[sdiy] Favorite VCO options

[David G. Dixon]

> > The feed to all of them is the output saw, not the raw integrator
> output.
> 
> cool. I suggest adding a place just before the shapers where you can
> cut the traces and add a header/connector.

There's already headers which feed the raw (low output impedance) saw and
triangle to the panel, so I can select between them for the pulse and sine
shapers.  You can't route the output (1k output impedance) waves around on
the board because you'll attenuate the output (a lesson I learned the hard
way some time ago).

> Sync out should be, from what I understand, the pulse from the
> comparator when it resets.

Oh, I get you now.  However, this pulse is -15V to ground to be compatible
with the gate of an N-channel JFET -- not necessarily all that useful
outside of this particular circuit.  In any case, any module which can be
synced will have an input capacitor to differentiate the incoming master
signal anyway, and other circuitry to make it compatible for that specific
device, so there's really no need to bring out the comparator pulse.

> Try and see. You'll never know!

No, I probably won't!

> Hey, some people are pedantic!

Boy, you ain't kiddin'!  ;-)

> Linear FM has a very different effect than modulating reset position:
> it changes the slope of the wave keeping the peak-to-peak voltage the
> same. Modulating the reset level, on the other hand, keeps a constant
> slope but the peak-to-peak voltage changes. This creates a different
> sound. It's an even bigger difference with other shapes: because of
> the way the waveform is imperfect, the shaped waveforms will have
> glitches and other stuff - which makes it real cool. This lets you get
> a controllable version of one aspect of what makes vintage oscillators
> sound so nice and fat.

Yes, I understand what you're saying, but I wonder how different the two
effects really are.  The main effect is to add randomness to the period of
the saw, which either one does, albeit by a slightly different mechanism.  I
believe that the magnitude change associated with a noisy reference voltage
would be quite inaudible.  Since we perceive loudness on the log scale, tiny
linear fluctuations in amplitude will go unnoticed.  Couple that with the
fact that what you propose would be a major pain in the ass, and I think
there is a good argument for sticking with good ol' Linear FM!

> What I meant was the multi-core trapez VCO core, there was a thread on
> it last year. A triangle core has less tracking error in the high
> frequencies, and the trapez vco core even less. And there are other
> things you can do :-)

Again, that's a different animal entirely.  Stay tuned for the 2164 Expo VCO
Tricore (rising like a phoenix from the ashes of my last failed attempt).

However, I have been thinking (in my increasingly rare idle moments) about a
sort of "Oscillator Helper" module -- kinda like Hamburger Helper, but with
fewer calories -- which would provide a whole pile of outboard waveshaping
and mixing functions without cluttering up the actual VCO panel or board.
In know that "dotcom" has something similar, but mine would have more stuff
on it.

> > I always assumed that the Moog slaves were just synced.  I didn't
realize
> > they used common current sources.
> 
> I thought they did. Didn't they? I'm fairly sure they did :-) I should
> check though..

I've no idea.  I've never looked into it.  It would be a bit of a challenge
to do well, though, I should think.  The only place where I've ever used
current mirrors to replicate an expo current is in my 4-pole LPF, and there
the acccuracy is probably not all that critical.

> When you sync the oscillator, the wave should be at a phase between 0
> and 2pi, settable. It is *very* useful.

A variable sync does something like this by varying the height of the sync
pulse, and therefore the point along the saw ramp where the sync can occur.
Other than that, I have no idea how to do what you're talking about.

> a linear fm offset pot is just for when you don't want to go all the
> way connecting a dc source to a VCA/gain then to the lin FM input.
> Instead of wasting 2 modules and 2 patch cords, you turn the pot :-)
> 
> same with FM offset.

If you're talking about input attenuators, they're already there.  Other
than that, I'm not sure why you would route a dc source to an FM input.
That's what the 1V/oct input is for!

My one firm rule of module design is this:  Every input is attenuated, no
outputs are attenuated.  To me, this gives maximum flexibility.

> >> 8. CV scaling (it is useful to have this separate for each VCO) - to
> >> make octaves larger or smaller than 1V
> >
> > Why?
> 
> It can make the chords *huge*. Try it! If you have windows, try the
> 'oatmeal' VST synth, it has a knob for stretch tuning (= size of
> octave), I think somewhere on the upper right of the GUI. Just
> pressing C3 and C4 together makes a nice phasing sound. So you start
> with C-F-A and it's fairly inanimated and then you press the C an
> octave higher, and it suddenly changes the sound completely.

This can also be achieved by putting a VCA between the keyboard's CV output
and the 1V/oct input.  I've done that.  You can then modulate the VCA with
an LFO too, which is pretty groovy, or use an AR envelope to give "stretch"
for realistic (or not) plucked sounds.  I will grant you, however, that
having the tuning controls accessible from the front panel (even through a
hole for a small screwdriver) would be nice (but, alas, mine aren't).

> is the sigmoid going to be the same thing as sin(x) from -pi/2 to
> pi/2, repeated over and over?

Essentially, yes.  It is half a sine wave with twice the period which resets
like a sawtooth (and sounds an awful lot like one too).

> I'm not sure what the centre/edge pwm switch does.

It aligns the pulse either centre-to-centre with the triangle, or
left-edge-to-left-edge with the saw.  This does very little unless you are
combining different waveforms from the same VCO in a waveshaper (like a ring
modulator), in which case it is pretty useful.  I got the inspiration for it
by looking at the Oakley VCO front panel on their website, and I figured
that it was such an easy thing to do, why not?