CC BY
56
THE HISTORY OF CONTINOUS MODULATION
DOI 10.24411/2072-8735-2018-10034
Boris P. Khromoy,
Moscow Technical Unisersity of Communication and Informatics, Moscow, Russia, [email protected]
Keywords: modulation,message, continuous, demodulation, transmission, characteristics, carrier, wave, parameters.
The term "modulation" comes from the Latin word "modulatio" (rhythm).This term is understood as the process of changing one or more parameters of the high-frequency carrier oscillation, according to the law of low-frequency information signal (mes-sage).Thus, the transmitted information is in the control (modulating) signal, and the role of carrier of information, performs high-frequency oscillation, called the carrier. Modulation is a process of transfer of information oscillation at the known carrier, with the aim of obtaining a new modulated signal. The term "continuous modulation" means the use of an analog signal. When the development of radio communication, a specific range of electromagnetic waves is divided into parts which are distributed among the radio stations. Each station worked on "their" frequency. As a result of the modulation range low frequency control signal is transferred in the high-frequency range. It allows broadcasting organizations to customize operation of the transceiver to different frequencies so that they do not "interfere" each other.
On the receiving end of the communication line should be the recovery process the control signal, i.e. the reverse process of modulation. This process of allocation information of the received modulated waves have been called demodulation, or detection of vibrations. Thus, modulation is performed on the transmitting end and demodulation at the receiving end. When broadcasting, transmitting radio microphone signal modulates a carrier oscillation, whose frequency allocated to this station, and all radio receivers configured to receive a given station, the process of demodulation (detection) and after amplification the signal is reproduced by the loudspeaker. Such separation devices, modulation and detection took place in the development of communications until the connection is carried out in one direction - from source to consumer. With the development of computer technology and the widespread use of the Internet communication has become two-way.
The consumer not only receives information from the Internet to your computer, but also transmits your information to the Internet. For this reason, it needs to have a device performing both operations: modulation and demodulation. This device is called a "modem". The number of types of modems used in practice is very large. A natural question arises: "Why do we need so many?".To answer this question is not simple. For this it is necessary to consider the transmission characteristics of the modulated signals, including noise immunity, wide spectrum, sensitivity to distortion in the channel, etc. The author gives information about the history of AMwith regard to the transmission of telephony signals, broadcasting, black-and-white television, color television SECAM and Quadrature Amplitude Modulation (QAM) to NTSC.
Для цитирования:
Хромой Б.П. История непрерывной модуляции // T-Comm: Телекоммуникации и транспорт. 2018. Том 12. №2. С. 70-75. For citation:
Khromoy B.P. (2018). The history of continous modulation. T-Comm, vol. 12, no.2, pр. 70-75
The term "modulation" comes from the Latin word "modu-iatio" (rhythm). This term is understood as the process of changing one or more parameters of the high-frequency carrier oscillation, according to the law of low-frequency information signal (message).Thus, the transmitted information is in the control (modulating) signal, and the role of carrier of information, performs high-frequency oscillation, called the carrier. Modulation is a process of transfer of information oscillation at the known carrier, with the aim of obtaining a new modulated signal. The term "continuous modulation" means the use of an analog signal.
When the development of radio communication, a specific range of electromagnetic waves is divided into parts which arc distributed among the radio stations. Each station worked 011 "their" frequency. As a result of the modulation range low frequency control signal is transferred in the high-frequency range. It allows broadcasting organizations to customize operation of the transceiver to different frequencies so that they do not "interfere" each other.
On the receiving end of the communication line should he the recovery process the control signal, i.e. the reverse process of modulation, This process of al location information of the received modulated waves have been called demodulation, or detection of vibrations. Thus, modulation is performed on the transmitting end and demodulation at the receiving end. When broadcasting, transmitting radio microphone signal modulates a carrier oscillation, whose frequency allocated to this station, and all radio receivers coniigured 10 receive a given station, the process of demodulation (detection) and after amplification the signal is reproduced by the loudspeaker 11 J.
