kedi kebba telecom engineering summary project report
faculty of technology
kedi wagobera edgar
it luks at telecom engineering and its applications in uganda specifically
Telecommunication refers to communication over long distances. In practice, something of the message may be lost in the process. 'Telecommunication' covers all forms of distance and/or conversion of the original communications, including radio, telegraphy, television, telephony, data communication and computer networking.
The elements of a telecommunication system are a transmitter, a medium (line) and possibly a channel imposed upon the medium (see baseband and broadband as well as multiplexing), and a receiver. The transmitter is a device that transforms or encodes the message into a physical phenomenon; the signal. The transmission medium, by its physical nature, is likely to modify or degrade the signal on its path from the transmitter to the receiver. The receiver has a decoding mechanism capable of recovering the message within certain limits of signal degradation. Sometimes, the final "receiver" is the human eye and/or ear (or in some extreme cases other sensory organs) and the recovery of the message is done by the brain (see psychoacoustics.)
Telecommunication can be point-to-point, point-to-multipoint or broadcasting, which is a particular form of point-to-multipoint that goes only from the transmitter to the receivers.
One of the roles of the telecommunications engineer is to analyse the physical properties of the line or transmission medium, and the statistical properties of the message in order to design the most effective encoding and decoding mechanisms.
When systems are designed to communicate through human sensory organs (mainly those for vision and hearing), physiological and psychological characteristics of human perception must be taken into account. This has important economic implications and engineers must research what defects can be tolerated in the signal and not significantly degrade the viewing or hearing experience.
- 1 Examples of human (tele)communications Edit
- 2 Contents
- 3 edit Etymology
- 4 edit Processes
- 5 edit See also
- 6 Contents
- 7 History
- 8 Geographical usage
- 9 Content
- 10 Social aspects
- 11 Environmental aspects
- 12 See also
- 13 References
- 14 Further reading
- 15 External links
- 16 Contents
- 17 edit History
- 18 edit Types
- 18.1 edit Conventional versus trunked
- 18.2 edit Simplex versus duplex channels
- 18.3 edit Push-to-talk
- 18.4 edit Analog versus digital
- 18.5 edit Data over two-way radio
- 18.6 edit Engineered versus not engineered
- 18.7 edit Options, duty cycle, and configuration
- 18.8 edit Life of equipment
- 19 edit Two-way radio frequencies
- 20 edit UHF versus VHF
- 21 edit Other Two-Way Radio Devices
- 22 edit See also
- 23 edit References
Examples of human (tele)communications Edit[edit | edit source]
In a simplistic example, consider a normal conversation between two people. The message is the sentence that the speaker decides to communicate to the listener. The transmitter is the language areas in the brain, the motor cortex, the vocal cords, the larynx, and the mouth that produce those sounds called speech. The signal is the sound waves (pressure fluctuations in air particles) that can be identified as speech. The channel is the air carrying those sound waves, and all the acoustic properties of the surrounding space: echoes, ambient noise, reverberation. Between the speaker and the listener, there might be other devices that do or do not introduce their own distortions of the original vocal signal (for example a telephone, a HAM radio, an IP phone, etc.) The receiver is the listener's ear and auditory system, the auditory nerve, and the language areas in the listener's brain that will "decode" the signal into meaningful information and filter out background noise.
All channels have noise. Another important aspect of the channel is called the bandwidth. A low bandwidth channel, such as a telephone, cannot carry all of the audio information that is transmitted in normal conversation, causing distortion and irregularities in the speaker's voice, as compared to normal, in-person speech.
Telecommunications engineering or telecom engineering is a major field within Electronic engineering. The work ranges from basic circuit design to strategic mass developments. A telecommunication engineer is responsible for designing and overseeing the installation of telecommunications equipment and facilities, such as complex electronic switching systems, copper telephone facilities, and fiber optics. Telecom engineering also overlaps heavily with broadcast engineering.
Telecommunication is a diverse field of engineering including electronics, civil, structural, and electrical engineering as well as being a political and social ambassador, a little bit of accounting and a lot of project management. Ultimately, telecom engineers are responsible for providing the method that customers can get telephone and high speed data services.
Telecom engineers use a variety of different equipment and transport media available from a multitude of manufacturers to design the telecom network infrastructure. The most common media, often referred to as plant in the telecom industry, used by telecommunications companies today are copper, coaxial cable, fiber, and radio.
Telecom engineers are often expected, as most engineers are, to provide the best solution possible for the lowest cost to the company. This often leads to creative solutions to problems that often would have been designed differently without the budget constraints dictated by modern society. In the earlier days of the telecom industry massive amounts of cable were placed that were never used or have been replaced by modern technology such as fiber optic cable and digital multiplexing techniques.
Telecom engineers are also responsible for keeping the records of the companies’ equipment and facilities and assigning appropriate accounting codes for purposes of taxes and maintenance. As telecom engineers responsible for budgeting and overseeing projects and keeping records of equipment, facilities and plant the telecom engineer is not only an engineer but an accounting assistant or bookkeeper (if not an accountant) and a project manager as well.
This article provides an overview of the major field, telecommunications engineering. Readers might be interested to have a look at electronic engineering and broadcast engineering which are heavily related to telecommunications engineering and often taught together in different academic institutes.
Radio is the transmission of signals by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space. Information is carried by systematically changing (modulating) some property of the radiated waves, such as amplitude, frequency, phase, or pulse width. When radio waves pass an electrical conductor, the oscillating fields induce an alternating current in the conductor. This can be detected and transformed into sound or other signals that carry information.
Contents[edit | edit source]
edit Etymology[edit | edit source]
Originally, radio or radiotelegraphy was called "wireless telegraphy", which was shortened to "wireless" by the British. The prefix radio- in the sense of wireless transmission, was first recorded in the word radioconductor, coined by the French physicist Édouard Branly in 1897 and based on the verb to radiate (in Latin "radius" means "spoke of a wheel, beam of light, ray"). This word also appears in a 1907 article by Lee De Forest, was adopted by the United States Navy in 1912 and became common by the time of the first commercial broadcasts in the United States in the 1920s. (The noun "broadcasting" itself came from an agricultural term, meaning "scattering seeds widely".) The term was then adopted by other languages in Europe and Asia. British Commonwealth countries continued to mainly use the term "wireless" until the mid 20th century, though the magazine of the BBC in the UK has been called Radio Times ever since it was first published in the early 1920s.
In recent years the term "wireless" has gained renewed popularity through the rapid growth of short-range computer networking, e.g., Wireless Local Area Network (WLAN), Wi-Fi, and Bluetooth, as well as mobile telephony, e.g., GSM and UMTS. Today, the term "radio" often refers to the actual transceiver device or chip, whereas "wireless" refers to the system and/or method used for radio communication, hence one talks about radio transceivers and Radio Frequency Identification (RFID), but about wireless devices and wireless sensor networks.
edit Processes[edit | edit source]
Radio systems used for communications will have the following elements. With more than 100 years of development, each process is implemented by a wide range of methods, specialized for different communications purposes.
Each system contains a transmitter. This consists of a source of electrical energy, producing alternating current of a desired frequency of oscillation. The transmitter contains a system to modulate (change) some property of the energy produced to impress a signal on it. This modulation might be as simple as turning the energy on and off, or altering more subtle properties such as amplitude, frequency, phase, or combinations of these properties. The transmitter sends the modulated electrical energy to a tuned resonant antenna; this structure converts the rapidly changing alternating current into an electromagnetic wave that can move through free space (sometimes with a particular polarization).
Electromagnetic waves travel through space either directly, or have their path altered by reflection, refraction or diffraction. The intensity of the waves diminishes due to geometric dispersion (the inverse-square law); some energy may also be absorbed by the intervening medium in some cases. Noise will generally alter the desired signal; this electromagnetic interference comes from natural sources, as well as from artificial sources such as other transmitters and accidental radiators. Noise is also produced at every step due to the inherent properties of the devices used. If the magnitude of the noise is large enough, the desired signal will no longer be discernible; this is the fundamental limit to the range of radio communications.
The electromagnetic wave is intercepted by a tuned receiving antenna; this structure captures some of the energy of the wave and returns it to the form of oscillating electrical currents. At the receiver, these currents are demodulated, which is conversion to a usable signal form by a detector sub-system. The receiver is "tuned" to respond preferentially to the desired signals, and reject undesired signals.
Early radio systems relied entirely on the energy collected by an antenna to produce signals for the operator. Radio became more useful after the invention of electronic devices such as the vacuum tube and later the transistor, which made it possible to amplify weak signals. Today radio systems are used for applications from walkie-talkie children's toys to the control of space vehicles, as well as for broadcasting, and many other applications.
edit Electromagnetic spectrum[edit | edit source]
Main article: Electromagnetic spectrum
Radio frequencies occupy the range from a few tens of hertz to three hundred gigahertz, although commercially important uses of radio use only a small part of this spectrum. Other types of electromagnetic radiation, with frequencies above the RF range, are microwave, infrared, visible light, ultraviolet, X-rays and gamma rays. Since the energy of an individual photon of radio frequency is too low to remove an electron from an atom, radio waves are classified as non-ionizing radiation. Early uses were maritime, for sending telegraphic messages using Morse code between ships and land. The earliest users included the Japanese Navy scouting the Russian fleet during the Battle of Tsushima in 1905. One of the most memorable uses of marine telegraphy was during the sinking of the RMS Titanic in 1912, including communications between operators on the sinking ship and nearby vessels, and communications to shore stations listing the survivors.
