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96-434: The plasma is typically in the form of a glow discharge and acts as a massless radiating element. The technique is a much later development of physics principles demonstrated by William Duddell 's "singing arc" of 1900, and Hermann Theodor Simon published the same phenomenon in 1898. The term ionophone was used by Dr. Siegfried Klein who developed a plasma tweeter that was licensed for commercial production by DuKane with

192-440: A digital-to-analog converter , typically at a frequency less than the desired RF-output frequency. The analog signal must then be shifted in frequency and linearly amplified to the desired frequency and power level (linear amplification must be used to prevent modulation distortion). This low-level method for AM is used in many Amateur Radio transceivers. AM may also be generated at a low level, using analog methods described in

288-424: A New York-based company, both offered their own variant of the plasma speaker as a DIY kit. The ExcelPhysics variant used a flyback transformer to step up voltage, a 555 timing chip to provide modulation and a 44 kHz carrier signal , and an audio amplifier. The kit is no longer marketed. A flame speaker uses a modulated flame for the driver and could be considered related to the plasma loudspeaker. This

384-505: A buzz in receivers. In effect they were already amplitude modulated. The first AM transmission was made by Canadian-born American researcher Reginald Fessenden on 23 December 1900 using a spark gap transmitter with a specially designed high frequency 10 kHz interrupter , over a distance of one mile (1.6 km) at Cobb Island, Maryland, US. His first transmitted words were, "Hello. One, two, three, four. Is it snowing where you are, Mr. Thiessen?". The words were barely intelligible above

480-638: A commercial plasma speaker that used a helium tank to provide the ionization gas. In 1978 Alan E. Hill of the Air Force Weapons Laboratory in Albuquerque, NM, designed the Plasmatronics Hill Type I , a commercial helium-plasma tweeter. This avoided the ozone and nitrogen oxides produced by radio frequency decomposition of air in earlier generations of plasma tweeters. But the operation of such speakers requires

576-436: A compromise in terms of bandwidth) in order to reduce the required channel spacing. Another improvement over standard AM is obtained through reduction or suppression of the carrier component of the modulated spectrum. In figure 2 this is the spike in between the sidebands; even with full (100%) sine wave modulation, the power in the carrier component is twice that in the sidebands, yet it carries no unique information. Thus there

672-642: A continuous supply of helium. In the 1950s, the pioneering DuKane Corporation produced the air-ionizing Ionovac , marketed in the UK as the Ionophone . Currently there remain manufacturers in Germany who use this design, as well as many do-it-yourself designs available on the Internet. To make the plasma speaker a more widely available product, ExcelPhysics, a Seattle-based company, and Images Scientific Instruments,

768-493: A faithful reproduction of the original program, including its varying modulation levels, is expected. In 1982, the International Telecommunication Union (ITU) designated the types of amplitude modulation: Amplitude modulation was used in experiments of multiplex telegraph and telephone transmission in the late 1800s. However, the practical development of this technology is identified with

864-472: A gas requires charge carriers, which can be either electrons or ions. Charge carriers come from ionizing some of the gas molecules. In terms of current flow, glow discharge falls between dark discharge and arc discharge. Below the breakdown voltage there is little to no glow and the electric field is uniform. When the electric field increases enough to cause ionization, the Townsend discharge starts. When

960-413: A glow discharge develops, the electric field is considerably modified by the presence of positive ions; the field is concentrated near the cathode. The glow discharge starts as a normal glow. As the current is increased, more of the cathode surface is involved in the glow. When the current is increased above the level where the entire cathode surface is involved, the discharge is known as an abnormal glow. If

1056-455: A great increase in the number of radio stations experimenting with AM transmission of news or music. The vacuum tube was responsible for the rise of AM broadcasting around 1920, the first electronic mass communication medium. Amplitude modulation was virtually the only type used for radio broadcasting until FM broadcasting began after World War II. At the same time as AM radio began, telephone companies such as AT&T were developing

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1152-408: A human voice for instance, the frequency content (horizontal axis) may be plotted as a function of time (vertical axis), as in figure 3. It can again be seen that as the modulation frequency content varies, an upper sideband is generated according to those frequencies shifted above the carrier frequency, and the same content mirror-imaged in the lower sideband below the carrier frequency. At all times,

