What Is Modulation?
Modulation means ‘to change or adjust’. The term is often used in traditional music to describe a change in musical key or to describe tremolo and vibrato. 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. Low rates of Amplitude Modulation make pulsing sounds. Higher rates of modulation create new and interesting textures.
This clip begins with a simple sine tone. Gradually increasing amounts of amplitude modulation are added.
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.
This sound begins as a simple tone, over time more Frequency Modulation is applied.
LFO (Low Frequency Oscillator)
Not anything to do with aliens, I’m afraid. An LFO is a Low Frequency Oscillator, an oscillator (a back and forth wave) that moves very slowly (at a low frequency). LFOs are modulating signals – they transform other sounds. They are very common in many synthesisers and used frequently in lots of different styles of music. They can be used to affect any number of parameters: frequency, amplitude or rate of change.
LFO Shifts Pitch
Here an LFO affects the pitch of an oscillator.
LFOs 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
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.