Radio was used to pass on orders and communications between armies and navies on both sides in World War I; Germany used radio communications for diplomatic messages once it discovered that its submarine cables had been tapped by the British. The United States passed on President Woodrow Wilson's Fourteen Points to Germany via radio during the war. Broadcasting began from San Jose, California in 1909, and became feasible in the 1920s, with the widespread introduction of radio receivers, particularly in Europe and the United States. Besides broadcasting, point-to-point broadcasting, including telephone messages and relays of radio programs, became widespread in the 1920s and 1930s. Another use of radio in the pre-war years was the development of detection and locating of aircraft and ships by the use of radar (RAdio Detection And Ranging).
Today, radio takes many forms, including wireless networks and mobile communications of all types, as well as radio broadcasting. Before the advent of television, commercial radio broadcasts included not only news and music, but dramas, comedies, variety shows, and many other forms of entertainment (the era from 1930 to the mid-1950s is commonly called radio's "Golden Age"). Radio was unique among methods of dramatic presentation in that it used only sound. For more, see radio programming.
edit Audio[edit | edit source]
A Fisher 500 AM/FM hi-fi receiver from 1959.
AM radio uses amplitude modulation, in which the amplitude of the transmitted signal is made proportional to the sound amplitude captured (transduced) by the microphone, while the transmitted frequency remains unchanged. Transmissions are affected by static and interference because lightning and other sources of radio emissions on the same frequency add their amplitudes to the original transmitted amplitude. In the early part of the 20th century, American AM radio stations broadcast with powers as high as 500 kW, and some could be heard worldwide; these stations' transmitters were commandeered for military use by the US Government during World War II. Currently, the maximum broadcast power for a civilian AM radio station in the United States and Canada is 50 kW, and the majority of stations that emit signals this powerful were grandfathered in (see List of 50kw AM radio stations in the USA). In 1986 KTNN received the last granted 50,000 watt license. These 50 kW stations are generally called "clear channel" stations (not to be confused with Clear Channel Communications), because within North America each of these stations has exclusive use of its broadcast frequency throughout part or all of the broadcast day.
FM broadcast radio sends music and voice with higher fidelity than AM radio. In frequency modulation, amplitude variation at the microphone causes the transmitter frequency to fluctuate. Because the audio signal modulates the frequency and not the amplitude, an FM signal is not subject to static and interference in the same way as AM signals. Due to its need for a wider bandwidth, FM is transmitted in the Very High Frequency (VHF, 30 MHz to 300 MHz) radio spectrum. VHF radio waves act more like light, traveling in straight lines; hence the reception range is generally limited to about 50–100 miles. During unusual upper atmospheric conditions, FM signals are occasionally reflected back towards the Earth by the ionosphere, resulting in long distance FM reception. FM receivers are subject to the capture effect, which causes the radio to only receive the strongest signal when multiple signals appear on the same frequency. FM receivers are relatively immune to lightning and spark interference.
High power is useful in penetrating buildings, diffracting around hills, and refracting in the dense atmosphere near the horizon for some distance beyond the horizon. Consequently, 100,000 watt FM stations can regularly be heard up to 100 miles (160 km) away, and farther (e.g., 150 miles, 240 km) if there are no competing signals. A few old, "grandfathered" stations do not conform to these power rules. WBCT-FM (93.7) in Grand Rapids, Michigan, USA, runs 320,000 watts ERP, and can increase to 500,000 watts ERP by the terms of its original license. Such a huge power level does not usually help to increase range as much as one might expect, because VHF frequencies travel in nearly straight lines over the horizon and off into space. Nevertheless, when there were fewer FM stations competing, this station could be heard near Bloomington, Illinois, USA, almost 300 miles (500 km) away.
FM subcarrier services are secondary signals transmitted in a "piggyback" fashion along with the main program. Special receivers are required to utilize these services. Analog channels may contain alternative programming, such as reading services for the blind, background music or stereo sound signals. In some extremely crowded metropolitan areas, the sub-channel program might be an alternate foreign language radio program for various ethnic groups. Sub-carriers can also transmit digital data, such as station identification, the current song's name, web addresses, or stock quotes. In some countries, FM radios automatically re-tune themselves to the same channel in a different district by using sub-bands.
Aviation voice radios use VHF AM. AM is used so that multiple stations on the same channel can be received. (Use of FM would result in stronger stations blocking out reception of weaker stations due to FM's capture effect). Aircraft fly high enough that their transmitters can be received hundreds of miles (or kilometres) away, even though they are using VHF.
Degen DE1103, an advanced world mini-receiver with single sideband modulation and dual conversion
Marine voice radios can use single sideband voice (SSB) in the shortwave High Frequency (HF—3 MHz to 30 MHz) radio spectrum for very long ranges or narrowband FM in the VHF spectrum for much shorter ranges. Narrowband FM sacrifices fidelity to make more channels available within the radio spectrum, by using a smaller range of radio frequencies, usually with five kHz of deviation, versus the 75 kHz used by commercial FM broadcasts, and 25 kHz used for TV sound.
Government, police, fire and commercial voice services also use narrowband FM on special frequencies. Early police radios used AM receivers to receive one-way dispatches.
Civil and military HF (high frequency) voice services use shortwave radio to contact ships at sea, aircraft and isolated settlements. Most use single sideband voice (SSB), which uses less bandwidth than AM. On an AM radio SSB sounds like ducks quacking, or the adults in a Charlie Brown cartoon. Viewed as a graph of frequency versus power, an AM signal shows power where the frequencies of the voice add and subtract with the main radio frequency. SSB cuts the bandwidth in half by suppressing the carrier and one of the sidebands. This also makes the transmitter about three times more powerful, because it doesn't need to transmit the unused carrier and sideband.
edit Telephony[edit | edit source]
Mobile phones transmit to a local cell site (transmitter/receiver) that ultimately connects to the public switched telephone network (PSTN) through an optic fiber or microwave radio and other network elements. When the mobile phone nears the edge of the cell site's radio coverage area, the central computer switches the phone to a new cell. Cell phones originally used FM, but now most use various digital modulation schemes. Recent developments in Sweden (such as DROPme) allow for the instant downloading of digital material from a radio broadcast (such as a song) to a mobile phone.
Satellite phones use satellites rather than cell towers to communicate.
edit Video[edit | edit source]
Television sends the picture as AM and the sound as AM or FM, with the sound carrier a fixed frequency (4.5 MHz in the NTSC system) away from the video carrier. Analog television also uses a vestigial sideband on the video carrier to reduce the bandwidth required.
Digital television uses 8VSB modulation in North America (under the ATSC digital television standard), and COFDM modulation elsewhere in the world (using the DVB-T standard). A Reed–Solomon error correction code adds redundant correction codes and allows reliable reception during moderate data loss. Although many current and future codecs can be sent in the MPEG transport stream container format, as of 2006 most systems use a standard-definition format almost identical to DVD: MPEG-2 video in Anamorphic widescreen and MPEG layer 2 (MP2) audio. High-definition television is possible simply by using a higher-resolution picture, but H.264/AVC is being considered as a replacement video codec in some regions for its improved compression. With the compression and improved modulation involved, a single "channel" can contain a high-definition program and several standard-definition programs.
[edit | edit source]
All satellite navigation systems use satellites with precision clocks. The satellite transmits its position, and the time of the transmission. The receiver listens to four satellites, and can figure its position as being on a line that is tangent to a spherical shell around each satellite, determined by the time-of-flight of the radio signals from the satellite. A computer in the receiver does the math.
Radio direction-finding is the oldest form of radio navigation. Before 1960 navigators used movable loop antennas to locate commercial AM stations near cities. In some cases they used marine radiolocation beacons, which share a range of frequencies just above AM radio with amateur radio operators. LORAN systems also used time-of-flight radio signals, but from radio stations on the ground. VOR (Very High Frequency Omnidirectional Range), systems (used by aircraft), have an antenna array that transmits two signals simultaneously. A directional signal rotates like a lighthouse at a fixed rate. When the directional signal is facing north, an omnidirectional signal pulses. By measuring the difference in phase of these two signals, an aircraft can determine its bearing or radial from the station, thus establishing a line of position. An aircraft can get readings from two VORs and locate its position at the intersection of the two radials, known as a "fix." When the VOR station is collocated with DME (Distance Measuring Equipment), the aircraft can determine its bearing and range from the station, thus providing a fix from only one ground station. Such stations are called VOR/DMEs. The military operates a similar system of navaids, called TACANs, which are often built into VOR stations. Such stations are called VORTACs. Because TACANs include distance measuring equipment, VOR/DME and VORTAC stations are identical in navigation potential to civil aircraft.
edit Radar[edit | edit source]
Radar (Radio Detection And Ranging) detects objects at a distance by bouncing radio waves off them. The delay caused by the echo measures the distance. The direction of the beam determines the direction of the reflection. The polarization and frequency of the return can sense the type of surface. Navigational radars scan a wide area two to four times per minute. They use very short waves that reflect from earth and stone. They are common on commercial ships and long-distance commercial aircraft.
General purpose radars generally use navigational radar frequencies, but modulate and polarize the pulse so the receiver can determine the type of surface of the reflector. The best general-purpose radars distinguish the rain of heavy storms, as well as land and vehicles. Some can superimpose sonar data and map data from GPS position.
Search radars scan a wide area with pulses of short radio waves. They usually scan the area two to four times a minute. Sometimes search radars use the Doppler effect to separate moving vehicles from clutter. Targeting radars use the same principle as search radar but scan a much smaller area far more often, usually several times a second or more. Weather radars resemble search radars, but use radio waves with circular polarization and a wavelength to reflect from water droplets. Some weather radar use the Doppler effect to measure wind speeds.
edit Data (digital radio)[edit | edit source]
2008 Pure One Classic digital radio
Most new radio systems are digital, see also: Digital TV, Satellite Radio, Digital Audio Broadcasting. The oldest form of digital broadcast was spark gap telegraphy, used by pioneers such as Marconi. By pressing the key, the operator could send messages in Morse code by energizing a rotating commutating spark gap. The rotating commutator produced a tone in the receiver, where a simple spark gap would produce a hiss, indistinguishable from static. Spark-gap transmitters are now illegal, because their transmissions span several hundred megahertz. This is very wasteful of both radio frequencies and power.
