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> From: Tkacs, Ken <ken.tkacs@...>
> To: 'MOTM Forum All' <MOTM@egroups.com>
> Subject: [motm] Module Idea -- Harmonic Bias Source
> Date: Thursday, April 13, 2000 3:43 PM
>
>
> Please bear with me on a long ramble (or you can just delete this Email

now,

> Mr. Phelps---it's always your choice).
>
> I've been thinking about an idea for a module that I'd like to throw out

to

> the group for merciless comment. This discussion probably wont interest

95%

> of you, and wont even make _sense_ to anyone that does not understand the
> relationships of partials (harmonics) and how they mix to create a sound

in

> additive/'Fourier-fashion'... but if created, I think many people would

see

> the value in it once they heard it.
>
> Anyway:
>
> This idea sprang out of Mr. Hendry's recently posted concept for an

octave

> switch a week or so ago. To refresh your memories, his idea was something
> like this:
>
> The MOTM VCO does not have an octave switch, so whenever you want to tune
> multiple VCOs to octaves-apart, it must be done carefully with the
> continuous pots. Now, if one wanted to add this feature, modifying the

VCO

> is a bad idea for several reasons. First of all, it ruins your warranty

<g>

> and collectibility, but also, having the VCO as it currently exists is

very

> often desirable. So you don't want to mess with it; a better solution is

to

> have an off-board module for the times when you need an octave switching
> function. By creating a bias voltage source with a rotary switch to
> switch-in trimmer resistors, and then feed this bias voltage into the

VCOs'

> FM inputs, you can make this happen. Just need some trim pots on the
> switching module to get the precise octave intervals on the VCOs.
>
> Okay, that got me thinking.
>
> Tuning VCOs to octaves is certainly something that is done often,

especially

> for that big fat analog sound. But what if you wanted to find ∗other∗

useful

> intervals. What would these be? The Partial relationships between the
> fundamental tone (VCO #1, let's say) and "harmonic #n" (played by VCO

#2),

> etc.! Reinforcing harmonically-related partials is very cool. After all,
> that's why tuning to ∗octaves∗ is so cool---the octave is a strong

harmonic

> relationship. But it's not the ∗only∗ one. There are very sophisticated
> harmonic tunings possible. A musical 'twelfth,' for example, because the
> twelfth reinforces the octave+fifth which is the third Partial (second
> harmonic). [See, I knew this would bore those not 'into' musical acoustic
> physics. But therein lies the basis of all we do as synthesists,

consciously

> or not.]
>
> AAAs some of you may have noticed from all my bitching about "partials"

on

> the list, > I'm really interested in working with the harmonic series in
> pseudo additive synthesis fashion. With a module like this, these very
> important relationships could be easily dialed in. So first you would

tune

> your VCOs to unison (for simplicity let's say there are only two VCOs)

with

> this Harmonic Biasing module set at the null position, #1. > At position

1,

> there is no bias, so VCO #2 > is at unison with VCO #1. Switching to
> position #2 would put VCO#2 up one > octave (the first harmonic). The

next

> position is octave-fifth (2nd > harmonic), then 2-octaves,

2-octaves-third,

> two-octaves-fifth.... etc, up to > as many harmonics as you have switch
> positions and trim pots for.
>
> [Using terminology like "twelfth" and "two-octaves-fifth" is technically
> erroneous, but it gets the idea across. Due to compromises made in Equal
> Temperament, as you go up the harmonic series you start to drift from the
> "notes on the piano." This module should be tuned to the true partial
> relationships, not their Equal Tempered counterparts. The intervals need

to

> be f/2, f/3, f/4...&c.]
>
> (Please hold your criticism until the end... I'm building this idea in

your

> minds one step at a time.)
>
> > This ability to easily dial in exact partials on secondary VCOs, to me,
> would be awesome. Most people detune oscillators by octaves to > get that
> big fat "organ" sound, but the really cool way to tune VCOs is to >
> emphasize the real positions of other "natural" harmonics.
> >
> (> In fact, there is a little known property of acoustic physics called >
> "sub-harmonics." I don't mean sub-octaves. But a sound can induce a

larger >

> object to resonate at frequencies LOWER than its fundamental by exciting

a >

> higher harmonic of the resonant properties of the second object. It is

very

> > subtle. Henry Cowell wrote about this in his book "New Musical

Resources"

