FM or Frequency Modulation Synthesis is based on the use of at least two oscillators; a Carrier and a Modulator. The Carrier Frequency is most related to the pitch or the fundamental frequency and is usually controllable with a keyboard or other MIDI input. The Modulating Frequency is used to alter the Carrier at a specific Modulation Rate and Modulation Intensity or Index - (Mantione. P, 2017)
The above image is a visual example of the way FM Synthesis works. Basically, the first waveform is the waveform that's going to be altered and is therefore called the "Carrier", and the second, larger waveform is used to alter the first, called the "Modulator". The third example is the result, the larger waveform has "spread" some sections of the frequency out. The maths of getting this kind of process is complicated, and to be honest it's a bit beyond me. If I spent a bit more time researching I could probably get a grasp of it but it doesn't feel very necessary for my production. I think my best course of action is to experiment! While researching a little bit about FM synthesis I came across a YouTube video about using FM synthesis in one of my favourite synths, Serum.
The FM options in Serum are as shown above. Each oscillator has the option to use either oscillator A or B, the noise oscillator, or sub oscillator as a modulation source. For example, I can can set the modulation source for OSC A as OSC B and then turn OSC B's level all the way down so we can only hear the modulation in OSC A rather than the actual wave form.
I'm going to try basic patch using OSC B to modulate OSC A, then I'm going to create a patch that uses OSC A as the carrier, OSC B as the modulator with the noise oscillator modulating OSC B. Then I'm going to try a patch with all sorts of weird combinations and see what I can get.
First Patch
I'm going to keep things simple for this first patch by just using a sine wave as the carrier and another sine wave as the modulator. The wave tables I've chosen for this to mimic a sine wave, but as we move through the wave table it changes the harmonic content slightly so that's what I've done for OSC B. Then I decided to see what would happen as I changed the pitch of the modulating oscillator. What I found is that it almost sounds like it changes the formant of the sound, and after some messing around I settled on having it two octaves and 5 semitones lower than the carrier. So far the sound is dissonant and erie which is exactly what I like, so I thought, why not make it a bit scarier by setting OSC B's unison control to 3 voices and modulating the detuning of it with an a slow LFO. This wasn't quite "scary sounding" enough so I took the carrier oscillator down by an octave and finally I arrived at the dark brooding, squealing siren sound I wanted, here's what it sounds like playing a low octave C with a nice big reverb added to it using Serum's FX panel.
Pretty basic to start with but good results, lets see what we can get with some different wave tables and FM from noise.
Second Patch
I'll be taking a similar approach to this patch as I did the last I'll have OSC A as the carrier and OSC B as the modulator, only this time I'm also going to modulate OSC B with some noise from Serum's noise oscillator. Basic set up almost exactly the same as before, only I haven't change the pitch or unison of OSC B as I'm relying on the noise I add to create some interest in the patch. I've also decided to a use a bit more of a complex wave table as my carrier, just to mix things up a little. I cycled through the many options in the noise panel until I came across one called "Inharmx 2", it sounds like a pretty basic noise but isn't quite white noise so I went with it. Noise modulating the modulator oscillator sounds crazy, and even crazier once I start modulating the pitch of the noise with a slow LFO like before, here's how it sounds.
Sounds a bit like R2D2 having a stroke, and I kind of love it. Making this patch has got me inspired, the next patch I make I'm going to try and get some really interesting out of these modulation techniques while incorporating some modulation from the bass oscillator as well.
Third Patch
I love the different textures I'm getting out of these patches, but I can't help but feel like they all start to sound quite similar. I'm wondering if this is because Serum isn't built to be a FM Synth; something like FM8 (which is actually made for FM Synthesis) would be a hell of a lot more interesting to mess with, but just wrapping my head around the concept of FM synthesis and what it does is a bit less daunting in a synth I'm already familiar with.
This patch is basically a combination of the last two, but instead of using the noise oscillator to modulate osc B I used the sub oscillator. The sub oscillator just generates basic wave forms like sine, triangle and square waves so I think it's probably useful as a modulator for subtle tweaks to a sound. With this I basically just changed it until I thought it was adding enough interesting texture to the original modulator, and to do so I've pitched up the sub oscillator by 4 octaves. This patch has kind of a percussive element to it and that's mostly to do with the LFO I have running on a low pass filter. The sound itself was just a washy, airy sort of sound and needed a little bit more variation.
Just by going through these different options and experimenting I think I've unlocked a new way of using one of my favourite synths, however I think there's more to be done with FM synthesis. Maybe my next step would be actually looking into the maths behind FM to try and really understand how it works. But for now, I have a new tool to use in my productions and I have a lot more experimenting to do.