The next advance was continuous wave telegraphy, or CW (Continuous Wave), in which a pure radio frequency, produced by a vacuum tube electronic oscillator was switched on and off by a key. A receiver with a local oscillator would "heterodyne" with the pure radio frequency, creating a whistle-like audio tone. CW uses less than 100 Hz of bandwidth. CW is still used, these days primarily by amateur radio operators (hams). Strictly, on-off keying of a carrier should be known as "Interrupted Continuous Wave" or ICW or on-off keying (OOK).
Radio teletypes usually operate on short-wave (HF) and are much loved by the military because they create written information without a skilled operator. They send a bit as one of two tones. Groups of five or seven bits become a character printed by a teletype. From about 1925 to 1975, radio teletype was how most commercial messages were sent to less developed countries. These are still used by the military and weather services.
Aircraft use a 1200 Baud radioteletype service over VHF to send their ID, altitude and position, and get gate and connecting-flight data. Microwave dishes on satellites, telephone exchanges and TV stations usually use quadrature amplitude modulation (QAM). QAM sends data by changing both the phase and the amplitude of the radio signal. Engineers like QAM because it packs the most bits into a radio signal when given an exclusive (non-shared) fixed narrowband frequency range. Usually the bits are sent in "frames" that repeat. A special bit pattern is used to locate the beginning of a frame.
Modern GPS receivers.
Communication systems that limit themselves to a fixed narrowband frequency range are vulnerable to jamming. A variety of jamming-resistant spread spectrum techniques were initially developed for military use, most famously for Global Positioning System satellite transmissions. Commercial use of spread spectrum began in the 1980s. Bluetooth, most cell phones, and the 802.11b version of Wi-Fi each use various forms of spread spectrum.
Systems that need reliability, or that share their frequency with other services, may use "coded orthogonal frequency-division multiplexing" or COFDM. COFDM breaks a digital signal into as many as several hundred slower subchannels. The digital signal is often sent as QAM on the subchannels. Modern COFDM systems use a small computer to make and decode the signal with digital signal processing, which is more flexible and far less expensive than older systems that implemented separate electronic channels. COFDM resists fading and ghosting because the narrow-channel QAM signals can be sent slowly. An adaptive system, or one that sends error-correction codes can also resist interference, because most interference can affect only a few of the QAM channels. COFDM is used for Wi-Fi, some cell phones, Digital Radio Mondiale, Eureka 147, and many other local area network, digital TV and radio standards.
edit Heating[edit | edit source]
Radio-frequency energy generated for heating of objects is generally not intended to radiate outside of the generating equipment, to prevent interference with other radio signals. Microwave ovens use intense radio waves to heat food. Diathermy equipment is used in surgery for sealing of blood vessels. Induction furnaces are used for melting metal for casting, and induction hobs for cooking.
edit Amateur radio service[edit | edit source]
Amateur radio station with multiple receivers and transceivers
Amateur radio, also known as "ham radio", is a hobby in which enthusiasts are licensed to communicate on a number of bands in the radio frequency spectrum non-commercially and for their own enjoyment. They may also provide emergency and public service assistance. This has been very beneficial in emergencies, saving lives in many instances. Radio amateurs use a variety of modes, including nostalgic ones like Morse code and experimental ones like Low-Frequency Experimental Radio. Several forms of radio were pioneered by radio amateurs and later became commercially important including FM, single-sideband (SSB), AM, digital packet radio and satellite repeaters. Some amateur frequencies may be disrupted by power-line internet service.
edit Unlicensed radio services[edit | edit source]
Unlicensed, government-authorized personal radio services such as Citizens' band radio in Australia, the USA, and Europe, and Family Radio Service and Multi-Use Radio Service in North America exist to provide simple, (usually) short range communication for individuals and small groups, without the overhead of licensing. Similar services exist in other parts of the world. These radio services involve the use of handheld units.
Free radio stations, sometimes called pirate radio or "clandestine" stations, are unauthorized, unlicensed, illegal broadcasting stations. These are often low power transmitters operated on sporadic schedules by hobbyists, community activists, or political and cultural dissidents. Some pirate stations operating offshore in parts of Europe and the United Kingdom more closely resembled legal stations, maintaining regular schedules, using high power, and selling commercial advertising time.
edit Radio control (R C)[edit | edit source]
Radio remote controls use radio waves to transmit control data to a remote object as in some early forms of guided missile, some early TV remotes and a range of model boats, cars and airplanes. Large industrial remote-controlled equipment such as cranes and switching locomotives now usually use digital radio techniques to ensure safety and reliability.
In Madison Square Garden, at the Electrical Exhibition of 1898, Nikola Tesla successfully demonstrated a radio-controlled boat. He was awarded U.S. patent No. 613,809 for a "Method of and Apparatus for Controlling Mechanism of Moving Vessels or Vehicles."
edit See also[edit | edit source]
TELEPHONEThe telephone (from the Greek: τῆλε, tēle, "far" and φωνή, phōnē, "voice"), often colloquially referred to as a phone, is a telecommunications device that transmits and receives sound, most commonly the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other. It is one of the most common appliances in the developed world, and has long been considered indispensable to businesses, households and governments. The word "telephone" has been adapted to many languages and is widely recognized around the world.
All telephones have a microphone to speak into, an earphone which reproduces the voice of the other person, a ringer which makes a sound to alert the owner when a call is coming in, and a keypad (or in older phones a telephone dial) to enter the telephone number of the telephone being called. The microphone and earphone are usually built into a handset which is held up to the face to talk. The keypad may be in the handset or in a separate part. A landline telephone is connected by a wire to the telephone network, while a mobile phone or cell phone is portable and communicates with the telephone network by radio. A cordless telephone has a portable handset which communicates by radio with a base station connected by wire to the telephone network, and can only be used within a limited range of the base station.
The microphone converts the sound waves to electrical signals, which are sent through the telephone network to the other phone, where they are converted back to sound waves by the earphone in the other phone's handset. Telephones are a duplex communications medium, meaning they allow the people on both ends to talk simultaneously. The telephone network, consisting of a worldwide net of telephone lines, fiberoptic cables, microwave transmission, cellular networks, communications satellites, and undersea telephone cables connected by switching centers, allows any telephone in the world to communicate with any other. Each telephone line has an identifying number called its telephone number. To initiate a telephone call, a conversation with another telephone, the user enters the other telephone's number into a numeric keypad on his/her phone. Graphic symbols used to designate telephone service or phone-related information in print, signage, and other media include
Main article: Digital Telephony
The Public Switched Telephone Network(PSTN) has gradually evolved towards digital telephony which has improved the capacity and quality of the network. End-to-end analog telephone networks were first modified in the early 1960s by upgrading transmission networks with T1 carrier systems, designed to support the basic 3 kHz voice channel by sampling the bandwidth-limited analog voice signal and encoding using PCM. While digitization allows wideband voice on the same channel, the improved quality of a wider analog voice channel did not find a large market in the PSTN.
Later transmission methods such as SONET and fiber optic transmission further advanced digital transmission. Although analog carrier systems existed that multiplexed multiple analog voice channels onto a single transmission medium, digital transmission allowed lower cost and more channels multiplexed on the transmission medium. Today the end instrument often remains analog but the analog signals are typically converted to digital signals at the (Serving Area Interface(SAI), central office(CO), or other aggregation point. Digital loop carriers(DLC) place the digital network ever closer to the customer premises, relegating the analog local loop to legacy status.
Main article: Voice over Internet Protocol
Hardware-based IP phone
Internet Protocol (IP) telephony (also known as Voice over Internet Protocol, VoIP), is a disruptive technology that is rapidly gaining ground against traditional telephone network technologies. As of January 2005, up to 10% of telephone subscribers in Japan and South Korea have switched to this digital telephone service. A January 2005 Newsweek article suggested that Internet telephony may be "the next big thing."As of 2006 many VoIP companies offer service to consumers and businesses.
IP telephony uses an Internet connection and hardware IP Phones or softphones installed on personal computers to transmit conversations encoded as data packets. In addition to replacing POTS (plain old telephone service), IP telephony services are also competing with mobile phone services by offering free or lower cost connections via WiFihotspots. VoIP is also used on private networks which may or may not have a connection to the global telephone network.
IP telephones have two notable disadvantages compared to traditional telephones. Unless the IP telephone's components are backed up with an uninterruptible power supply or other emergency power source, the phone will cease to function during a power outage as can occur during an emergency or disaster, exactly when the phone is most needed. Traditional phones connected to the older PSTN network do not experience that problem since they are powered by the telephone company's battery supply, which will continue to function even if there's a prolonged power black-out. A second distinct problem for an IP phone is the lack of a 'fixed address' which can impact the provision of emergency services such as police, fire or ambulance, should someone call for them. Unless the registered user updates the IP phone's physical address location after moving to a new residence, emergency services can be, and have been, dispatched to the wrong location.
Fixed telephone lines per 100 inhabitants 1997-2007
By the end of 2009, there were a total of nearly 6 billion mobile and fixed-line subscribers worldwide. This included 1.26 billion fixed-line subscribers and 4.6 billion mobile subscribers. 
Telephone operating companies
Television (TV) is the most widely used telecommunication medium for transmitting and receiving moving images that are either monochromatic ("black and white") or color, usually accompanied by sound. "Television" may also refer specifically to a television set, television programming or television transmission. The word is derived from mixed Latin and Greek roots, meaning "far sight": Greek tele (τῆλε), far, and Latin visio, sight (from video, vis- to see, or to view in the first person).