1248-478: A longer mean free path allows a charged particle to gain more energy before colliding with another particle. The cell is typically filled with neon, but other gases can also be used. An electric potential of several hundred volts is applied between the two electrodes. A small fraction of the population of atoms within the cell is initially ionized through random processes, such as thermal collisions between atoms or by gamma rays . The positive ions are driven towards

1344-547: A nearly ideal reproduction of the sound source when the electric or magnetic field is modulated with the audio signal. Conventional loudspeaker transducer designs use the input electrical audio frequency signal to vibrate a significant mass: In a dynamic loudspeaker this driver is coupled to a stiff speaker cone —a diaphragm which pushes air at audio frequencies. But the inertia inherent in its mass resists acceleration —and all changes in cone position. Additionally, speaker cones will eventually suffer tensile fatigue from

1440-430: A negative DC-bias voltage on the sample surface. The DC-bias is the result of an alternating current waveform that is centered about negative potential; as such it more or less represent the average potential residing on the sample surface. Radio-frequency has ability to appear to flow through insulators (non-conductive materials). Both radio-frequency and direct-current glow discharges can be operated in pulsed mode, where

1536-473: A plasma speaker, as member of the family of massless speakers, these limitations do not exist. The low-inertia driver has exceptional transient response compared to other designs. The result is an even output, accurate even at higher frequencies beyond the human audible range. Such speakers are notable for accuracy and clarity, but not lower frequencies because plasma is composed of tiny molecules and with such low mass are unable to move large volumes of air unless

1632-468: A precise carrier frequency reference signal (usually as shifted to the intermediate frequency ) from a greatly reduced "pilot" carrier (in reduced-carrier transmission or DSB-RC) to use in the demodulation process. Even with the carrier eliminated in double-sideband suppressed-carrier transmission , carrier regeneration is possible using a Costas phase-locked loop . This does not work for single-sideband suppressed-carrier transmission (SSB-SC), leading to

1728-673: A problem. Early experiments in AM radio transmission, conducted by Fessenden, Valdemar Poulsen , Ernst Ruhmer , Quirino Majorana , Charles Herrold , and Lee de Forest , were hampered by the lack of a technology for amplification . The first practical continuous wave AM transmitters were based on either the huge, expensive Alexanderson alternator , developed 1906–1910, or versions of the Poulsen arc transmitter (arc converter), invented in 1903. The modifications necessary to transmit AM were clumsy and resulted in very low quality audio. Modulation

1824-584: A rather small (or zero) remaining carrier amplitude. Modulation circuit designs may be classified as low- or high-level (depending on whether they modulate in a low-power domain—followed by amplification for transmission—or in the high-power domain of the transmitted signal). In modern radio systems, modulated signals are generated via digital signal processing (DSP). With DSP many types of AM are possible with software control (including DSB with carrier, SSB suppressed-carrier and independent sideband, or ISB). Calculated digital samples are converted to voltages with

1920-499: A single sine wave, as treated above. However, by the principle of Fourier decomposition , m(t) can be expressed as the sum of a set of sine waves of various frequencies, amplitudes, and phases. Carrying out the multiplication of 1 + m(t) with c(t) as above, the result consists of a sum of sine waves. Again, the carrier c(t) is present unchanged, but each frequency component of m at f i has two sidebands at frequencies f c + f i and f c – f i . The collection of

2016-403: A special modulator produces such a waveform at a low level followed by a linear amplifier . What's more, a standard AM receiver using an envelope detector is incapable of properly demodulating such a signal. Rather, synchronous detection is required. Thus double-sideband transmission is generally not referred to as "AM" even though it generates an identical RF waveform as standard AM as long as

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2112-466: Is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave . In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to that of the message signal, such as an audio signal . This technique contrasts with angle modulation , in which either the frequency of the carrier wave is varied, as in frequency modulation , or its phase , as in phase modulation . AM

2208-400: Is a carrier with a frequency of 0 Hz. It is modulated by a microphone ( transmitter ) in the telephone set according to the acoustic signal from the speaker. The result is a varying amplitude direct current, whose AC-component is the speech signal extracted at the central office for transmission to another subscriber. An additional function provided by the carrier in standard AM, but which

