[sdiy] Re: Spiral Waveforms

[harrybissell]

Without regard to anyone's personal issues...  I don't think any apologies are
in order here at synth-diy.  We have a wide spectrum of people, with different
personalities.. but we share one thing in common - an interest in technical
discussion
about synth-diy.

I took the spiral waveform issue VERY seriously at first on Analogue Heaven.
Because
I subscribe in DIGEST mode... I was not aware of the responses to people who found
the ascii-matic posted there.

I made an offlist suggestion that an actual schematic, perhaps hosted on someone's
page
if necessary... would really help in the understanding of the concept. Several
other people
added links for those who (like me) never heard of a "spiral waveform".  I think I
speak
for SOMEONE (at least) on this list, who might have graciously posted a schematic
to serve
as the basis for intelligent discussion.

Mutual respect is the basis for any civilized discussion. Alleging that persons on
this list
are not intelligent enough to read an ascii-matic is a poor way to open a
dialogue... especially
when it seems that a large number of people, reasonably skilled in the art... found
the posted
schematics to be hard to read at best.

Concensus is an important concept... If I am the only one who holds an idea I'd
consider whether I was in fact... wrong...  if everyone else thought so.  That
would not of course MAKE
me wrong.   I wanted to see / understand the concept that Steven J proposed... it
could be/have been as significant as the idea that the earth is a sphere.  Seeing
all the other posts about the
ascii-matic problems (too late) gave me solace that I'm not the only "false
portrayer" on the list.

Then the feeding frenzy began.  Those who thought that synth-diy started on the
wrong foot have
to understand that a major 'occurence' had just happened on AH... to which many of
us subscribe as well.

OK, that said we can "play nice".

The spirals that Magnus referrs to in the SVF example have some similarities to the

"lorenz attractor". Looked at on a scope in x-y mode we have a sort of pseudo-3D
plot of them

As Scott pointed out - our normal methods of waveform synthesis start with linear
sources...
simple because they are the easiest to produce - lot of bang for little bucks (USA
- your
currency may vary).  Linear charging is not the only possible method... we could
use some
arbitrary curve as well. The natural RC constant is one possibility (done that) and
we could
generate charging curves to any arbitrary function we liked.

The advantage of linear... is that most of the nasty side effects of other methods
are not there /
cancel out.  We make oscillators the way we do, because we can.

A nice idea might be to try and synthesize a spiral wave using some digital
method... and then
make an MP3 or so and listen to it.  Does it sound interesting ???

H^) harry

Magnus Danielson wrote:

> From: Scott Gravenhorst <music.maker at gte.net>
> Subject: [sdiy] Re: Spiral Waveforms
> Date: Sun, 11 Apr 2004 07:49:58 -0700
> Message-ID: <200404111449.i3BEnwg13072 at linux6.lan>
>
> Hi Scott,
>
> > First, I think that everyone was caught off guard by Steven.  No one knew
> > until days later why he responded the way he did.  After reading about AS,
> > his posts make more psychological sense.  I doubt the group reaction would
> > have been as it was had we known, I don't believe that anyone here is
> > intentionally cruel.
>
> Now you've got me off track here. This thread has been so full of feelings that
> I've stayed out of it so far. What am I missing?
>
> (BTW. I'm sad to see so much reactions here, this used to be a quite little
> heaven but if people are going to continue like this... no, now.. behave!)
>
> > That said, here are my random synapse firings...
> >
> > Ian's description notes that a constant angular velocity yeilds a constant
> > output frequency,
>
> Which in my mind is really the definition of frequency. Given the phase-state
> you can get all other aspects of time-variance from derivations of more or less
> complex style.
>
> > which is a good thing for tonal music.  But this doesn't
> > seem all that easy to do in analog electronics.  Sawtooth and triangle
> > oscillators try to do this by using a linear current to charge a capacitor.
> >  The sawtooth has the nasty reset time that perturbs constant angular
> > velocity.
>
> Well, we try to compensate that out.
>
> >  I'm not sure, but triangle oscillators may also exhibit this to
> > a lesser degree at ramp direction reversal time due to the finite amount of
> > time it takes to switch the current source's polarity.
>
> Yes, but usually you should be able to balance the positve and negative
> reversal times fairly well. It is now the difference between them which is
> important for outgoing frequency, so the first-degree error is by default
> compensated out.
>
> > Once we get away from linear ramps, things get difficult.  Someone correct me
> > if I am wrong, but sinewave oscillators don't operate by following a dot as
> > it goes around a circle with constant angular velocity.
>
> You really *SHOULD* take a look at how a sine/cosine oscillator built around
> a state-variable works, since it actually does exactly that, follow the dot
> around. Here there is an error, and that error is in the difference of gain in
> the first and second integrator. Also, the integrators isn't perfect
> integrators but has some leakage. This causes the circle to be slightly
> elliptic and slightly titled.
>
> > Of the difficulties involved, it seems to me that there are two main
> > requirements: 1) a circuit to establish and maintain the spiral and
> > 2) a circuit that can provide a projection of the spiral placing a "light
> > source" at a given angle.  One way to approach this might be the use of
> > quadrature, would simple panning provide the projection?  If one distorts the
> > quadrature outputs, the angular velocity is maintained, but would panning
> > still give a (variable) projection?  It seems that this idea might be more
> > easily implemented using DSP or other digitally computed methods.  I can't
> > envision how much different this would be from what we already do with
> > oscillators and waveshapers.
>
> I have an old article on how to make oscilloscope art using Paia modules.
> In essence you hook up the scope at the different outputs of a Paia filter
> module (a state-variable). Creating a spiral waveform isn't very difficult,
> since you just send in a tick and then the high-Q setting allows the filter to
> ring and the two out of phase outputs will provide the X and Y waveforms and
> forming a spiral.
>
> The higher Q, the less energy the ringing loses per cycle, and thus the denser
> will the spirals be.
>
> You don't need any DSP to do this, I could have my SEMs do it for me! Or for
> that matter, the ASM-1. If you have an ARP 2500 standing in a corner collecting
> dust I'm sure you can do it with that too, since it too has a SVF filter.
>
> Is this what you guys have been fighting about or what?
>
> Cheers,
> Magnus - puzzled