Commercially available since the late 1920s, the television set has become ubiquitous in homes, businesses and institutions, particularly as a source of entertainment and news. Since the 1970s the availability of video cassettes, laserdiscs, DVDs and now Blu-ray Discs, have resulted in the television set frequently being used for viewing recorded as well as broadcast material. In recent years Internet television has seen the rise of television available via the Internet, e.g. iPlayer and Hulu.
Although other forms such as closed-circuit television (CCTV) are in use, the most common usage of the medium is for broadcast television, which was modeled on the existing radio broadcasting systems developed in the 1920s, and uses high-powered radio-frequency transmitters to broadcast the television signal to individual TV receivers.
Broadcast TV is typically disseminated via radio transmissions on designated channels in the 54–890 MHz frequency band. Signals are now often transmitted with stereo and/or surround sound in many countries. Until the 2000s broadcast TV programs were generally transmitted as an analog signal, but in recent years public and commercial broadcasters have been progressively introducing digital television broadcasting technology.
A standard television set comprises multiple internal electronic circuits, including those for receiving and decoding broadcast signals. A visual display device which lacks a tuner is properly called a monitor, rather than a television. A television system may use different technical standards such as digital television (DTV) and high-definition television (HDTV). Television systems are also used for surveillance, industrial process control, and guiding of weapons, in places where direct observation is difficult or dangerous.
Amateur television (ham TV or ATV) is also used for experimentation, pleasure and public service events by amateur radio operators. Ham TV stations were on the air in many cities before commercial TV stations came on the air.
Contents[edit | edit source]
History[edit | edit source]
Main article: History of television
In its early stages of development, television employed a combination of optical, mechanical and electronic technologies to capture, transmit and display a visual image. By the late 1920s, however, those employing only optical and electronic technologies were being explored. All modern television systems rely on the latter, although the knowledge gained from the work on electromechanical systems was crucial in the development of fully electronic television.
American family watching TV, 1958
The first images transmitted electrically were sent by early mechanical fax machines, including the pantelegraph, developed in the late nineteenth century. The concept of electrically powered transmission of television images in motion was first sketched in 1878 as the telephonoscope, shortly after the invention of the telephone. At the time, it was imagined by early science fiction authors, that someday that light could be transmitted over wires, as sounds were.
The idea of using scanning to transmit images was put to actual practical use in 1881 in the pantelegraph, through the use of a pendulum-based scanning mechanism. From this period forward, scanning in one form or another has been used in nearly every image transmission technology to date, including television. This is the concept of "rasterization", the process of converting a visual image into a stream of electrical pulses.
In 1884 Paul Gottlieb Nipkow, a 23-year-old university student in Germany, patented the first electromechanical television system which employed a scanning disk, a spinning disk with a series of holes spiraling toward the center, for rasterization. The holes were spaced at equal angular intervals such that in a single rotation the disk would allow light to pass through each hole and onto a light-sensitive selenium sensor which produced the electrical pulses. As an image was focused on the rotating disk, each hole captured a horizontal "slice" of the whole image.
Nipkow's design would not be practical until advances in amplifier tube technology became available. The device was only useful for transmitting still "halftone" images—represented by equally spaced dots of varying size—over telegraph or telephone lines. Later designs would use a rotating mirror-drum scanner to capture the image and a cathode ray tube (CRT) as a display device, but moving images were still not possible, due to the poor sensitivity of the selenium sensors. In 1907 Russian scientist Boris Rosing became the first inventor to use a CRT in the receiver of an experimental television system. He used mirror-drum scanning to transmit simple geometric shapes to the CRT.
Typical modern plasma-screen television set.
Scottish inventor John Logie Baird demonstrated the transmission of moving silhouette images in London in 1925, and of moving, monochromatic images in 1926. Baird's scanning disk produced an image of 30 lines resolution, just enough to discern a human face, from a double spiral of lenses. This demonstration by Baird is generally agreed to be the world's first true demonstration of television, albeit a mechanical form of television no longer in use. Remarkably, in 1927 Baird also invented the world's first video recording system, "Phonovision": by modulating the output signal of his TV camera down to the audio range, he was able to capture the signal on a 10-inch wax audio disc using conventional audio recording technology. A handful of Baird's 'Phonovision' recordings survive and these were finally decoded and rendered into viewable images in the 1990s using modern digital signal-processing technology.
In 1926, Hungarian engineer Kálmán Tihanyi designed a television system utilizing fully electronic scanning and display elements, and employing the principle of "charge storage" within the scanning (or "camera") tube.
Also in 1927, Herbert E. Ives of Bell Labs transmitted moving images from a 50-aperture disk producing 16 frames per minute over a cable from Washington, DC to New York City, and via radio from Whippany, New Jersey. Ives used viewing screens as large as 24 by 30 inches (60 by 75 centimeters). His subjects included Secretary of Commerce Herbert Hoover.
In 1927, Philo Farnsworth made the world's first working television system with electronic scanning of both the pickup and display devices, which he first demonstrated to the press on 1 September 1928.
The first practical use of television was in Germany. Regular television broadcasts began in Germany in 1929 and in 1936 the Olympic Games in Berlin were broadcast to television stations in Berlin and Leipzig where the public could view the games live.
Mexican inventor Guillermo González Camarena also played an important role in early television. His experiments with television (known as telectroescopía at first) began in 1931 and led to a patent for the "trichromatic field sequential system" color television in 1940, as well as the remote control.
Although television was first introduced to the general public at the 1939 World's Fair, the outbreak of World War II prevented it from being manufactured on a large scale until after the end of the war. True regular commercial network television programming did not begin in the U.S. until 1948. During that year, legendary conductor Arturo Toscanini made his first of ten TV appearances conducting the NBC Symphony Orchestra, and Texaco Star Theater, starring comedian Milton Berle, became television's first gigantic hit show.
Geographical usage[edit | edit source]
Television introduction by country 1930 to 1939 1940 to 1949 1950 to 1959 1960 to 1969 1970 to 1979 1980 to 1989 1990 to 1999 No data
Main article: Geographical usage of television
Content[edit | edit source]
Programming[edit | edit source]
Getting TV programming shown to the public can happen in many different ways. After production the next step is to market and deliver the product to whatever markets are open to using it. This typically happens on two levels:
- Original Run or First Run: a producer creates a program of one or multiple episodes and shows it on a station or network which has either paid for the production itself or to which a license has been granted by the producers to do the same.
- Broadcast syndication: this is the terminology rather broadly used to describe secondary programming usages (beyond original run). It includes secondary runs in the country of first issue, but also international usage which may not be managed by the originating producer. In many cases other companies, TV stations or individuals are engaged to do the syndication work, in other words to sell the product into the markets they are allowed to sell into by contract from the copyright holders, in most cases the producers.
First run programming is increasing on subscription services outside the U.S., but few domestically produced programs are syndicated on domestic free-to-air (FTA) elsewhere. This practice is increasing however, generally on digital-only FTA channels, or with subscriber-only first-run material appearing on FTA.
Unlike the U.S., repeat FTA screenings of a FTA network program almost only occur on that network. Also, affiliates rarely buy or produce non-network programming that is not centred around local events.
Funding[edit | edit source]
Television sets per 1000 people of the world 1000+ 500–1000 300–500 200–300 100–200 50–100 0–50 No data
Around the globe, broadcast television is financed by either government, advertising, licensing (a form of tax), subscription or any combination of these. To protect revenues, subscription TV channels are usually encrypted to ensure that only subscription payers receive the decryption codes to see the signal. Unencrypted channels are known as free to air or FTA.
In 2009 the global TV market represented 1,217.2 million TV households with at least one television, and total revenues of 268.9 billion EUR (declining 1.2% compared to 2008). North America had the biggest TV revenue market share with 39%, followed by Europe (31%), Asia-Pacific (21%), Latin America (8%) and Africa and the Middle East (2%).
Advertising[edit | edit source]
Television's broad reach makes it a powerful and attractive medium for advertisers. Many television networks and stations sell blocks of broadcast time to advertisers ("sponsors") in order to fund their programming.
[edit | edit source]
This is a Worldwide TV Shipments by Technology as of Q2 2010.
This is Worldwide Flat Panel TV Brand Rankings by Revenue Share as of Q2 2010.
United States[edit | edit source]
Since inception in the U.S. in 1940, TV commercials have become one of the most effective, persuasive, and popular method of selling products of many sorts, especially consumer goods. During the 1940s and into the 1950s, programs were hosted by single advertisers. This, in turn, gave great creative license to the advertisers over the content of the show. Due to the Quiz show scandals in the 1950s, networks shifted to the magazine concept introducing commercial breaks with multiple advertisers. The networks effectively ended advertisers influence over television programming with this introduction.
U.S. advertising rates are determined primarily by Nielsen ratings. The time of the day and popularity of the channel determine how much a television commercial can cost. For example, the highly popular American Idol can cost approximately $750,000 for a thirty second block of commercial time; while the same amount of time for the World Cup and the Super Bowl can cost several million dollars. Conversely, lesser-viewed time slots, such as early mornings and weekday afternoons, are often sold in bulk to producers of infomercials.
In recent years, the paid program or infomercial has become common, usually in lengths of 30 minutes or one hour. Some drug companies and other businesses have even created "news" items for broadcast, known in the industry as video news releases, paying program directors to use them.
Some TV programs also weave advertisements into their shows, a practice begun in film and known as product placement. For example, a character could be drinking a certain kind of soda, going to a particular chain restaurant, or driving a certain make of car. (This is sometimes very subtle, where shows have vehicles provided by manufacturers for low cost, rather than wrangling them.) Sometimes a specific brand or trade mark, or music from a certain artist or group, is used. (This excludes guest appearances by artists, who perform on the show.)