Such separation devices, modulation and detection took place in the development of communications until the connection is carried out in one direction - from source to consumer. With the development of computer technology and the w idespread use of the Internet communication has become two-way. The consumer not only receives information from the Internet to your computer, but also transmits your information to the Internet. For this reason, it needs to have a device performing both operations: modulation and demodulation. This device is called a "modem". The number of types of modems used in practice is very large. A natural question arises: "Why do we need so many?". To answer this question is not simple. For this it is necessary to consider the transmission characteristics of the modulated signals, including noise immunity, wide spectrum, sensitivity to distortion in the channel, etc. As an example, it is advisable to consider the features of the simple analog types of modulation: amplitude modulation (AM), (eng. amplitude modulation).
Amplitude modulation
The process of amplitude modulation, converts the carrier wave. Under the carrier oscillation is a function of voltage changes in ac-cordancc with the expression:
»,(')= t/t.cos(w„/ + (?„) (| (
As can be seen from this expression for two values of Wo and (p(1 are fixed values, and the value of U„ can be variable. Since U„ is the amplitude of the sinusoidal oscillations of its change in time is called amplitude modulation (AM). Because it applies to AM, the amplitude in the expression (1) becomes a function of time:
In the foniiuia (2) Vc is the initial value of amplitude, u, < t) is the time vary ing voltage of the transmitted signal, k is the coefficient of proportionality. VJt) in the radio is called the envelope of the AM signal. Making the substitution (2) into (1), we obtain the function expression of the signal transmitted by modulating a carrier,
UM, (ty^U, (/)cos(<V + (fo) = [i?, + ATM,(i)]cos(ay + ) • (3)
For graphic presentation (3) consider the original single-tone AM, when the modulating signal is a harmonic oscillation (4)
w,(/) = £;,cos(Q/ + 3() (4)
where E0 is the amplitude; Q =s In¡T^— 2/rF - angular
frequency of modulation; F- cyclic frequency modulation; T{ — period of the modulation; (9 is the initial phase.
Substituting formula (4) in relation (3), we obtain the expression for the AM signal (5)
«Ail (') = + cos(£i/ + 04)]cos(<v +
Designating by AU = KAE0 - the maximum deviation of the
amplitude of AM signal of carrier amplitude Un and after making some simple calculations, we get (6)
"am {t) = Vc[} +M cos{£îî + ^0)]cos(o0/+<p„)
(6)
where M = Kßn/Ue = AU/Uc
- the ratio or depth of amplitude modulation.
Spectrum of AM signal. Applying the expression (5) trigonometric formula for the product of cosines, after simple calculations we gel (7):
"am (0 = Vc cos (û>„t + (pti ) + UcM cos (Q/ + )cos(rv + <p„ ) = = Vc cos( (0^ + <plt ) + " cos [( ma +Ci)l + <pK +61,] +
—-cos (i»0 - Q)i + % -
2 <7)
Formula (7) represents the spectrum of the AM signal. From the formula (7) shows that when single-tone amplitude modulation spectrum of AM signal consists of three high frequency components. Graphical form of the transmitted (baseband) signal and the carrier shown in Fig. I and 2.
Fig. I
Fig. 2
As can he seen from Fig. I and 2, the carrier signal has a frequency substantially greater than transmitted. This corresponds to the basic principle of AM, namely, to transfer the transmitted signal to the RF for transmission through electromagnetic radiation.
"jucCO ■
Fig.3
The waveform representing the result of modulating in accordance with the formula (6) shown in Fig.3. As can be seen from Fig.3 the envelope of the oscillations has the form of the modulating signal.To assess the technical characteristics of AM it is advisable to consider the spectra of three of the signals described above.
Spectra of the transmitted carrier signals shown in Fig. 4a,b. Fach of them contains one spectral lines with frequencies Q and w,, .The spectrum of resulting signal (Fig.3) in accordance with the formula (7) shown in Fig. 4c.
Um
О
«0
a)
b)
■ï г .
Olo-iï
+
(Dû
\ (ÛO +П
ca
C) Fig. 4
The magnitude of these spectral components depends on the modulation depth, which is in accordance with the formula (6) cannot be greater than unity. Thus, in the spectrum of the modulated signal the main energy is not the transmitted signal and the carrier oscillation, In practice, carrier signal contains up to 64% of the energy emitted by the transmitter, and an information sidebar for 18%.his factor has a very negative value in systems with frequency seal.
