1.
Modulation
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Most radio systems in the 20th century used frequency modulation or amplitude modulation to make the carrier carry the radio broadcast. Modulation of a sine waveform transforms a narrow frequency range baseband message signal into a passband signal, a modulator is a device that performs modulation. A demodulator is a device that performs demodulation, the inverse of modulation, a modem can perform both operations. The aim of digital modulation is to transfer a digital bit stream over an analog bandpass channel, in music synthesizers, modulation may be used to synthesise waveforms with an extensive overtone spectrum using a small number of oscillators. In this case the carrier frequency is typically in the order or much lower than the modulating waveform. In analog modulation, the modulation is applied continuously in response to the information signal. Where an AM carrier also carries a variable phase f. TM is f In digital modulation, a carrier signal is modulated by a discrete signal. Digital modulation methods can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as analog-to-digital conversion, the changes in the carrier signal are chosen from a finite number of M alternative symbols. A simple example, A telephone line is designed for transferring audible sounds, for example tones, computers may however communicate over a telephone line by means of modems, which are representing the digital bits by tones, called symbols. If there are four symbols, the first symbol may represent the bit sequence 00, the second 01, the third 10. If the modem plays a melody consisting of 1000 tones per second, since each tone represents a message consisting of two digital bits in this example, the bit rate is twice the symbol rate, i. e.2000 bits per second. This is similar to the used by dialup modems as opposed to DSL modems. According to one definition of digital signal, the signal is a digital signal. According to another definition, the modulation is a form of digital-to-analog conversion, most textbooks would consider digital modulation schemes as a form of digital transmission, synonymous to data transmission, very few would consider it as analog transmission. The most fundamental digital modulation techniques are based on keying, PSK, FSK, a finite number of frequencies are used. ASK, a number of amplitudes are used. QAM, a number of at least two phases and at least two amplitudes are used
2.
Amplitude modulation
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Amplitude modulation is a modulation technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. In amplitude modulation, the amplitude of the wave is varied in proportion to the waveform being transmitted. That waveform may, for instance, correspond to the sounds to be reproduced by a loudspeaker and this technique contrasts with frequency modulation, in which the frequency of the carrier signal is varied, and phase modulation, in which its phase is varied. AM was the earliest modulation method used to transmit voice by radio and it was developed during the first two decades of the 20th century beginning with Roberto Landell De Moura and Reginald Fessendens radiotelephone experiments in 1900. It remains in use today in many forms of communication, for example it is used in two way radios, VHF aircraft radio, Citizens Band Radio, and in computer modems. AM is often used to refer to mediumwave AM radio broadcasting, when it reaches its destination, the information signal is extracted from the modulated carrier by demodulation. In amplitude modulation, the amplitude or strength of the oscillations is what is varied. For example, in AM radio communication, a continuous wave signal has its amplitude modulated by an audio waveform before transmission. The audio waveform modifies the amplitude of the wave and determines the envelope of the waveform. In the frequency domain, amplitude modulation produces a signal with power concentrated at the carrier frequency, each sideband is equal in bandwidth to that of the modulating signal, and is a mirror image of the other. Standard AM is thus sometimes called double-sideband amplitude modulation to distinguish it more sophisticated modulation methods also based on AM. One disadvantage of all amplitude modulation techniques is that the receiver amplifies and detects noise, increasing the received signal to noise ratio, say, by a factor of 10, thus would require increasing the transmitter power by a factor of 10. For this reason AM broadcast is not favored for music and high fidelity broadcasting, another disadvantage of AM is that it is inefficient in power usage, at least two-thirds of the power is concentrated in the carrier signal. The carrier signal contains none of the information being transmitted. However its presence provides a means of demodulation using envelope detection, providing a frequency. The receiver may regenerate a copy of the frequency from a greatly reduced pilot carrier to use in the demodulation process. Even with the carrier totally eliminated in double-sideband suppressed-carrier transmission, carrier regeneration is possible using a Costas phase-locked loop and this doesnt work however for single-sideband suppressed-carrier transmission, leading to the characteristic Donald Duck sound from such receivers when slightly detuned. Single sideband is used widely in amateur radio and other voice communications both due to its power efficiency and bandwidth efficiency
3.
Frequency modulation
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In telecommunications and signal processing, frequency modulation is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. This contrasts with amplitude modulation, in which the amplitude of the wave varies. This modulation technique is known as frequency-shift keying, FSK is widely used in modems and fax modems, and can also be used to send Morse code. Frequency modulation is used for FM radio broadcasting. For this reason, most music is broadcast over FM radio, frequency modulation has a close relationship with phase modulation, phase modulation is often used as an intermediate step to achieve frequency modulation. Mathematically both of these are considered a case of quadrature amplitude modulation. While most of the energy of the signal is contained within fc ± fΔ, the frequency spectrum of an actual FM signal has components extending infinitely, although their amplitude decreases and higher-order components are often neglected in practical design problems. Mathematically, a baseband modulated signal may be approximated by a continuous wave signal with a frequency fm. This method is also named as Single-tone Modulation. As in other systems, the modulation index indicates by how much the modulated variable varies around its unmodulated level. e. The maximum deviation of the frequency from the carrier frequency. For a sine wave modulation, the index is seen to be the ratio of the peak frequency deviation of the carrier wave to the frequency of the modulating sine wave. If h ≪1, the modulation is called narrowband FM, sometimes modulation index h<0.3 rad is considered as Narrowband FM otherwise Wideband FM. In the case of digital modulation, the carrier f c is never transmitted, rather, one of two frequencies is transmitted, either f c + Δ f or f c − Δ f, depending on the binary state 0 or 1 of the modulation signal. If h ≫1, the modulation is called wideband FM, if the frequency deviation is held constant and the modulation frequency increased, the spacing between spectra increases. The carrier and sideband amplitudes are illustrated for different modulation indices of FM signals, for particular values of the modulation index, the carrier amplitude becomes zero and all the signal power is in the sidebands. Since the sidebands are on sides of the carrier, their count is doubled, and then multiplied by the modulating frequency to find the bandwidth. For example,3 kHz deviation modulated by a 2.2 kHz audio tone produces an index of 1.36. Suppose that we limit ourselves to only those sidebands that have an amplitude of at least 0.01
4.
Quadrature amplitude modulation
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Quadrature amplitude modulation is both an analog and a digital modulation scheme. The modulated waves are summed, and the waveform is a combination of both phase-shift keying and amplitude-shift keying, or, in the analog case, of phase modulation. In the digital QAM case, a number of at least two phases and at least two amplitudes are used. PSK modulators are often designed using the QAM principle, but are not considered as QAM since the amplitude of the carrier signal is constant. QAM is used extensively as a scheme for digital telecommunication systems. Arbitrarily high spectral efficiencies can be achieved with QAM by setting a suitable size, limited only by the noise level. QAM is being used in optical systems as bit rates increase, QAM16. Like all modulation schemes, QAM conveys data by changing some aspect of a signal, or the carrier wave. In the case of QAM, the amplitude of two waves of the frequency, 90° out-of-phase with each other are changed to represent the data signal. Amplitude modulating two carriers in quadrature can be viewed as both amplitude modulating and phase modulating a single carrier. Phase modulation and phase-shift keying can be regarded as a case of QAM. This can also be extended to frequency modulation and frequency-shift keying, at the receiver, these two modulating signals can be demodulated using a coherent demodulator. Such a receiver multiplies the received signal separately with both a cosine and sine signal to produce the received estimates of I and Q respectively, because of the orthogonality property of the carrier signals, it is possible to detect the modulating signals independently. This filtered signal is unaffected by Q, showing that the component can be received independently of the quadrature component. Similarly, we may multiply s by a wave and then low-pass filter to extract Q. Analog QAM suffers from the problem as Single-sideband modulation, the exact phase of the carrier is required for correct demodulation at the receiver. If the demodulating phase is even a little off, it results in crosstalk between the modulated signals and this issue of carrier synchronization at the receiver must be handled somehow in QAM systems. The coherent demodulator needs to be exactly in phase with the received signal and this is achieved typically by transmitting a burst subcarrier or a Pilot signal
5.