United Kingdom[edit | edit source]
The TV regulator oversees TV advertising in the United Kingdom. Its restrictions have applied since the early days of commercially funded TV. Despite this, an early TV mogul, Roy Thomson, likened the broadcasting licence as being a "licence to print money". Restrictions mean that the big three national commercial TV channels: ITV, Channel 4, and Five can show an average of only seven minutes of advertising per hour (eight minutes in the peak period). Other broadcasters must average no more than nine minutes (twelve in the peak). This means that many imported TV shows from the US have unnatural breaks where the UK company has edited out the breaks intended for US advertising. Advertisements must not be inserted in the course of certain specific proscribed types of programs which last less than half an hour in scheduled duration; this list includes any news or current affairs program, documentaries, and programs for children. Nor may advertisements be carried in a program designed and broadcast for reception in schools or in any religious service or other devotional program, or during a formal Royal ceremony or occasion. There also must be clear demarcations in time between the programs and the advertisements.
The BBC, being strictly non-commercial is not allowed to show advertisements on television in the UK, although it has many advertising-funded channels abroad. The majority of its budget comes from TV licencing (see below) and the sale of content to other broadcasters.
Republic of Ireland[edit | edit source]
The Broadcasting Commission of Ireland (BCI) (Irish: Coimisiún Craolacháin na hÉireann) oversees advertising on television and radio within the Republic of Ireland on both private and state owned broadcasters. Similar to other European countries, advertising is found on both private and state owned broadcasters. There are some restrictions based on advertising, especially in relation to the advertising of alcohol. Such advertisements are prohibited until after 7pm. Broadcasters in the Republic of Ireland adhere to broadcasting legislation implemented by the Broadcasting Commission of Ireland and the European Union. Sponsorship of current affairs programming is prohibited at all times.
As of October 1, 2009 the responsibilities held by the BCI are gradually being transferred to the Broadcasting Authority of Ireland.
Taxation or license[edit | edit source]
Television services in some countries may be funded by a television licence or a form of taxation which means advertising plays a lesser role or no role at all. For example, some channels may carry no advertising at all and some very little, including:
The BBC carries no advertising on its UK channels and is funded by an annual licence paid by all households owning a television. This licence fee is set by government, but the BBC is not answerable to or controlled by government and is therefore genuinely independent.
The two main BBC TV channels are watched by almost 90 percent of the population each week and overall have 27 per cent share of total viewing. This in spite of the fact that 85% of homes are multichannel, with 42% of these having access to 200 free to air channels via satellite and another 43% having access to 30 or more channels via Freeview. The licence that funds the seven advertising-free BBC TV channels currently costs £139.50 a year (about US$215) irrespective of the number of TV sets owned. When the same sporting event has been presented on both BBC and commercial channels, the BBC always attracts the lion's share of the audience, indicating viewers prefer to watch TV uninterrupted by advertising.
The Australian Broadcasting Corporation (ABC) carries no advertising (except for internal promotional material) as it is banned under the ABC Act 1983. The ABC receives its funding from the Australian Government every three years. In the 2008/09 Federal Budget the ABC received A$1.13 Billion. The funds assist in providing the ABC's Television, Radio, Online and International outputs. The ABC also receives funds from its many ABC Shops across Australia. However funded by the Australian Government the editorial independence of the ABC is ensured through law.
In Japan, NHK is paid for by license fees (known in Japanese as reception fee (受信料, Jushinryō?)). The Broadcast Law which governs NHK’s funding stipulates that any television equipped to receive NHK is required to pay. The fee is standardized, with discounts for office workers and students who commute, as well a general discount for residents of Okinawa prefecture.
Subscription[edit | edit source]
Some TV channels are partly funded from subscriptions therefore the signals are encrypted during broadcast to ensure that only the paying subscribers have access to the decryption codes. Most subscription services are also funded by advertising.
Genres[edit | edit source]
Television genres include a broad range of programming types that entertain, inform, and educate viewers. The most expensive entertainment genres to produce are usually drama and dramatic miniseries. However, other genres, such as historical Western genres, may also have high production costs.
Popular entertainment genres include action-oriented shows such as police, crime, detective dramas, horror, or thriller shows. As well, there are also other variants of the drama genre, such as medical dramas and daytime soap operas. Science fiction shows can fall into either the drama or action category, depending on whether they emphasize philosophical questions or high adventure. Comedy is a popular genre which includes situation comedy (sitcom) and animated shows for the adult demographic such as South Park.
The least expensive forms of entertainment programming are game shows, talk shows, variety shows, and reality TV. Game shows show contestants answering questions and solving puzzles to win prizes. Talk shows feature interviews with film, television and music celebrities and public figures. Variety shows feature a range of musical performers and other entertainers such as comedians and magicians introduced by a host or Master of Ceremonies. There is some crossover between some talk shows and variety shows, because leading talk shows often feature performances by bands, singers, comedians, and other performers in between the interview segments. Reality TV shows "regular" people (i.e., not actors) who are facing unusual challenges or experiences, ranging from arrest by police officers (COPS) to weight loss (The Biggest Loser). A variant version of reality shows depicts celebrities doing mundane activities such as going about their everyday life (The Osbournes, Snoop Dogg's Father Hood) or doing manual labor (Simple Life).
Social aspects[edit | edit source]
Main article: Social aspects of television
Television has played a pivotal role in the socialization of the 20th and 21st centuries. There are many aspects of television that can be addressed, including media violence research. In 2010 the iPlayer incorporated a social media aspect to its internet television service, including facebook and twitter.
Environmental aspects[edit | edit source]
With high lead content in CRTs, and the rapid diffusion of new, flat-panel display technologies, some of which (LCDs) use lamps which contain mercury, there is growing concern about electronic waste from discarded televisions. Related occupational health concerns exist, as well, for disassemblers removing copper wiring and other materials from CRTs. Further environmental concerns related to television design and use relate to the devices' increasing electrical energy requirements..
See also[edit | edit source]
- Content Discovery Platform
- Handheld television
- How television works
- Satellite television
- Information-action ratio
- Internet television
- List of countries by number of television broadcast stations
- List of television manufacturers
- List of years in television
- Media psychology
- Outdoor television
- Computer monitor/VDU units
- Sign language on television
- Technology of television
References[edit | edit source]
- ^ Television Frequency Table, CSGNetwork.com., a Division of Computer Support Group.
- ^ Kowalewski, Anthony, "An Amateur's Television Transmitter", Radio News, April 1938. Early Television Museum and Foundation Website. Retrieved 2009-07-19.
- ^ "History of the Cathode Ray Tube". About.com. http://inventors.about.com/od/cstartinventions/a/CathodeRayTube.htm. Retrieved 2009-10-04.
- ^ R. W. Burns, John Logie Baird: television pioneer, IET, 2000 ISBN 0852967977 pp. 73, 88
- ^ Mr ali283280 says: (2009-10-08). "World's First TV Recordings". Tvdawn.com. http://www.tvdawn.com/. Retrieved 2010-06-18.
- ^ "Hungary –Kálmán Tihanyi's 1926 Patent Application 'Radioskop'". Memory of the World. United Nations Educational, Scientific and Cultural Organization (UNESCO). http://portal.unesco.org/ci/en/ev.php-URL_ID=23240&URL_DO=DO_TOPIC&URL_SECTION=201.html. Retrieved 2008-02-22.
- ^ United States Patent Office, Patent No. 2,133,123, Oct. 11, 1938.
- ^ United States Patent Office, Patent No. 2,158,259, May 16, 1939
- ^ "Vladimir Kosma Zworykin, 1889–1982". Bairdtelevision.com. http://www.bairdtelevision.com/zworykin.html. Retrieved 2009-04-17.
- ^ Glinsky, Albert. Theremin: ether music and espionage. University of Illinois Press, 2000. pg. 46.
- ^ a b "Philo Taylor Farnsworth (1906–1971)", The Virtual Museum of the City of San Francisco
- ^ Farnsworth, Elma G., Distant Vision: Romance and Discovery on an Invisible Frontier, Salt Lake City, PemberlyKent, 1989, p. 108.
- ^ "TV History". Gadgetrepublic. 2009-05-01. http://www.tvhistory.tv. Retrieved 2009-05-01.
- ^ http://ewh.ieee.org/r2/johnstown/downloads/20090217_IEEE_JST_Trivia_Answers.pdf
- ^ http://www.scitech.mtesz.hu/52tihanyi/flat-panel_tv_en.pdf
- ^ http://www.google.com/patents?id=sQBkAAAAEBAJ&dq=2296019 Patent 2296019 Chromoscopic Adapter for Television Adapter. Google patents
- ^ Global TV 2010 – Markets, Trends Facts & Figures (2008–2013) International Television Expert Group
- ^ Global TV Revenues (2008–2009) International Television Expert Group
- ^ iDate's Global TV Revenue Market Shares International Television Expert Group
- ^ OFCOM's Global TV Market Report 2009 International Television Expert Group
- ^ Karen Hornick "That Was the Year That Was" American Heritage, Oct. 2006.
- ^ Worldwide TV Shipments by Technology. DisplaySearch, Inc. http://www.displaysearch.com/cps/rde/xchg/displaysearch/hs.xsl/100831_q2_10_tv_shipments_show_continued_growth_but_some_regions_weakening.asp. Retrieved 2010-10-25
- ^ WorldwideFlat Panel TV Brand Rankings by Revenue Share. DisplaySearch, Inc. http://www.displaysearch.com/cps/rde/xchg/displaysearch/hs.xsl/100831_q2_10_tv_shipments_show_continued_growth_but_some_regions_weakening.asp. Retrieved 2010-10-25
- ^ Jon Stewart of "The Daily Show" was mock-outraged at this, saying, "That's what we do!", and calling it a new form of television, "infoganda".
