for the use of a high-speed alternator that generated "pure sine waves" and produced "a continuous train of radiant waves of uniform strength", or, in modern terminology, a continuous-wave transmitter. Fessenden began his research on audio transmissions while doing developmental work for the United States Weather Service on Cobb Island, Maryland; because he did not yet have a continuous-wave transmitter he worked with an experimental "high-frequency spark" transmitter, taking advantage of the fact that the higher the spark rate, the closer a spark-gap transmission comes to producing continuous waves. He reported that, in the fall of 1900, he transmitted speech over a distance of about 1.6 kilometers, which appears to have been the first successful audio transmission using radio signals.
However, at this time the sound was far too distorted to be commercially practical. For a time he continued working with more sophist
Gospel music is a genre of Christian music. The creation, performance and the definition of gospel music varies according to culture and social context. Gospel music is composed and performed for many purposes, including aesthetic pleasure, religious or ceremonial purposes, as an entertainment product for the marketplace. Gospel music has dominant vocals with Christian lyrics. Gospel music can be traced with roots in the black oral tradition. Hymns and sacred songs were repeated in a call and response fashion. Most of the churches relied on hand clapping and foot stomping as rhythmic accompaniment. Most of the singing was done a cappella; the first published use of the term "gospel song" appeared in 1874. The original gospel songs were written and composed by authors such as George F. Root, Philip Bliss, Charles H. Gabriel, William Howard Doane, Fanny Crosby. Gospel music publishing houses emerged; the advent of radio in the 1920s increased the audience for gospel music. Following World War II, gospel music moved into major auditoriums, gospel music concerts became quite elaborate.
Gospel blues is a blues-based form of gospel music. Southern gospel used all tenor-lead-baritone-bass quartet make-up. Progressive Southern gospel is an American music genre that has grown out of Southern gospel over the past couple of decades. Christian country music, sometimes referred to as country gospel music, is a subgenre of gospel music with a country flair, it peaked in popularity in the mid-1990s. Bluegrass gospel music is rooted in American mountain music. Celtic gospel music infuses gospel music with a Celtic flair, is quite popular in countries such as Ireland. British black gospel refers to Gospel music of the African diaspora, produced in the UK; some proponents of "standard" hymns dislike gospel music of the late 19th and early 20th centuries. Today, with historical distance, there is a greater acceptance of such gospel songs into official denominational hymnals. Gospel music features Christian lyrics; some modern gospel music, isn't explicitly Christian and just utilizes the sound.
Subgenres include contemporary gospel, urban contemporary gospel, Southern gospel, modern gospel music. Several forms of gospel music utilize choirs, use piano or Hammond organ, drums, bass guitar and electric guitar. In comparison with hymns, which are of a statelier measure, the gospel song is expected to have a refrain and a more syncopated rhythm. Several attempts have been made to describe the style of late 19th and early 20th century gospel songs in general. Christ-Janer said "the music was tuneful and easy to grasp... rudimentary harmonies... use of the chorus... varied metric schemes... motor rhythms were characteristic... The device of letting the lower parts echo rhythmically a motive announced by the sopranos became a mannerism". Patrick and Sydnor emphasize the notion that gospel music is "sentimental", quoting Sankey as saying, "Before I sing I must feel", they call attention to the comparison of the original version of Rowley's "I Will Sing the Wondrous Story" with Sankey's version.
Gold said, "Essentially the gospel songs are songs of testimony, religious exhortation, or warning. The chorus or refrain technique is found." According to Yale University music professor Willie Ruff, the singing of psalms in Gaelic by Presbyterians of the Scottish Hebrides evolved from "lining out" – where one person sang a solo and others followed – into the call and response of gospel music of the American South. Coming out of the African-American religious experience, American gospel music can be traced to the early 17th century, with foundations in the works of Dr. Isaac Watts and others. Gospel music has roots in the black oral tradition, utilizes a great deal of repetition, which allows those who could not read the opportunity to participate in worship. During this time and sacred songs were lined and repeated in a call and response fashion, Negro spirituals and work songs emerged. Repetition and "call and response" are accepted elements in African music, designed to achieve an altered state of consciousness we sometimes refer to as "trance", strengthen communal bonds.