The second feature of AM is the presence of two spectral components related to the transmitted signal. They correspond to the frequencies wO -ii and raO + fi. The actual signal transmitted using AM, can have a wide range of frequencies. For example, the width of the spectrum of a musical signal is 15 kHz, and the range of television analog signal of 6.5 MHz. Full information about the transmitted signal is contained in one oT the lateral component of the spectrum. Thus, in addition to the energy inefficiency of AM, its application leads to the need to have excess bandwidth of the communication channel.
The third feature of AM is the low noise immunity. Since the value of the useful signal is determined by the amplitude of the carrier, a simple summation of signal interference leads to a deterioration in the quality of information transfer.
The history of AM
The above mentioned disadvantages of AM is a natural question about applying this kind of modulation in the historical aspect.
It is considered that the transmission of speech by AM. the first by one of the pioneers of radio, the American engineer R. Fessendcn in 1906.Modulation was carried out by turning on the microphone, changing the attenuation in the circuit connecting the transmitting antenna and machine a high-frequency generator. This kind of modulation in 1920 became the main sound broadcasting in the range of low, medium and high frequencies (LF, MF and I IF) and a network of analogue AM broadcasting, which is almost hundred years developed in all countries of the world. The 40 years of this kind of modulation was used not only in broadcasting but also in all other types of radio communication.
In addition to the use of AM for specific tasks, carried out its improvement with the aim of eliminating the above disadvantages. Of great importance for telecommunications was the invention of the American scientists R. Carson amplitude modulation, single side band (SSB) made in 1915.This method of modulation allows very efficient use of bandwidth of the communication channel. System with OBP widely used still in multichannel communication systems and in television (TV) broadcasting.
In the Soviet Union in 1939 by 1, A. Kozovym was invented another method of analog modulation, called polar modulation
(PM).The essence of this method lies in the fact that the positive half wave of the carrier frequency is modulated in amplitude with a single message, and a negative to others. In the USSR this method was chosen to create a system of stereo broadcasting [4J.
The big disadvantages of amplitude modulation is the presence of a component of the carrier signal in the spectrum of the modulated signal, which accounts for a significant portion of power. The elimination of this disadvantage it is possible by the application of balanced modulation,. If balanced modulation is the formation of a modulated signal without the carrier signal component. For this use a special balanced or l ing modulator.
The use ofAM balanced modulation and signal transmission with single side-band was useful in multichannel communication systems with frequency division of channels. It is known that in the early stages of development communications engineering signal transmission of telephony, telegraphy and radio broadcasting was carried out I the effective use of expensive linear structures preferably in the frequency band provided by the cable, to mark as many channels. This was made possible by the use of AM balanced modulation and signal transmission with single side band.
The transfer efficiency of single side-band is obvious. The frequency seal this allows you to double the number of transmitted line signals. As for balanced modulation, Ihe reason for Ihe efficiency of its use is self-explanatory. When transmitting signals through metal cables is the attenuation due to energy losses. Superimposed on the useful signal-to-noise contrast and at a certain signal/noise signal reception becomes impossible. For example, symmetric cable length of 100 km has a loss of signal is equal to ¡75 dR. You must provide either a large signal power at the input lines, or to carry out the signal amplification at intermediate stations. Construction of intermediate stations requires large financial costs.
The power increase on the transmitting end can be done within ccrtain limits. The reason is that if excessive signal power in the transmission path occur nonlinear distortions, which are generated mutual interference between the channels. Since the power signals of individual channels are summed, the maximum power imported one channel has a small magnitude. But in the AM modulation of the main power falls on the carrier frequency. Remove the same carrier using balanced modulation can significantly increase the power of the useful signal in each channel without increasing the total signal power in a linear path.
For these reasons, when the frequency sealing is applied to the balanced modulation and the removal of one of the side bands in the spectrum with the aim of' increasing the number of transmission channels. Basic telephone channel is called channel voice frequency (PM).The widlh of the spectrum of the telephone signal is equal to: 3100 Hz. Channel PM were suitable for the transmission of voice, data and Telegraph signals, facsimile signals and systems.