Single-sideband modulation
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Amplitude modulation produces an output signal that has twice the bandwidth of the original baseband signal. Single-sideband modulation avoids this bandwidth doubling, and the power wasted on a carrier, at the cost of increased device complexity, radio transmitters work by mixing a radio frequency signal of a specific frequency, the carrier wave, with the signal to be broadcast. That is, the signal has a spectrum with twice the bandwidth of the original input signal. In conventional AM radio, this signal is sent to the radio frequency amplifier. Due to the nature of the process, the quality of the resulting signal can be defined by the difference between the maximum and minimum signal energy. Normally the maximum signal energy will be the carrier itself, perhaps twice as powerful as the mixed signals, SSB takes advantage of the fact that the entire original signal is encoded in either one of these sidebands. It is not necessary to broadcast the entire mixed signal, a receiver can extract the entire signal from either the upper or lower sideband. This means that the amplifier can be used more efficiently. A transmitter can choose to only the upper or lower sideband. By doing so, the amplifier only has to work effectively on one half the bandwidth, as a result, SSB transmissions use the available amplifier energy more efficiently, providing longer-range transmission with little or no additional cost. The first U. S. patent for SSB modulation was applied for on December 1,1915 by John Renshaw Carson, the U. S. Navy experimented with SSB over its radio circuits before World War I. SSB first entered service on January 7,1927 on the longwave transatlantic public radiotelephone circuit between New York and London. The high power SSB transmitters were located at Rocky Point, New York and Rugby, the receivers were in very quiet locations in Houlton, Maine and Cupar Scotland. SSB was also used over long distance lines, as part of a technique known as frequency-division multiplexing. FDM was pioneered by telephone companies in the 1930s and this enabled many voice channels to be sent down a single physical circuit, for example in L-carrier. SSB allowed channels to be spaced just 4,000 Hz apart, amateur radio operators began serious experimentation with SSB after World War II. The Strategic Air Command established SSB as the standard for its aircraft in 1957. It has become a de facto standard for voice radio transmissions since then
6.
Amplitude-shift keying
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Amplitude-shift keying is a form of amplitude modulation that represents digital data as variations in the amplitude of a carrier wave. In an ASK system, the binary symbol 1 is represented by transmitting a carrier wave. If the signal value is 1 then the signal will be transmitted, otherwise. Any digital modulation scheme uses a number of distinct signals to represent digital data. ASK uses a number of amplitudes, each assigned a unique pattern of binary digits. Usually, each amplitude encodes a number of bits. Each pattern of bits forms the symbol that is represented by the particular amplitude, frequency and phase of the carrier are kept constant. Like AM, an ASK is also linear and sensitive to noise, distortions, propagation conditions on different routes in PSTN. Both ASK modulation and demodulation processes are relatively inexpensive, the ASK technique is also commonly used to transmit digital data over optical fiber. For LED transmitters, binary 1 is represented by a pulse of light. Laser transmitters normally have a bias current that causes the device to emit a low light level. This low level represents binary 0, while a higher-amplitude lightwave represents binary 1, the simplest and most common form of ASK operates as a switch, using the presence of a carrier wave to indicate a binary one and its absence to indicate a binary zero. For instance, an encoding scheme can represent two bits with each shift in amplitude, an eight-level scheme can represent three bits, and so on. These forms of amplitude-shift keying require a high ratio for their recovery. ASK system can be divided into three blocks, the first one represents the transmitter, the second one is a linear model of the effects of the channel, the third one shows the structure of the receiver. These impulses are sent to the filter ht to be sent through the channel, in other words, for each symbol a different carrier wave is sent with the relative amplitude. After the A/D conversion the signal z can be expressed in the form, z = n r + v g + ∑ n ≠ k v g In this relationship, the second term represents the symbol to be extracted. The others are unwanted, the first one is the effect of noise and it is shown in cyan for just one of them
7.
Frequency-shift keying
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Frequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal. The technology is used for systems such as amateur radio, caller ID. The simplest FSK is binary FSK, BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the 1 is called the mark frequency, the time domain of an FSK modulated carrier is illustrated in the figures to the right. Reference implementations of FSK modems exist and are documented in detail, the demodulation of a binary FSK signal can be done using the Goertzel algorithm very efficiently, even on low-power microcontrollers. In principle FSK can be implemented by using completely independent free-running oscillators, in general, independent oscillators will not be at the same phase and therefore the same amplitude at the switch-over instant, causing sudden discontinuities in the transmitted signal. In practice, many FSK transmitters use only a single oscillator, the elimination of discontinuities in the phase reduces sideband power, reducing interference with neighboring channels. This filter has the advantage of reducing power, reducing interference with neighboring channels. It is used by DECT, Bluetooth, Cypress WirelessUSB, Nordic Semiconductor, Texas Instruments LPRF, Z-Wave, for basic data rate Bluetooth the minimum deviation is 115 kHz. Gaussian filtering is a way for reducing spectral width, it is called pulse shaping in this application. In ordinary non-filtered FSK, at a jump from −1 to +1 or +1 to −1, the modulated waveform changes rapidly, if we change the pulse going from −1 to +1 as −1, −.98, −.93. +.93, +.98, +1, and we use this smoother pulse to determine the carrier frequency, minimum frequency-shift keying or minimum-shift keying is a particular spectrally efficient form of coherent FSK. In MSK, the difference between the higher and lower frequency is identical to half the bit rate, consequently, the waveforms that represent a 0 and a 1 bit differ by exactly half a carrier period. The maximum frequency deviation is δ =0.25 fm, as a result, the modulation index m is 0.5. This is the smallest FSK modulation index that can be such that the waveforms for 0 and 1 are orthogonal. A variant of MSK called Gaussian minimum shift keying is used in the GSM mobile phone standard, normally, the transmitted audio alternates between two tones, one, the mark, represents a binary one, the other, the space, represents a binary zero. AFSK differs from regular frequency-shift keying in performing the modulation at baseband frequencies, in radio applications, the AFSK-modulated signal normally is being used to modulate an RF carrier for transmission. AFSK is not always used for high-speed data communications, since it is far less efficient in both power and bandwidth than most other modulation modes
8.
Minimum-shift keying
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In digital modulation, minimum-shift keying is a type of continuous-phase frequency-shift keying that was developed in the late 1950s and 1960s. Similar to OQPSK, MSK is encoded with bits alternating between quadrature components, with the Q component delayed by half the symbol period, however, instead of square pulses as OQPSK uses, MSK encodes each bit as a half sinusoid. This results in a signal, which reduces problems caused by non-linear distortion. In addition to being viewed as related to OQPSK, MSK can also be viewed as a phase frequency shift keyed signal with a frequency separation of one-half the bit rate. In MSK the difference between the higher and lower frequency is identical to half the bit rate, consequently, the waveforms used to represent a 0 and a 1 bit differ by exactly half a carrier period. Thus, the frequency deviation is δ =0.25 fm where fm is the maximum modulating frequency. As a result, the index m is 0.5. This is the smallest FSK modulation index that can be such that the waveforms for 0 and 1 are orthogonal. A variant of MSK called GMSK is used in the GSM mobile phone standard, a I has its pulse edges on t = and a Q on t =. The carrier frequency is f c, therefore, the signal is modulated in frequency and phase, and the phase changes continuously and linearly. In digital communication, Gaussian minimum shift keying or GMSK is a continuous-phase frequency-shift keying modulation scheme and this has the advantage of reducing sideband power, which in turn reduces out-of-band interference between signal carriers in adjacent frequency channels. GMSK has high efficiency, but it needs a higher power level than QPSK, for instance. GMSK is most notably used in the Global System for Mobile Communications, constellation diagram used to examine the modulation in signal space. J. Vol.1, Number 1,1976, F. Amoroso, Pulse and Spectrum Manipulation in the Minimum Shift Keying Format, IEEE Trans. Document from the University of Hull giving a description of GMSK
9.
On-off keying
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On-off keying denotes the simplest form of amplitude-shift keying modulation that represents digital data at the presence or absence of a carrier wave. In its simplest form, the presence of a carrier for a specific duration represents a binary one, some more sophisticated schemes vary these durations to convey additional information. It is analogous to unipolar encoding line code, on-off keying is most commonly used to transmit Morse code over radio frequencies, although in principle any digital encoding scheme may be used. OOK has been used in the ISM bands to transfer data between computers, for example, OOK is more spectrally efficient than frequency-shift keying, but more sensitive to noise when using a regenerative receiver or a poorly implemented superheterodyne receiver. For a given rate, the bandwidth of a BPSK signal. In addition to RF carrier waves, OOK is also used in communication systems. Unipolar encoding Application Note - Im OOK
10.
Phase-shift keying
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Phase-shift keying is a digital modulation scheme that conveys data by changing the phase of a reference signal. The modulation occurs by varying the sine and cosine inputs at a precise time and it is widely used for wireless LANs, RFID and Bluetooth communication. Any digital modulation scheme uses a number of distinct signals to represent digital data. PSK uses a number of phases, each assigned a unique pattern of binary digits. Usually, each phase encodes a number of bits. Each pattern of bits forms the symbol that is represented by the particular phase and this requires the receiver to be able to compare the phase of the received signal to a reference signal — such a system is termed coherent. Alternatively, instead of operating with respect to a constant reference wave, changes in phase of a single broadcast waveform can be considered the significant items. In this system, the demodulator determines the changes in the phase of the signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying, in exchange, it produces more erroneous demodulation. In the case of PSK, the phase is changed to represent the data signal, a convenient method to represent PSK schemes is on a constellation diagram. This shows the points in the plane where, in this context. Such a representation on perpendicular axes lends itself to straightforward implementation, the amplitude of each point along the in-phase axis is used to modulate a cosine wave and the amplitude along the quadrature axis to modulate a sine wave. By convention, in-phase modulates cosine and quadrature modulates sine, in PSK, the constellation points chosen are usually positioned with uniform angular spacing around a circle. This gives maximum phase-separation between adjacent points and thus the best immunity to corruption and they are positioned on a circle so that they can all be transmitted with the same energy. In this way, the moduli of the numbers they represent will be the same. Two common examples are binary phase-shift keying which uses two phases, and quadrature phase-shift keying which uses four phases, although any number of phases may be used. Since the data to be conveyed are usually binary, the PSK scheme is designed with the number of constellation points being a power of 2. It is a form of the complementary Gaussian error function
11.
