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| quikcafe.com | History of radio | Discovery and development | Wireless age | 20th century | Uses of radio | Audio | Data | Digital Radio |
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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 100Hz 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. |
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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 radio teletype 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. Usually the bits are sent in "frames" that repeat. A special bit pattern is used to locate the beginning of a frame. Systems that need reliability, or that share their frequency with other services, may use "corrected 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 sub channels. 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 WiFi, some cell phones, Digital Radio Mondiale, Eureka 147, and many other local area network, digital TV and radio standards. Most new radio systems are digital, see also: Digital TV, Satellite Radio, Digital Audio Broadcasting. HeatingRadio-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. (Note: It is a common misconception that the radio waves are tuned to the resonant frequency of water molecules. The microwave frequencies used are actually about a factor of 10 below the resonant frequency.) Diathermy equipment is used in surgery for sealing of blood vessels. Induction furnaces are used for melting metal for casting. Mechanical force Tractor beams: Radio waves exert small electrostatic and magnetic forces. These are enough to perform station-keeping in microgravity environments. Conceptually, spacecraft propulsion: Radiation pressure from intense radio waves has been proposed as a propulsion method for an interstellar probe called Star wisp. Since the waves are long, the probe could be a very light metal mesh, and thus achieve higher accelerations than if it were a solar sail. OtherAmateur radio is a hobby where enthusiasts who purchase or build their own equipment and use radio for their own enjoyment. They may also provide an emergency and public-service radio service. This has been of great use, saving lives in many instances. Radio amateurs are able to use frequencies in a large number of narrow bands throughout the radio spectrum. Radio amateurs use all forms of encoding, including obsolete and experimental ones. Several forms of radio were pioneered by radio amateurs and later became commercially important, including FM, single-sideband AM, digital packet radio and satellite repeaters.Personal radio services such as Citizens' Band Radio, Family Radio Service, Multi-Use Radio Service and others exist in North America 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. Wireless energy transfer: A number of schemes have been proposed that transmit power using microwaves, and the technique has been demonstrated. (See Microwave power transmission). These schemes include, for example, solar power stations in orbit beaming energy down to terrestrial users. Radio remote control: Use of 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 aero planes. Large industrial remote-controlled equipment such as cranes and switching locomotives now usually use digital radio techniques to ensure safety and reliability. Energy autarkic radio technology consists of a small radio transmitter powered by environmental energy (push of a button, temperature differences, light, vibrations, etc.). |
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| quikcafe.com | History of radio | Discovery and development | Wireless age | 20th century | Uses of radio | Audio | Data | Digital Radio |