To transmit audio broadcasting signal (Af = 6 -M5 кГц) highspeed data transmission or in transmission of the set of papers require a wider strip, which is obtained by aggregation in the standard group, in Fig. 5 shows a schematic representation of the spectrum of the AM signal telephone. The carrier frequency of 12 kllz corresponds to the first channel of PM. At this frequency do not overlap the lower half of the spectrum of the modulated signal with the spectrum of the original signal.
From the presented spectrum formed the spectrum that is passed by cable. The carrier spectral line is removed by using a special circuit device to the balanced modulation, and the lower half of the spectrum using a special filter. The result is a signal for ! channel. The signal has the abbreviation SSB single sideband. Since the number of organized channels through a single cable can exceed several hundred, for their realization will require a large range of fillers on both the transmitter and receiver ends.
8 kHz 12 «Hit 16 kHz
Fig. 5
To the variety of filters was less first join in group three channel frequency carrier modulation to 12, 16 and 20 kHz. Lower sideband suppressed. The combined spectrum of the three channels is represented in Fig. 6. Block diagram of a device that combines the three channels into a single group, is represented in Fig, 7.
12 kHz
16 kHz
20 kHz
24 kHz
standard TV channel has a bandwidth of 8 MHz (Fig. 8), and its use lower sideband spectrum of the amplitude-modulated signal is suppressed, but only partly.
The reason is that you cannot create a filler with the desired frequency response, which is necessary to separate the lower spectral component, distant from the carrier frequency of only 25 Hz. Additionally, the process is carried out in AM TV transmitter in the output stage, so the filter must be designed to work with the signal of high power.
ÎMHi
fuHt
Fig. 6
si
us2 ">
»S3
Fitter
12-1fi кГц
tfc- 12 кГц J *
Modulator Filter
2 16-20 кГц —'
Î&» 16 кГц t \
Modulator Filter
3 ) 20-24 кГц
fci ~ 20 кГц
Fig. 7
Absolutely similarly transform 4-6 channels, 7-12 channels. To combine further, alt 12 channels, use the second con vers ion. Take the modulating frequency is 84, 96, 108 and 120 kHz. Filters to suppress the upper sideband.The first group of 12 channels is called the primary path. Further, from the primary paths are formed secondary and tertiary paths [5]. Thus, the use of analog AM for the transmission system with frequency seal required the development of a number of original technical solutions that allowed for several decades to effectively use the cabie line.
Another interesting solution to the application of AM is "polar modulation".By using polar modulation solves the problem of transmission of one channel of the two signals simultaneously, which requires, for example, in stereoviewing or transfer of stereochemical.To solve this problem, the positive components of the sinusoidal oscillations of the carrier are modulated by one signal and the negative components others.
At the receiving end using two detectors with different polarity of the inclusion of diodes stand out both signals.
Of interest is the method of using SSB AM to transmit television (TV) signals.
The television signal has a spectrum in a frequency range of 25 Hz to 6 MHz. This corresponds to the standard adopted in Russia (625 lines, frame rate 25 Hz). When you use AM to transmit TV signal over the air range is expanded two times, reaching 12 MHz. A
Fig.»
In TV broadcasting bottom side of the AM spectrum is suppressed and partially around the carrier remains part of the spectrum with a width of 1.25 MHz (Fig. 8a).When detecting such a signal in the frequency range from 25 Hz to 1.25 MHz signal amplitude exceed the amplitude of other frequency components.
To align the coefficients of transmission of various spectral components in the high frequency path of all TV receivers by the correction spectrum through the use of high-frequency amplifier with a frequency characteristic with a linear sloping land (m - n), as shown in Fig. 8b.When detecting spectral components located to the left of the carrier, will be summed with the spectral components in the range fCWlfv-n will happen the accurate reproduction of the spectrum ofTV signal.
With the introduction of color TV has the problem of transmission in the same frequency range in which the transmitled black -and white TV signals, in order to form a color image. This task was carried out in the United States in the development of the NTSC system, by using quadrature modulation. The method of quadrature modulation in addition to the NTSC system is also used ill the PAL system.
