Fm and mod_fm divisor and depth opts

Hello everyone. I’ve always wondered about these opts as the description says “Don’t worry too much about these”!

Can anyone provide a quick and a little more detailed explanation? Is it a simple decimal multiplier (divider), hz, midi note or something else? I have been recording examples to try and figure it out by myself but am confused by what I see in the spectrum analyzer. I’m also having trouble identifying how the depth option modifies / interacts with the divisor and carrier (note).

The sound research I’m doing relies on being able to control / predict harmonic content.

Thanks for any clues!

Hi @coodge

The divisor is the modulator and, in FM terms the divisor is also known as ratio.
Depth can be thought of as strength i.e. how much of the modulator (divisor) affects the carrier (the oscillator you hear).

Sound On Sound’s intro to FM might be a good initial read. The type of FM synthesis using SPi is a sort of two operator (2 OP) carrier/modulator FM.

I’ve covered a bit of this in a patch here. If you watch the vid then you’ll see I go through divisor/depth as well as some of the different env_curves which, I suspect, you need to look at for your research as well.

You might want to consider using a more comprehensive visualizer but you will need to record the audio and open it in another application here.

Hope that helps.


Thanks @Hussein Very good explanation and links. I’ve read the Sound on Sound intro (thanks for that) but will have to read again to absorb more. I was a radio tech in a previous life but that was 30 - 40 years ago. Your video is also very helpful. I’ll experiment some more and see how I go.

I’m currently using a couple of the free visualizers from Melda Production FreeFXBundle:
MAnalyzer and MStereoScope, after loading the recording into AudioMulch.

Thanks for taking the time to reply, much appreciated.

Hi @coodge glad it helped.

The Melda plugins are great. I only recommended Sonic Visualiser as I wondered if you were using the visualiser in SPI. Envelopes are really important with FM, as is managing amplitude (VCAs) of not just the carrier but also the modulator. Glad the video helped.

The mod_fm synth gives you greater control over these and more parameters, so possibly more useful for your research? From my perspective I’ve tended to find the collection of mod_ synths to be less useful for my aims.Might be different for you.

Good luck. The joy of FM and PM!



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