FM synths - Anti-aliasing/Oversampling

DSP, Plug-in and Host development discussion.
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Smashed Transistors
KVRist
132 posts since 10 Oct, 2014

Post Mon Feb 26, 2018 3:33 pm

Integration/derivation also induce phase changes.
They are not critical with sine waves and considering the f scaling is enough.

I think that phase matters very much with complex waveforms as modulators.

For example, using a sawtooth as a modulator leads to something that is similar to osc sync sweeps. If you use a waveform with a similar spectrum as the sawtooth but with different harmonic phases, the result is much different.
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mystran
KVRAF
5032 posts since 12 Feb, 2006 from Helsinki, Finland

Re: FM synths - Anti-aliasing/Oversampling

Post Mon Feb 26, 2018 4:53 pm

Smashed Transistors wrote:Integration/derivation also induce phase changes.
They are not critical with sine waves and considering the f scaling is enough.
Well, right.. so multiply (or divide) the spectrum with i/f instead. :P
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S0lo
KVRian
629 posts since 31 Dec, 2008

Re: FM synths - Anti-aliasing/Oversampling

Post Tue Feb 27, 2018 10:16 am

Another thing that can cause severe aliasing is feedback modulation. That is, OP1 modulating OP2, then OP2 modulating OP1. This can very quickly go too wild and end up producing just useless noise. I personally prevent this by limiting the modulation amount in one direction and allowing it to be large in the other direction. But someone else may have better ideas :idea:

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Smashed Transistors
KVRist
132 posts since 10 Oct, 2014

Re: FM synths - Anti-aliasing/Oversampling

Post Tue Feb 27, 2018 10:57 am

The feedback aliasing problem is sometimes called "hunting", it occurs even when there is only one oscillator when the feedback coefficient is big enough so that the system becomes chaotic with bifurcations.
To limit this effect you can use a simple "anti hunting" filter (as described in Yamaha's patent...) try to put a basic filter like y = 0.5f * (x + y) in the loop.
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S0lo
KVRian
629 posts since 31 Dec, 2008

Re: FM synths - Anti-aliasing/Oversampling

Post Tue Feb 27, 2018 12:27 pm

Smashed Transistors wrote:The feedback aliasing problem is sometimes called "hunting", it occurs even when there is only one oscillator when the feedback coefficient is big enough so that the system becomes chaotic with bifurcations.
To limit this effect you can use a simple "anti hunting" filter (as described in Yamaha's patent...) try to put a basic filter like y = 0.5f * (x + y) in the loop.
hmmm, bit confused :? filtering an arbitrary wave (modulator) will obviously change it's shape. it's no longer THAT wave any more.

And if they're filtering sin waves!!! well, thats just equivalent to lowering it's amplitude, which is the same as lowering the modulation amount.

I haven't read the patent, so I'm probably missing something here.

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Smashed Transistors
KVRist
132 posts since 10 Oct, 2014

Re: FM synths - Anti-aliasing/Oversampling

Post Wed Feb 28, 2018 12:30 pm

It only filters the high end of the spectrum.
As it is fed back, it is no more a sine wave.
For example, when you feed back one oscillator on itself, as you increase the feedback the sine morphs into sort of a sawtooth.
Without the filter, some HF ringing happens.
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S0lo
KVRian
629 posts since 31 Dec, 2008

Re: FM synths - Anti-aliasing/Oversampling

Post Thu Mar 01, 2018 12:01 am

Smashed Transistors wrote:As it is fed back, it is no more a sine wave.
Your right. I don't know what I was thinking :dog:

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