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Difficulty recreating traditional FM waveforms

Moonbot7000 Member Posts: 61 Member
edited April 2022 in Reaktor

Im trying to follow some DX7 guides to learn more about how to create FM sounds, and I'm having trouble right out of the gate. I know PM is different from FM, but my understanding is that they are functionally the same. However, when I follow the guides for creating a Sawtooth (1:1 ratio) and Square (2:1 ratio) I don't get anything close. See screenshots below, are there ways to recreate these "synth recipes" using PM in reaktor?



  • drb
    drb Member Posts: 2 Member


    1) An oscilloscope is not as useful with FM as it is with subtractive synthesis. A spectrum analyzer is more useful with FM. The reason is the common shapes of subtractive synthesis (saw, square, triangle) depend on phase relationships among the partials of a tone. But these phase relationships are not in general the same with FM. Also, they do not in general make an audible difference. The tone (of a steady tone) is basically the partial frequencies and amplitudes, not phases.

    2) Take a look a the Player (NI, not user) / Blocks Base / Racks / Tutorial / Frequency Modulation.nksr . This is a simple two oscillator FM setup as close as possible to DX7 (using only two operators). Note that the oscillator on the right (the carrier) has "LIN TZ" under "FM" on the left side of the module. This is critical for acting like DX7 FM.

    3) I fairly sure the Bento Box Oscillator does FM, not PM. This does not matter much for a simple two oscillator synth. The DX does PM for the sake of cascaded modulators and feedback.


  • Moonbot7000
    Moonbot7000 Member Posts: 61 Member

    @drb Thank you so much for your response, very helpful :)

  • Studiowaves
    Studiowaves Member Posts: 449 Advisor

    fm produces harmonics, no one really cares what the wave form ends up looking like. dx7's never used subtractive synthesis.

  • Moonbot7000
    Moonbot7000 Member Posts: 61 Member

    Yes, learning all the time, I thought the guides literally meant it creates a saw/square, but you all cleared that up, thx!

  • Studiowaves
    Studiowaves Member Posts: 449 Advisor

    Basically any waveform is made up of sine waves if the frequency spectrum. Theoretically you can add sine waves with the proper amplitude and phase relationship and replicate any wave form. It's pretty interesting how an oscilloscope shows a picture of the wave form and a spectrum analyzer shows the amplitudes of the sine waves that make up the wave form. Have fun learning.

  • colB
    colB Member Posts: 754 Guru
    edited April 2022

    I know PM is different from FM, but my understanding is that they are functionally the same

    FM and PM are very closely related

    Phase modulation is directly modulating the phase, Frequency Modulation is modulating the (per audio tick) change in phase...

    So to get the same result from FM as you would from PM you would have to integrate the modulating signal before applying it (...I think, maybe you differentiate it - I haven't had my coffee yet)

  • gentleclockdivider
    gentleclockdivider Member Posts: 101 Helper
    edited April 2022

    You 'de get a lot closer to phase modualtion FM if you set it to linear FM , but bento FM does NOT do phase modulation

    Also , to get a perfectly straight saw or square on yamaha fm synths , some feedback on the modulator is required

    Here's anexample using the wonderfull plogue ops7 .

    And if you phase shift the modulator with 90degrees you get triangle waves (same harmonics as square ) , too lazy to fire up my tg77 :)

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