An Introduction to FM Synthesis

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A synthesist’s arsenal is only as deep as their knowledge of technique, and we’ve covered a lot of technique. From the hilarious to the challenging, sometimes a dive into why something sounds the way it does can transform a session and take a song to a completely different level.

Today’s technique of choice is a primer on the oft-misunderstood FM Synthesis. And we’re going to talk about that. But first, don’t forget to check out Soundfly’s online course, Advanced Synths and Patch Design for Producers, and learn how to move beyond presets to create a wide array of scintillating synth sounds for your productions.

Surely you’ve heard of the Yamaha DX7 synth — or, if you haven’t heard of it, you’ve definitely heard it:

The DX7 was the first synthesizer to incorporate FM, and its release essentially single-handedly killed the era of dominance of analog synths. Now, that’s a lot to live up to for one synth based on one type of synthesis. As you will soon see, however, FM synthesis can be extremely powerful if used in the right way and is capable of making everything from smooth, glassy pads, to filthy rave basses, to glitchy percussion sounds, and a whole lot more in between.

In principle, FM synthesis is very straightforward. On traditional analog synths, the Low Frequency Oscillator (LFO) is a standard control that will modulate other parts of the synth (e.g., the pitch of the main oscillator). Now, imagine that that LFO, which only operates in the sub-audio range, were to just be a regular oscillator modulating the pitch of another oscillator. That’s all there is to it: one oscillator modulating another oscillator at a variety of rates to create an infinite amount of timbral variation.

So, even though there are two oscillators involved, you actually only ever hear the final output of the second oscillator, the one that is being modulated. This simple premise means that the modulating oscillator can be moving in all kinds of non-musical ways but still creating musical output via the oscillator that is being modulated.

Now that you have the two-oscillator concept down, let’s get into some terminology.

The oscillator that carries out the modulation is called the signal or modulator. Simple enough, right? The modulator modulates the second oscillator, which is called the carrier — called such because it “carries” the modulator’s effect. One more time: The modulator modulates the carrier, and we only hear the output from the carrier. The carrier’s pitch is usually controlled from a MIDI source such as a keyboard, which allows us to make all the brassy pads and glassy leads we all know and love, rather than uncontrolled inharmonic chaos.

Speaking of harmonics — harmonic ratios are the key to controlling FM synthesis. Without going too deeply into even and odd harmonics, the concept can be boiled down to something as simple as if you want to add something “pleasant,” akin to a filter sweep to your sound, stick to whole number ratios; if you want to get noisy and out of tune with your FM, add uneven ratios.

What do I mean by ratios? Say the carrier is tuned to A 440hz. Tune the modulator to 2:1 or 3:1, which would be 880hz and 1320hz, respectively (and so on). You’ll get pleasant, ethereal overtones that make your synth shimmer. Add something in between and listen to the synth howl.

I can already hear you asking, “But wait, I hear these moving patches on the DX7, but you’re telling me that static tuning is the key! How does that synth create tones that move like that?”

Here’s where the architecture of this classic synth starts to shine: Just like in good, old analog synthesis, amplitude and pitch envelopes can be applied to the modulator oscillator to create sounds wholly unique.

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Imagine that when you strike a key on the keyboard that it triggers a velocity-sensitive envelope that controls the amplitude of the modulator. All of a sudden, the leading edge of the carrier has a great, plucked sound to it! Or even better, patch an LFO to the pitch of the modulator, and all of a sudden, a one-note bass line has a massive depth and thickness to it.

The real key to unlocking the magic of FM synthesis is understanding this combination of oscillators, envelopes, and amplitude. Very subtle shifts in modulator amplitude can yield vastly different results at the final output. Again, this is where the DX7 shines, because Yamaha packaged these three key components together into something called an Operator and designed presets with as many as six Operators interacting in all sorts of musically interrelated ways.

If you want to get your hands dirty and try some of these techniques yourself, there are a wide variety of options available in almost every DAW. In Ableton, the Operator VST is, as you might have now guessed, comprised of several Operators that work in tandem, just like the DX7. Massive actually has a variety of cross-mod options that can be wired to create DX7-style patches, and Native Instruments’ FM8 is another good resource to try. Logic’s ES 2, EFM 1, and Retro Synth are all great options as well.

One final point: Because the most popular applications of FM have been digital from the get-go, you don’t even need to splash the cash to dive into the physical version of the old synths! You can get sounds just as glassy and chaotic out of all the VSTs that I mentioned above. So there’s even less of a barrier to entry here.

Armed with this knowledge, you can now open up your DAW and make the same patches that Madonna, Peter Gabriel, and Pink Floyd would all love to get their hands on! So go on, dive in.

Want more?

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