2304-448: Is a great advantage in efficiency in reducing or totally suppressing the carrier, either in conjunction with elimination of one sideband ( single-sideband suppressed-carrier transmission ) or with both sidebands remaining ( double sideband suppressed carrier ). While these suppressed carrier transmissions are efficient in terms of transmitter power, they require more sophisticated receivers employing synchronous detection and regeneration of

2400-451: Is always positive for undermodulation. If m > 1 then overmodulation occurs and reconstruction of message signal from the transmitted signal would lead in loss of original signal. Amplitude modulation results when the carrier c(t) is multiplied by the positive quantity (1 + m(t)/A) : In this simple case m is identical to the modulation index , discussed below. With m = 0.5 the amplitude modulated signal y ( t ) thus corresponds to

2496-456: Is based, heterodyning , and invented one of the first detectors able to rectify and receive AM, the electrolytic detector or "liquid baretter", in 1902. Other radio detectors invented for wireless telegraphy, such as the Fleming valve (1904) and the crystal detector (1906) also proved able to rectify AM signals, so the technological hurdle was generating AM waves; receiving them was not

2592-417: Is flat bottom (that is, so that the depth analyzed over the crater area is uniform). In bulk measurement, a rough or rounded crater bottom would not adversely impact analysis. Under the best conditions, depth resolution in the single nanometer range has been achieved (in fact, within-molecule resolution has been demonstrated). The chemistry of ions and neutrals in vacuum is called gas phase ion chemistry and

2688-412: Is lost in either single or double-sideband suppressed-carrier transmission, is that it provides an amplitude reference. In the receiver, the automatic gain control (AGC) responds to the carrier so that the reproduced audio level stays in a fixed proportion to the original modulation. On the other hand, with suppressed-carrier transmissions there is no transmitted power during pauses in the modulation, so

2784-584: Is made. This process is called sputtering and it gradually ablates the cathode. Sputtering is useful when using spectroscopy to analyze the composition of the cathode, as is done in Glow-discharge optical emission spectroscopy . However, sputtering is not desirable when glow discharge is used for lighting, because it shortens the life of the lamp. For example, neon signs have hollow cathodes designed to minimize sputtering, and contain charcoal to continuously remove undesired ions and atoms. In

2880-451: Is no universal mechanism explaining the striations for all conditions of gas and pressure producing them, but recent theoretical and modelling studies, supported with experimental results, mention the importance of the Dufour effect . In addition to causing secondary emission, positive ions can strike the cathode with sufficient force to eject particles of the material from which the cathode

2976-461: Is part of the analytical study that includes glow discharge. In analytical chemistry , glow discharges are usually operated in direct-current mode. For direct-current, the cathode (which is the sample in solids analysis) must be conductive. In contrast, analysis of a non conductive cathode requires the use of a high frequency alternating current. The potential, pressure, and current are interrelated. Only two can be directly controlled at once, while

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3072-404: Is shown in the first waveform, below. For m = 1.0 {\displaystyle m=1.0} , it varies by 100% as shown in the illustration below it. With 100% modulation the wave amplitude sometimes reaches zero, and this represents full modulation using standard AM and is often a target (in order to obtain the highest possible signal-to-noise ratio ) but mustn't be exceeded. Increasing

3168-412: Is sometimes called Crookes dark space, and sometimes referred to as the cathode fall , because the largest voltage drop in the tube occurs in this region. The ionization in the cathode dark space results in a high electron density, but slower electrons, making it easier for the electrons to recombine with positive ions, leading to intense light, through a process called bremsstrahlung radiation . As

3264-417: Is strongly reduced so long as the received signal is well above the threshold for reception. For this reason AM broadcast is not favored for music and high fidelity broadcasting, but rather for voice communications and broadcasts (sports, news, talk radio etc.). AM is also inefficient in power usage; at least two-thirds of the power is concentrated in the carrier signal. The carrier signal contains none of

3360-418: Is that the receiver amplifies and detects noise and electromagnetic interference in equal proportion to the signal. Increasing the received signal-to-noise ratio , say, by a factor of 10 (a 10 decibel improvement), thus would require increasing the transmitter power by a factor of 10. This is in contrast to frequency modulation (FM) and digital radio where the effect of such noise following demodulation

