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In the case of Raspberry Pi, which is a board with an SoC, page 5 of a Raspberry Pi 4 compilation (BCM2835 ARM Peripherals data sheet) shows that the Pi's hardware uses multiple memory mapping schemes for its board: 1) system bus addresses 2) physical addresses 3) virtual addresses in Linux (kernel-mode, and user-mode). This SoC on a board implements a Linux OS in its hardware; while we think of the software on our own terms, the manufacturer is integrating (ARM/Broadcom/other devices) on the Raspberry Pi board. The data sheet (p.6) states that the peripheral addresses are system bus addresses; the software which accesses peripherals directly must translate the bus addresses into physical addresses or virtual addresses.
One could postulate SoCs out there, which might accomplish a higher level of integration than a Raspberry Pi 4 board, with gigabytes of on-chip memory and wireless ethernet talking to our phone/laptops as peripheral devices, but right now that kind of SoC would be much more expensive than a Raspberry Pi 4 board, which was designed for students/enthusiasts during the past decade. In the future, this kind of SoC might become commonplace. We can't buy such an SoC off-the-shelf right now.


The reformats are affecting the footnotes. When I use NavPop to read the footnotes, for example (Lakatos 1976) I can no longer read the note at the same time as the text. --Ancheta Wis   (talk | contribs) 01:50, 18 August 2021 (UTC) I do see the CS1 error, instead. Here is some history on the lack of a page number on Lakatos 1976: The ideas took some time for me (at the time, I checked out the book from the library and re-read the passages until they clicked), but what I finally understood are in the Proofs and Refutations article. I just found a pdf with some page numbers: 10-11, which pertain to Proofs and Refutations. I have learned that <ref name= someName group= Note> can be used to separate notes and refs. --Ancheta Wis   (talk | contribs) 03:20, 18 August 2021 (UTC) — I have a proposal: in parallel with the [1] work, when a CS1 flagged error in a NavPop appears, that I re-format the flagged footnote into types. Those types could be: Notes | Citations | Sources, etc. The refs themselves would use the <ref name= aRefName group= aNote >etc.</ref>{{rp|page number, etc,}} technique. This would keep the re-formatting effort intact, while footnotes and links remain readable. (The proposed types come from the formatting of today's Featured article.) If this proposal interferes with the re-formatting, I could wait.[reply]

Modern developments

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Emergence of radio and electronics
  • Nikola Tesla developed means to reliably produce radio frequencies, publicly demonstrated the principles of radio, and transmitted long distant signals. He holds the US patent for the invention of the radio, as defined as "wireless transmission of data".
  • Reginald Fessenden [1] and Lee de Forest invented amplitude-modulated (AM) radio, so that more than one station can send signals (as opposed to spark-gap radio, where one transmitter covers the entire bandwidth of the spectrum).
  • Edwin H. Armstrong invented frequency-modulated (FM) radio, so that an audio signal can avoid "static", that is, interference from electrical equipment and atmospherics.

Early radios ran the entire power of the transmitter through a carbon microphone. While some early radios used some type of amplification through electric current or battery, until the mid 1920s the most common type of receiver was the crystal set. In the 1920s, amplifying vacuum tube radio receivers and transmitters came into use.

Brief history

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Main article: History of radio

In 1893 in St. Louis, Missouri, Tesla made devices for his experiments with electricity. Addressing the Franklin Institute in Philadelphia and the National Electric Light Association, he described and demonstrated in detail the principles of his work. [2] The descriptions contained all the elements that were later incorporated into radio systems before the development of the vacuum tube. He initially experimented with magnetic receivers, unlike the coherers (detecting devices consisting of tubes filled with iron filings which had been invented by Temistocle Calzecchi-Onesti at Fermo in Italy in 1884) used by Guglielmo Marconi and other early experimenters. [3]. Tesla is usually considered the first to apply the mechanism of electrical conduction to wireless practices.

In 1896 Marconi was awarded the British patent 12039, Improvements in transmitting electrical impulses and signals and in apparatus there-for, for radio. In 1897 he established the world's first radio station on the Isle of Wight, England. Marconi opened the world's first "wireless" factory in Hall Street, Chelmsford, England in 1898, employing around 50 people. Around 1900, Tesla opened the Wardenclyffe Tower facility and advertised services. By 1903, the tower structure neared completion. Various theories exist on how Tesla intended to achieve the goals of this wireless system (reportedly, a 200 kW system). Tesla claimed that Wardenclyffe, as part of a world system of transmitters, would have allowed secure multichannel transceiving of information, universal navigation, time synchronization, and a global location system.

