[sdiy] Generating acyclic waveforms?

[cheater cheater]

Hi guys,
thanks for the emails. I'm really happy this has drawn such
interesting discourse from you guys.

Magnus,
I am wondering whether this feedback delay you mention cannot be
simulated (to a degree good enough for our purposes) with a simple
capacitor working as a simple trigger delay. If so, then it could be
varied by changing the amplitude of the triggers going into it. Of
course this is not ideal but we're not looking for the ideal string
model, just something that will do anharmonic oscillation.

I'll give those papers a read, thanks for the links. They look very
interesting; however don't seem to mention the analog network that you
describe - can you point to some resources for this thing? I am
assuming you mean something similar to the last two images here:
http://paws.kettering.edu/~drussell/Demos/reflect/reflect.html
I was unable to find information about an analog electronic way of
simulating this.

I am wondering whether a form of self-resonating filter would not be
able to do stretched harmonic waveforms. If you look at it from the
frequency perspective, it can be thought of as a phase delay, the
value of which can rise with frequency (for an LPF). In that case, the
higher partials fed back would get delayed over and over with every
cycle of the feedback and therefore would end up being pitch shifted
down, right? And similarly you could probably arrive at something that
pitch shifts the partials up to simulate the harmonic sharpening that
Veronica mentions. However, I cannot think of a filter that would
preserve higher harmonics like that; plus, of course this requires a
different mode of resonation as the usual immediate feedback, because
that only gives resonance at the pi phase shift point (if I remember
correctly being pre-breakfast, please someone correct me if I'm wrong)

Maybe this bears a clue of some sort, disconnected from the physical
simulation approach:
http://en.wikipedia.org/wiki/Resonance
"A physical system can have as many resonance frequencies as it has
degrees of freedom; each degree of freedom can vibrate as a harmonic
oscillator."
The question is open, how do you create an analogue filter with
multiple degrees of freedom?

Thanks
D.

On Mon, Mar 22, 2010 at 05:10, Magnus Danielson
<magnus at rubidium.dyndns.org> wrote:
> cheater cheater wrote:
>>
>> Magnus,
>> How does the effect you describe work? Does it have a name? Do you
>> think that it can be used for audio-rate oscillators, even if not
>> necessarily with voltage controlled frequency?
>
> I think you need to look into physical modeling.
>
> The issue is that the phase-delay or for that matter group delay at the end
> of the resonant cavity isn't perfectly matching up, such that the effective
> resonant length is somewhat different or for that matter the resonant
> period.
>
> The impedance error at the end of an open cavity and the way the effective
> reflection
> behaves in the time-plane shifts over frequency. The shape of the mouth is
> of interest in the acoustical sense.
>
> Physical modeling using two delays (to simulate the two directions of the
> wave) and then analogue networks for reflection impedance simulation would
> pull it off. It is however not very near out normal VCOs in architecture.
>
> Scott Gravenhorst have look at similar things:
> http://home1.gte.net/res0658s/FPGA_synth/
> http://www.fpga.synth.net/pmwiki/pmwiki.php?n=FPGASynth.DigitalWaveguide
>
> https://ccrma.stanford.edu/~jos/swgt/swgt.html
> https://ccrma.stanford.edu/~jos/swgt/Wind_Instruments.html
>
> If you want to go in deep:
> https://ccrma.stanford.edu/STANM/stanms/stanm39/
> (185 pages, acoustical impedances and all)
>
> In essence, for the fundamental and for instance the 3rd overtone to be
> perfectly harmonic to each other, their round-trip delay needs to be the
> same. If the group delay is different (relevant for noise input when
> blowing), they will not be perfectly in tune but somewhat shifted.
>
> Cheers,
> Magnus
>