Author Topic: general FM synthesis question  (Read 2482 times)

fori

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general FM synthesis question
« on: March 09, 2021, 04:21:37 PM »
Hi All,

This is my first FM synth with non-sinusoidal operators. I am trying to wrap my head around how it impacts the harmonics when you have something like a sawtooth wave as a modulator. I wondered if you all have general advice for how to use / think about other wave forms as modulators? I'm guessing there are common tricks/uses for them?

For example, let's say I have a simple 2-operator patch with a sawtooth wave modulating a sine wave at a 1:1 modulator:carrier frequency ratio. A sawtooth wave has every harmonic above the fundamental with progressively decreasing amplitudes. Am I then right to think of this modulator as a bunch of sine modulators in parallel with frequency ratios of 1:1, 2:1, 3:1, etc. and progressively decreasing amplitudes?

matrix12x

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Re: general FM synthesis question
« Reply #1 on: March 10, 2021, 02:56:18 AM »
I think that is an accurate way of thinking about it.


Xavier

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Re: general FM synthesis question
« Reply #2 on: March 10, 2021, 08:29:19 AM »

Hi,
It's not easy to guess what "non sinusoidal" waveform does in FM.
If you can, plug the preenfm2 audio jack in your computer's soundcard, and use something like Voxengo Span to see the spectrum as you change you modulation.

fori

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Re: general FM synthesis question
« Reply #3 on: March 10, 2021, 05:51:28 PM »
If you can, plug the preenfm2 audio jack in your computer's soundcard, and use something like Voxengo Span to see the spectrum as you change you modulation.

That's a good suggestion, but I have not been able to get VST plugins to work for me. I don't have a DAW.

Are you saying that the effecct of non-sinusoidal waveforms as modulators is more complicated than I was suggesting?

TanaBarbier

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Re: general FM synthesis question
« Reply #4 on: April 21, 2021, 10:33:15 AM »
Hi

Basically it is what you are saying, but keep in mind that when using ratios like 1:1, 1:2, 1:3 a lot of sidebands fall against each other, and as a result what you hear at one frequency is the sum of two sidebands of different amplitudes and opposite polarity, as they come from two different pairs. So now you add harmonics to the fundamental of the modulator, those will add new sidebands that will fall against those generated by the fundamental, so the resulting amplitude is even more complex.

Trying to calculate is going to be really hard, but rule of thumb will work:
As you add harmonics to the modulator (or the carrier) it is as if you had a new FM interaction creating its own sidebands. As a matter of fact those harmonics are always of higher frequencies that the fundamentals, and because of that the new sidebands tend to appear at higher frequencies too. Typically a saw tooth has energy up until quite a few harmonics, and those will generate sidebands "at" their own frequencies, so higher.
If you see the sawtooth or square wave (or whatever wave) in an additive way, it is a lot clearer I think. Each harmonic of the mod "does" its own FM with each harmonic of the carrier, and each one of those interaction has exactly the sound you expect, but the result is the sum of a LOT of those FM interaction falling against each other, so it gets hard to predict where you'll have peaks and valleys in the resulting spectrum.

Something I find interesting is that higher harmonic content for mod or carrier tend to add a lot of higher frequencies sidebands, which tends to get "noisy", but on the other hand when you use non integers ratios like say 1 : 2,37 and use two "rich" waveforms, it tends to kind of hide a little bit the "noisy" side of sidebands repartition due to the inharmonic ratio.
« Last Edit: April 21, 2021, 10:35:46 AM by TanaBarbier »

served

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Re: general FM synthesis question
« Reply #5 on: April 28, 2021, 08:47:22 AM »
I am using Virtual Analizer for spectrum at my workshop. Works great! Try it, its free.