Synth Quickie – FM Bass


Since we’re so nice, we wrote down the synth recipes for your convenience:

Simple & modern:

  • Type of Synthesis: FM
  • Operator Routing: 2 -> 1
  • Operator 1
    • Waveform: Sine
  • Operator 2
    • Waveform: Sine
    • Pitch Ratio: 2:1
    • Amp Envelope: Sustain between 0 and 80%, quick Decay
    • Level: Between 80% and Max

Bit more complex, and classic

  • Type of Synthesis: FM
  • Operator Routing: 2&3 -> 1
  • Operator 1
    • Waveform: Sine
  • Operator 2 will give us our electric bass tone
    • Waveform: Sine
    • Pitch Ratio: 1:1
    • Amp Envelope: Max Sustain
    • Level: Between 60-80%
  • Operator 3 will give us our attack transient
    • Waveform: Sine
    • Pitch Ratio: 7:1 (adjust to taste)
    • Amp Envelope: Sustain at 0, quick Decay
    • Level: Between 70-80%

The Dexed download link is right here. No, not the word “here”, the orange ones. You missed them, go back. Yeah.

As per usual, the patch challenge is up and running so you can upload your version of an FM bass to the forum and discuss whatever you want with fellow Syntorians. Upload away!


FM bass, this is a pretty basic FM synthesis patch. It’s a great way to learn how FM synthesis works. So you don’t have to have any prior knowledge of FM synthesis. I’m gonna show you how it works as we go.

This synth is dexed. It’s a free VST/AU for Mac and PC. Link to download is in the description below. So what do we have here? Well, we’ve got six separate oscillators, or as they call ’em in the FM world, operators. And they can only generate a sine wave. You can’t change the waveform like you can with subtractive and wavetable synths.

So how do we get interesting sounds when all we have is a sine wave to work with? Well, that’s where FM comes into play, frequency modulation. Operator 2 is routed so that it modulates operator 1. We can’t actually hear its audio. If I turn its level up, we won’t hear another sine wave. Instead, as we increase this level, it modulates operator 1 more, and ends up changing its tone. Listen. It makes it brighter, but it does it in a very unique FM way, in this kind of metallic, stringy way.

And we can change the nature of that brightness with operator 2’s pitch. By raising it, you’ll hear that tone become more metallic and higher up, listen. Now FM synths control pitch in a bit of a different way than what you’re probably used to with subtractive and wavetable synths. They use pitch ratios, which very briefly means they are multiplying the frequency of the pitch.

So F equals two means we’ve doubled the frequency, which equates to one octave. Three means we’ve tripled the frequency, which equates to an octave and a fifth, and it goes up and up through this thing called a harmonic series, which you can look up. There’s plenty of information on that. I’m not gonna talk about it, ’cause you don’t need to know it. All you need to know is that this Coarse knob, as you raise it, it changes the nature of that brightness, makes it more metallic, makes it higher.

I’m gonna keep it at one right now. So it’s the same pitch as operator 1. Where things get interesting is when we modulate the level of this operator. So this level is its volume. As we increase its volume, it modulates operator 1 more. Therefore, all we need is an amp envelope to modulate this level, and that’s what this is right here!

So I’m gonna take the sustain level, right here, turn it all the way down, and I’m gonna take its decay rate, which is right below, and right now, it’s so fast that it just jumps down. That’s why there’s nothing here. I’m gonna slow it down. And what do we get? Right, so it’s taking this level and it’s shooting it down. It’s creating our basic starting FM bass.

Now, if you’re listening closely, you kinda hear some noise in there, this crackling sound. That’s just something this synth is doing. I don’t think other FM synths do that so much. It might just be a small shortcoming of this synth, but hey, it’s free. And you can kinda reduce it a little bit, by bringing our low pass cutoff down. Just takes some of the highs off. It’s still kinda there, but… Okay, so that’s a basic FM bass, right there!

Now it modulates all the way down to zero, leaving us with a sine wave. Here we have a sine wave, but maybe you don’t want that. What if you want there to be a little bit more in your sound after that attack transient? Well, you could raise that sustain level back up. Oh, yeah, maybe right there. Eh, maybe there. You get to decide exactly where it ends.

And watch what happens when I raise this pitch up one octave. That might sound more familiar. This is a very sort of modern approach to FM bass. It’s nice that it’s simple. Maybe I don’t want it to be as bright. There we go. All right! So that’s our first FM bass. Kind of modern-sounding, real basic approach. I wanna show you one more approach, more complex approach, that’s gonna get us more of like a classic ’80s-style FM bass.

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I’m gonna remove this modulation here. Go back down to same pitch as operator 1. And I’m just gonna use this to get our timbre. I wanna get kind of like an electric bass, like an actual string electric bass tone. Yeah, maybe there. And if you wanna get rid of those pops and clicks, put a little bit of an attack on operator 1. That’s not bad, how ’bout, and then the release as well, we wanna have a little bit of a release.

Okay, now, I want to take operator 3 and modulate operator 1 with it as well, use that to create our attack transient. ‘Cause I’m missing that at the beginning, all right? It’s just kind of a muted sound. Now you could stick with this if you just wanted this simple, plain electric bass tone; it’s perfect. But I want some attack transient on there, and I’m kinda going for that more classic-sounding FM bass.

The problem is, by default right now, operator 2 is modulating operator 1, but 3 is not operating modulator 1. That’s where we come down here to the routing of an FM synth. So the way they do it in dexed is, the operator numbers that are on the bottom row are the ones we can actually hear. So we can hear 1, we can also hear 3. And then, up from top, we have a sort of cascading modulation, so 2 modulates 1, 6 modulates 5, modulates 4, modulates 3. We don’t need all that. I need 2 and 3 to both be modulating operator 1. So I’m gonna change the algorithm. There’s a lot of ’em.

The one I found was right around here. 2, 3, and 5 are all modulating operator 1. I don’t need 5, but I got what I want out of this, 2 and 3. Okay, so right now, we still just have 2 modulating operator 1. I’m gonna increase 3, and use the amp envelope to create an attack transient, just like we did before. Let’s go up to two. It doesn’t quite sound like it did when we were using this operator, right? Because now we have two operators modulating operator 1, so it’s gonna change the nature. They’re gonna interact in an interesting way.

I’m gonna go up to seven now. Now what do we got? Let’s go a little faster. Back off on this a little bit, there we go. And maybe ease off here. There it is! A little bit more of a classic-sounding FM bass. And of course, I could try other pitches.

So there you have it, and as you can see, there’s a million ways you can create an FM bass with an FM synth, all with varying tones! Let me show you one example that came with the presets, FM bass. This uses all six operators. Different routing here, so we’re actually hearing 1 and 3, they’re sort of layering two sounds together and using the other operators to create the tones for each of those.

All right, so there is a Patch Challenge over on our forum, Go there, and I wanna hear your take on FM bass. Download dexed or use whatever synth you have, and then post your patch there and talk about it a little bit. I wanna hear what you can do. And while you’re at that forum you can also create new topics and request other patches. So if there’s a particular patch from a song you wanna know how to make, post there, and I or someone else will help you make that patch. And of course, subscribe to our YouTube channel to get more videos like this.