modulation (redirected from Demod)

modulation, in communications

modulation, in communications, process in which some characteristic of a wave (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 radiation, 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

Amplitude modulation (AM) is the modulation method used in the AM radio 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 sidebands extending above and below the carrier frequency by an amount equal to the highest modulating frequency. If the modulated carrier is rectified (see rectifier) 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 noise 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 radio 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

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 disc 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).

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modulation, in music

modulation, in music, shift in the key 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.

Bibliography

See C. Zöller, The Art of Modulation (1930); M. Reger, On the Theory of Modulation (tr. 1948).

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modulation

In electronics, a technique for impressing information (voice, music, picture, or data) on a radio-frequency carrier wave by varying one or more characteristics of the wave in accordance with the signal. There are various forms of modulation, each designed to alter a particular characteristic of the carrier wave. The most commonly altered characteristics include amplitude (see AM), frequency (see FM), phase, pulse sequence, and pulse duration.

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modulation

In music, the transition from one mode or key to another. There are three principal methods of modulation in classical harmony: diatonic, in which a pivot chord is common to both keys; chromatic, in which the notes of the pivot chord are altered by a semitone; and enharmonic, in which the notes of the pivot chord, while retaining their original tones, simply assume different names. Modulation may be transitory, as in the course of thematic development, or structural, contributing to the harmonic definition of the form.

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modulation

Typically refers to the altering of a carrier wave in order to transmit a data signal (text, voice, audio, video, etc.) from one location to another via a discrete channel (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 analog telephony, modulation also refers to the altering of a DC current in order to transmit voice electronically. 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 (QAM), both the amplitude and phase of the carrier are modified (see QAM).