Trellis modulation
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In telecommunication, trellis modulation is a modulation scheme that transmits information with high efficiency over band-limited channels such as telephone lines. Gottfried Ungerboeck invented trellis modulation while working for IBM in the 1970s and it went largely unnoticed, however, until he published a new, detailed exposition in 1982 that achieved sudden and widespread recognition. In the late 1980s, modems operating over plain old telephone service typically achieved 9.6 kbit/s by employing four bits per symbol QAM modulation at 2,400 baud,14 kbit/s is only 40% of the theoretical maximum bit rate predicted by Shannons theorem for POTS lines. The name trellis derives from the fact that a diagram of the technique closely resembles a trellis lattice. The scheme is basically a convolutional code of rates, Ungerboecks unique contribution is to apply the parity check for each symbol, instead of the older technique of applying it to the bit stream then modulating the bits. He called the key idea mapping by set partitions and this idea groups symbols in a tree-like structure, then separates them into two limbs of equal size. At each limb of the tree, the symbols are further apart, though hard to visualize in multiple dimensions, a simple one-dimension example illustrates the basic procedure. Suppose the symbols are located at, place all odd symbols in one group, and all even symbols in the second group. Take every other symbol in each group and repeat the procedure for each tree limb and he next described a method of assigning the encoded bit stream onto the symbols in a very systematic procedure. Once this procedure was described, his next step was to program the algorithms into a computer. Even the most simple code produced error rates nearly one one-thousandth of an equivalent uncoded system, for two years Ungerboeck kept these results private and only conveyed them to close colleagues. Finally, in 1982, Ungerboeck published a paper describing the principles of trellis modulation, a flurry of research activity ensued, and by 1990 the International Telecommunication Union had published modem standards for the first trellis-modulated modem at 14.4 kilobits/s. Over the next several years further advances in encoding, plus a symbol rate increase from 2,400 to 3,429 baud. Today, the most common trellis-modulated V.34 modems use a 4-dimensional set partition—achieved by treating two two-dimensional symbols as a single lattice. This set uses 8,16, or 32 state convolutional codes to squeeze the equivalent of 6 to 10 bits into each symbol the modem sends. Once manufacturers introduced modems with trellis modulation, transmission rates increased to the point where interactive transfer of multimedia over the telephone became feasible, sharing a floppy disk via a BBS could be done in just a few minutes, instead of an hour. Thus Ungerboecks invention played a key role in the Information Age, modems, for the history of various encoding modulations from 0. G. Ungerboeck, Channel coding with multilevel/phase signals, IEEE Trans, G. Ungerboeck, Trellis-coded modulation with redundant signal sets part I, introduction, IEEE Communications Magazine, vol
12.
Hierarchical modulation
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It has been widely proven and included in various standards, such as DVB-T, MediaFLO, UMB, and is under study for DVB-H. Hierarchical modulation is also taken as one of the implementations of superposition precoding. When hierarchical-modulated signals are transmitted, users with good reception and advanced receivers can demodulate multiple layers, for a user with a conventional receiver or poor reception, it may only demodulate the data stream embedded in the base layer. With hierarchical modulation, an operator can target users of different types with different services or QoS. However, traditional hierarchical modulation suffers from serious inter-layer interference with impact on the symbol rate. For example, the figure depicts a layering scheme with QPSK base layer, the first layer is 2 bits. The signal detector only needs to establish which quadrant the signal is in, in better signal conditions, the detector can establish the phase and amplitude more precisely, to recover four more bits of data. Thus, the base layer carries 10, and the enhancement layer carries 1101, but unlayered 16QAM with the same SNR would approach full throughput. This means, due to ILI, about 1. 5/4 =37. 5% loss of the base-layer achievable throughput, furthermore, due to ILI and the imperfect demodulation of base-layer symbols, the demodulation error rate of higher-layer symbols increases too. Link adaptation Mipmap, Pyramid – similar techniques in image processing H. Méric, J. Lacan, F. Arnal, G. Lesthievent, boucheret, Combining Adaptive Coding and Modulation With Hierarchical Modulation in Satcom Systems, IEEE Transactions on Broadcasting, Vol.59, No. H. Jiang and P. Wilford, A hierarchical modulation for upgrading digital broadcast systems, IEEE Transactions on broadcasting, Vol.51, shu Wang, Soonyil Kwon and Yi, B. K. On enhancing hierarchical modulation, IEEE International Symposium on Broadband Multimedia Systems and Broadcasting, March 31 2008-April 22008, Las Vegas, NV, Hierarchical Modulation Explained Hierarchical Modulation under DVB
13.
Spread spectrum
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This is a technique in which a telecommunication signal is transmitted on a bandwidth considerably larger than the frequency content of the original information. Frequency hopping is a modulation technique used in spread spectrum signal transmission. This technique decreases the potential interference to other receivers while achieving privacy, spread spectrum generally makes use of a sequential noise-like signal structure to spread the normally narrowband information signal over a relatively wideband band of frequencies. The receiver correlates the received signals to retrieve the information signal. Frequency-hopping spread spectrum, direct-sequence spread spectrum, time-hopping spread spectrum, chirp spread spectrum, ultra-wideband is another modulation technique that accomplishes the same purpose, based on transmitting short duration pulses. Wireless standard IEEE802.11 uses either FHSS or DSSS in its radio interface, techniques known since the 1940s and used in military communication systems since the 1950s spread a radio signal over a wide frequency range several magnitudes higher than minimum requirement. DS is good at resisting continuous-time narrowband jamming, while FH is better at resisting pulse jamming, by contrast, in narrowband systems where the signal bandwidth is low, the received signal quality will be severely lowered if the jamming power happens to be concentrated on the signal bandwidth. Multiple access capability, known as code-division multiple access or code-division multiplexing and their approach was unique in that frequency coordination was done with paper player piano rolls - a novel approach which was never put into practice. A synchronous digital system is one that is driven by a signal and. In fact, a clock signal would have all its energy concentrated at a single frequency. It has become a technique to gain regulatory approval because it requires only simple equipment modification. It is even more popular in portable electronics devices because of faster clock speeds, since these devices are designed to be lightweight and inexpensive, traditional passive, electronic measures to reduce EMI, such as capacitors or metal shielding, are not viable. Active EMI reduction techniques such as spread-spectrum clocking are needed in these cases, however, spread-spectrum clocking, like other kinds of dynamic frequency change, can also create challenges for designers. Principal among these is clock/data misalignment, or clock skew, note that this method does not reduce total radiated energy, and therefore systems are not necessarily less likely to cause interference. Spreading energy over a larger bandwidth effectively reduces electrical and magnetic readings within narrow bandwidths, typical measuring receivers used by EMC testing laboratories divide the electromagnetic spectrum into frequency bands approximately 120 kHz wide. If the system under test were to all its energy in a narrow bandwidth. Distributing this same energy into a larger bandwidth prevents systems from putting enough energy into any one narrowband to exceed the statutory limits, FCC certification testing is often completed with the spread-spectrum function enabled in order to reduce the measured emissions to within acceptable legal limits. However, the spread-spectrum functionality may be disabled by the user in some cases and this might be considered a loophole, but is generally overlooked as long as spread-spectrum is enabled by default
14.
Chirp spread spectrum
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In digital communications, chirp spread spectrum is a spread spectrum technique that uses wideband linear frequency modulated chirp pulses to encode information. A chirp is a signal whose frequency increases or decreases over time. In the picture is an example of an upchirp in which the frequency increases linearly over time, as with other spread spectrum methods, chirp spread spectrum uses its entire allocated bandwidth to broadcast a signal, making it robust to channel noise. Further, because the chirps utilize a broad band of the spectrum, additionally, chirp spread spectrum is resistant to the Doppler effect, which is typical in mobile radio applications. Chirp spread spectrum was originally designed to compete with ultra-wideband for precision ranging, however, since the release of IEEE802.15. 4a, it is no longer actively being considered by the IEEE for standardization in the area of precision ranging. Currently, Nanotron Technologies, which produces real-time location devices and was the force behind getting CSS added to IEEE802.15. 4a, is the only seller of wireless devices using CSS. In particular, their product, the nanoLOC TRX transceiver, uses CSS and is marketed as a network device with real-time location. Some areas where this type of technology can be useful are medical applications, logistics, chirp spread spectrum is ideal for applications requiring low power usage and needing relatively low data rates. In particular, IEEE802.15. 4a specifies CSS as a technique for use in Low-Rate Wireless Personal Area Networks, Nanotrons CSS implementation was actually seen to work at a range of 570 meters between devices. Further, Nanotrons implementation can work at rates of up to 2 Mbit/s - higher than specified in 802.15. 4a. Finally, the IEEE802.15. 4a PHY standard actually mixes CSS encoding techniques with differential phase shift keying modulation to achieve data rates. Chirp spread spectrum may also be used in the future for military applications as it is difficult to detect. Very similar frequency swept waveforms are used in frequency modulated continuous wave radars to measure range, fM-CW radars are very widely used as radio altimeters in aircraft
15.
