In music, modulation is the process of changing from one key (tonal center) to another. If a piece of music starts out in the key of F major but then changes, either immediately or gradually, to the key of Bb major, we would say it modulates from F major to Bb major. A piece is considered to be in a "key" if the root of the key is the tonic, also called "home" or the "gravitational center".

In some definitions, a modulation is not considered to have taken place until the new key has been confirmed with a perfect authentic cadence. More loosely though, you have changed key if a new tonal center feels now like the tonic.

This is the process that pieces of music go through to change the tonal centre from one key note to another. For example, a piece of music may modulate from C major to G major so that G takes over (albeit temporarily) from C as the keynote.

Modulation is central to compositional technique in classical music but to a lesser extent in modern popular music. Much has been written in composition text books, harmony books and theoretical texts about modulation, probably because the process of key change is reasonably well understood as it is a consciously learned process for composers. This contrasts with the organisation of chord progressions into syntactic structures which is largely subconscious.

Why Modulation?
The question here is what is the purpose behind modulation in a piece of music or point for modulation?

One reason is to add interest and variety to our music. Especially if we repeat the same sections many times, changing the keys will, at the very least, give our listener something interesting to hear. The contrast of the first key with the second, or an increase or decrease in intensity, can be interesting even on a subtle level.

Modulations often provide a sense of forward propulsion and drive the music into new levels of energy. In the classical repertoire a majority of pieces modulate to the key of the dominant, and the introduction of an extra sharp (for example introducing the pitch F# to modulate from C to G major) can give the music lift and direction.

From a practical standpoint, a film composer might need to write a piece of music that fits between two already existing songs in different keys to create an effect that change the mode of the scene. In this act, the middle piece will need a smoothly transition from the key of the first piece to the key of the second. Or if there is no transition, the composer needs to at least be aware of the effect of a sudden change in key and make sure it is welcome.

Other purpose of modulation is to help give music structure, direction and create a define mode. Modulation can trigger a variety of emotional responses from the listener. Depending on the technique or combination of techniques used, modulation can help music to more effectively evoke a mood. Modulation enables limitless harmonic and melodic variety while preserving unity. A successful modulation provides the brain with a new orientation of tones and chords, a leap into a musical parallel universe.

Act of Modulation
Modulation in a piece of music is often associated with the formal structure of a piece. A piece in a major key often modulates to its dominant key and there is often a cadence in the dominant at the end of the first section of the piece. For example, a piece starting in C major often ends its first section with a cadence in G major. The second section of a piece then takes the music back to the tonic key either directly or via other related keys.

Pieces in the minor key often move to a cadence in the relative major key at the end of the first section. For example, a piece in A-minor may end its first section with a cadence in C major. The second section then takes the piece back to the original tonic key.

The distance of the key change varies between periods in musical history and from composer to composer. The length of the stay in each new key can also vary considerably from a brief stop off (sometimes referred to as transient modulation or tonicization) to a whole section in a new key (sometimes referred to as transposition although this can also mean the transfer of a whole piece into a new key). Sometimes pieces can pass through several keys before settling on a new key centre or returning to the home key.

Modulation Techniques
Several factors govern the effects of modulation. But simply put, major keys generally produce a positive or happy effect; minor keys, a sad one. Tempo is also a factor. Depending on whether the move to the new key is harmonically prepared or unprepared, for example, modulations may be smooth or abrupt. In addition to modulations to major and minor keys, it is possible to modulate between various modes and keys and to create some interesting modal-tonal combinations.

There are various techniques of modulation. Each one employs a slightly different compositional technique, which in turn generates a distinct sound. Be that as it may, all forms of modulation share the same purpose: to change key.

Some are more complex than others, and each can be heard in certain genres more than others. We will explore a few different types of modulation to give you an idea. Today, however, we are going to take a look at modulation in its most three basic basic form. While modulation is not absolutely necessary in three- or four-minute pop songs, it certainly helps to add new life and maintain the listener’s curiosity. In longer works such as symphonies, however, modulation is of the essence. Even the greatest composers would have a difficult time keeping things fresh and interesting without modulating to new keys.

In this article, we will confine the discussion to actual key changes and explore five techniques for modulating:
1 Direct modulation,
2 Pivot-chord modulation, and
3 Parallel modulation

No matter which technique you use to create a modulation, you need to ask three questions:
(1) What is the desired effect?
(2) What is the destination key? And
(3) Which technique will get you there?

In using modulation, it must be a carefully sculptured musical event that involves a series of harmonic and melodic choices that conform to the style and context of the music at hand.