The next great invention was the vacuum tube detector, invented by a team of Westinghouse engineers. On Christmas Eve, 1906, Reginald Fessenden used a synchronous rotary-spark transmitter for the first radio program broadcast, from Brant Rock, Massachusetts. Ships at sea heard a broadcast that included Fessenden playing O Holy Night on the violin and reading a passage from the Bible. The first radio news program was broadcast August 31, 1920 by station 8MK in Detroit, Michigan. The first regular entertainment broadcasts commenced in 1922 from the Marconi Research Centre at Writtle, near Chelmsford, England.

One of the first developments in the early 20th century (1900-1959) was that Aircraft used commercial AM radio stations for navigation. This continued until the early 1960s when VOR systems finally became widespread (though AM stations are still marked on U.S. aviation charts). In the early 1930s, single sideband and frequency modulation were invented by amateur radio operators. By the end of the decade, they were established commercial modes. Radio was used to transmit pictures visible as television as early as the 1920s. Standard analog transmissions started in North America and Europe in the 1940s. In 1954, Regency introduced a pocket transistor radio, the TR-1, powered by a "standard 22.5 V Battery".

In 1960, Sony introduced their first transistorized radio, small enough to fit in a vest pocket, and able to be powered by a small battery. It was durable, because there were no tubes to burn out. Over the next 20 years, transistors replaced tubes almost completely except for very high-power uses. In 1963 color television was commercially transmitted, and the first (radio) communication satellite, TELSTAR, was launched. In the late 1960s, the U.S. long-distance telephone network began to convert to a digital network, employing digital radios for many of its links. In the 1970s, LORAN became the premier radio navigation system. Soon, the U.S. Navy experimented with satellite navigation, culminating in the invention and launch of the GPS constellation in 1987. In the early 1990s, amateur radio experimenters began to use personal computers with audio cards to process radio signals. In 1994, the U.S. Army and DARPA launched an aggressive, successful project to construct a software radio that could become a different radio on the fly by changing software. Digital transmissions began to be applied to broadcasting in the late 1990s.

Uses of radio

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A Truetone console radio
A 1990s Bush reproduction of a 1930s cathedral style radio
Top-of-the-line, modern Sangean model PR-D3L radio receiver, sold by Digitor in Australia.

Many of radio's 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 its submarine cables were cut by the British. The United States passed on President Woodrow Wilson's Fourteen Points to Germany via radio during the war.

Broadcasting began to become 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 detecting and locating aircraft and ships by the use of radar (RAdio Detection And Ranging).

Today, radio takes many forms, including wireless networks, mobile communications of all types, as well as radio broadcasting. Read more about radio's history.

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. Radio was unique among dramatic presentation that it used only sound. For more, see radio programming.

There are a number of uses of radio:

Audio

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AM broadcast radio sends music and voice in the Medium Frequency (MF—0.300 MHz to 3 MHz) radio spectrum. AM radio uses amplitude modulation, in which louder sounds at the microphone causes wider fluctuations in the transmitter power while the transmitter frequency remains unchanged. Transmissions are affected by static because lightning and other sources of radio add their radio waves to the ones from the transmitter.

FM broadcast radio sends music and voice, with higher fidelity than AM radio. In frequency modulation, louder sounds at the microphone cause the transmitter frequency to fluctuate farther, the transmitter power stays constant. FM is transmitted in the Very High Frequency (VHF—30 MHz to 300 MHz) radio spectrum. FM requires more radio frequency space than AM and there are more frequencies available at higher frequencies, so there can be more stations, each sending more information. Another effect is that shorter VHF radio waves act more like light, travelling 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.

FM Subcarrier services are secondary signals transmitted "piggyback" 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 subchannel program might be an alternate foreign language radio program for various ethnic groups. Subcarriers 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 retune 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 (kilometres) away, even though they are using VHF.

