Know Sound Design: Revisiting FM - KeyboardMag

Know Sound Design: Revisiting FM

Learning FM synthesis through analog analogies
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Thirteen years ago, I contributed a few FM tutorials to Keyboard’s 20-year commemoration of the Yamaha DX7. Back then, Ableton’s Operator was still a few years away and Native Instruments’ FM8 was just a twinkle in Skrillex’s eye. Since then, FM has made a huge comeback thanks to those softsynths, as well as more recent products such as Arturia’s new Synclavier V, which, depending on the musical context, can sound either vintage or absolutely modern.

One of the best things about modern FM synths is that their user interfaces make them easier to program, despite the cryptic terminology and mathematical overtones of the technology. In fact, there’s a way to approach FM using concepts that every analog fan already knows, and this month, we’ll use Ableton Operator’s wonderfully concise interface to demystify this powerful form of synthesis.

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Algorithms. Getting a handle on FM requires an understanding of algorithm selection, which determines how the operators are configured. In Ableton Operator, the best algorithm for experimentation consists of two simple carrier/modulator pairs, which can be used for a basic two-layer synth patch. For this tutorial, switch off Oscillators C and D, and focus your experimentation on Oscillators A and B (yellow and green, respectively), making sure that both are set to sine waveforms. To make things easier, follow this rule of thumb: Oscillator A controls overall pitch and amplitude, while Oscillator B governs the harmonic content and timbre.

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Levels. Translating Oscillator A and B’s level parameters is the easiest way to get started. Because A (the carrier) controls volume, changing its level affects loudness, whereas changing the level of Oscillator B introduces FM. With Operator’s default settings, leaving Oscillator B’s level at 0 will produce a pure sine wave from A, much like a lowpass filter with a very low cutoff frequency. Raising the level of B gradually adds more harmonics, like opening up the filter. Thus, the levels for A and B can be thought to function like volume and cutoff knobs, respectively.

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Tuning. Without getting into the minutiae of harmonic interaction, there’s a quick and easy technical metaphor that can translate FM tuning parameters to common analog terms. In our selected algorithm, the Coarse parameter for Oscillator A (carrier) is similar to the tuning parameter of an analog oscillator; however, each number corresponds to a harmonic, so a value of 2 will be an octave higher than 1, while a value of 3 will be an octave and a fifth higher, and so on. The Coarse parameter for Oscillator B (the modulator) changes the harmonic relationship between the two, resulting in behavior that’s reminiscent of an analog oscillator’s waveform selector.

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Envelopes.Now that we’ve translated the above parameters to volume, cutoff, pitch, and waveform, the purpose of each oscillator’s envelope should be more obvious: The carrier envelope (Oscillator A) is like a VCA envelope, while the modulator envelope (Oscillator B) behaves like a filter envelope. The only thing to remember here is that if your modulator release time is shorter than your carrier release time, lifting the key will result in a quick jump back to a sine wave. With FM, it’s often best to set your modulator envelopes to maximum release, until you get the hang of it.