Frequency-hopping spread spectrum
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It is used as a multiple access method in the code division multiple access scheme frequency-hopping code division multiple access. FHSS is a technology that spreads its signal over rapidly changing frequencies. Each available frequency band is divided into sub-frequencies, signals rapidly change among these in a pre-determined order. Interference at a frequency will only affect the signal during that short interval. FHSS can, however, cause interference with adjacent direct-sequence spread spectrum systems, a sub-type of FHSS used in Bluetooth wireless data transfer is adaptive frequency hopping spread spectrum. A spread-spectrum transmission offers three main advantages over a transmission, Spread-spectrum signals are highly resistant to narrowband interference. The process of re-collecting a spread signal spreads out the interfering signal, Spread-spectrum signals are difficult to intercept. A spread-spectrum signal may simply appear as an increase in the noise to a narrowband receiver. An eavesdropper may have difficulty intercepting a transmission in real time if the sequence is not known. Spread-spectrum transmissions can share a band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the communications. As a result, bandwidth can be used more efficiently, Spread-spectrum signals are highly resistant to deliberate jamming, unless the adversary has knowledge of the spreading characteristics. Military radios use cryptographic techniques to generate the sequence under the control of a secret Transmission Security Key that the sender and receiver share in advance. By itself, frequency hopping provides only limited protection against eavesdropping and jamming, most modern military frequency hopping radios also employ separate encryption devices such as the KY-57. U. S. military radios that use frequency hopping include the JTIDS/MIDS family, HAVE QUICK, some walkie-talkies that employ frequency-hopping spread spectrum technology have been developed for unlicensed use on the 900 MHz band. Several such radios were marketed under the name eXtreme Radio Service, despite the names similarity to the FRS allocation, the system is a proprietary design, rather than an official FCC allocated service. Motorola has deployed a business-banded, license-free digital radio that uses FHSS technology, the overall bandwidth required for frequency hopping is much wider than that required to transmit the same information using only one carrier frequency. However, because transmission occurs only on a portion of this bandwidth at any given time
16.
Time-hopping
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Time-hopping is a communications signal technique which can be used to achieve anti-jamming or low probability of intercept. It can also refer to pulse-position modulation, which in its simplest form employs 2k discrete pulses to transmit k bit per pulse, to achieve LPI, the transmission time is changed randomly by varying the period and duty cycle of the pulse using a pseudo-random sequence. The transmitted signal will then have intermittent start and stop times, although often used to form hybrid spread-spectrum systems, TH is strictly speaking a non-SS technique. Spreading of the spectrum is caused by other factors associated with TH, an example of hybrid SS is TH-FHSS or hybrid TDMA. Spread spectrum Frequency-hopping spread spectrum Direct-sequence spread spectrum Ultra-wideband Frenzel, Louis E, time hopping and frequency hopping in ultrawideband systems
17.
Line code
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In telecommunication, a line code is a code chosen for use within a communications system for transmitting a digital signal down a line. Line coding is used for digital data transport. Some line codes are digital baseband modulation or digital baseband transmission methods, Line coding consists of representing the digital signal to be transported, by a waveform that is appropriate for the specific properties of the physical channel. The pattern of voltage, current or photons used to represent the data on a transmission link is called line encoding. The common types of line encoding are unipolar, polar, bipolar, after line coding, the signal is put through a physical channel, either a transmission medium or data storage medium. Sometimes the characteristics of two very different-seeming channels are similar enough that the line code is used for them. The most common physical channels are, the signal can directly be put on a transmission line. The line-coded signal undergoes further pulse shaping and then modulated to create an RF signal that can be sent through free space, the line-coded signal can be used to turn on and off a light source in free-space optical communication, most commonly used in an infrared remote control. The line-coded signal can be printed on paper to create a bar code, the line-coded signal can be converted to magnetized spots on a hard drive or tape drive. The line-coded signal can be converted to pits on an optical disc, some of the more common or binary line codes include, Each line code has advantages and disadvantages. The running disparity is the total of the disparity of all previously transmitted words. Unfortunately, most long-distance communication channels cannot reliably transport a DC component, the DC component is also called the disparity, the bias, or the DC coefficient. The simplest possible line code, unipolar, gives too many errors on such systems, most line codes eliminate the DC component – such codes are called DC-balanced, zero-DC, or DC-free. There are three ways of eliminating the DC component, Use a constant-weight code, for example, Manchester code and Interleaved 2 of 5. In other words, the transmitter has to make sure that every word that averages to a negative level is paired with another code word that averages to a positive level. Therefore, it must keep track of the running DC buildup, the receiver is designed so that either code word of the pair decodes to the same data bits. For example, AMI, 8B10B, 4B3T, etc, for example, the scrambler specified in RFC2615 for 64b/66b encoding. This is important if the signal must pass through a transformer or a transmission line
18.
Modem
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A modem is a network hardware device that modulates one or more carrier wave signals to encode digital information for transmission and demodulates signals to decode the transmitted information. The goal is attempting to produce a signal that can be transmitted easily, modems can be used with any means of transmitting analog signals, from light emitting diodes to radio. Modems are generally classified by the amount of data they can send in a given unit of time, usually expressed in bits per second. Modems can also be classified by their rate, measured in baud. The baud unit denotes symbols per second, or the number of times per second the modem sends a new signal. For example, the ITU V.21 standard used audio frequency shift keying with two frequencies, corresponding to two distinct symbols, to carry 300 bits per second using 300 baud. By contrast, the original ITU V.22 standard, which could transmit and receive four distinct symbols, news wire services in the 1920s used multiplex devices that satisfied the definition of a modem. However, the function was incidental to the multiplexing function, so they are not commonly included in the history of modems. S. SAGE modems were described by AT&Ts Bell Labs as conforming to their newly published Bell 101 dataset standard, while they ran on dedicated telephone lines, the devices at each end were no different from commercial acoustically coupled Bell 101,110 baud modems. The 201A and 201B Data-Phones were synchronous modems using two-bit-per-baud phase-shift keying, the famous Bell 103A dataset standard was also introduced by AT&T in 1962. It provided full-duplex service at 300 bit/s over normal phone lines, frequency-shift keying was used, with the call originator transmitting at 1,070 or 1,270 Hz and the answering modem transmitting at 2,025 or 2,225 Hz. The readily available 103A2 gave an important boost to the use of remote low-speed terminals such as the Teletype Model 33 ASR and KSR, AT&T reduced modem costs by introducing the originate-only 113D and the answer-only 113B/C modems. For many years, the Bell System maintained a monopoly on the use of its phone lines, however, the seminal Hush-a-Phone v. FCC case of 1956 concluded it was within the FCCs jurisdiction to regulate the operation of the Bell System. The FCC found that as long as a device was not electronically attached to the system and this led to a number of devices that mechanically connected to the phone through a standard handset. Since most handsets were supplied by Western Electric and thus of a standard design and this type of connection was used for many devices, such as answering machines. Acoustically coupled Bell 103A-compatible 300 bit/s modems were common during the 1970s, well-known models included the Novation CAT and the Anderson-Jacobson, the latter spun off from an in-house project at Stanford Research Institute. An even lower-cost option was the Pennywhistle modem, designed to be built using parts from electronics scrap, in December 1972, Vadic introduced the VA3400, notable for full-duplex operation at 1,200 bit/s over the phone network. Like the 103A, it used different frequency bands for transmit, in November 1976, AT&T introduced the 212A modem to compete with Vadic
19.
Pulse-amplitude modulation
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Pulse-amplitude modulation, is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulse. It is an analog pulse modulation scheme in which the amplitudes of a train of pulses are varied according to the sample value of the message signal. Demodulation is performed by detecting the level of the carrier at every single period. There are two types of pulse amplitude modulation, Single polarity PAM, In this a suitable fixed DC bias is added to the signal to ensure all the pulses are positive. Double polarity PAM, In this the pulses are positive and negative. In particular, all telephone modems faster than 300 bit/s use quadrature amplitude modulation, the number of possible pulse amplitudes in analog PAM is theoretically infinite. Digital PAM reduces the number of pulse amplitudes to some power of two, some versions of the Ethernet communication standard are an example of PAM usage. Pulse-amplitude modulation has also developed for the control of light-emitting diodes. Pulse-amplitude modulation LED drivers are able to synchronize pulses across multiple LED channels to enable perfect colour matching. Due to the inherent nature of PAM in conjunction with the switching speed of LEDs it is possible to use LED lighting as a means of wireless data transmission at high speed. The North American Advanced Television Systems Committee standards for digital television uses a form of PAM to broadcast the data that makes up the television signal. This system, known as 8VSB, is based on a three-level PAM like 100BASE-TX, using a single 6 MHz channel allocation, as defined in the previous NTSC analog standard, 8VSB is capable of transmitting 32 Mbit/s. After accounting for error correcting codes and other overhead, the rate in the signal is 19.39 Mbit/s
20.