- ^ "Kenneth Roy Thomson". Press Gazette. 7 July 2006. http://www.pressgazette.co.uk/story.asp?storyCode=34783§ioncode=1. Retrieved 24 April 2010.
- ^ "BCI :: Introduction to the BCI". Bci.ie. 2009-10-01. http://www.bci.ie/. Retrieved 2010-06-18.
- ^ "viewing statistics in UK". Barb.co.uk. http://www.barb.co.uk/viewingsummary/weekreports.cfm?report=multichannel&requesttimeout=500&flag=viewingsummary. Retrieved 2009-04-17.
- ^ "The Communications Market: Digital Progress Report – Digital TV, Q3 2007"(PDF). Archived from the original on June 25, 2008. http://web.archive.org/web/20080625013029/http://www.ofcom.org.uk/research/tv/reports/dtv/dtv_2007_q3/dtvq307.pdf. Retrieved 2010-06-18.
- ^ [dead link]
- ^ Ministry of Finance[dead link]
- ^ New BBC iPlayer: Integration with Facebook and Twitter
- ^ "The Rise of the Machines: A Review of Energy Using Products in the Home from the 1970s to Today"(PDF). Energy Saving Trust. July 3, 2006. http://www.energysavingtrust.org.uk/uploads/documents/aboutest/Riseofthemachines.pdf. Retrieved 2007-08-31.
Further reading[edit | edit source]
- Albert Abramson, The History of Television, 1942 to 2000, Jefferson, NC, and London, McFarland, 2003, ISBN 0786412208.
- Pierre Bourdieu, On Television, The New Press, 2001.
- Tim Brooks and Earle March, The Complete Guide to Prime Time Network and Cable TV Shows, 8th ed., Ballantine, 2002.
- Jacques Derrida and Bernard Stiegler, Echographies of Television, Polity Press, 2002.
- David E. Fisher and Marshall J. Fisher, Tube: the Invention of Television, Counterpoint, Washington, DC, 1996, ISBN 1887178171.
- Steven Johnson, Everything Bad is Good for You: How Today's Popular Culture Is Actually Making Us Smarter, New York, Riverhead (Penguin), 2005, 2006, ISBN 1594481946.
- Jerry Mander, Four Arguments for the Elimination of Television, Perennial, 1978.
- Jerry Mander, In the Absence of the Sacred, Sierra Club Books, 1992, ISBN 0871565099.
- Neil Postman, Amusing Ourselves to Death: Public Discourse in the Age of Show Business, New York, Penguin US, 1985, ISBN 0670804541.
- Evan I. Schwartz, The Last Lone Inventor: A Tale of Genius, Deceit, and the Birth of Television, New York, Harper Paperbacks, 2003, ISBN 0060935596.
- Beretta E. Smith-Shomade, Shaded Lives: African-American Women and Television, Rutgers University Press, 2002.
- Alan Taylor, We, the Media: Pedagogic Intrusions into US Mainstream Film and Television News Broadcasting Rhetoric, Peter Lang, 2005, ISBN 3631518528.
[edit | edit source]
Find more about Television on Wikipedia's sister projects:
- A History of Television at the Canada Science and Technology Museum
- The Encyclopedia of Television at the Museum of Broadcast Communications
- The Evolution of TV, A Brief History of TV Technology in Japan NHK
- Television's History – The First 75 Years
- Worldwide Television Standards
- Global TV Market Data
- Television in Color, April 1944 one of the earliest magazine articles detailing the new technology of color television
- NEC Lab – NEC Lab is a tool to design a test antennas for television broadcasting.
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Two-way radios are available in mobile, stationary base and hand-held portable configurations. Hand-held radios are often called walkie-talkies or handie-talkies. A push-to-talk or Press To Transmit button is often present to activate the transmitter.
A mobile phone or cellular telephone is an example of a two-way radio that both transmits and receives at the same time (or full-duplex). It uses two different radio frequencies to carry the two directions of the conversation simultaneously.
edit History[edit | edit source]
Installation of receivers and transmitters at the same fixed location allowed exchange of messages wirelessly. As early as 1907, two-way telegraphy traffic across the Atlantic Ocean was commercially available. By 1912 commercial and military ships carried both transmitters and receivers, allowing two-way communication in close to real-time with a ship that was out of sight of land.
The first truly mobile two-way radio was developed in Australia in 1923 by Senior Constable Frederick William Downie of the Victorian Police. The Victoria Police were the first in the world to use wireless communication in cars, putting an end to the inefficient status reports via public telephone boxes which had been used until that time. The first sets took up the entire back seat of the Lancia patrol cars.
As radio equipment became more powerful, compact, and easier to use, smaller vehicles had two-way radio communication equipment installed. Installation of radio equipment in aircraft allowed scouts to report back observations in real-time, not requiring the pilot to drop messages to troops on the ground below or to land and make a personal report.
In 1933, the Bayonne, New Jersey police department successfully operated a two-way system between a central fixed station and radio transceivers installed in police cars; this allowed rapidly directing police response in emergencies. During World War II hand-held radio transceivers were extensively used by air and ground troops.
Early two-way schemes allowed only one station to transmit at a time while others listened, since all signals were on the same radio frequency - this was called "simplex" mode. Code and voice operations required a simple communication protocol to allow all stations to cooperate in using the single radio channel, so that one station's transmissions were not obscured by another's. By using receivers and transmitters tuned to different frequencies, and solving the problems introduced by operation of a receiver immediately next to a transmitter, simultaneous transmission and reception was possible at each end of a radio link, in so-called "full duplex" mode.
Early two-way schemes required training operators to learn and use Morse code; in ship-board installations the radio operating officer typically had no other duties than handling radio messages. When voice transmission became possible, dedicated operators were no longer required and two-way use became more common. Today's two-way mobile radio equipment is nearly as simple to use as a household telephone, from the point of view of operating personnel, thereby making two-way communications a useful tool in a wide range of personal, commercial and military roles.
edit Types[edit | edit source]
There is an array of two-way radio technologies, systems, and types. There are families of radio types and each family has differing sub-groups and specific radio models. Some of these types are listed below.
edit Conventional versus trunked[edit | edit source]
edit Conventional[edit | edit source]
Conventional radios operate on fixed RF channels. In the case of radios with multiple channels, they operate on one channel at a time. The proper channel is selected by a user. The user operates a channel selector or buttons on the radio control panel to pick the channel.
In multi-channel systems, channels are used for separate purposes. A channel may be reserved for a specific function or for a geographic area. In a functional channel system, one channel may allow City of Springfield road repair crews to talk to the City of Springfield's road maintenance office. A second channel may allow road repair crews to communicate with state highway department crews. In a geographic system, a taxi company may use one channel to communicate in the Boston, Massachusetts area and a second channel when taxis are in Providence, Rhode Island. In marine radio operations, one channel is used as an emergency and calling channel, so that stations may make contact then move to a separate working channel for continued communication.
Motorola uses the term mode to refer to channels on some conventional two-way radio models. In this use, a mode consists of a radio frequency channel and all channel-dependent options such as selective calling.
edit Scanning in conventional radios[edit | edit source]
Some conventional radios scan more than one channel. That is, the receiver searches more than one channel for a valid transmission. A valid transmission may be a radio channel with any signal or a combination of a radio channel with a specific CTCSS (or Selective calling) code.
There are a wide variety of scan configurations which vary from one system to another. Some radios have scan features that receive the primary selected channel at full volume and other channels in a scan list at reduced volume. This helps the user distinguish between the primary channel and others without looking at the radio control panel. An overview:
- A scanning feature can be defined and preset: when in scanning mode, a predetermined set of channels is scanned. Channels are not changeable by the radio user.
- Some radios allow an option for user-selected scan: this allows either lockout of pre-selected channels or adding channels to a scan list by the operator. The radio may revert to a default scan list each time it is powered off or may permanently store the most recent changes.
In professional radios, scan features are programmable and have many options. Scan features can affect system latency. If the radio has a twenty channel scan list and some channels have CTCSS, it can take several seconds to search the entire list. The radio must stop on each channel with a signal and check for a valid CTCSS before resuming scanning. This can cause missed messages.
For this reason, scan features are either not used or scan lists are intentionally kept short in emergency applications. Part of APCO Project 16 set standards for channel access times and delays caused by system overhead. Scan features can further increase these delays. One study said delays of longer than 0.4 seconds (400 milliseconds) in emergency services are not recommended. No delay from user push-to-talk until the user's voice is heard in your radio's speaker is an unattainable ideal.
edit Talk-back on scan[edit | edit source]
Some conventional radios use, or have an option for, a talk-back-on-scan function. If the user transmits when the radio is in a scan mode, it may transmit on the last channel received instead of the selected channel. This may allow users of multi-channel radios to reply to the last message without looking at the radio to see which channel it was on. Without this feature, the user would have to use the channel selector to switch to the channel where the last message occurred. (This option can cause confusion and users must be trained to understand this feature.)
This is an incomplete list of some conventional radio types:
edit Trunked[edit | edit source]
Main article: Trunked radio system
In a trunked radio system, the system logic automatically picks the physical radio frequency channel. There is a protocol that defines a relationship between the radios and the radio backbone which supports them. The protocol allows channel assignments to happen automatically.
Digital trunked systems may carry simultaneous conversations on one physical channel. In the case of a digital trunked radio system, the system also manages time slots on a single physical channel. The function of carrying simultaneous conversations over a single channel is called multiplexing.
Instead of channels, radios are related by groups which may be called, groups, talk groups, or divided into a hierarchy such as fleet and subfleet, or agency-fleet-subfleet. These can be thought of as virtual channels which appear and disappear as conversations occur.
Systems make arrangements for handshaking and connections between radios by one of these two methods:
- A computer assigns channels over a dedicated control channel. The control channel sends a continual data stream. All radios in the system monitor the data stream until commanded by the computer to join a conversation on an assigned channel.
