I
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.
II
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.