1 .Direct modulation:
This (also referred to as phrase, shift, static, or abrupt modulation) is by far the simplest, easiest, and most dramatic type of modulation to construct. Theoretically speaking, direct modulation involves the transition between two keys without any harmonic or melodic preparation. It is pretty amateur technique, and sound quite corny.

This type of modulation is widely used in pop and country music (is often called a "truck driver's modulation" in pop music because it's like the song has kicked into another gear) and has two main applications: These are suddenly shifting to a new key between sections of a song and repeating a section of a song in a higher key. The simple analogy of direct modulation is jumping form “square space A” to “square space B” with both legs at the same time.

Listen to Barry Manilow in I can’tsmile without you. Though he uses the truck driver modulation often for the last chorus of his songs. Also John Mayer used this technique in City love and Steve Wonder in You Are the Sunshine of MyLife

Direct modulation is a great go-to modulatory technique because it’s both simple and effective. It works well for short progressions, lengthy passages, or entire sections of music. You can play a progression once or several times before modulating. Your choices are virtually endless, so experiment with them. At the end of the day, trust your ear. If it sounds good, then it works.

2. Pivot chord Modulation:
This is also refers to as common chord modulation and is a great method of moving from one key to another, without drawing much attention. Unlike direct modulation, it can often go unnoticed by the untrained ear. This allows the composer to make drastic changes through subtle nuance. If you’re looking for a way to ease the song into a new key area, you will probably want to try using a “pivot chord.”

Pivot chord modulation is Diatomic modulation in the sense that it is usually used to travel or move to the key that are closely related (as in circle of fifth). In this modulation technique, while going along in one key, you will hit a pivot chord (or modulating chord), and then suddenly you will be in another key. Basically, the pivot chord is one that's diatonic (made only out of notes in the key's scale) in both keys.

This is the real cool type of modulation that classical composer used. It involves finding two common chords in two different keys and using those chords to modulate through. The simple analogy to this is moving from one room to another through a common door to both.  For example, here's a basic pivot chord modulation from key C major to G major. In this choral example by Bach, the C major triad is the common chord in modulating from C major to G major:

The `I and IV’ are the same chord (a C chord), and it belongs to both keys. It is the pivot chord of this modulation. You could continue in G major (by changing the key signature at 3 bar to G major) for the rest of the song; there's no requirement to modulate back to C unless you want to.
Another example below, the A minor triad (the second chord) is vi in C major and i in A minor.

A pivot chord modulation is only possible if the two keys are closely enough related on the circle of fifths (or by relative or parallel minor) to share a common triad. There are some guidelines for which modulations sound best. Take a look at the circle of fifths with each major key's relative minor on the inside.

Note that the Common chords are merely the chords that two keys have in common while the Pivot chords are the chords that we’ve actually chosen to use in our modulation.

A study of modulation based on common chords (pivot chords) is a useful starting point in the understanding of the process of modulation but, in practice, composers use a variety of techniques for modulation and a better understanding can be made of this subject by studying the chord progressions used.

3. Parallel modulation:
This is when you change mode without changing the root. In a parallel modulation, the music moves from major to minor or minor to major without changing the tonic, an example is when we modulate from C major to C minor, or F minor to F major, etc.
The idea of a key being "parallel" comes from the fact we are changing the tonic (1 or I) chord from major-minor or minor-major on the same root. This means if we started in C major, its parallel key would be C minor. We could therefore call Cm the "parallel minor" of Cmaj and Cmaj the "parallel major" of Cm.

These transitions are smooth because the tonic has not changed, but the shift from the bright tonal quality of the major key to the darker minor key and vice versa can be very dramatic. Thus, the main function of this modulation is to change the overall mood of the music with a simple chord change from minor to major or major to minor because major keys generally being upbeat and "light" while minor keys generally being ominous and "dark". It's naturally very smooth because the tonic remains the same, but is also very colorful because the character of the music changes quite dramatically.

This can be done directly or facilitated by the various modulation techniques described above. Depending on the length of the modulation and whether or not it returns to the original key, it may or may not be designated by a change of key signature.

Here is a parallel modulation from C major to C minor. Even though this modulation uses a common chord in both keys (G major chord), it is called a parallel modulation because the two keys share the same tonic.

Mozart was very fond of parallel modulations, one example can be heard in his famous Rondo alla Turca. The A section is in A minor, the B section is in A major. Notice how bright and triumphant the B section feels by suddenly lifting the mood from minor to major!

Changing key is a great way to inject a bit of energy into music. But it’s got to be done well, or it can just sound confusing.

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