Marine voice radios can use AM 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. Government, police, fire and commercial voice services use narrowband FM on special frequencies. Fidelity is sacrificed to use a smaller range of radio frequencies, usually five kHz of deviation, rather than the 75 kHz used by FM broadcasts and 25 kHz used by TV sound.

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. 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 (usually) lower sideband. This also makes the transmitter about three times more powerful, because it doesn't need to transmit the unused carrier and sideband.

TETRA, Terrestrial Trunked Radio is a digital cell phone system for military, police and ambulances. Commercial services such as XM, WorldSpace and Sirius offer encrypted digital Satellite radio.

Telephony

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Cell phones transmit to a local cell transmitter/receiver site, which connects to the public service telephone network through an optic fiber or microwave radio. When the phone leaves the cell radio's area, the central computer switches the phone to a new cell. Cell phones originally used FM, but now most use various digital encodings. Satellite phones come in two types: INMARSAT and Iridium. Both types provide world-wide coverage. INMARSAT uses geosynchronous satellites, with aimed high-gain antennas on the vehicles. Iridium provides cell phones, with the cells being satellites in orbit.

Video

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Television sends the picture as AM and the sound as FM, with the sound carrier a fixed frequency (4.5 Mhz in the NTSC system) away from the video Carrier. Analog televison also uses a vestigial sideband on the video carrier to reduce the bandwith required.

Digital television uses quadrature amplitude modulation. 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-2 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.

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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 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 from the station. An aircraft can get readings from two VORs, and locate its position at the intersection of the two beams.

Radar

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Radar detects things 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 to measure wind speeds.

Emergency services

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Emergency Position-Indicating Radio Beacons (EPIRBs), Emergency Locating Transmitters (ELTs) or Personal Locator Beacons (PLBs) are small radio transmitters that satellites can use to locate a person or vehicle needing rescue. Their purpose is to help rescue people in the first day, when survival is most likely. There are several types, with widely-varying performance.

Data (digital radio)

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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.

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. 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 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 WiFi, some cell phones, Digital Radio Mondiale, Eureka 147, and many other local area network, digital TV and radio standards.

Heating

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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. (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 ten 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

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Tractor beams can use radio waves which 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 Starwisp. 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.

Other

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Amateur radio is a hobby in which enthusiasts 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. They 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. A number of schemes have been proposed Wireless energy transfer. Various plans included transmitting 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 sof 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 aeroplanes. 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.).

See also

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References

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  • A História da Rádio em Datas (1819-1997) (in Portuguese) - notes on etymology
  • Leigh White, Buck Fuller and the Dymaxion World (refers to Waldo Warren as the inventor of the word radio), in: The Saturday Evening Post, 14 October 1944, cited in: Joachim Krausse and Claude Lichtenstein (eds.), Your Private Sky, Lars Müller Publishers, Baden/Switzerland, 1999, page 132. ISBN 3-907044-88-6
  • L. de Forest, article in Electrical World 22 June 1270/1 (1907), early use of word "radio".

Further reading

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  • Aitkin Hugh G. J. The Continuous Wave: Technology and the American Radio, 1900-1932 (Princeton University Press, 1985).
  • Briggs Asa. The History of Broadcasting in the United Kingdom (Oxford University Press, 1961).
  • Ewbank Henry and Lawton Sherman P. Broadcasting: Radio and Television (Harper & Brothers, 1952).
  • Maclaurin W. Rupert. Invention and Innovation in the Radio Industry (The Macmillan Company, 1949).
  • Ray William B. FCC: The Ups and Downs of Radio-TV Regulation (Iowa State University Press, 1990).
  • Scannell, Paddy, and Cardiff, David. A Social History of British Broadcasting, Volume One, 1922-1939 (Basil Blackwell, 1991).
  • Schwoch James. The American Radio Industry and Its Latin American Activities, 1900-1939 (University of Illinois Press, 1990).
  • Sterling Christopher H. Electronic Media, A Guide to Trends in Broadcasting and Newer Technologies 1920-1983 (Praeger, 1984).
  • White Llewellyn. The American Radio (University of Chicago Press, 1947).

Primary sources

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  • De Lee Forest. Father of Radio: The Autobiography of Lee de Forest (1950).
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Wikimedia Commons has media related to Category:Radio.