Pulse-code modulation
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Pulse-code modulation is a method used to digitally represent sampled analog signals. It is the form of digital audio in computers, compact discs, digital telephony. In a PCM stream, the amplitude of the signal is sampled regularly at uniform intervals. Linear pulse-code modulation is a type of PCM where the quantization levels are linearly uniform. This is in contrast to PCM encodings where quantization levels vary as a function of amplitude, though PCM is a more general term, it is often used to describe data encoded as LPCM. Early electrical communications started to sample signals in order to interlace samples from multiple telegraphy sources, the American inventor Moses G. Farmer conveyed telegraph time-division multiplexing as early as 1853. Electrical engineer W. M. Miner, in 1903, used an electro-mechanical commutator for time-division multiplexing multiple telegraph signals and he obtained intelligible speech from channels sampled at a rate above 3500–4300 Hz, lower rates proved unsatisfactory. This was TDM, but pulse-amplitude modulation rather than PCM, in 1920 the Bartlane cable picture transmission system, named after its inventors Harry G. Bartholomew and Maynard D. In 1926, Paul M. Rainey of Western Electric patented a machine which transmitted its signal using 5-bit PCM. The machine did not go into production, british engineer Alec Reeves, unaware of previous work, conceived the use of PCM for voice communication in 1937 while working for International Telephone and Telegraph in France. He described the theory and advantages, but no practical application resulted, Reeves filed for a French patent in 1938, and his US patent was granted in 1943. By this time Reeves had started working at the Telecommunications Research Establishment, the first transmission of speech by digital techniques, the SIGSALY encryption equipment, conveyed high-level Allied communications during World War II. In 1943 the Bell Labs researchers who designed the SIGSALY system became aware of the use of PCM binary coding as already proposed by Alec Reeves. In 1949 for the Canadian Navys DATAR system, Ferranti Canada built a working PCM radio system that was able to transmit digitized radar data over long distances, PCM in the late 1940s and early 1950s used a cathode-ray coding tube with a plate electrode having encoding perforations. The plate collected or passed the beam, producing current variations in binary code, rather than natural binary, the grid of Goodalls later tube was perforated to produce a glitch-free Gray code, and produced all bits simultaneously by using a fan beam instead of a scanning beam. Patent 2,801,281 filed in 1946 and 1952, another patent by the same title was filed by John R. Pierce in 1945, and issued in 1948, U. S. The three of them published The Philosophy of PCM in 1948, PCM is the method of encoding generally used for uncompressed audio, although there are other methods such as pulse-density modulation. The 4ESS switch introduced time-division switching into the US telephone system in 1976, LPCM is used for the lossless encoding of audio data in the Compact disc Red Book standard, introduced in 1982
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Pulse-width modulation
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Pulse-width modulation, or pulse-duration modulation, is a modulation technique used to encode a message into a pulsing signal. In addition, PWM is one of the two principal algorithms used in photovoltaic solar battery chargers, the other being maximum power point tracking. The average value of voltage fed to the load is controlled by turning the switch between supply and load on and off at a fast rate, the longer the switch is on compared to the off periods, the higher the total power supplied to the load. The PWM switching frequency has to be higher than what would affect the load. The term duty cycle describes the proportion of on time to the interval or period of time. Duty cycle is expressed in percent, 100% being fully on, the main advantage of PWM is that power loss in the switching devices is very low. When a switch is off there is no current. Power loss, being the product of voltage and current, is thus in both close to zero. PWM also works well with controls, which, because of their on/off nature. PWM has also used in certain communication systems where its duty cycle has been used to convey information over a communications channel. It was an inefficient scheme, but tolerable because the power was low. While the rheostat was one of methods of controlling power. This mechanism also needed to be able to drive motors for fans, pumps and robotic servos, PWM emerged as a solution for this complex problem. One early application of PWM was in the Sinclair X10, a 10 W audio amplifier available in kit form in the 1960s, at around the same time PWM started to be used in AC motor control. Pulse-width modulation uses a pulse wave whose pulse width is modulated resulting in the variation of the average value of the waveform. As f is a wave, its value is y max for 0 < t < D ⋅ T and y min for D ⋅ T < t < T. The above expression then becomes, y ¯ =1 T =1 T = D ⋅ y max + y min and this latter expression can be fairly simplified in many cases where y min =0 as y ¯ = D ⋅ y max. From this, it is obvious that the value of the signal is directly dependent on the duty cycle D
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Delta-sigma modulation
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Delta-sigma modulation is a method for encoding analog signals into digital signals as found in an analog-to-digital converter. In a conventional ADC, a signal is integrated, or sampled, with a sampling frequency. This process introduces quantization error noise, the first step in a delta-sigma modulation is delta modulation. In delta modulation the change in the signal is encoded, rather than the absolute value, the result is a stream of pulses, as opposed to a stream of numbers as is the case with PCM. Both ADCs and DACs can employ delta-sigma modulation, a delta-sigma ADC first encodes an analog signal using high-frequency delta-sigma modulation, and then applies a digital filter to form a higher-resolution but lower sample-frequency digital output. In both cases, the use of a lower-resolution signal simplifies circuit design and improves efficiency. The coarsely-quantized output of a modulator is occasionally used directly in signal processing or as a representation for signal storage. For example, the Super Audio CD stores the output of a delta-sigma modulator directly on a disk, in brief, because it is very easy to regenerate pulses at the receiver into the ideal form transmitted. The only part of the transmitted waveform required at the receiver is the time at which the pulse occurred, given the timing information the transmitted waveform can be reconstructed electronically with great precision. In contrast, without conversion to a stream but simply transmitting the analog signal directly, all noise in the system is added to the analog signal. Each pulse is made up of a step up followed after an interval by a step down. It is possible, even in the presence of noise, to recover the timing of these steps. Then the accuracy of the process reduces to the accuracy with which the transmitted pulse stream represents the input waveform. Delta-sigma modulation converts the voltage into a pulse frequency and is alternatively known as Pulse Density modulation or Pulse Frequency modulation. The substitution of frequency for voltage is thus entirely natural and carries in its train the transmission advantages of a pulse stream, the different names for the modulation method are the result of pulse frequency modulation by different electronic implementations, which all produce similar transmitted waveforms. This interval can be chosen to give any desired resolution or accuracy, the method is cheaply produced by modern methods, and it is widely used. Because P may take any designed value it may be large enough to give any desired resolution or accuracy. Each pulse of the stream has a known, constant amplitude V and duration d t
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Orthogonal frequency-division multiplexing
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Orthogonal frequency-division multiplexing is a method of encoding digital data on multiple carrier frequencies. OFDM is a frequency-division multiplexing scheme used as a digital modulation method. A large number of closely spaced orthogonal sub-carrier signals are used to carry data on several parallel data streams or channels, each sub-carrier is modulated with a conventional modulation scheme at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth. The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe conditions without complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly modulated narrowband signals rather than one rapidly modulated wideband signal. e, the following list is a summary of existing OFDM-based standards and products. For further details, see the Usage section at the end of the article, High spectral efficiency as compared to other double sideband modulation schemes, spread spectrum, etc. Can easily adapt to severe conditions without complex time-domain equalization. This greatly simplifies the design of both the transmitter and the receiver, unlike conventional FDM, a filter for each sub-channel is not required. The orthogonality requires that the spacing is Δ f = k T U Hertz, where TU seconds is the useful symbol duration. Therefore, with N sub-carriers, the passband bandwidth will be B ≈ N·Δf. The orthogonality also allows high spectral efficiency, with a symbol rate near the Nyquist rate for the equivalent baseband signal. Almost the whole available frequency band can be utilized, OFDM generally has a nearly white spectrum, giving it benign electromagnetic interference properties with respect to other co-channel users. A simple example, A useful symbol duration TU =1 ms would require a sub-carrier spacing of Δ f =11 m s =1 k H z for orthogonality, N =1,000 sub-carriers would result in a total passband bandwidth of NΔf =1 MHz. For this symbol time, the bandwidth in theory according to Nyquist is B W =1 / =0.5 M H z. If a guard interval is applied, Nyquist bandwidth requirement would be even lower, the FFT would result in N =1,000 samples per symbol. If no guard interval was applied, this would result in a base band complex valued signal with a rate of 1 MHz. However, the passband RF signal is produced by multiplying the signal with a carrier waveform resulting in a passband bandwidth of 1 MHz. A single-side band or vestigial sideband modulation scheme would achieve almost half that bandwidth for the symbol rate
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Frequency-division multiplexing