3456-416: Is the most common. In this arrangement, the sample is used as the cathode. As mentioned earlier, gas ions and atoms striking the sample surface knock atoms off of it, a process known as sputtering. The sputtered atoms, now in the gas phase, can be detected by atomic absorption , but this is a comparatively rare strategy. Instead, atomic emission and mass spectrometry are usually used. Collisions between

3552-465: The envelope of the transmitted waveform. In the frequency domain , amplitude modulation produces a signal with power concentrated at the carrier frequency and two adjacent sidebands . 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" (DSBAM). A disadvantage of all amplitude modulation techniques, not only standard AM,

3648-412: The cathode by the electric potential, and the electrons are driven towards the anode by the same potential. The initial population of ions and electrons collides with other atoms, exciting or ionizing them. As long as the potential is maintained, a population of ions and electrons remains. Some of the ions' kinetic energy is transferred to the cathode. This happens partially through the ions striking

3744-506: The instantaneous phase deviation ϕ ( t ) {\displaystyle \phi (t)} . This description directly provides the two major groups of modulation, amplitude modulation and angle modulation . In angle modulation, the term A ( t ) is constant and the second term of the equation has a functional relationship to the modulating message signal. Angle modulation provides two methods of modulation, frequency modulation and phase modulation . In amplitude modulation,

3840-418: The "flame". Glow discharge A glow discharge is a plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the striking voltage , the gas ionization becomes self-sustaining, and the tube glows with a colored light. The color depends on

3936-517: The AGC must respond to peaks of the transmitted power during peaks in the modulation. This typically involves a so-called fast attack, slow decay circuit which holds the AGC level for a second or more following such peaks, in between syllables or short pauses in the program. This is very acceptable for communications radios, where compression of the audio aids intelligibility. However it is absolutely undesired for music or normal broadcast programming, where

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4032-549: The Ionovac and Fane Acoustics with the Ionofane in the late 1940s and 1950s. The effect takes advantage of several physical principles: First, ionization of a gas creates a highly conductive plasma, which responds to alternating electric and magnetic fields . Second, this low- density plasma has a negligibly small mass. Thus, the air remains mechanically coupled with the essentially massless plasma, allowing it to radiate

4128-487: The angle term is held constant and the first term, A ( t ), of the equation has a functional relationship to the modulating message signal. The modulating message signal may be analog in nature, or it may be a digital signal, in which case the technique is generally called amplitude-shift keying . For example, in AM radio communication, a continuous wave radio-frequency signal has its amplitude modulated by an audio waveform before transmission. The message signal determines

4224-411: The background buzz of the spark. Fessenden was a significant figure in the development of AM radio. He was one of the first researchers to realize, from experiments like the above, that the existing technology for producing radio waves, the spark transmitter, was not usable for amplitude modulation, and that a new kind of transmitter, one that produced sinusoidal continuous waves , was needed. This

4320-401: The bandwidth of an AM signal is narrower than one using frequency modulation (FM), it is twice as wide as single-sideband techniques; it thus may be viewed as spectrally inefficient. Within a frequency band, only half as many transmissions (or "channels") can thus be accommodated. For this reason analog television employs a variant of single-sideband (known as vestigial sideband , somewhat of

4416-549: The carrier frequency. Single-sideband modulation uses bandpass filters to eliminate one of the sidebands and possibly the carrier signal, which improves the ratio of message power to total transmission power , reduces power handling requirements of line repeaters, and permits better bandwidth utilization of the transmission medium. AM remains in use in many forms of communication in addition to AM broadcasting : shortwave radio , amateur radio , two-way radios , VHF aircraft radio , citizens band radio , and in computer modems in

4512-451: The carrier frequency. For that reason, standard AM continues to be widely used, especially in broadcast transmission, to allow for the use of inexpensive receivers using envelope detection . Even (analog) television, with a (largely) suppressed lower sideband, includes sufficient carrier power for use of envelope detection. But for communications systems where both transmitters and receivers can be optimized, suppression of both one sideband and

4608-454: The carrier frequency. Passing the modulated signal through another nonlinear device can extract the original baseband signal. His analysis also showed that only one sideband was necessary to transmit the audio signal, and Carson patented single-sideband modulation (SSB) on 1 December 1915. This advanced variant of amplitude modulation was adopted by AT&T for longwave transatlantic telephone service beginning 7 January 1927. After WW-II, it