Quadrature Amplitude Modulation (QAM) - amplitude modulation of a signal which represents the sum of the two carrier oscillations of the same frequency but shifted in phase relative to each other by 90°, each of which is modulated by the amplitude of its modulating signal. In NTSC for the first lime to transfer color was applied color-difference signals Ub-yHUr.Y. These signals are combined with luminance signal which was required for the formation of even-white image in the conventional TV receiver. For combining formed signal at the color subcarrier frequency. This frequency was chosen in the range of TV signal and modulated two color-difference signals. This used a method of quadrature modulation, the principle of which is shown in Fig. 9.
As can be seen from Fig. 9, there are two modulator inputs to which the signals ER.V and Eu.v. Carrier frequency fs is generated by the generator and supplied to the modulator signal Eg-y and with a phase shift of 90" to the modulator ER.y.
Thus, the resulting signal Us is composed of two AM signals having a phase shift of 90".As Us is transmitted in the frequency band of the luminance signal, it needs to have perhaps a lower power that is achieved by using modulators of the balanced type.
Fig. 9
In the result, the resulting signal contains the carrier frequencies and its value is determined by the formula:
and phase sliill (p of the vector Us on one of the oscillations
<p = arctg^ld- (9)
U B-Y
As follows from the formulas, the amplitude of the resulting signal is determined by the sum of squares of difference signals, and the phase of their relationship. From the theory of calorimetry and subject to the availability of balanced modulation, it follows that the formula (8) determines the vector length and color saturation, and (9) the color tone. The chromavcctors is visualized graphically in the diagram of colors in polar coordinates (Fig. 10),
On the color chart (Fig. 10) the saturation is a fixed value, defined by a circle, and the vectors depending on the angle characterize color tone: purple (45°), red (113° ), yellow (173"), green (225°), and blue (353u ).
In a TV receiver color-difference signals must be separated. A device for the separation of quadrature components must consist of two synchronous detectors and a reference oscillator subcarrier frequency that is synchronized in frequency and phase with the transmitter oscillator. Signal generator reference subcarrier should be used for the operation of the two synchronous detectors and to have a phase of 0" for allocation of color-difference signal 13-Y and a phase 90", the selection signal R-Y. However, such information is not contained in the received TV signal, since balanced modulation sub-carrier itself is suppressed, and the side frequencies are the product of modulation, depending on the transmitted color (and hence, phase shifts, differing from the unmodulated values of fs) [2].
The formation of the signals for the synchronous detection in the NTSC system is carried out using the transmit clock signal in the form of a "package" oiS periods (Fig. 1!). This packet is transmitted during the flyback the horizontal and is placed at flyback blanking pulse (FBP) after passing the line sync pulse (LSP).
In addition to the tasks of image reproduction in both color and black-and-white TV receivers solves the problem of sound reproduction. As shown in Fig.SA, for the transmission of sound is used
fcarr.sound, spaced 6,5 MHz from the carrier image. The sound is transmitted by the method of frequency modulation (FM). This decision is connected with the need for sound reproduction with high quality in the presence of interference.
FM gives the opportunity to solve this problem, as it allows to obtain a broadband spectrum of the modulated signal. Frequency modulation was patented by an American radio engineer E. H. Armstrong in 1933. In 1924, the M.A. Bonch-Bruevieh reported on the invention of the new method of telephony based on the change of the oscillation period. Demonstration of frequency modulation was carried out using a laboratory model. High quality transmission of the audio signal in the TV is because when EM is applied large (compared to the width of the spectrum AM) the deviation of the carrier signal, and a TV receiver uses a limiter amplitude of the signal to eliminate the pulse noise. FM is used in radio communication with a small frequency deviation of the carrier signal.
In frequency modulation the amplitude of the signal remains constant while its frequency changes relative to the average value in accordance with the magnitude of the modulating signal. The difference: a/ - ¡/rlll, -/[-|/-/mi„|' between the unmodulated and frequency modulated oscillations is called the deviation or frequency deviation that depends on the amplitude of the modulating signal.According to the existing national standard, the maximum deviation of frequency for radio broadcasting in the Russian Federation adopted Afmax = 50 kHz.
