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May 20, 2002
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FM Synthesis

It's not just for electric pianos

By Peter Gorges 

Free SoundForum Synth Download from Native Instruments!



 

 
SoundForum Synth: Windows

SoundForum Synth: Mac

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In this installment of our tone-bending tutorial we're going to start looking beyond the horizon of simple analog synthesis and examine some of the more remote and exotic possibilities of today's analog synthesizers. The possibilities of which I speak are frequency modulation, ring modulation, and sync (oscillator synchronization, to be precise).

More mumbo-jumbo, you say. Indeed, but don't let the techno-babble intimidate you. Hiding behind these terms are some hip features that extend the sonic spectrum of the Soundforum synthesizer considerably.

This month and next, we're going to look at frequency modulation. We'll delve into ring mod and sync in the November and December issues. By that time you will have put every parameter of the Soundforum synth to constructive use.

What Is Frequency Modulation?

Back in the taste-challenged '80s, frequency modulation (FM) was the sexy new kid on the synthesizer block. Courtesy of Yamaha DX synthesizers, its ascent to world domination was swift, spelling doom for the first era of analog synths. Analog didn't recover until the early '90s with the advent of techno, the various offshoots of which still bank on the stylistic devices of analog synthesis.

During the '80s virtually every vocal track from California to Berlin was enveloped in a bed of chimey electric pianos and metallic basses. Occasionally adding some sonic variety was the maddening twitter of FM marimbas racing through the stereo panorama like so many amphetamine-crazed woodpeckers.

Think of FM as a spice rather than the stew, and you'll come to enjoy its taste. Unlike analog synthesis, where you can twist just about any knob and get a good sound, you'll need a recipe for FM if you want to cook up a specific timbre. If FM is your personal X-Files, fear not, for the following primer will air its mystery with hands-on exercises.

The Sound of Simple FM

The FM abilities of the Soundforum synthesizer are nowhere near as sophisticated as those of the good old DX7 and its successors. In a triumph of architectural austerity, the entire FM functionality of the Soundforum synthesizer is packed into a single knob, imaginatively labeled "FM." Select snapshot 16, "Basic FM," and play a couple of notes. What you're hearing is a rather metallic bell-like sound.

Now how is that accomplished?

Turn the Oscillator 1 FM knob down to 0 and slowly back up to 4500 as you play.

When the knob is set to 0, all you're hearing (and seeing in the scope, as in Figure 1) is a simple sinusoidal wave coming from Oscillator 2. As you rotate the knob, you're gradually dialing in a bell-like timbre, which will look something like Figure 2 or 3. That tells us the sound has something to do with this knob. But what?
Vibrato Is FM, FM Is Vibrato

Set the FM knob to 50 and turn down the Oscillator 1 Interval knob to -60. Play notes lower than C3.

What's that, you say? You're hearing a sine tone, but suddenly with vibrato? Me too, and that is as it should be.

Frequency modulation means that the moment-to-moment amplitude of one oscillator (in this case, Oscillator 1) is modulating -- controlling, in other words -- the frequency of another (Oscillator 2). What this means in terms of the Soundforum synth is that we're not listening to Oscillator 1 directly: Its output is set to 0 in the Mixer section. Instead, Oscillator 1's output waveform acts like a pitchbend wheel or lever, which is constantly being wiggled back and forth by a phantom hand, varying Oscillator 2's pitch.

But how do we get from here to the sound of Quasimodo tolling his pain? Simply copy everything I do -- it's easy, pro sound programmers do it all the time -- and in each step pay close attention to the change in sound. Ask yourself if you're hearing what I'm describing.

Turn the FM knob all the way up. The vibrato increases, morphing into a cheap laser effect.

Increase Oscillator 1's Interval value in semitone steps and play a couple of notes on the computer keyboard's G key so we share a common pitch reference.

At a value of about -24, the vibrato is no longer audible as a modulated sine wave oscillation. The sound and the modulation are no longer separate, they seem to have converged to create a new sound.

Allow me an analogy to explain this phenomenon. When still images are flashed by the eye, at a certain speed it begins seeing them as an animated sequence. Likewise, at a certain modulation rate the ear no longer perceives the individual beats of a vibrato effect, hearing instead a new animated sound. This is precisely what happen when the oscillator doing the modulating has a frequency within the range of human hearing, as is the case here with Oscillator 1.

But don't take my word for it:

Set the mixer's Osc1 knob to 1 and the Osc2 knob to 0.

As you can hear, the oscillator enters the audible frequency range at just about the value at which our perception of the frequency modulation changes from vibrato to sound. 

Timbre

Connoisseurs of fine sound have surely noted that this example uses two sine wave oscillations -- in other words, tuning fork-type tones sans any vestige of overtones. That's the forte of FM: Creating very complex spectra from a simple source. The timbre (sonority or tone color by any other name) is shaped by two properties, the interval between the two oscillators and the level of the modulator.

To put this to the test:

Set both oscillators' Interval knobs to 0. Set the FM knob to about 3500. You should hear a sound rich in sawteeth but poor in other overtones.

Try the following settings for oscillator 1 and see if you hear the timbres they suggest:

1: fundamental tone of an e-piano
12: square wave
17: grandfather clock
19: harp
24: more aggressive, angular square wave
30-40: mallet (e.g. vibraphone)
41-50: electric piano attack

Now try different combinations of Interval values for Oscillators 1 and 2, and fiddle with the Detune knob.

By tweaking the relationship of frequencies between the two (in FM theory this is called the ratio). If you choose an interval from the overtone series (for example, a perfect fifth or an octave) for Oscillators 1 and 2, you'll end up with a harmonically pleasing sound.

The second factor -- the number of harmonics and thus the sound's overtone content -- is determined by the intensity (or depth) of the modulation. The Soundforum synthesizer's FM knob controls that. You could think of it as the send knob for oscillator 2's "phantom" pitch control circuit.

FM also lets you modulate the harmonic content via time-based functions. In other words, you can control modulation depth by means of an automated process. The idea is similar to that of controlling the filter cutoff frequency as in classic analog synthesis, but the sonic results are different. In the Soundforum synthesizer you can do this with an envelope.

Call up the "Basic FM" snapshot again.

Set the Interval values to 19 for Oscillator 1 and to 0 for Oscillator 2.

You get a harp-like sound.

Slowly back off the Decay value for the filter envelope as you play. 

The FM knob is automated by the envelope, so the modulation cycle becomes shorter and shorter as the decay value decreases. This modulation is switched on or off in the "Filter Env->Osc" section. In this section you can see that the filter envelope controls the amplitude of Oscillator 1. (The knob has no effect on the amplitude modulation routing in this section, but does affect the pitch and symmetry modulation.) 
 


PETER GORGES is a professional studio keyboardist, a sound designer and consultant for synth manufacturers, and an author. He owns and runs the book and CD-ROM publisher Wizoo (www.wizoo.com). The SoundForum column appears in a different form in the German-language magazine Keyboards.
 

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