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This allows a single transmission medium such as the radio spectrum, a cable or optical fiber to be shared by multiple independent signals. Another use is to carry separate serial bits or segments of a higher signal in parallel. The most natural example of frequency-division multiplexing is radio and television broadcasting, another example is cable television, in which many television channels are carried simultaneously on a single cable. The multiple separate information signals that are sent over an FDM system, at the source end, for each frequency channel, an electronic oscillator generates a carrier signal, a steady oscillating waveform at a single frequency that serves to carry information. The carrier is much higher in frequency than the baseband signal, the carrier signal and the baseband signal are combined in a modulator circuit. The modulator alters some aspect of the signal, such as its amplitude, frequency, or phase, with the baseband signal. The result of modulating the carrier with the signal is to generate sub-frequencies near the carrier frequency, at the sum. The information from the signal is carried in sidebands on each side of the carrier frequency. Therefore, all the information carried by the channel is in a band of frequencies clustered around the carrier frequency. Similarly, additional signals are used to modulate carriers at other frequencies. The carriers are spaced far apart in frequency that the band of frequencies occupied by each channel. All the channels are sent through the medium, such as a coaxial cable, optical fiber. As long as the frequencies are spaced far enough apart that none of the passbands overlap. Thus the available bandwidth is divided into slots or channels, each of which can carry a modulated signal. The frequencies subtract, producing the signal for that channel again. The resulting baseband signal is filtered out of the other frequencies, for shorter distances, cheaper balanced pair cables were used for various systems including Bell System K- and N-Carrier. By the end of the 20th Century, FDM voice circuits had become rare, modern telephone systems employ digital transmission, in which time-division multiplexing is used instead of FDM. Since the late 20th century digital subscriber lines have used a Discrete multitone system to divide their spectrum into frequency channels, the concept corresponding to frequency-division multiplexing in the optical domain is known as wavelength-division multiplexing
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Multiplexing
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In telecommunications and computer networks, multiplexing is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share an expensive resource, for example, in telecommunications, several telephone calls may be carried using one wire. Multiplexing originated in telegraphy in the 1870s, and is now applied in communications. In telephony, George Owen Squier is credited with the development of telephone carrier multiplexing in 1910, the multiplexed signal is transmitted over a communication channel such as a cable. The multiplexing divides the capacity of the channel into several logical channels. A reverse process, known as demultiplexing, extracts the original channels on the receiver end, a device that performs the multiplexing is called a multiplexer, and a device that performs the reverse process is called a demultiplexer. Multiple variable bit rate digital bit streams may be transferred efficiently over a fixed bandwidth channel by means of statistical multiplexing. This is an asynchronous mode time-domain multiplexing which is a form of time-division multiplexing, digital bit streams can be transferred over an analog channel by means of code-division multiplexing techniques such as frequency-hopping spread spectrum and direct-sequence spread spectrum. In wired communication, space-division multiplexing, also known as Space-division multiple access is the use of separate point-to-point electrical conductors for each transmitted channel, in wireless communication, space-division multiplexing is achieved with multiple antenna elements forming a phased array antenna. Examples are multiple-input and multiple-output, single-input and multiple-output and multiple-input and single-output multiplexing, different antennas would give different multi-path propagation signatures, making it possible for digital signal processing techniques to separate different signals from each other. These techniques may also be utilized for space diversity or beamforming rather than multiplexing Frequency-division multiplexing is inherently an analog technology, FDM achieves the combining of several signals into one medium by sending signals in several distinct frequency ranges over a single medium. In FDM the signals are electrical signals, one of the most common applications for FDM is traditional radio and television broadcasting from terrestrial, mobile or satellite stations, or cable television. Only one cable reaches a customers residential area, but the provider can send multiple television channels or signals simultaneously over that cable to all subscribers without interference. Receivers must tune to the frequency to access the desired signal. A variant technology, called wavelength-division multiplexing is used in optical communications, time-division multiplexing is a digital technology which uses time, instead of space or frequency, to separate the different data streams. TDM involves sequencing groups of a few bits or bytes from each individual input stream, one after the other, if done sufficiently quickly, the receiving devices will not detect that some of the circuit time was used to serve another logical communication path. Consider an application requiring four terminals at an airport to reach a central computer, each terminal communicated at 2400 baud, so rather than acquire four individual circuits to carry such a low-speed transmission, the airline has installed a pair of multiplexers. A pair of 9600 baud modems and one dedicated analog communications circuit from the ticket desk back to the airline data center are also installed
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Electromagnetic radiation
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In physics, electromagnetic radiation refers to the waves of the electromagnetic field, propagating through space carrying electromagnetic radiant energy. It includes radio waves, microwaves, infrared, light, ultraviolet, X-, classically, electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields that propagate at the speed of light through a vacuum. The oscillations of the two fields are perpendicular to other and perpendicular to the direction of energy and wave propagation. The wavefront of electromagnetic waves emitted from a point source is a sphere, the position of an electromagnetic wave within the electromagnetic spectrum can be characterized by either its frequency of oscillation or its wavelength. Electromagnetic waves are produced whenever charged particles are accelerated, and these waves can interact with other charged particles. EM waves carry energy, momentum and angular momentum away from their source particle, quanta of EM waves are called photons, whose rest mass is zero, but whose energy, or equivalent total mass, is not zero so they are still affected by gravity. Thus, EMR is sometimes referred to as the far field, in this language, the near field refers to EM fields near the charges and current that directly produced them, specifically, electromagnetic induction and electrostatic induction phenomena. In the quantum theory of electromagnetism, EMR consists of photons, quantum effects provide additional sources of EMR, such as the transition of electrons to lower energy levels in an atom and black-body radiation. The energy of a photon is quantized and is greater for photons of higher frequency. This relationship is given by Plancks equation E = hν, where E is the energy per photon, ν is the frequency of the photon, a single gamma ray photon, for example, might carry ~100,000 times the energy of a single photon of visible light. The effects of EMR upon chemical compounds and biological organisms depend both upon the power and its frequency. EMR of visible or lower frequencies is called non-ionizing radiation, because its photons do not individually have enough energy to ionize atoms or molecules, the effects of these radiations on chemical systems and living tissue are caused primarily by heating effects from the combined energy transfer of many photons. In contrast, high ultraviolet, X-rays and gamma rays are called ionizing radiation since individual photons of high frequency have enough energy to ionize molecules or break chemical bonds. These radiations have the ability to cause chemical reactions and damage living cells beyond that resulting from simple heating, Maxwell derived a wave form of the electric and magnetic equations, thus uncovering the wave-like nature of electric and magnetic fields and their symmetry. Because the speed of EM waves predicted by the wave equation coincided with the speed of light. Maxwell’s equations were confirmed by Heinrich Hertz through experiments with radio waves, according to Maxwells equations, a spatially varying electric field is always associated with a magnetic field that changes over time. Likewise, a varying magnetic field is associated with specific changes over time in the electric field. In an electromagnetic wave, the changes in the field are always accompanied by a wave in the magnetic field in one direction
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Amplitude
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The amplitude of a periodic variable is a measure of its change over a single period. There are various definitions of amplitude, which are all functions of the magnitude of the difference between the extreme values. In older texts the phase is called the amplitude. Peak-to-peak amplitude is the change between peak and trough, with appropriate circuitry, peak-to-peak amplitudes of electric oscillations can be measured by meters or by viewing the waveform on an oscilloscope. Peak-to-peak is a measurement on an oscilloscope, the peaks of the waveform being easily identified and measured against the graticule. This remains a common way of specifying amplitude, but sometimes other measures of amplitude are more appropriate. In audio system measurements, telecommunications and other areas where the measurand is a signal that swings above and below a value but is not sinusoidal. If the reference is zero, this is the absolute value of the signal, if the reference is a mean value. Semi-amplitude means half the peak-to-peak amplitude, some scientists use amplitude or peak amplitude to mean semi-amplitude, that is, half the peak-to-peak amplitude. It is the most widely used measure of orbital wobble in astronomy, the RMS of the AC waveform. For complicated waveforms, especially non-repeating signals like noise, the RMS amplitude is used because it is both unambiguous and has physical significance. For example, the power transmitted by an acoustic or electromagnetic wave or by an electrical signal is proportional to the square of the RMS amplitude. For alternating current electric power, the practice is to specify RMS values of a sinusoidal waveform. One property of root mean square voltages and currents is that they produce the same heating effect as direct current in a given resistance, the peak-to-peak value is used, for example, when choosing rectifiers for power supplies, or when estimating the maximum voltage that insulation must withstand. Some common voltmeters are calibrated for RMS amplitude, but respond to the value of a rectified waveform. Many digital voltmeters and all moving coil meters are in this category, the RMS calibration is only correct for a sine wave input since the ratio between peak, average and RMS values is dependent on waveform. If the wave shape being measured is greatly different from a sine wave, true RMS-responding meters were used in radio frequency measurements, where instruments measured the heating effect in a resistor to measure current. The advent of microprocessor controlled meters capable of calculating RMS by sampling the waveform has made true RMS measurement commonplace
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Frequency
