FM Oscillator Shootout on Linear mutual FM

On another thread we were talking about the difference between the FM sound of different oscillators when they modulate each others.

As an exercise I put together a patch with 4 different VCOs in this ‘one operator feedback loop’ topology. You can solo or mute these each to compare their sound.

The discussion - on my part - was to talk about how hardware oscillators sound one 2 oscillators modulate each other’s Frequency. Ultimately my goal was to make work :smile: for @modlfo use his own tools (and giant brain) to model analog oscllators with mutual linear FM, so they would sound closer to actual Analog oscillators.

You can solo each and try and judge how simiilar & different they are. My judgement is that Kitchen Sink, FM-Op and Pigeon Pink have similar sounds in this topology, but each has it’s own flavor. The VCV VCO is different, in that unlike the other 3, you get pitch changes in addition to adding harmonics with FM.

Which leads me to believe that whatever VCV VCO thinks is linear FM, it doesn’t sound like what other modules think it should.

Not one of them is better or worse, just different. FM on the VCV VCO isn’t as useful musically - where you want the pitch to be steady as you increase the mod index. The VCV VCO is better at the kind of wonky FM sounds that are good for synthetic percussion.

The funniest ones to compare are Kitchen Sink and Pigeon Pink, which is based on Squinky Kitchen sink, but sounds noticeably different even with similar settings for the mod index.

FM-Test.vcv (6.7 KB)

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I always wondered why the FM I get in Serum sounds quite different to the FM I get in Rack… and I’m just talking about simple FM with a modulator and a carrier.

What you’ve found is that among those oscillators, VCV VCO is the only one that actually performs linear frequency modulation. Most modules that advertise linear FM actually perform “FM” as popularized by Yamaha, which is really phase modulation and not frequency modulation.

A few oscillators can do both FM and PM, such as Valley Terrorform (enable the True FM button to switch).

Check out my “FM Test Patch” thread for lots of audio comparisons comparisons along these same lines:

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Phase modulation tends to sound 'better" than true FM. But even the PM in Rack sounds quite different to the PM in Serum and I’m interested as to why that would be.

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Chowning’s whole implementation is so simple it fits in a few kilobytes and runs on 80s vintage cpus. Other more sophisticated implementation might sound very different.

One big difference between most FM VCOs and the fundamental is that most use phase modulation, whereas Fundamental uses FM. FM is (mathematically) PM if you rolled off the modulator with a 6db / octave lowpass. So FM does sound quite a bit different, even it it’s rarely used. I used to have a long paper on this topic, but unfortunately I took it down in a fit of pique.

Another reason ppl rarely use “FM” is that it tends to go really out of tune if there is any DC at all on the modulator, which there almost always is. You can easily see this yourself. Let me see if I can find an illo from that old paper… oh, here it is, out of context:

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Of course aside from the tuning issue you can see that the spectra from PM is quite different than from FM.

that was a sad fit that made you take down those highly educational posts. Hope you may add em again when you are over said fit :wink:

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