Since the amplitude of the side frequencies decreases with increasing deviation from the carrier, when determining the bandwidth is taken into account only that portion of the spectrum, in which each lateral component of the spectrum has an amplitude of at least 1% of the amplitude of the unmodulated carrier.
Experience established that all significant lateral components of the spectrum pass, if the high frequency band of the receiver path at the level of 0.7 is equal to
nm/^2(Afmm/FB+2)FB (10)
When frequency modulation for radio broadcasting, the maximum frequency modulation is adopted Fu = 15 kHz. then //„„= 2(50-10 V 15-10' i 2) 15-10 = 160 kI 'ii. For almost one station is assigned the channel with a width of up to 250 kHz. A broad band of frequencies occupied by the signal, the frequency modulation allows the use of this method of modulation for radio broadcasting in ranges long, medium and short waves, which can lit only a few radio stations. For these purposes, the selected higher frequency VHF.
The feasibility of the FM signal in the VHF range is connected with its high interference resistance, which is due to the fact that all interference and noise contribute spurious amplitude modulation which can be eliminated by bilateral clipping of the useful signal. In radar found use a special kind of world LFM w hich is that the emitted radio pulses are filled with vibrations the frequency of which varies in time according to a linear law. Such signals with linear frequency modulation is commonly referred to chirp LFM.
LFM signals have one important property. If the signal on the frequency-dependent delay line, the delay time of the signal is high at small frequencies and decreases with increasing frequency in the chirp signal, the output of this line there is "compression" of the signal in one period of the high frequency oscillations by summing the amplitude values of all the periods of the signal. This conversion of the output signal amplitude reduces the effect of fluctuation noise, as summed simultaneously for the same periods, the noises are not correlated. The sinusoidal carrier could change as the method of phase modulation (PM).FM - mode oscillations, in which the transmitted signal controls the phase of the RF carrier oscillations. If the modulating signal is sinusoidal, the range and shape of the signals at FM and PM are the same.
The differences are manifested în more complex forms of the modulating signai. FM is used primarily as an intermediate transformation in the world PM with high stability of the carrier frequency. When a sinusoidal carrier phase increases proportional to time (On t. In the presence of FM phase is changed on the more difficult law, in accordance with the formula {11 ):
t/ (/) - i/0 sin (cy + 0> (/)+4>0 ) (11)
where i/o - amplitude modulated oscillation, <p() is the initial phase <p(t) is the component of the total phase fluctuations, changing in the process of modulation.
The instantaneous frequency of a phase modulated signal u(t) is a time derivative value of the total phase fluctuations: £/0(/)
dt
(12)
The modulation of the harmonic signal with frequency il the phase change of the signal occurs in accordance with the formula (13):
® (/) = w sin fir (13)
The amplitude of the phase change t is called the index of the comer of modulate, or phase deviation. The maximum frequency deviation, defined by the relations (12) and (13) the frequency deviation. The difference between FM and FM manifests itself in a modulation signal containing the frequency spectrum.When the FM modulation index does not depend on the modulation frequency and the frequency deviation is proportional to the modulation frequency. Using FM frequency deviation does not depend on the modulation frequency, and is determined by the amplitude of the modulating
signal. This is the difference between FM and I'M. Phase modulation properties close to the frequency. When the FM modulation index does not depend on the modulation frequency and the frequency deviation is proportional to the modulation frequency. Using FM frequency deviation does not depend on the modulation frequency, and is determined by the amplitude of the modulating signal. It also has a high efficiency and noise immunity. In electronic systems, it has received limited application, but photovoltaic systems are used widely.
The phase change of the carrier signal in the apparatus can be carried out by changing the capacitance of the oscillatory circuit or by switching the generators rotor oscillation. Phase modulation is not associated with the initial phase of the carrier signal, is called the relative phase modulation {OFM),]n the case where the information signal is discrete, we use the term "phase shift keying". However, this term is typically used when transmitting digital signals.
Rcfcrences
1. Kulcshov V.N., Udalov N.N., Bogachcv V.M. et al. The Generation {foscillations and formation of radio signals. Moscow: MEI, 2008.416 p.