4704-400: The carrier itself remains constant, and of greater power than the total sideband power. The RF bandwidth of an AM transmission (refer to figure 2, but only considering positive frequencies) is twice the bandwidth of the modulating (or " baseband ") signal, since the upper and lower sidebands around the carrier frequency each have a bandwidth as wide as the highest modulating frequency. Although

4800-473: The carrier represent a net advantage and are frequently employed. A technique used widely in broadcast AM transmitters is an application of the Hapburg carrier, first proposed in the 1930s but impractical with the technology then available. During periods of low modulation the carrier power would be reduced and would return to full power during periods of high modulation levels. This has the effect of reducing

4896-436: The cathode directly. The primary mechanism, however, is less direct. Ions strike the more numerous neutral gas atoms, transferring a portion of their energy to them. These neutral atoms then strike the cathode. Whichever species (ions or atoms) strike the cathode, collisions within the cathode redistribute this energy resulting in electrons ejected from the cathode. This process is known as secondary electron emission. Once free of

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4992-467: The cathode region a white or blue color, while in the rest of the tube, radiation is only from the carrier gas and tends to be more monochromatic. Electrons near the cathode are less energetic than the rest of the tube. Surrounding the cathode is a negative field, which slows electrons as they are ejected from the surface. Only those electrons with the highest velocity are able to escape this field, and those without enough kinetic energy are pulled back into

5088-406: The cathode, the electric field accelerates electrons into the bulk of the glow discharge. Atoms can then be excited by collisions with ions, electrons, or other atoms that have been previously excited by collisions. Once excited, atoms will lose their energy fairly quickly. Of the various ways that this energy can be lost, the most important is radiatively, meaning that a photon is released to carry

5184-507: The cathode. Once outside the negative field, the attraction from the positive field begins to accelerate these electrons toward the anode. During this acceleration electrons are deflected and slowed down by positive ions speeding toward the cathode, which, in turn, produces bright blue-white bremsstrahlung radiation in the negative glow region. Glow discharges can be used to analyze the elemental, and sometimes molecular, composition of solids, liquids, and gases, but elemental analysis of solids

5280-507: The characteristic "Donald Duck" sound from such receivers when slightly detuned. Single-sideband AM is nevertheless used widely in amateur radio and other voice communications because it has power and bandwidth efficiency (cutting the RF bandwidth in half compared to standard AM). On the other hand, in medium wave and short wave broadcasting, standard AM with the full carrier allows for reception using inexpensive receivers. The broadcaster absorbs

5376-473: The context of sputtering, the gas in the tube is called "carrier gas," because it carries the particles from the cathode. Because of sputtering occurring at the cathode, the colors emitted from regions near the cathode are quite different from the anode. Particles sputtered from the cathode are excited and emit radiation from the metals and oxides that make up the cathode. The radiation from these particles combines with radiation from excited carrier gas, giving

5472-413: The current is increased still further, other factors come into play and an arc discharge begins. The simplest type of glow discharge is a direct-current glow discharge. In its simplest form, it consists of two electrodes in a cell held at low pressure (0.1–10 torr ; about 1/10000th to 1/100th of atmospheric pressure). A low pressure is used to increase the mean free path ; for a fixed electric field,

5568-456: The discharge horizontally will result in fewer regions. The positive column will be compressed while the negative glow will remain the same size, and, with small enough gaps, the positive column will disappear altogether. In an analytical glow discharge, the discharge is primarily a negative glow with dark region above and below it. The cathode layer begins with the Aston dark space, and ends with

5664-426: The electrons keep losing energy, less light is emitted, resulting in another dark space. The anode layer begins with the positive column, and ends at the anode. The anode layer has a negative space charge and a moderate electric field. With fewer ions, the electric field increases, resulting in electrons with energy of about 2 eV, which is enough to excite atoms and produce light. With longer glow discharge tubes,

5760-422: The energy away. In optical atomic spectroscopy , the wavelength of this photon can be used to determine the identity of the atom (that is, which chemical element it is) and the number of photons is directly proportional to the concentration of that element in the sample. Some collisions (those of high enough energy) will cause ionization. In atomic mass spectrometry , these ions are detected. Their mass identifies