Beginning of the 20th century

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During the development of radio, many scientists and inventors contributed to radio technology and electronics. In his classic UHF experiments of 1888, Heinrich Hertz transmitted (via a spark-gap transmitter) and detected radio waves using electrical equipment. In 1895, Nikola Tesla was able to detect signals from the transmissions of his New York lab at West Point (a distance of 80.4 km) [2]. In 1896, Alexander Popov made the wireless transmissions across 60 m and Guglielmo Marconi, around the same time, made a transmission across 2.4 km. John Fleming invented the first radio tube, the diode, in 1904.

Reginald Fessenden recognized that a continuous wave transmission was required for speech for radio and he continued the work of Nikola Tesla, John Stone Stone, and Elihu Thomson on this subject. By the end of 1906, Fessenden sent the first radio broadcast of voice. Also in 1906, Robert von Lieben and Lee De Forest independently developed the amplifier tube, called the triode. [3] Edwin Howard Armstrong developed in 1914 the FM radio. Manfred von Ardenne later introduced the cathode ray tube, a crucial enabling technology for electronic television, in 1931.[4]

Second World War years

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The second world war saw tremendous advances in the field of electronics; especially in RADAR and with the invention of the magnetron by Randle and Boot at the University of Birmingham in 1940. Radio location, radio communication and radio guidance of aircraft were all developed in Britain at this time. An early electronic computing device, Colossus was built by Tommy Flowers of the GPO to decipher the coded messages of the German Lorenz cipher machine. Also developed at this time were advanced clandestine radio transmitters and receivers for use by secret agents. An American invention at the time was a device to scramble the telephone calls between Churchill and Roosevelt. This was called the Green Hornet system and worked by inserting noise into the signal. The noise was then extracted at the receiving end. This system was never broken by the Germans. A great amount of work was undertaken in the United States as part of the War Training Program in the areas of radio direction finding, pulsed linear networks, frequency modulation, vacuum tube circuits, transmission line theory and fundamentals of electromagnetic engineering. These studies were published shortly after the war in what became known as the 'Radio Communication Series' published by McGraw hill 1946. In 1941 Konrad Zuse presented the Z3, the world's first fully functional and programmable computer. [5]

Post war developments

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Prior to the second world war, the subject was commonly known as 'radio engineering' and basically was restricted to aspects of communications and RADAR, commercial radio and early television. At this time, study of radio engineering at universities could only be undertaken as part of a physics degree.

Later, in post war years, as consumer devices began to be developed, the field broadened to include modern TV, audio systems, Hi-Fi and latterly computers and microprocessors. In 1946 the ENIAC (Electronic Numerical Integrator and Computer) of John Presper Eckert and John Mauchly followed, beginning the computing era. The arithmetic performance of these machines allowed engineers to develop completely new technologies and achieve new objectives, including the Apollo missions and the NASA moon landing. "The ENIAC Museum Online". Retrieved January 18, 2006.

The invention of the transistor in 1947 by William B. Shockley, John Bardeen and Walter Brattain opened the door for more compact devices and led to the development of the integrated circuit in 1958 by Jack Kilby and independently in 1959 by Robert Noyce. [6] In the mid to late 1950s, the term radio engineering gradually gave way to the name electronics engineering, which then became a stand alone university degree subject, usually taught alongside electrical engineering with which it had become associated due to some similarities. In 1968 Marcian Hoff invented the first microprocessor at Intel and thus ignited the development of the personal computer. The first realization of the microprocessor was the Intel 4004, a 4-bit processor developed in 1971, but only in 1973 did the Intel 8080, an 8-bit processor, make the building of the first personal computer, the Altair 8800, possible.[7]

References

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Citations
  1. ^ etc.
  2. ^ Leland Anderson, "Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony, and Transmission of Power", Sun Publishing Company, LC 92-60482, ISBN 0-9632652-0-2 (ed. excerpts available online)
  3. ^ "History of Amateur Radio". What is Amateur Radio?. Retrieved January 18, 2006.
  4. ^ "History of TV". Retrieved January 18, 2006.
  5. ^ "The Z3". Retrieved January 18, 2006.
  6. ^ "Electronics Timeline". Greatest Engineering Achievements of the Twentieth Century. Retrieved January 18, 2006.
  7. ^ "Computing History (1971 - 1975)". Retrieved January 18, 2006.
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