modulation(redirected from Demod)
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modulation,in communications, process in which some characteristic of a wavewave,
in physics, the transfer of energy by the regular vibration, or oscillatory motion, either of some material medium or by the variation in magnitude of the field vectors of an electromagnetic field (see electromagnetic radiation).
..... Click the link for more information. (the carrier wave) is made to vary in accordance with an information-bearing signal wave (the modulating wave); demodulation is the process by which the original signal is recovered from the wave produced by modulation. The original, unmodulated wave may be of any kind, such as sound or, most often, electromagnetic radiationelectromagnetic radiation,
energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an electric field.
..... Click the link for more information. , including optical waves. The carrier wave can be a direct current, an alternating current, or a pulse chain. In modulation, it is processed in such a way that its amplitude, frequency, or some other property varies.
Amplitude modulation (AM) is the modulation method used in the AM radioradio,
transmission or reception of electromagnetic radiation in the radio frequency range. The term is commonly applied also to the equipment used, especially to the radio receiver.
..... Click the link for more information. broadcast band. In this system the intensity, or amplitude, of the carrier wave varies in accordance with the modulating signal. When the carrier is thus modulated, a fraction of the power is converted to sidebandssideband,
any frequency component of a modulated carrier wave other than the frequency of the carrier wave itself, i.e., any frequency added to the carrier as a result of modulation; sidebands carry the actual information while the carrier contributes none at all.
..... Click the link for more information. extending above and below the carrier frequency by an amount equal to the highest modulating frequency. If the modulated carrier is rectified (see rectifierrectifier,
component of an electric circuit used to change alternating current to direct current. Rectifiers are made in various forms, all operating on the principle that current passes through them freely in one direction but only slightly or not at all in the opposite
..... Click the link for more information. ) and the carrier frequency filtered out, the modulating signal can be recovered. This form of modulation is not a very efficient way to send information; the power required is relatively large because the carrier, which contains no information, is sent along with the information.
In a variant of amplitude modulation, called single sideband modulation (SSB), the modulated signal contains only one sideband and no carrier. The information can be demodulated only if the carrier is used as a reference. This is normally accomplished by generating a wave in the receiver at the carrier frequency. SSB modulation is used for long-distance telephony (such as in the amateur radio bands) and telegraphy over land and submarine cables.
Frequency and Phase Modulation
In frequency modulation (FM), the frequency of the carrier wave is varied in such a way that the change in frequency at any instant is proportional to another signal that varies with time. Its principal application is also in radio, where it offers increased noisenoise,
any signal that does not convey useful information. Electrical noise consists of electrical currents or voltages that interfere with the operation of electronic systems.
..... Click the link for more information. immunity and decreased distortion over the AM transmissions at the expense of greatly increased bandwidth. The FM band has become the choice of music listeners because of its low-noise, wide-bandwidth qualities; it is also used for the audio portion of a television broadcast.
Digital radiodigital radio,
audio broadcasting in which an analog audio signal is converted into a digital signal before being transmitted; also known as digital audio broadcasting (DAB) and high-definition radio.
..... Click the link for more information. is based on frequency division multiplexing (FDM), which allows transmission of multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within its own unique frequency range (carrier), which is modulated by the data (audio, video, etc.). Orthogonal frequency-division multiplexing (OFDM) takes this concept further, separating an individual transmission into multiple low-frequency signals with a high resistance to interference. A further extension of the technology, coded orthogonal frequency-division multiplexing (COFDM), is widely used in Europe and elsewhere where the digital audio broadcast (DAB) standard has been adopted. OFDM and COFDM offer the benefits of high spectral efficiency, resilience to radio-frequency (RF) interference, and lower multipath distortion.
Phase modulation, like frequency modulation, is a form of angle modulation (so called because the angle of the sinewave carrier is changed by the modulating wave). The two methods are very similar in the sense that any attempt to shift the frequency or phase is accomplished by a change in the other.
