feedback fm


    Nov 12 2007 | 12:20 am
    Hello all,
    I've been trying to create a one-oscillator feedback FM patch based on the Roads book, but obviously I'm missing something. I scoured the Internet, and haven't found any clearer explanation either.
    Any suggestions? A patch is below, it produces something, but I'm sure it's not what's intended! Thanks, Inouk

    • Nov 12 2007 | 6:07 am
      On Nov 11, 2007, at 4:20 PM, Inouk Demers wrote:
      >
      > Hello all,
      >
      > I've been trying to create a one-oscillator feedback FM patch based
      > on the Roads book, but obviously I'm missing something. I scoured
      > the Internet, and haven't found any clearer explanation either.
      One thing to bear in mind is that, in the digital domain, there is no
      feedback without at least some latency, and this latency is going to
      put strange effects into the phase relationship of the feedback FM
      that are totally different to the analogue domain.
      In your patch, there are two points of latency - one is the send~/
      receive~ that will in this case introduce one vector size of latency,
      and the other is the fft~ which will incur your fft size of latency,
      in samples. If your vector size is e.g. 128 then there's 1152
      samples total, which corresponds to 26ms. That means a 360 phase
      delay at 38Hz, double that delay at each octave above.
      Not sure what sonic effect this will have though.
      An option to get around this (or minimize the delay at least) is to
      run the feedback fm inside a poly~ with vectorsize set to 2, and
      reading your phase directly from a driving phasor~ rather than using
      fft.
    • Nov 12 2007 | 8:37 am
      > I've been trying to create a one-oscillator feedback FM patch based on the Roads book, but obviously I'm missing something. I scoured the Internet, and haven't found any clearer explanation either.
      Does roads actually suggest to phase information of an fft and use that in the signal domain? Since you name yamaha, I think that model below comes close to what the dx7 does. Also I think that vector delay is not a problem at all for this application.
      _
      johan
    • Nov 12 2007 | 3:01 pm
      Here's another effort. I don't have the Roads book in front of me, so I can't be sure this is what he specifies. I would bet $$ this is a simplified version of a patch previously posted to the list.
    • Nov 12 2007 | 9:50 pm
      Why?
      Try running your patch with pitch at, say, 20, and offset at say 0.2,
      feedback at 0.25.
      Now change the vector size and hear the differences.
      They sound kind of cool... but since the vector delay has such a
      strong impact on the sound, I think it should be incorporated as a
      parameter itself; e.g. place a comb~ or tapin~ tapout~ in the
      feedback loop.
      Try this:
      max v2;
      Hey, why not also make the delay time variable by feedback too to
      make it doubly nonlinear...
      On Nov 12, 2007, at 12:37 AM, jvkr wrote:
      > Also I think that vector delay is not a problem at all for this
      > application.
    • Nov 12 2007 | 11:10 pm
      Thanks so much for the replies everyone.
      Johan, you seem to have two things being fed in to the phase input of cycle~ (thanks for that: I was feeding back into the primary input), yet each produces different things.
      The Roads diagram only shows the feedback being multiplied by a modulation index before being added to the 'offset' frequency. That seems to match the right hand portion of your patch, which works on its own. The middle portion seems like a version of simple fm using the phase input. Do I have that right?