- Electronics embedded in each radio communicate using a protocol of tones or data in order to establish a conversation, (scan-based).
If all physical channels are busy, some systems include a protocol to queue or stack pending requests until a channel becomes available.
Some trunked radios scan more than one talk group or agency-fleet-subfleet.
Visual clues a radio may be trunked include the 1) lack of a squelch knob or adjustment, 2) no monitor button or switch, and 3) a chirp (made infamous by Nextel) showing the channel is available and ready at the moment the push-to-talk is pressed.
This is an incomplete list of some trunked radio types:
edit Simplex versus duplex channels[edit | edit source]
edit Simplex[edit | edit source]
Simplex channel systems use a single channel for transmit and receive. This is typical of aircraft VHF AM and marine radios. Simplex systems are often legacy systems that have existed for years or decades. The architecture allows old radios to work with new ones in a single network. In the case of all ships worldwide or all aircraft worldwide, the large number of radios installed, (the installed base,) can take decades to upgrade. Simplex systems often use open architectures that allow any radio meeting basic standards to be compatible with the entire system.
- Advantage: as the simplest system configuration, there is reliability from the fact that only two radios are needed to establish communication between them.
- Disadvantages: The simplex configuration offers communication over the shortest range or distance because mobile units must be in effective range of each other. The available channel bandwidth limits the number of simultaneous conversations, since "dead" air time cannot be easily used for additional communication.
edit Duplex[edit | edit source]
Duplex means two channels are used: one in each direction.
Duplex channel systems transmit and receive on different discrete channels. This defines systems where equipment cannot communicate without some infrastructure such as a repeater, base station or Talk-Through Base. Most common in the US is a repeater configuration where a base station is configured to re-transmit the audio received from mobile units. This makes the mobiles, or hand-helds, able to communicate amongst one another anywhere within reception range of the base station or repeater. Typically the base or repeater station has a high antenna, which allows greater range, compared with a ground vehicle or hand-held transceiver.
Duplex systems can be divided into two types. The term half-duplex refers to systems where use of a push-to-talk switch is required to communicate. Full duplex refers to systems like mobile telephones with a capability to simultaneously receive and transmit.
- Advantage: duplex channels usually allow repeater operation which extends range (in most cases due to increased transmit power and improved aerial location / height) - especially where hand-held radios are in use.
- Disadvantage: If a radio cannot reach the repeater, it cannot communicate.
edit Hybrid simplex/duplex[edit | edit source]
Some systems use a mix of the two where radios use duplex as a default but can communicate simplex on the base station channel if out-of-range. In the US, the capability to talk simplex on a duplex channel with a repeater is sometimes called talk-around, direct, or car-to-car.
edit Push-to-talk[edit | edit source]
In one Motorola system, a Special Products microphone was created with a rocker-style push-to-talk button. The microphone looked like a normal mobile microphone except that the button rocked either up or down instead of pressing in. Rocking the switch in one direction transmitted duplex on a repeater; the other transmitted simplex on car-to-car.
In two-way radios with headsets, a push-to-talk button may be included on a cord or wireless electronics box clipped to the user's clothing. In an ambulance or aircraft, a button may be present where the corded headset plugs in to the radio wiring. Dispatch consoles often have a hand-operated push-to-talk buttons along with a foot switch or pedal. If the dispatcher's hands are on a computer keyboard, the user can step on the foot pedal to transmit. Some systems have muting so the dispatcher can be on a telephone call and the caller cannot hear what is said over the radio. Their headset microphone will mute if they transmit. This relieves the dispatcher of explaining every radio message to a caller.
In some circumstances, voice-operated transmit (VOX) is used in place of a push-to-talk button. Possible uses are handicapped users who cannot push a button, Amateur radio operators, firefighters, crane operators, or others performing critical tasks where hands must be free but communication is still necessary.
edit Analog versus digital[edit | edit source]
One example of analog radios are AM aircraft radios used to communicate with control towers and air traffic controllers. Another is a Family Radio Service walkie talkie. Equipment is less complex than digital.
- Advantage: In high-quality equipment, better ability to communicate in cases where a received signal is weak or noisy.
- Disadvantage: Only one conversation at a time can occur on each channel.
- Advantage: More simultaneous talking paths are possible and information such as unit ID, status buttons, or text messages can be embedded into a single digital radio channel.
- Disadvantage: Radios must be designed to the same, compatible standard, radios can become obsolete quickly, cost more to purchase, and are more complicated.
edit Data over two-way radio[edit | edit source]
In some cases, two-way radio is used to communicate analog or digital data. Systems can be simplex or duplex and may employ selective calling features such as CTCSS. In full-duplex systems, data can be sent real-time between two points. In simplex or half-duplex, data can be sent with a time lag between many points.
Some two-way digital systems carry both audio and data over a single data stream. Systems of this type include NXDN and APCO Project 25. The method of encoding and decoding the audio stream is called a codec, such as the AMBE family of codecs.
After market GPS tracking and mobile messaging devices can be interfaced with popular two-way radio models providing a range of features.
edit Analog[edit | edit source]
Analog systems may communicate a single condition, such as water level in a livestock tank. A transmitter at the tank site continually sends a signal with a constant tone. The tone would change in pitch to indicate the tank's water level. A meter at the remote end would vary, corresponding to the pitch, to indicate the amount of water present in the livestock tank. Similar methods can be used to telemeter any analog condition. This type of radio system serves a purpose equivalent to a four-to-twenty milliampere loop. In the US, mid-band 72-76 MHz or UHF 450-470 MHz interstitial channels are often used for these systems. Some systems multiplex telemetry of several analog conditions by limiting each to a separate range of tone pitches, for example.
edit Digital[edit | edit source]
Digital systems may communicate text from computer-aided dispatch (CAD). For example, a display in a tow truck may give a textual location for a call and any related details. The tow truck driver may press an acknowledge button, sending data in the opposite direction and flagging the call as received by the driver. They can be used for analog telemetry systems, such as livestock tank levels, as described above. Analog conditions are translated into data words. Some systems send radio paging messages which can either 1) beep a paging receiver, 2) send a numeric message, or 3) send a text message.
Digital systems typically use data rates in the 1,200-19,200 kilobit-per-second rates and may employ modulation schemes such as frequency shift keying, audio frequency shift keying, or quadrature phase shift keying to encode characters. Modern equipment have the same capabilities to carry data as are found in Internet Protocol. Working within the system's protocol constraints, virtually anything can be sent or received.
edit Engineered versus not engineered[edit | edit source]
Engineered systems are designed to perform close to a specification or standard. They are designed as systems with all equipment matched to perform together. For example, a modern, local government two-way radio system in the US may be designed to provide 95% area coverage in an urban area. System designers use radio frequency models, terrain models, and signal propagation modeling software in an attempt to accurately estimate where radios will work within a defined geographic area. The models help designers choose equipment, equipment locations, antennas, and estimate how well signals will penetrate buildings. These models will be backed-up by drive testing and actual field signal level measurements. Designers adjust antenna patterns, add or move equipment sites, and design antenna networks in a way that will accomplish the intended level of performance.
Some systems are not engineered. Legacy systems are existing systems which were never designed to meet a system performance objective. They may have started with a base station and a group of mobile radios. Over a period of years, they have equipment added on in a building block style. Legacy systems may perform adequately even though they were not professionally designed as a coherent system. A user may purchase and locate a base station with an expectation that similar systems used in the past worked acceptably. A City Road Department may have a system that works acceptably, so the Parks Department may build a new similar system and find it equally usable. General Mobile Radio Service systems are not usually engineered.
edit Options, duty cycle, and configuration[edit | edit source]
Example of control arrangement on a configured P25-capable hand-held radio.
1940s tube-type land mobile two way radios often had one channel and were carrier squelch. Because radios were costly and there were fewer radio users, it might be the case that no one else nearby used the same channel. A transmit and receive crystal had to be ordered for the desired channel frequency, then the radio had to be tuned or aligned to work on the channel. 12-volt mobile, tube-type radios drew several amperes on standby and tens-of-amperes on transmit. Equipment worked ideally when new. The performance of vacuum tubes gradually degraded over time. US regulations required an indicator lamp showing the transmitter had power applied and was ready to transmit and a second indicator, (usually red,) that showed the transmitter was on. In radios with options, wire jumpers and discrete components were used to select options. To change a setting, the technician soldered an option jumper wire then made any corresponding adjustments.
The trend is toward increasing complexity. Modern radios can have capacities over 100 channels and are synthesized: the internal electronics in modern radios operate over a range of frequencies with no tuning adjustments. High-end models may have several hundred optional settings and require a computer and software to configure. Sometimes, controls on the radio are referred to as programmable. By changing configuration settings, a system designer could choose to set up a button on the radio's control panel to either:
- turn scan on or off,
- alert another mobile radio, (selective calling),
- turn on an outside speaker, or
- select repeater locations.
Microprocessor-based radios can draw less than 0.2 amperes on standby and up to tens-of-amperes on high-powered, 100 watt transmitters.
Base stations, repeaters, and high-quality mobile radios often have specifications that include a duty cycle. A repeater should always be continuous duty. This means the radio is designed to transmit in a continuous broadcast without transmitter overheating and failure. Mobile radios used in emergency equipment are rated for continuous duty use. This is necessary because any one of an entire fleet of ambulances, for example, could be pressed into service as command post at a major incident.
edit Life of equipment[edit | edit source]
Though the general life term for the two way radio is 5 to 7 years and 1 to 2 years for its accessories but still the usage, atmosphere and environment plays a major role to decide its life term. There are so many speculations on the life term of two way radios and their accessories i.e. batteries, chargers, head set etc.