5856-444: The extra power cost to greatly increase potential audience. A simple form of digital amplitude modulation which can be used for transmitting binary data is on–off keying , the simplest form of amplitude-shift keying, in which ones and zeros are represented by the presence or absence of a carrier. On–off keying is likewise used by radio amateurs to transmit Morse code where it is known as continuous wave (CW) operation, even though

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5952-400: The form of QAM . In electronics , telecommunications and mechanics , modulation means varying some aspect of a continuous wave carrier signal with an information-bearing modulation waveform, such as an audio signal which represents sound, or a video signal which represents images. In this sense, the carrier wave, which has a much higher frequency than the message signal, carries

6048-493: The former frequencies above the carrier frequency is known as the upper sideband, and those below constitute the lower sideband. The modulation m(t) may be considered to consist of an equal mix of positive and negative frequency components, as shown in the top of figure 2. One can view the sidebands as that modulation m(t) having simply been shifted in frequency by f c as depicted at the bottom right of figure 2. The short-term spectrum of modulation, changing as it would for

6144-442: The gas used. Glow discharges are used as a source of light in devices such as neon lights , cold cathode fluorescent lamps and plasma-screen televisions . Analyzing the light produced with spectroscopy can reveal information about the atomic interactions in the gas, so glow discharges are used in plasma physics and analytical chemistry . They are also used in the surface treatment technique called sputtering . Conduction in

6240-419: The gas-phase sample atoms and the plasma gas pass energy to the sample atoms. This energy can excite the atoms, after which they can lose their energy through atomic emission. By observing the wavelength of the emitted light, the atom's identity can be determined. By observing the intensity of the emission, the concentration of atoms of that type can be determined. Energy gained through collisions can also ionize

6336-497: The ground state, emitting light at a wavelength corresponding to the difference between the energy bands of the atoms. This glow is seen very near the cathode. As electrons from the cathode gain more energy, they tend to ionize, rather than excite atoms. Excited atoms quickly fall back to ground level emitting light, however, when atoms are ionized, the opposite charges are separated, and do not immediately recombine. This results in more ions and electrons, but no light. This region

6432-441: The information. At the receiving station, the message signal is extracted from the modulated carrier by demodulation . In general form, a modulation process of a sinusoidal carrier wave may be described by the following equation: A(t) represents the time-varying amplitude of the sinusoidal carrier wave and the cosine-term is the carrier at its angular frequency ω {\displaystyle \omega } , and

6528-399: The longer space is occupied by a longer positive column, while the cathode layer remains the same. For example, with a neon sign, the positive column occupies almost the entire length of the tube. An electric field increase results in the anode glow. Fewer electrons results in another dark space. Bands of alternating light and dark in the positive column are called striations . There

6624-489: The modulating signal beyond that point, known as overmodulation , causes a standard AM modulator (see below) to fail, as the negative excursions of the wave envelope cannot become less than zero, resulting in distortion ("clipping") of the received modulation. Transmitters typically incorporate a limiter circuit to avoid overmodulation, and/or a compressor circuit (especially for voice communications) in order to still approach 100% modulation for maximum intelligibility above

6720-443: The modulation amplitude and carrier amplitude, respectively; the modulation amplitude is the peak (positive or negative) change in the RF amplitude from its unmodulated value. Modulation index is normally expressed as a percentage, and may be displayed on a meter connected to an AM transmitter. So if m = 0.5 {\displaystyle m=0.5} , carrier amplitude varies by 50% above (and below) its unmodulated level, as

6816-409: The modulation index is below 100%. Such systems more often attempt a radical reduction of the carrier level compared to the sidebands (where the useful information is present) to the point of double-sideband suppressed-carrier transmission where the carrier is (ideally) reduced to zero. In all such cases the term "modulation index" loses its value as it refers to the ratio of the modulation amplitude to

6912-435: The negative glow region. The cathode layer shortens with increased gas pressure. The cathode layer has a positive space charge and a strong electric field. Electrons leave the cathode with an energy of about 1 eV, which is not enough to ionize or excite atoms, leaving a thin dark layer next to the cathode. Electrons from the cathode eventually attain enough energy to excite atoms. These excited atoms quickly fall back to

