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i'm trying to study for my A levels and struggling to understand the purpose of modulation? I'm getting the idea that it is simply something that enables a effects to change the timbre of a raw instrument?
So it allows you to add effects to a guitar or synth, etc...
Is this correct?
Modulation is not an effect. Modulation is the process of varying one or more properties of something, in this case the sound of a guitar.
Effects are achieved through modulation (variation, change) of some property of the sound (like the phase, harmonics, frequency, amplitude, etc), but modulation itself is not an effect. This is very important to note to clear your confusion.
If you modulate the phase of a sound and mix that with the original signal you create the phaser effect.
If you modulate the time of a sound (using a delay) and mix that with the original signal you create the flanger effect.
If you modulate the amplitude (volume) of a sound you create the tremolo effect.
If you modulate the frequency (pitch, note) of a sound you create the vibrato effect.
Modulation refers to an entire category of different effects that all have this in common: a dry audio signal is combined with another signal of the same sound that is delayed in time, slowed down or sped up. This makes use of the property that physics describes as interference or wave propagation. This creates phase cancellation and reinforcement. In most cases the degree of change in time itself is constantly and periodically changing, or being modulated. Hence the name "modulation" for this class of effects.
The classic modulation effect is the flanger. This sound was originally created by taking a tape recording of a musical instrument, making two copies of the tape, and loading it onto two different independent tape machines for playback. While one tape machine played back at a constant rate of speed, the second tape machine's speed was subtly slowed down and sped up in a periodic fashion by the engineer applying his thumb to the flange, or the outer perimeter, of the metal tape reel, to slow down and speed up the mechanism. The resulting sweeping sound was the result of the two tapes going in and out of phase with each other. Nowadays the same effect is achieved using analog or digital electronics, not recording tape.
Here is a YouTube video demostrating how the effect was originally achieved.
