Modulation is to change from one key, or tonal area, to another in the middle of a piece of music using a recognizable harmonic progression. There are a number of rules and conventions designed to make the key change as smooth as possible.
Every major key is related to a minor key. The relative minor key begins on the 6th note of the major scale, and the two scales share the same notes. They also share the same key signature. For this reason, the following explanation of modulation need deal only with major keys.
As the circle of fifths demonstrates every major key is closely related to two other major keys. Only one note in their scales differs each time. These two other keys begin on the subdominant (fourth) and dominant (fifth) notes of the original key. Take the key of C major as an example: the two keys closest to it are F major and G major. F is the 4th note in the scale of C major, and G is the 5th. Only one note in the scale of F major differs from that of C major – B♭. And only one note in the scale of G major – F♯. Moreover, comparing the three major scales of C, F, and G reveals that the three primary intervals (I, IV, and V) of all three keys are related by common notes. The note C is the root in C major, the subdominant (4th) in G major, and the dominant (5th) in F major. The note F is the root in F major and the subdominant (4th) in C major. The note G is the root in G major and the dominant (5th) in C major. This pattern is repeated throughout the twelve keys, and these primary relationships allow a smooth natural transition.
Modulation in sound synthesis
Modulation, in a different sense, is a key concept in bringing expressiveness to synthesized sounds. Many different aspects of a sound can be modulated. For example, the pitch of an oscillator can be modulated yo produce a vibrato effect or the cut-off frequency of a filter can be modulated to create a characteristic sweeping sound. Modulation can be achieved by the player operating synthesizer controls such as the modulation wheel or by increasing pressure on an after-touch-sensitive keyboard.