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FM Synthesis - n00b question

Postby OscBombs » 30 May 2012, 04:02

Hello all!

I'm a n00b with FM Synthesis. Trying to learn the principles involved with this, saw this vid which was a nice intro to the game:

http://www.sonicstate.com/articles/article.cfm?id=148

I'm curious on one thing that comes up in the vid around 3:45 as relates to the Wave... On the Wave, it looks as though when I adjust the "Shape" knob, that's effectively increasing the volume of the modulator. However, is there also a way to change the pitch so it changes which overtones are boosted?

Just to clarify, I know that by using Osc Mod > FM, I can achieve this by adjusting the pitch of oscillator 2. However, just using the FM algorithms selectable on the LED, are there any ways to adjust the pitch of the modulator in, say, one of the 2-operator algorithms to that I can produce inharmonic frequencies in the overtones?

Also, can anyone recommend a good resource--book, site, video, etc--sharing some FM tricks (EG how to create FM strings, FM brass, etc).

Thanks!
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FM Synthesis - n00b question


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Re: FM Synthesis - n00b question

Postby mjbrands » 30 May 2012, 21:09

I think the video you linked is one of the better sources. For me FM is a bit like voodoo. I just toyed around on my Stage 2 but the sounds I got sounded a bit like a combination of a whale song and a small dog being sat on by its fat owner. Horrible enough for my cat to decide to leave the room rather than having to endure it. :lol:

I think the shape knob changes how much the modulator influences the carrier (and how much feedback there is) for that particular oscillator; unless you use Osc Mod, the shape knob for OSC2 will never have any influence on OSC1 and the shape knob for OSC1 will never have any influence on OSC2. This is what I get from the diagram on the bottom-left of page 22.

Image

Directly above this diagram it says: The Shape parameter is used for setting the amount of modulation and/or feedback from none to maximum.

I think this is the same as what you describe (increasing the volume of the modulator).

If you use FM on an oscillator, you will actually have a carrier, a carrier + modulator or a carrier + 2 modulators running on a single oscillator. So you could actually have two FM sounds in a single slot (one on each oscillator) or four by using both slots. If you set OSC1 to a sine wave (either using 1-OP FM or by using the shapable sine options from MISC) and use Osc Mod, I think you're actually running 2 operator FM (OSC2 does not need to be a FM sound itself, but it could be). However, from this diagram it appears you influence just the carrier, not the modulator. So if you set OSC1 to 2-OP FM, you'd probably have two modulators modifying the same carrier instead of OSC2 modifying the modulator of the carrier on OSC1.

The diagram in the section on Osc Mod (page 24) just shows OSC2 modulating OSC1; it doesn't show or suggest what it modulates (the carrier or the modulator(s)).

Btw. have you looked at the Mod envelope? Could that be of any use?
Last edited by mjbrands on 31 Jul 2012, 12:32, edited 2 times in total.
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Re: FM Synthesis - n00b question

Postby OscBombs » 31 May 2012, 21:22

Haha. Hilarious. I guess it's true that the Dutch speak English better than Americans! :) I've become decent at producing tortured FM whalesongs as well.

The best I can figure out so far is that, going through the FM algorithms, you tend to create higher overtones when increasing the ratio of carrier to modulator. But I'm still on the hunt for any tips on how and why to apply any of the particular ratios, when to use feedback, etc.

You're right that the shape knob only effects the particular oscillator, unless you're using Osc Mod, in which case the shape, semi tone control, and fine tune control of osc 2 all effect the tone of osc 1.

Per your question, I believe that, when you're using Osc Mod, Osc 2 influences the "carrier" of Osc 1, since the "carrier" refers to what you actually hear, and you can set up Osc Mod regardless of whether you have an FM algorithm selected as your wave type for Osc 1 (and using FM terminology, all of the wave types have "carriers", but only the FM algorithms would also have "modulators"). I believe there is no way to effect the modulator volume or frequency of the modulators within the FM algorithms... although since I'm not very familiar with FM synthesis, I don't know if this is a relevant limitation.

I did try out the modulation envelope to help control FM, and it does help add expression. BUT, one limitation of the Wave is that the envelope can only be routed directly to Shape 2 (and not Shape 1). You CAN use LFO 2 in Single Cycle mode to do something similar, but you have less control over Attack/Decay than with the envelope where you can set those values independently of each other.

Something nice I discovered (and many probably already have) is that, when using Osc Mod > FM, and then detuning the Fine Tune of Osc 2, it creates a pulse (probably a natural phase effect), almost like an LFO is modulating the sound.
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Re: FM Synthesis - n00b question

Postby mjbrands » 01 Jun 2012, 09:05

As I understand it (from the video you linked) a modulator can affect either another modulator or the carrier on a dedicated FM synth like the DX7. The Nord synths (and probably most other virtual analogs that do FM) are more restricted and a modulator can only modulate the carrier (and via feedback itself, which can be turned on/off).

My DSI Poly Evolver also does FM on the digital oscillators and while it has even less features than the Wave when it comes to FM, you can create some pretty cool sounds when using feedback. I toyed around with that a bit yesterday, as it can do more FM tricks than my the Stage 2.

Edit: it seems there is a trial (demo) version of Native Instruments' FM8 synthesizer. It has a really easy to use interface and is a really powerful FM synth. Might be interesting to look at. http://www.native-instruments.com/en/su ... -versions/
Last edited by mjbrands on 31 Jul 2012, 12:32, edited 3 times in total.
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Re: FM Synthesis - n00b question

Postby OscBombs » 01 Jun 2012, 16:31

Thanks for the link! I've heard about that soft synth, wanted to check it out. Have to put it on the back burner, so I can focus on recording (the eternal struggle with gear vs work!!). But I think something like this is the only way to understand FM. Till then, my working knowledge is: (for the most part) that the higher the ratio of modulator to carrier, the higher frequency the overtone is, and the more "amount" you add, the more you boost that overtone. This isn't always the case, and I still have almost no idea what's going on in the 3-Op algorithms--whether it's a linear link from modulator 1 to modulator 2 to the carrier, or if there are direct links from both modulators to the carrier...so more investigation needed.

Also, I've been reading this *excellent* series to help understand synthesis in general, and here's an installment about FM: http://www.soundonsound.com/sos/apr00/a ... ecrets.htm

It's all very theoretical and often gets down to trigonometry to explain the math behind the sounds, but it's really great, and there are 40-50 articles like this.

BTW, love the Poly Evolver. I spent a few weeks deciding between that and the Wave, but ultimately picked the Wave because of the samples and polyphony. I don't regret that decision at all, but I do love that blue synth! Now, if they only come out with a desktop version of that...
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Re: FM Synthesis - n00b question

Postby mjbrands » 04 Jun 2012, 01:06

OscBombs wrote:Now, if they only come out with a desktop version of that...

Image
Or did you mean with the 4-voice polyfony?
Last edited by mjbrands on 31 Jul 2012, 12:32, edited 2 times in total.
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Re: FM Synthesis - n00b question

Postby OscBombs » 05 Jun 2012, 00:13

Yeah, I meant the poly. But nice thought!
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