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FM Synthesis is very confusing to me as I am a beginner. I'm just started learning ABLETON's OPERATOR, which I assume is an FM synth. The first OSC works which I am sure but cannot figure out if the rest of the OSC above it are only modulating the sound but nothing else?

So can we call the rest of the so called OSCs fake, which are really just LFOs modulating the main OSCs FREQUENCY?

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  • I guess it depends on what you consider a real or fake oscillator. As FM synthesis is & always was entirely calculated in the digital domain, none of the oscillators are 'real'.
    – Tetsujin
    Jan 10 at 18:39

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In an FM algorithm, all oscillators are capable of producing audio output.

In a regular FM algorithm diagram, the oscillators that are audible appear at the bottom.

Table of 4-operator FM algorithms

In this simple 4-operator chart, you can see that in the first algorithm (1), only oscillator 1 is audible while the rest are modulating its output. However, in the last algorithm (8), all oscillators are audible together.

All of the oscillators still operate within the audible range, whether directly outputting sound or modulating another oscillator. This is what enables FM synths to produce those signature combinations of complex shifting waves and harmonics. The bottom oscillator in each algorithm can be thought of as the fundamental frequency, key in producing the pitch quality in the sound. Any other oscillators above the fundamental can be assigned to either track the pitch similar to a harmonic or stay fixed like a ring modulator.

On top of that, each oscillator can be assigned independent envelopes along with a few other parameters, like a very limited mini synthesiser all by itself. These mini synths are what we are referring to when we talk about "Operators" - They're not just oscillators but more complex units.

A single operator in Dexed VST looks like this:

Single operator controls take from Dexed VST

You can see that this is not just a simple oscillator, yet this is probably one of the most simple implementations of an FM operator.

An FM synth may also have one or more LFOs that operate below audible range and can be used to modulate pitch, amplitude or filter width in a more familiar way to give vibrato, tremolo, etc.

This is very similar to the way "West Coast" synthesis works.

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LFO means Low Frequency Oscillator (less than a couple of Hertz). Usually, with FM synthesis, modulating oscillators have an audio range frequency. Then we can’t name modulating oscillators « LFO » even if they modulate the carrier in the same way a LFO modulate the pitch of an oscillator.

To be exact, most FM synthesisers are Phase Modulation, then modulators modulate the phase and not directly the pitch of the carrier oscillators. (Then not necessarily exactly like the way a LFO modulates).

These oscillators are computing values which vary in alternance between -100% and 100% (sine wave or other curve), then we can name them oscillators.

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