2. Samoilov V.F., Khromoy B.P. TV, Moscow, 1975. 400 p,
3. Gonorovsky I.S. Frequency modulation and its application. Moscow: Svyazist, 1948. 284 p.
4. Guscv K.G., Filatov A.D., Sobolev A.P. Polarization modulation. Moscow: Soviet radio, 1974. 288 p.
5. Vertunov M.V. Single-sideband modulation in radio communications. Moscow: Voenizdat, 1 972. 296 p.
история непрерывной модуляции
Хромой Борис Петрович, МТУСИ, Москва, Россия, mtuci@ mtuci.ru
Aннотaция
Термин "модуля?ция" произошел от латинского слова "modulatio" (размеренность, ритмичность). Под этим термином понимают процесс изменения одного или нескольких параметров высокочастотного несущего колебания по закону низкочастотного информационного сигнала (сообщения). Таким образом, передаваемая информация заложена в управляющем (модулирующем) сигнале, а роль переносчика информации выполняет высокочастотное колебание, называемое несущим (модулируемым). Модуляция, представляет собой процесс "посадки" информационного колебания на заведомо известную несущую, с целью получения нового, модулированного сигнала. Термин "непрерывная модуляция" означает использование аналогового сигнала. Когда происходило развитие радиосвязи, определенный диапазон электромагнитных волн делился на части, которые распределялись между радиостанциями. Каждая станция работала на "своей" частоте. В результате модуляции спектр низкочастотного управляющего сигнала переносился в область высоких частот. Это позволяет при организации вещания настроить функционирование всех приёмо-передающих устройств на разных частотах с тем, чтобы они "не мешали" друг другу. На приемном конце линии связи должен осуществляться процесс восстановления управляющего сигнала, т.е.процесс обратный модуляции. Этот процесс выделения информации из принятых модулированных колебаний получил название демодуляции, или детектирования колебаний. Таким образом, модуляция осуществляется на передающем конце линии связи, а демодуляция на приёмном конце. При радиовещании, на передающей радиостанции сигнал с микрофона модулирует несущее колебание, частота которого выделена для данной станции, а во всех радиоприемниках настроенных на прием данной станции, осуществляется процесс демодуляции (детектирования) и после усиления сигнал воспроизводится громкоговорителем. Такое разделение устройств модуляции и детектирования имело место в развитии связи, пока связь осуществлялась в одном направлении - от источника к потребителю. С развитием компьютерных технологий и широким применением интернета связь стала двухсторонней. Потребитель не только принимает информацию из интернета на компьютер, но и передает свою информацию в интернет. По этой причине он должен иметь устройство, осуществляющее обе операции: модуляцию и демодуляцию.Это устройство называется "модем". Количество типов модемов, применяемых на практике весьма велико. Возникает естественный вопрос: "Зачем нужно такое количество?". Ответить на этот вопрос не просто. Для этого следует рассмотреть особенности передачи модулированных сигналов, в том числе: помехоустойчивость, ширину спектра, чувствительность к искажениям в канале и т.п. Далее автор даёт информацию об истории Амплитудной модуляции (АМ) применительно к передаче сигналов телефонии, радиовещания, черно-белого телевидения, цветного телевидения SECAM и Квадратурной Амплитудной Модуляции (QAM) в NTSC.
Ключевые слова: модуляция, сообщение, продолжение, демодуляция передача, характеристики, несущая parameters. Литература
1. Кулешов В.Н., Удалов Н.Н., Богачев В.М. и др. Генерирование колебаний и формирование радиосигналов. М.: МЭИ, 2008. 416 с.
2. Самойлов В.Ф., Хромой Б.П. Телевидение. М.: Связь, 1975. 400 с.
3. Гоноровский И.С. Частотная модуляция и её применение. М.: Связьиздат, 1948. 284 с.
4. Гусев К.Г., Филатов А.Д., Сополев А.П. Поляризационная модуляция. М.: Советское радио, 1974. 288 с.
5. Верзунов М.В. Однополосная модуляция в радиосвязи. М.: Воениздат, 1972. 296 с.
Информация об авторе:
Хромой Борис Петрович, профессор МТУСИ, Москва, Россия