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Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as frequency, which emphasizes the contrast to spatial frequency. The period is the duration of time of one cycle in a repeating event, for example, if a newborn babys heart beats at a frequency of 120 times a minute, its period—the time interval between beats—is half a second. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as vibrations, audio signals, radio waves. For cyclical processes, such as rotation, oscillations, or waves, in physics and engineering disciplines, such as optics, acoustics, and radio, frequency is usually denoted by a Latin letter f or by the Greek letter ν or ν. For a simple motion, the relation between the frequency and the period T is given by f =1 T. The SI unit of frequency is the hertz, named after the German physicist Heinrich Hertz, a previous name for this unit was cycles per second. The SI unit for period is the second, a traditional unit of measure used with rotating mechanical devices is revolutions per minute, abbreviated r/min or rpm. As a matter of convenience, longer and slower waves, such as ocean surface waves, short and fast waves, like audio and radio, are usually described by their frequency instead of period. Spatial frequency is analogous to temporal frequency, but the axis is replaced by one or more spatial displacement axes. Y = sin = sin d θ d x = k Wavenumber, in the case of more than one spatial dimension, wavenumber is a vector quantity. For periodic waves in nondispersive media, frequency has a relationship to the wavelength. Even in dispersive media, the frequency f of a wave is equal to the phase velocity v of the wave divided by the wavelength λ of the wave. In the special case of electromagnetic waves moving through a vacuum, then v = c, where c is the speed of light in a vacuum, and this expression becomes, f = c λ. When waves from a monochrome source travel from one medium to another, their remains the same—only their wavelength. For example, if 71 events occur within 15 seconds the frequency is, the latter method introduces a random error into the count of between zero and one count, so on average half a count. This is called gating error and causes an error in the calculated frequency of Δf = 1/, or a fractional error of Δf / f = 1/ where Tm is the timing interval. This error decreases with frequency, so it is a problem at low frequencies where the number of counts N is small, an older method of measuring the frequency of rotating or vibrating objects is to use a stroboscope
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Sine wave
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A sine wave or sinusoid is a mathematical curve that describes a smooth repetitive oscillation. It is named after the sine, of which it is the graph. It occurs often in pure and applied mathematics, as well as physics, engineering, signal processing and many other fields. Its most basic form as a function of time is, y = A sin = A sin where, A = the amplitude, F = the ordinary frequency, the number of oscillations that occur each second of time. ω = 2πf, the frequency, the rate of change of the function argument in units of radians per second φ = the phase. When φ is non-zero, the entire waveform appears to be shifted in time by the amount φ /ω seconds, a negative value represents a delay, and a positive value represents an advance. The sine wave is important in physics because it retains its shape when added to another sine wave of the same frequency and arbitrary phase. It is the only periodic waveform that has this property and this property leads to its importance in Fourier analysis and makes it acoustically unique. The wavenumber is related to the frequency by. K = ω v =2 π f v =2 π λ where λ is the wavelength, f is the frequency, and v is the linear speed. This equation gives a wave for a single dimension, thus the generalized equation given above gives the displacement of the wave at a position x at time t along a single line. This could, for example, be considered the value of a wave along a wire, in two or three spatial dimensions, the same equation describes a travelling plane wave if position x and wavenumber k are interpreted as vectors, and their product as a dot product. For more complex such as the height of a water wave in a pond after a stone has been dropped in. This wave pattern occurs often in nature, including wind waves, sound waves, a cosine wave is said to be sinusoidal, because cos = sin , which is also a sine wave with a phase-shift of π/2 radians. Because of this start, it is often said that the cosine function leads the sine function or the sine lags the cosine. The human ear can recognize single sine waves as sounding clear because sine waves are representations of a frequency with no harmonics. Presence of higher harmonics in addition to the fundamental causes variation in the timbre, on the other hand, if the sound contains aperiodic waves along with sine waves, then the sound will be perceived noisy as noise is characterized as being aperiodic or having a non-repetitive pattern. In 1822, French mathematician Joseph Fourier discovered that sinusoidal waves can be used as building blocks to describe and approximate any periodic waveform
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Mathematical analysis
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Mathematical analysis is the branch of mathematics dealing with limits and related theories, such as differentiation, integration, measure, infinite series, and analytic functions. These theories are studied in the context of real and complex numbers. Analysis evolved from calculus, which involves the elementary concepts and techniques of analysis, analysis may be distinguished from geometry, however, it can be applied to any space of mathematical objects that has a definition of nearness or specific distances between objects. Mathematical analysis formally developed in the 17th century during the Scientific Revolution, early results in analysis were implicitly present in the early days of ancient Greek mathematics. For instance, a geometric sum is implicit in Zenos paradox of the dichotomy. The explicit use of infinitesimals appears in Archimedes The Method of Mechanical Theorems, in Asia, the Chinese mathematician Liu Hui used the method of exhaustion in the 3rd century AD to find the area of a circle. Zu Chongzhi established a method that would later be called Cavalieris principle to find the volume of a sphere in the 5th century, the Indian mathematician Bhāskara II gave examples of the derivative and used what is now known as Rolles theorem in the 12th century. In the 14th century, Madhava of Sangamagrama developed infinite series expansions, like the power series and his followers at the Kerala school of astronomy and mathematics further expanded his works, up to the 16th century. The modern foundations of analysis were established in 17th century Europe. During this period, calculus techniques were applied to approximate discrete problems by continuous ones, in the 18th century, Euler introduced the notion of mathematical function. Real analysis began to emerge as an independent subject when Bernard Bolzano introduced the definition of continuity in 1816. In 1821, Cauchy began to put calculus on a firm logical foundation by rejecting the principle of the generality of algebra widely used in earlier work, instead, Cauchy formulated calculus in terms of geometric ideas and infinitesimals. Thus, his definition of continuity required a change in x to correspond to an infinitesimal change in y. He also introduced the concept of the Cauchy sequence, and started the theory of complex analysis. Poisson, Liouville, Fourier and others studied partial differential equations, the contributions of these mathematicians and others, such as Weierstrass, developed the -definition of limit approach, thus founding the modern field of mathematical analysis. In the middle of the 19th century Riemann introduced his theory of integration, the last third of the century saw the arithmetization of analysis by Weierstrass, who thought that geometric reasoning was inherently misleading, and introduced the epsilon-delta definition of limit. Then, mathematicians started worrying that they were assuming the existence of a continuum of numbers without proof. Around that time, the attempts to refine the theorems of Riemann integration led to the study of the size of the set of discontinuities of real functions, also, monsters began to be investigated
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Radio
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When radio waves strike an electrical conductor, the oscillating fields induce an alternating current in the conductor. The information in the waves can be extracted and transformed back into its original form, Radio systems need a transmitter to modulate some property of the energy produced to impress a signal on it, for example using amplitude modulation or angle modulation. Radio systems also need an antenna to convert electric currents into radio waves, an antenna can be used for both transmitting and receiving. The electrical resonance of tuned circuits in radios allow individual stations to be selected, the electromagnetic wave is intercepted by a tuned receiving antenna. Radio frequencies occupy the range from a 3 kHz to 300 GHz, a radio communication system sends signals by radio. The term radio is derived from the Latin word radius, meaning spoke of a wheel, beam of light, however, this invention would not be widely adopted. The switch to radio in place of wireless took place slowly and unevenly in the English-speaking world, the United States Navy would also play a role. Although its translation of the 1906 Berlin Convention used the terms wireless telegraph and wireless telegram, the term started to become preferred by the general public in the 1920s with the introduction of broadcasting. Radio systems used for communication have the following elements, with more than 100 years of development, each process is implemented by a wide range of methods, specialised for different communications purposes. Each system contains a transmitter, This consists of a source of electrical energy, the transmitter contains a system to modulate 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 such as amplitude, frequency, phase. Amplitude modulation of a carrier wave works by varying the strength of the signal in proportion to the information being sent. For example, changes in the strength can be used to reflect the sounds to be reproduced by a speaker. It was the used for the first audio radio transmissions. Frequency modulation varies the frequency of the carrier, the instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. FM has the capture effect whereby a receiver only receives the strongest signal, Digital data can be sent by shifting the carriers frequency among a set of discrete values, a technique known as frequency-shift keying. FM is commonly used at Very high frequency radio frequencies for high-fidelity broadcasts of music, analog TV sound is also broadcast using FM. Angle modulation alters the phase of the carrier wave to transmit a signal
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Information