7008-534: The next section. High-power AM transmitters (such as those used for AM broadcasting ) are based on high-efficiency class-D and class-E power amplifier stages, modulated by varying the supply voltage. Older designs (for broadcast and amateur radio) also generate AM by controlling the gain of the transmitter's final amplifier (generally class-C, for efficiency). The following types are for vacuum tube transmitters (but similar options are available with transistors): The simplest form of AM demodulator consists of

7104-542: The noise. Such circuits are sometimes referred to as a vogad . However it is possible to talk about a modulation index exceeding 100%, without introducing distortion, in the case of double-sideband reduced-carrier transmission . In that case, negative excursions beyond zero entail a reversal of the carrier phase, as shown in the third waveform below. This cannot be produced using the efficient high-level (output stage) modulation techniques (see below) which are widely used especially in high power broadcast transmitters. Rather,

7200-467: The original information being transmitted (voice, video, data, etc.). However its presence provides a simple means of demodulation using envelope detection , providing a frequency and phase reference to extract the modulation from the sidebands. In some modulation systems based on AM, a lower transmitter power is required through partial or total elimination of the carrier component, however receivers for these signals are more complex because they must provide

7296-404: The other large application for AM: sending multiple telephone calls through a single wire by modulating them on separate carrier frequencies, called frequency division multiplexing . In 1915, John Renshaw Carson formulated the first mathematical description of amplitude modulation, showing that a signal and carrier frequency combined in a nonlinear device creates a sideband on both sides of

7392-489: The overall power demand of the transmitter and is most effective on speech type programmes. Various trade names are used for its implementation by the transmitter manufacturers from the late 80's onwards. The AM modulation index is a measure based on the ratio of the modulation excursions of the RF signal to the level of the unmodulated carrier. It is thus defined as: where M {\displaystyle M\,} and A {\displaystyle A\,} are

7488-518: The period between 1900 and 1920 of radiotelephone transmission, that is, the effort to send audio signals by radio waves. The first radio transmitters, called spark gap transmitters , transmitted information by wireless telegraphy , using pulses of the carrier wave to spell out text messages in Morse code . They could not transmit audio because the carrier consisted of strings of damped waves , pulses of radio waves that declined to zero, and sounded like

7584-409: The plasma are in large number. So these designs are more effective as tweeters . Plasma speaker designs ionize ambient air which contains the gases nitrogen and oxygen . In an intense electrical field these gases can produce reactive by-products, and in closed rooms these can reach a hazardous level. The two predominant gases produced are ozone and nitrogen dioxide . Plasmatronics produced

7680-417: The potential is turned on and off. This allows higher instantaneous powers to be applied without excessively heating the cathode. These higher instantaneous powers produce higher instantaneous signals, aiding detection. Combining time-resolved detection with pulsed powering results in additional benefits. In atomic emission, analyte atoms emit during different portions of the pulse than background atoms, allowing

7776-415: The repeated shaking of sonic vibration. Thus conventional speaker output, or the fidelity of the device, is distorted by physical limitations inherent in its design. These distortions have long been the limiting factor in commercial reproduction of strong high frequencies. To a lesser extent square wave characteristics are also problematic; the reproduction of square waves most stress a speaker cone. In

7872-475: The sample atoms. The ions can then be detected by mass spectrometry. In this case, it is the mass of the ions that identify the element and the number of ions that reflect the concentration. This method is referred to as glow discharge mass spectrometry (GDMS) and it has detection limits down to the sub-ppb range for most elements that are nearly matrix-independent. Both bulk and depth analysis of solids may be performed with glow discharge. Bulk analysis assumes that

7968-411: The sample is fairly homogeneous and averages the emission or mass spectrometric signal over time. Depth analysis relies on tracking the signal in time, therefore, is the same as tracking the elemental composition in depth. Depth analysis requires greater control over operational parameters. For example, conditions (current, potential, pressure) need to be adjusted so that the crater produced by sputtering

8064-516: The system as follows: The approach itself provides a novel visible analog computing approach for solving a wide class of maze searching problems based on the properties of lighting up of a glow discharge in a microfluidic chip. In the mid-20th century, prior to the development of solid state components such as Zener diodes , voltage regulation in circuits was often accomplished with voltage-regulator tubes , which used glow discharge. Amplitude modulation Amplitude modulation ( AM )