Most of the churches relied on foot-stomping as rhythmic accompaniment. Guitars and tambourines were sometimes available, but not frequently. Church choirs became a norm only after emancipation. Most of the singing was done a cappella; the most famous gospel-based hymns were composed in the 1760s and 1770s by English writers John Newton and Augustus Toplady, members of the Anglican Church. Starting out as lyrics only, it took decades for standardized tunes to be added to them. Although not directly connected with African-American gospel music, they were adopted by African-Americans as well as white Americans, Newton's connection with the abolition movement provided cross-fertilization; the first published use of the term "Gospel Song" appeared in 1874 when Philip Bliss released a songbook entitled Gospel Songs. A Choice Collection of Hymns and Tunes, it was used to describe a new style of church music, songs that were easy to grasp and more singable than the traditional church hymns, which came out of the mass revival movement starting with Dwight L. Moody, whose musician was Ira D. Sankey, as well as the Holiness-Pentecostal movement.
Prior to the meeting of Moody and
In telecommunications, a repeater is an electronic device that receives a signal and retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side of an obstruction; some types of repeaters broadcast an identical signal, but alter its method of transmission, for example, on another frequency or baud rate. There are several different types of repeaters. A broadcast relay station is a repeater used in broadcast television; when an information-bearing signal passes through a communication channel, it is progressively degraded due to loss of power. For example, when a telephone call passes through a wire telephone line, some of the power in the electric current which represents the audio signal is dissipated as heat in the resistance of the copper wire; the longer the wire is, the more power is lost, the smaller the amplitude of the signal at the far end. So with a long enough wire the call will not be audible at the other end.
The farther from a radio station a receiver is, the weaker the radio signal, the poorer the reception. A repeater is an electronic device in a communication channel that increases the power of a signal and retransmits it, allowing it to travel further. Since it amplifies the signal, it requires a source of electric power; the term "repeater" originated with telegraphy in the 19th century, referred to an electromechanical device used to regenerate telegraph signals. Use of the term has continued in data communications. In computer networking, because repeaters work with the actual physical signal, do not attempt to interpret the data being transmitted, they operate on the physical layer, the first layer of the OSI model; this is used to increase the range of telephone signals in a telephone line. Land line repeaterThey are most used in trunklines that carry long distance calls. In an analog telephone line consisting of a pair of wires, it consists of an amplifier circuit made of transistors which use power from a DC current source to increase the power of the alternating current audio signal on the line.
Since the telephone is a duplex communication system, the wire pair carries two audio signals, one going in each direction. So telephone repeaters have to be bilateral, amplifying the signal in both directions without causing feedback, which complicates their design considerably. Telephone repeaters were the first type of repeater and were some of the first applications of amplification; the development of telephone repeaters between 1900 and 1915 made long distance phone service possible. Now, most telecommunications cables are fiber optic cables. Before the invention of electronic amplifiers, mechanically coupled carbon microphones were used as amplifiers in telephone repeaters. After the turn of the 20th century it was found that negative resistance mercury lamps could amplify, they were used; the invention of audion tube repeaters around 1916 made transcontinental telephony practical. In the 1930s vacuum tube repeaters using hybrid coils became commonplace, allowing the use of thinner wires.
In the 1950s negative impedance gain devices were more popular, a transistorized version called the E6 repeater was the final major type used in the Bell System before the low cost of digital transmission made all voiceband repeaters obsolete. Frequency frogging repeaters were commonplace in frequency-division multiplexing systems from the middle to late 20th century. Submarine cable repeaterThis is a type of telephone repeater used in underwater submarine telecommunications cables; this is used to increase the range of signals in a fiber optic cable. Digital information travels through a fiber optic cable in the form of short pulses of light; the light is made up of particles called photons, which can be scattered in the fiber. An optical communications repeater consists of a phototransistor which converts the light pulses to an electrical signal, an amplifier to increase the power of the signal, an electronic filter which reshapes the pulses, a laser which converts the electrical signal to light again and sends it out the other fiber.
However, optical amplifiers are being developed for repeaters to amplify the light itself without the need of converting it to an electric signal first. This is used to extend the range of coverage of a radio signal; the history of radio relay repeaters began in 1898 from the publication by Johann Mattausch in Austrian Journal Zeitschrift für Electrotechnik. But his proposal "Translator" was not suitable for use; the first relay system with radio repeaters, which functioned, was that invented in 1899 by Emile Guarini-Foresio. A radio repeater consists of a radio receiver connected to a radio transmitter; the received signal is amplified and retransmitted on another frequency, to provide coverage beyond the obstruction. Usage of a duplexer can allow the repeater to use one antenna for both receive and transmit at the same time. Broadcast relay station, rebroadcastor or translator: This is a repeater used to extend the coverage of a radio or television broadcasting station, it consists of a secondary television transmitter.
The signal from the main transmitter comes over leased telephone lines or by microwave relay. Microwave relay: This is a specialized point-to-point telecommunications link, consisting of a microwave receiver that receives information over a beam of microwaves from an
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