Pulse modulation involves modulating a carrier that is a train of regularly recurrent pulses. The modulation might vary the amplitude (PAM or pulse amplitude modulation), the duration (PDM or pulse duration modulation), or the presence of the pulses (PCM or pulse code modulation). PCM can be used to send digital data; audio signals on a compact disccompact disc
(CD), a small plastic disc used for the storage of digital data. As originally developed for audio systems, the sound signal is sampled at a rate of 44,100 times a second, then each sample is measured and digitally encoded on the 4 3-4 in (12 cm) disc as a series of
..... Click the link for more information. use pulse code modulation. Developed in 1939 by the English inventor Alec H. Reeves, pulse code modulation is the most important form of pulse modulation because it can be used to transmit information over long distances with hardly any interference or distortion; for this reason it has become increasingly important in the transmission of data in the space program and between computers. Although PCM transmits digital instead of analog signals, the modulating wave is continuous. Digital modulation begins with a digital modulating signal. The two most common digital modulating techniques are phase-shift keying (PSK) and frequency-shift keying (FSK).
modulation,in music, shift in the keykey.
1 In music, term used to indicate the scale from which the tonal material of a given composition is derived. To say, for example, that a composition is in the key of C major means that it uses as its basic tonal material the tones of that scale which is associated
..... Click the link for more information. center of a composition. For its accomplishment use is made of the fact that each chord figures in the harmonic relationships of several keys. In modulating from one key to another, a chord that is common to both keys is used as a pivot chord. If there is no chord common to the two keys, a passage may move through several keys before the desired modulation has been effected. Modulation is commonly employed as a means of achieving variety in a composition and has been in use since the late 15th cent.
See C. Zöller, The Art of Modulation (1930); M. Reger, On the Theory of Modulation (tr. 1948).
in music, a change of key by displacement of the tonic (tonal modulation). In ordinary functional modulation the connection between two keys is established by a chord common to both, the pivot chord, the function of which changes with the appearance of a harmonic shift that is characteristic of the new key. The modulating chord, with the corresponding alteration, is of decisive importance.
A special type of functional modulation is enharmonic modulation, in which the pivot chord is common to both the old and the new key, owing to the enharmonic interpretation of its structure. This kind of modulation easily connects distant keys and often produces the impression of an unexpected, abrupt modulatory shift.
Of great importance in modulation are the melodic connections of chords, or natural voice leading, which can play a major role, pushing aside and even completely replacing the functional connections of chords. Melodic harmonic modulation that lacks a common chord is most typical of the direct jump to a distant key, in which only the modulating chord that is melodically leading serves as a link from one key to the other. Melodic modulation per se (without harmony), which can also go to a remote key, is encountered in one-voice (or octave) movement.
Modulation that occurs without any preparation and with the immediate establishment of the new tonic is called a confrontation of tonalities. Usually, it is used in the transition to a new section of a musical form, but occasionally it is encountered within a particular structure.
Tonal modulation is distinguished from modal modulation, in which the tonic is not displaced but the key changes from the major to the minor of the same name, or vice versa.
REFERENCESRiemann, H. Sistematicheskoe uchenie o moduliatsii... Moscow, 1929. (Translated from German.)
Rimsky-Korsakov, N. A. “Uchebnik garmonii.” Poln. sobr. soch., vol. IV. Moscow, 1960.
Tiulin, lu. Uchebnik garmonii, part 2. Moscow, 1959.
IU. N. TIULIN
(in physics and technology), the variation with respect to time of quantities that describe some regular process. It is produced by an external influence. The modulation of electromagnetic oscillations in the radio-frequency and optical bands is of great practical value. The operation of all electron devices is based on the modulation of an electron flux. Thus, in electron tubes the density of the electron flux is modulated, and in television picture tubes the intensity of the electron beam that bombards the screen is modulated. In klystrons and other superhigh-frequency electron devices, modulation of the electron velocity is used. Modulation is extensively used in measurement technology; preliminary modulation of the quantity being measured can increase the sensitivity of the apparatus and improve the accuracy of measurement.
modulationTypically refers to the altering of a fixed-frequency carrier wave (radio or TV channel) in order to transmit data, audio or video from one location to another (see carrier). At the receiving end, a tuner latches onto the particular carrier frequency and a demodulator circuit isolates the data from the carrier.
In traditional analog telephony, modulation alters a DC current. In such a conversation, both parties are modulating the current simultaneously.
Amplitude, Frequency and Phase
There are three basic types of modulation: amplitude modulation (AM) varies the voltage; frequency modulation (FM) varies the frequency, and phase modulation (PM) varies the angle of the wave (see amplitude modulation, frequency modulation and phase modulation). In quadrature amplitude modulation (see QAM), both the amplitude and phase of the carrier are modified.