modulation The process of varying one signal, called the carrier, according to the pattern provided by another signal. The carrier is usually an analog signal selected to match the characteristics of a particular transmission system. Modulation signals and techniques may be combined to produce composite signals carrying many independent channels of information (see multiplexing).

The primary types of modulation are as follows:(a)Amplitude modulation (AM) – the strength or amplitude of the carrier signal is varied. This form of modulation is not often directly used in computer communication except in some modems as quadrative amplitude modulation (QAM).(b)Frequency modulation (FM) – the frequency of the carrier is varied. This technique is often used by modems. See also frequency shift keying.(c)Phase modulation (PM) – the phase of the carrier wave is varied. This technique is often used together with amplitude modulation in high-speed modems. See also phase shift keying.(d)Pulse code modulation (PCM) – an analog signal is encoded as a series of pulses in a digital data stream. This technique is used by codecs.The term shift keying, as in frequency shift keying, denotes specialized modulation techniques in which the modulating signal is digital rather than analog.

mod·u·late / ˈmäjəˌlāt/ • v. [tr.] exert a modifying or controlling influence on: the state attempts to modulate private business's cash flow. ∎  vary the strength, tone, or pitch of (one's voice): we all modulate our voice by hearing it. ∎  alter the amplitude or frequency of (an electromagnetic wave or other oscillation) in accordance with the variations of a second signal, typically one of a lower frequency: radio waves are modulated to carry the analog information of the voice. ∎  [intr.] Mus. change from one key to another: the first half of the melody, modulating from E minor to G. ∎  [intr.] (modulate into) change from one form or condition into (another): ideals and opinions are not modulated into authoritative journalese.DERIVATIVES: mod·u·la·tion / ˌmäjəˈlāshən/ n.mod·u·la·tor / -ˌlātər/ n.

modulation. The changing from one key to another in the course of a section of a comp. by evolutionary mus. means (not just by stopping and starting anew in another key) and as a part of the work's formal organization. The simplest and most natural modulations are to the related keys (or attendant keys) i.e. to the relative minor or major, to the dominant and its relative major or minor and to the subdominant and its relative minor or major. The tonic major and minor are also related keys, modulation from one to the other being simple, but they are not usually so described. Chromatic modulation, found frequently in Wagner, Franck, and Strauss, in general means altering a chord by means of a chromatic change. It can also be achieved by moving basses up or down major or minor 3rds. Enharmonic modulation covers the use of chords altered by enharmonic means, e.g. turning a dominant 7th chord to a Ger. 6th. Modulation becomes less of a feature in atonal mus. because of the enlargement of the scale. First composers to use modulation may have been Obrecht and Desprès. Chromatic modulation occurs in madrigals of Gesualdo and Monteverdi. John Bull's organ fantasia Ut, re, mi, fa, sol, la modulates a whole tone upward successively into different keys. With J. S. Bach, modulation became integral part of fugue.

modulation In physics, the process of varying the characteristics of one wave system in accordance with those of another. It is basic to radio broadcasting. In amplitude modulation (AM), the amplitude of a high-frequency radio carrier wave varies in accordance with the frequency of a current generated by a sound wave. This means the wave will vary with a broadcast sound, such as a voice or music. Frequency modulation (FM), in which the frequency of the carrier wave is modulated, is used for static-free, short-range broadcasting.