FM Operator modulation index?

Hi! Does anyone know how the “FM amount” param in the FM operator works, and how it relates to the modulation index, and what the maximum modulation index is. I noticed that to get aggressive metallic sounds I have to put a “Scale” block before the FM input.

For FM8 for example, the info was posted in here:


  • No one knows? @giku? After some tests I would assume that the FM input signal is simply multiplied with the "FM amount" and then added as a phase offset to the FM oscillator - is that right? If so that would mean that the maximum modulation index would be equal to 1. For modulator -> carrier FM we can get higher modulation index using a scale module, but this workaround is not possible for the feedback. It would be great if the range of the FM feedback (and amount) could be increased - this would probably require making the scaling nonlinear to get a useful behavior.

    One simple alternative: Since FM amount and and feedback can be modulated, perhaps just remove the range checks for the modulation of the parameters? In that way we could get higher FM feedback (and FM amount) by modulating using a fader - including our own preferred non-linear behavior. :)

  • edited October 21

    Yes you are right FM amount is FM index. I can increase range if you feel its not enough.

    As about self feedback. Yes going further would result in a digital / harsh noise. There is one problem, this noise sounds very different depending on sample rate. (its not a problem in hardware like Digitakt, when SR is constant). It may sound trivial, but needs some extra work and CPU :) ( working at constant sample rating = resampling)

  • Im very interested in getting the digitone FM noise in these extended ranges as FM noise is in tune unlike noise. However the developer as he mentioned to you has some technical hurdles.

  • edited October 21

    Thanks for the answer! Yes, I see that the feedback would be problematic since Drambo need to support different sampling rates. Probably overkill to run at fixed rate and always do sample rate conversion on main out - but might be the only way to guarantee identical sound. :-/

    Regarding the range, yes a higher range of modulation index would be very useful - for the DX7, the maximum is ~13 - but the range is not linear (its quadratic), with 70% of the parameter range covering index between 0-1. For the DX21, the maximum index is even higher at ~25! If you have access to Chownings "FM Theory and Application" the details are in Appendix 2.

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