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In other words, it is the answer to a question of some kind. It is thus related to data and knowledge, as data represents values attributed to parameters, as it regards data, the informations existence is not necessarily coupled to an observer, while in the case of knowledge, the information requires a cognitive observer. At its most fundamental, information is any propagation of cause, Information can be encoded into various forms for transmission and interpretation. It can also be encrypted for safe storage and communication, the uncertainty of an event is measured by its probability of occurrence and is inversely proportional to that. The more uncertain an event, the information is required to resolve uncertainty of that event. The bit is a unit of information, but other units such as the nat may be used. Example, information in one fair coin flip, log2 =1 bit, the concept that information is the message has different meanings in different contexts. The English word was derived from the Latin stem of the nominative. Inform itself comes from the Latin verb informare, which means to give form, eidos can also be associated with thought, proposition, or even concept. The ancient Greek word for information is πληροφορία, which transliterates from πλήρης fully and it literally means fully bears or conveys fully. In modern Greek language the word Πληροφορία is still in use and has the same meaning as the word information in English. In addition to its meaning, the word Πληροφορία as a symbol has deep roots in Aristotles semiotic triangle. In this regard it can be interpreted to communicate information to the one decoding that specific type of sign, from the stance of information theory, information is taken as an ordered sequence of symbols from an alphabet, say an input alphabet χ, and an output alphabet ϒ. Information processing consists of a function that maps any input sequence from χ into an output sequence from ϒ. The mapping may be probabilistic or deterministic and it may have memory or be memoryless. Often information can be viewed as a type of input to an organism or system, inputs are of two kinds, some inputs are important to the function of the organism or system by themselves. In his book Sensory Ecology Dusenbery called these causal inputs, other inputs are important only because they are associated with causal inputs and can be used to predict the occurrence of a causal input at a later time. Some information is important because of association with information but eventually there must be a connection to a causal input
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Morse code
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Morse code is a method of transmitting text information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment. It is named for Samuel F. B, Morse, an inventor of the telegraph. Because many non-English natural languages use more than the 26 Roman letters, each Morse code symbol represents either a text character or a prosign and is represented by a unique sequence of dots and dashes. The duration of a dash is three times the duration of a dot, each dot or dash is followed by a short silence, equal to the dot duration. The letters of a word are separated by an equal to three dots, and the words are separated by a space equal to seven dots. The dot duration is the unit of time measurement in code transmission. To increase the speed of the communication, the code was designed so that the length of each character in Morse varies approximately inversely to its frequency of occurrence in English. Thus the most common letter in English, the letter E, has the shortest code, Morse code is used by some amateur radio operators, although knowledge of and proficiency with it is no longer required for licensing in most countries. Pilots and air controllers usually need only a cursory understanding. Aeronautical navigational aids, such as VORs and NDBs, constantly identify in Morse code, compared to voice, Morse code is less sensitive to poor signal conditions, yet still comprehensible to humans without a decoding device. Morse is, therefore, an alternative to synthesized speech for sending automated data to skilled listeners on voice channels. Many amateur radio repeaters, for example, identify with Morse, in an emergency, Morse code can be sent by improvised methods that can be easily keyed on and off, making it one of the simplest and most versatile methods of telecommunication. The most common signal is SOS or three dots, three dashes, and three dots, internationally recognized by treaty. Beginning in 1836, the American artist Samuel F. B, Morse, the American physicist Joseph Henry, and Alfred Vail developed an electrical telegraph system. This system sent pulses of current along wires which controlled an electromagnet that was located at the receiving end of the telegraph system. A code was needed to transmit natural language using only these pulses, around 1837, Morse, therefore, developed an early forerunner to the modern International Morse code. Around the same time, Carl Friedrich Gauss and Wilhelm Eduard Weber as well as Carl August von Steinheil had already used codes with varying lengths for their telegraphs. In 1837, William Cooke and Charles Wheatstone in England began using a telegraph that also used electromagnets in its receivers
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Wireless telegraphy
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Wireless telegraphy is the transmission of electric telegraphy signals without wires. Wireless telegraphy came to mean Morse code transmitted by radio waves, initially called Hertzian waves, the primitive spark radio transmitters used until World War 1 could not transmit audio. The code sounded like musical beeps in the earphone of the receiver, the receiving radio operator would translate the beeps back into text. The first practical wireless telegraphy transmitters and receivers were developed by Guglielmo Marconi beginning in 1895, by 1910 communication by Hertzian waves was universally referred to as radio, and the term wireless telegraphy has been largely replaced by the more modern term radiotelegraphy. The transmission of sound began to displace wireless telegraphy by the 1920s for many applications, continuous wave radiotelegraphy is regulated by the International Telecommunication Union as emission type A1A. Today, due to modern text transmission methods, Morse code radiotelegraphy for commercial use has become obsolete. On shipboard the computer and satellite linked GMDSS system has largely replaced Morse as a means of communication, Telegraphy is taught on a very limited basis by the military. A CW coastal station, KSM, still exists in California, run primarily as a museum by volunteers, Radio beacons, particularly in the aviation service, but also as placeholders for commercial ship-to-shore systems, also transmit Morse but at very slow speeds. The US Federal Communications Commission does still issue a lifetime commercial Radiotelegraph Operator License and this requires passing a simple written test on regulations, a more complex written exam on technology, and demonstrating Morse reception at 20 words per minute plain language and 16 wpm code groups. Wireless telegraphy is still used today by amateur radio hobbyists where it is commonly referred to as radio telegraphy, continuous wave. However its knowledge is not required to obtain any class of amateur license, a number of wireless electrical signaling schemes including electric currents through water and the ground were investigated for telegraphy before practical radio systems became available. The original telegraph used two wires between two stations to form an electrical circuit or loop. This led to speculation that it might be possible to both wires and therefore transmit telegraph signals through the ground without any wires connecting the stations. Other attempts were made to send the current through bodies of water, in order to span rivers. The first wireless voice telecommunication device, invented in 1880, was the photophone, both electrostatic and electromagnetic induction were used to develop wireless telegraph systems that saw limited commercial application. This system was successful technically but not economically, as there turned out to be little interest by train travelers in a telegraph service. During the Great Blizzard of 1888, this system was used to send, the disabled trains were able to maintain communications via their Edison induction wireless telegraph systems, perhaps the first successful use of wireless telegraphy to send distress calls. The most successful creator of an electromagnetic induction telegraph system was William Preece in the United Kingdom, beginning with tests across the Bristol Channel in 1892, Preece was able to telegraph across gaps of about 5 kilometres
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Damped wave
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A damped wave is a wave whose amplitude of oscillation decreases with time, eventually going to zero. This term also refers to a method of radio transmission produced by spark gap transmitters. Information was carried on this signal by telegraphy, turning the transmitter on, damped waves were the first practical means of radio communication, used during the wireless telegraphy era which ended around 1920. In radio engineering it is now referred to as Class B emission. However, such transmissions have a bandwidth and generate electrical noise which interferes with other radio transmissions. Continuous wave Damping On-off keying Amplitude modulation Types of radio emissions Sparks Telegraph Key Review A complete listing with photos of damped wave telegraph keys by manufacturer, 47cfr2.201 FCC rules where Type B damped wave emissions are forbidden
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Spark-gap transmitter
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A spark-gap transmitter is a device that generates radio frequency electromagnetic waves using a spark gap. Spark gap transmitters were the first devices to demonstrate practical radio transmission, later, more efficient transmitters were developed based on rotary machines like the high-speed Alexanderson alternators and the static Poulsen Arc generators. Even when vacuum tube based transmitters had been installed, many retained their crude. However, by 1940, the technology was no longer used for communication, use of the spark-gap transmitter led to many radio operators being nicknamed Sparks long after they ceased using spark transmitters. Even today, the German verb funken, literally, to spark, the effects of sparks causing unexplained action at a distance, such as inducing sparks in nearby devices, had been noticed by scientists and experimenters well before the invention of radio. Extensive experiments were conducted by Joseph Henry, Thomas Edison and David Edward Hughes, with no other theory to explain the phenomenon, it was usually written off as electromagnetic induction. Hertz used a spark gap transmitter and a tuned spark gap detector located a few meters from the source. Many experimenters used the spark gap setup to further investigate the new Hertzian wave phenomenon, including Oliver Lodge, the Italian inventor Guglielmo Marconi used a spark-gap transmitter in his experiments to develop the radio phenomenon into a wireless telegraphy system in the early 1890s. In 1895 he succeeded in transmitting over a distance of 1 1/4 miles and his first transmitter consisted of an induction coil connected between a wire antenna and ground, with a spark gap across it. Every time the induction coil pulsed, the antenna was momentarily charged up to tens of thousands of volts until the gap started to arc. This acted as a switch, essentially connecting the antenna to ground. While the various systems of spark transmitters worked well enough to prove the concept of wireless telegraphy. The biggest problem was that the power that could be transmitted was directly determined by how much electrical charge the antenna could hold. Because the capacitance of practical antennas is quite small, the way to get a reasonable power output was to charge it up to very high voltages. However, this made transmission impossible in rainy or even damp conditions, also, it necessitated a quite wide spark gap, with a very high electrical resistance, with the result that most of the electrical energy was used simply to heat up the air in the spark gap. Another problem with the transmitter was a result of the shape of the waveform produced by each burst of electromagnetic radiation. These transmitters radiated an extremely dirty wide band signal that could interfere with transmissions on nearby frequencies. Receiving sets relatively close to such a transmitter had entire sections of a band masked by this wide band noise, reginald Fessendens first attempts to transmit voice employed a spark transmitter operating at approximately 10,000 sparks/second