In government systems, equipment may be replaced based on budgeting rather than any plan or expected service life. Funding in government agencies may be cyclical or sporadic. Managers may replace computing systems, vehicles, or budget computer and vehicle support costs while ignoring two-way radio equipment. Equipment may remain in use even though maintenance costs are unreasonable when viewed from an efficiency standpoint.
Different system elements will have differing service lifetimes. These may be affected by who uses the equipment. An individual contacted at one county government agency claimed equipment used by 24-hour services wears out much faster than equipment used by those who work in positions staffed eight hours a day.
One document says "seven years" is beyond the expected lifetime of walkie-talkies in police service. Batteries are cited as needing replacement more often. Twelve-year-old dispatch consoles mentioned in the same document were identified as usable. These were compared to problematic 21-year-old consoles used elsewhere in the same system.
Another source says system backbone equipment like consoles and base stations are expected to have a fifteen year life. Mobile radios are expected to last ten years. Walkie talkies typically last eight. In a State of California document, the Department of General Services reports expected service life for a communications console used in the Department of Forestry and Fire Protection is 10 years.
edit Two-way radio frequencies[edit | edit source]
Two-way radios can operate on many different frequencies, and these frequencies are assigned differently in different countries. Typically channelized operations are used, so that operators need not tune equipment to a particular frequency but instead can use one or more pre-seleted frequencies, easily chosen by a pushbutton or other means. For example, in the United States, there is a block of 22 channels (pre-selected radio frequencies) assigned, collectively, to the General Mobile Radio Service and Family Radio Service.
In an analog, conventional system, (the simplest type of system,) a frequency or channel serves as a physical medium or link carrying communicated information. The performance of a radio system is partly dependent on the characteristics of frequency band used. The selection of a frequency for a two-way radio system is affected, in part, by:
- government licensing and regulations.
- local congestion or availability of frequencies.
- terrain, since radio signals travel differently in forests and urban viewsheds.
- the presence of noise, interference, or intermodulation.
- sky wave interference below 50-60 MHz and troposhperic bending at VHF.
- in the US, some frequencies require approval of a frequency coordination committee.
A channel number is just a shorthand notation for a frequency. It is, for instance, easier to remember "Channel 1" than to remember "26.965 MHz" (CB Channel 1) or "462.5625 MHz" (FRS/GMRS channel 1), or "156.05 MHz" (Marine channel 1). It is necessary to identify which radio service is under discussion when specifying a frequency by its channel number. Organizations, such as electric power utilities or police departments, may have several assigned frequencies in use with arbitrarily assigned channel numbers. For example, one police department's "Channel 1" might be known to another department as "Channel 3" or may not even be available. Public service agencies have an interest in maintaining some common frequencies for inter-area coordination in emergencies.
Each country allocates radio frequencies to different two-way services, in accordance with international agreements. In the United States some examples of two-way services are: Citizen's Band, FRS, GMRS, MURS, and BRS.
- Amateur radio operators nearly always use frequencies rather than channel numbers, since there is no regulatory or operating requirement for fixed channels in this context. Even amateur radio equipment will have "memory" features to allow rapidly setting the transmitter and receiver to favorite frequencies.
edit UHF versus VHF[edit | edit source]
There are two major formats for two-way radios. They are Ultra High Frequency (UHF) radio and Very High Frequency (VHF) radio. Neither frequency band is inherently better than the other. Both formats are effective ways to communicate with another person so the right radio depends on the application.
The wavelength of a UHF and VHF signal plays a big role in which radio technology to use. UHF has a shorter wavelength which makes it easier for the signal to find its way through rugged terrain or the inside of a building. The longer wavelength of VHF means it can transmit further under ideal conditions. For most applications, lower radio frequencies are better for longer range. A broadcasting TV station illustrates this. A typical VHF station operates at about 100,000 watts and has a coverage radius range of about 60 miles. A UHF station with a 60-mile coverage radius requires transmitting at 3,000,000 watts.
If an application requires working mostly outdoors, a VHF radio is probably the best choice, especially if a base station radio indoors is used and an external antenna is added. The higher the antenna is placed, the further the radio can transmit and receive. One exception to using a VHF radio outdoors is if it is used it in a heavily wooded or rugged area. Under these conditions a UHF radio may be able to transmit better though the terrain (unless the VHF antenna is raised above the terrain).
If the radios are used mainly inside buildings, then UHF is likely the best solution since its shorter wavelength travels through the building better. There are also repeaters that can be installed that relay a UHF signal to increase the communication distance.
There are more available channels with UHF so in more populated areas UHF may be less likely to have interference from other systems. Since the range of UHF is also not as far as VHF under most conditions, there is less chance of distant radios interfering with the signal.
All UHF radios require a license, except for those operating in the Family Radio Service(or PMR446 in Europe). Most VHF frequencies except for MURS also require a license.
edit Other Two-Way Radio Devices[edit | edit source]
Example of the types of two-way radio devices available.
Not all two way radios are hand-held devices. The same technology that is used in two way radios can be placed in other radio
forms. An example of this is a wireless callbox. A wireless callbox is a device that can be used for voice communication at security gates and doors. Not only can they be used to talk to people at these entry points, personnel can remotely unlock the door so the visitor can enter. There are also customer service callboxes that can be placed around a business that a customer can use to summon help from a two way radio equipped store employee.
Another use of two-way radio technology is for a wireless PA system. A wireless PA is essentially a one-way two way radio that enables broadcasting messages from handheld two-way radios or base station intercoms.
edit See also[edit | edit source]
edit References[edit | edit source]
- ^ Haldane, Robert. (1995) The People's Force, A history of the Victoria Police. Melbourne University Press. ISBN 0-522-84674-2 1995
- ^ IEEE History Milestones retrieved Oct. 2, 2007
- ^ One example of purpose-specific channel assignments is described in Ivanov, D. A., V. P. Savelyev, and P. V. Shemanski, "Organization of Communications," Fundamentals of Tactical Command and Control: A Soviet View, Soviet Military Thought Series #18, (Washington, D.C.: Superintendent of Documents, 1977) Library of Congress Control Number: 84602565. This is a US Air Force translation of a Soviet-era, Russian-language book. See also, "Inadequate System Capacity," Special Report: Improving FirefighterCommunications, USFA-TR-099/January 1999, (Emmitsburg, Maryland: U.S. Fire Administration, 1999) pp. 18–19 and "5.2 Present System," The California Highway PatrolCommunications Technology Research Project on 800 MHz, 80-C477, (Sacramento, California: Department of General Services, Communications Technology Division, 1982,) pp. V-4 - V-6.
- ^ "3.4.1 User Equipment General Deficiencies," San Rafael Police Radio Committee: Report to Mayor and City Council, (San Rafael, California: City of San Rafael, 1995,) pp. 12.
- ^ For an example of talk around use, see "Problem Reporting," Special Report: Improving Firefighter Communications, USFA-TR-099/January 1999, (Emmitsburg, Maryland: U.S. Fire Administration, 1999) pp. 25–26. This article also confirms the definition of the phrase talk around.
- ^ For examples, see, Mikhailov, K. E. "Communications Facilities on the Volga-Moscow Transmission Line," Long-Distance Electrical Transmission between the V. I. Lenin Hydroelectric Station and Moscow, (Jerusalem: Israeli Program for Scientific Translations, 1965).
- ^ For an electrocardiogram telemetry example, see Planning Emergency Medical Communications: Volume 2, Local/Regional-Level Planning Guide, (Washington, D.C.: National Highway Traffic Safety Administration, US Department of Transportation, 1995) pp. 48.
- ^ For two examples of drive testing and field measurements of received signal levels, see:
- "Section II: Radio Propagation Studies," The California Highway Patrol Communications Technology Research Project on 800 MHz, 80-C477, (Sacramento, California: Department of General Services, Communications Technology Division, 1982,) pp. II-1 - II-34.
- Ossanna, Jr., Joseph F., "A Model For Mobile Radio Fading Due to Building Reflections: Theoretical and Experimental Fading Waveform Power Spectra," Bell SystemTechnical Journal, November 1964, pp. 2935–2971. 800 MHz trivia: this article shows that signal fades occur at audio frequencies near CTCSS tones, explaining why only DCS was used in Motorola 800 MHz systems in the 1970s.
- ^ For one example, see: "Plan Element S-7: Rationalized Funding" and "Plan Element L-2: Permanent Contra Costa Public Safety Radio Authority," Contra Costa County Public Safety Mobile Radio Master Plan, (Fairfax, Virginia: Federal Engineering, Inc., 2002,) pp. 45, 49.
- ^ For one example, see: "220.127.116.11 Current System Problems," Trunked Radio System: Request For Proposals, (Oklahoma City, Oklahoma: Oklahoma City Municipal Facilities Authority, Public Safety Capital Projects Office, 2000) pp. 56.
- ^ "2.4 Equipment Inventory," San Rafael Police Radio Committee: Report to Mayor and City Council, (San Rafael, California: City of San Rafael, 1995,) pp. 8.
- ^ "8000 Exhibits:Equipment Replacement Costs for a Typical Three Position CDF Command and Control Center," 8000 Telecommunications Manual, (Sacramento, California: State of California, Department of Forestry and Fire Protection, 2006) Adobe PDF file on console costs.
- ^ See, "Appendix B - FCC Regulations," California EMS Communications Plan: Final Draft, (Sacramento, California: State of California EMS Authority, September 2000) pp. 38. and Arizona Phase II Final Report: Statewide Radio Interoperability Needs Assessment, Macro Corporation and The State of Arizona, 2004.
- ^ "Two-Way Radio Success: How to Choose Two-Way Radios, Commercial Intercoms, and Other Wireless Communication Devices for Your Business" page of .
- ^ "Two-Way Radio Success: How to Choose Two-Way Radios, Commercial Intercoms, and Other Wireless Communication Devices for Your Business" page of .
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