Modulation
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The process by which one signal is used to modify another. The controlled signal is referred to as the 'carrier', and the controlling signal as the 'modulator'. It is of the greatest importance in both sound synthesis and signal processing.
The two main parameters of a sound to which modulation is commonly applied are those of amplitude (loudness) and frequency. Amplitude modulation of a signal causes changes in loudness in that signal. If the control signal is a slow regular waveform (e.g. a sine wave) the result is a tremolo - similar to (and often confused with) vibrato, except that there is no pitch deviation. Once the frequency of the modulator is raised above 10 to 15 Hz the tremolo amalgamates into a complex sound with added frequency components derived from the pitches of carrier and modulator. An irregular or transient control signal is generally associated with envelope generation.
Frequency modulation causes the pitch of the carrier to rise and fall according to the shape of the modulating waveform. If this is, as above, a slow regular waveform, the result is vibrato if the depth of the modulation is low, otherwise becoming a wide siren-like effect. If the modulating waveform is a series of discrete steps, the result is a series of discrete pitches. Particularly complex sounds arise when the frequency of the modulator is close to or even higher than that of the carrier. (Source - Richard Dobson (1992). A Dictionary of Electronic and Computer Music Technology. Oxford University Press.)
Ring modulation allows for the sum and difference of two input frequencies to be combined to create a new sound. Often one of these frequencies is at a sub-audio level allowing the creation of a sound of two similar frequencies, frequently with noisy textural characteristics.
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When sounds are added together they can create new textures. But we can also use one sound to control the properties of another creating patterns or rhythms.
What is Modulation?
Modulation means ‘to change or adjust’. The term is often used in traditional music to describe a change in musical key.
But in reality anything can be modulated.
For example: When I open a bottle of tomato sauce my hand modulates the lid from closed to open.
The technical process of analogue soundwave modulation involves two signals:
•Carrier signal – the main signal that is affected by the process.
•Modulating signal – the signal that affects and changes the carrier signal.
In the following examples we will observe how the modulating signal affects the carrier signal.
Amplitude Modulation and Frequency Modulation
Two common examples of modulation used in audio are Amplitude Modulation (AM) and Frequency Modulation (FM).
Amplitude Modulation (AM)
In Amplitude Modulation the amplitude (loudness) of a carrier signal is affected over time.
The original carrier signal is changed from a constant loudness to a fluctuating pulse by the modulating signal.
Frequency Modulation (FM)
In Frequency Modulation the frequency of an original carrier signal is affected.
In this case the modulating signal does not affect the loudness (or amplitude of the sound) this remains constant. But it does change the frequency (number of waves per second) of the original signal.
LFO (Low Frequency Oscillator)
Not anything to do with aliens, I’m afraid. An LFO is an oscillator (back and forth wave) that moves very slowly (at a Low Frequency). LFO’s are very frequently used to modulate other sounds.
They are very common in many synthesisers and used frequently in lots of different styles of music.
LFO shifts pitch
Here an LFO affects the pitch of an oscillator.
Composition Tip
LFO’s allow musicians and composers to slowly change the quality of their sounds, while still remaining relatively constant. This means that they can add interest to static or constant sounds.
An LFO controls the rate of change for a Phase, Flanger or Chorus effects.
LFO controls a Filter
Here an LFO controls the cutoff point of a filter.
Fact
Dubstep is a style of music that makes significant use of modulation. The ‘wobble’ that is the most recognisable feature of Dubstep is caused by low frequency modulation of a filter affecting the main synthesiser tone.
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