8160-481: The third must be allowed to vary. The pressure is most typically held constant, but other schemes may be used. The pressure and current may be held constant, while potential is allowed to vary. The pressure and voltage may be held constant while the current is allowed to vary. The power (product of voltage and current) may be held constant while the pressure is allowed to vary. Glow discharges may also be operated in radio-frequency. The use of this frequency will establish

8256-422: The top graph (labelled "50% Modulation") in figure 4. Using prosthaphaeresis identities , y ( t ) can be shown to be the sum of three sine waves: Therefore, the modulated signal has three components: the carrier wave c(t) which is unchanged in frequency, and two sidebands with frequencies slightly above and below the carrier frequency f c . A useful modulation signal m(t) is usually more complex than

8352-425: The transmission is not strictly "continuous". A more complex form of AM, quadrature amplitude modulation is now more commonly used with digital data, while making more efficient use of the available bandwidth. A simple form of amplitude modulation is the transmission of speech signals from a traditional analog telephone set using a common battery local loop. The direct current provided by the central office battery

8448-402: The transmitting antenna, where voltages in the tens of thousands volts are involved. The ionized air is heated in direct relationship to the modulating signal with surprisingly high fidelity over a wide area. Due to the destructive effects of the (self-sustaining) discharge this cannot be permitted to persist, and automatic systems momentarily shut down transmission within a few seconds to quench

8544-469: The two to be discriminated. Analogously, in mass spectrometry, sample and background ions are created at different times. An interesting application for using glow discharge was described in a 2002 scientific paper by Ryes, Ghanem et al. According to a Nature news article describing the work, researchers at Imperial College London demonstrated how they built a mini-map that glows along the shortest route between two points. The Nature news article describes

8640-496: The type of atoms and their quantity reveals the amount of that element in the sample. The illustrations to the right shows the main regions that may be present in a glow discharge. Regions described as "glows" emit significant light; regions labeled as "dark spaces" do not. As the discharge becomes more extended (i.e., stretched horizontally in the geometry of the illustrations), the positive column may become striated . That is, alternating dark and bright regions may form. Compressing

8736-427: Was a cheap source of continuous waves and could be easily modulated to make an AM transmitter. Modulation did not have to be done at the output but could be applied to the signal before the final amplifier tube, so the microphone or other audio source didn't have to modulate a high-power radio signal. Wartime research greatly advanced the art of AM modulation, and after the war the availability of cheap tubes sparked

8832-475: Was a radical idea at the time, because experts believed the impulsive spark was necessary to produce radio frequency waves, and Fessenden was ridiculed. He invented and helped develop one of the first continuous wave transmitters – the Alexanderson alternator , with which he made what is considered the first AM public entertainment broadcast on Christmas Eve, 1906. He also discovered the principle on which AM

8928-402: Was developed for military aircraft communication. The carrier wave ( sine wave ) of frequency f c and amplitude A is expressed by The message signal, such as an audio signal that is used for modulating the carrier, is m ( t ), and has a frequency f m , much lower than f c : where m is the amplitude sensitivity, M is the amplitude of modulation. If m < 1, (1 + m(t)/A)

9024-422: Was explored using the combustion of natural gas or candles to produce a plasma through which current is then passed. These combustion designs do not require high voltages to generate a plasma field, but there has been no commercial products using them. A similar effect is occasionally observed in the vicinity of high-power amplitude-modulated radio transmitters when a corona discharge (inadvertently) occurs from

9120-415: Was the earliest modulation method used for transmitting audio in radio broadcasting. It was developed during the first quarter of the 20th century beginning with Roberto Landell de Moura and Reginald Fessenden 's radiotelephone experiments in 1900. This original form of AM is sometimes called double-sideband amplitude modulation ( DSBAM ), because the standard method produces sidebands on either side of

9216-652: Was usually accomplished by a carbon microphone inserted directly in the antenna or ground wire; its varying resistance varied the current to the antenna. The limited power handling ability of the microphone severely limited the power of the first radiotelephones; many of the microphones were water-cooled. The 1912 discovery of the amplifying ability of the Audion tube , invented in 1906 by Lee de Forest , solved these problems. The vacuum tube feedback oscillator , invented in 1912 by Edwin Armstrong and Alexander Meissner ,

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