> back in 1911. I think Walter Piston even addresses it in one of is famous
> books on harmony and/or orchestration. The first sub-harmonic of C is a
> lower F. The sounding board of a > piano actually adds these

sub-harmonics

> in VERY subtle amounts. It would be > neat to be able to play with these
> easily.
> >
> >
> > Now, to make this ∗more∗ of a nightmare project...
> >
> >
> > What if, instead of using rotary switches for selecting the offset
> relationship, you use pots with smooth, internal > electronic switching?

As

> you turned the pot, the voltage would jump the VCO's pitch in >

quantized,

> harmonic steps. The first benefit is that you don't need a 64 position >
> rotary to be able to get 64 harmonics on the dial, but the ∗BEST∗ reason

for

> > this is... <tah-dah!> ∗Voltage control!!∗ A CV could change the

quantized

> > biasing as well as the front panel control. What could you do with

this?

> You could have a CV, like an LFO, force a VCO to > sweep the harmonic
> series! This is an effect people try to get with Serge > waveshapers,
> high-resonance bandpass filters, etc. but you could have two > VCOs

tracking

> each other with the second one sweeping its frequency, but > always its
> pitch is a true "natural" harmonic of the first VCO! Or an ∗EG∗ could

sweep

> it---on a percussive envelope, the EG would push the second VCO up to

some

> higher harmonic of the first VCO, and then VCO #2 would fall to some

other,

> still harmonic relationship. This is similar to what happens in nature

with

> pitched percussive instruments, but can be just different enough to be
> really wild.
> >
> This to me is a VERY exciting idea. With the second VCO producing a sine
> wave and the above effect being patched up, I can only imagine that it

would

> sound like the first VCO's signal was being split and the side-chain was
> going through an impossibly tight bandpass filter that only emphasized

one

> single harmonic at a time. Of course, no real bandpass filter could be so
> flexible and selective as to single out individual harmonics like this,
> especially relatively high in the harmonic series where they get very

tight.

>
> And, of course, it need not be used just with VCOs. If instead of

producing

> a bias voltage for the VCO FM input it actually altered a 1V/OCT voltage
> passing through it, it could be used with the VCLFO, the filters,

anything

> that works in the pitch domain. How would a filter sound with a peaky
> resonance tracking the keyboard but also being swept with
> harmonically-related control voltages? I bet it would be very cool, and

at

> the same time subtle because you are dealing with the natural harmonic
> series, not just crude 'organ stop' relationships. The difference would

be

> the difference between equal temperament and just intonation, which is
> simultaneously subtle and yet very dramatic!
>
> (Wendy Carlos comments on "Secrets of Synthesis" that analog tends to
> deteriorate "into that quasi-organ sound that most synthesizer work
> eventually degenerates into." That's because most people tune square

waves

> to simple octaves and fifths to build a sound, which does start to sound

the

> same after a while. This need not be the case, and this module works
> ∗against∗ that organ-ish tendency.)
> >
> > I can imagine a 2u module. The four pots on the left side set the

harmonic

> > offsets for four VCOs. The four pots on the right are attenuators for

the

> incoming CVs to modulate them. Four > jacks are CV ins, and 4 are the

biased

> outs.
> > >
> Okay, next.
>
> The weak link is the bank of trim pots, one per harmonic relationship.

Now,

> I know NOTHING about > PIC processors, but I'm told they are cheap and
> really useful. I'm wondering > if the DACs in these are good enough so

that

> the harmonic relationships > could be programmed into a PIC chip. That

way

> they can be exactly specified > and theoretically would not drift. And,

of

> course, no calibration would be needed---you could lose the 64 trim pots

per

> channel, a huge expense and pain in the ass. > > Several people on this

list

> are PIC gurus who may be able to answer this question. >
>
> As the above idea evolved, it resembled less of the original octave

switch

> and more of a really unique, specific kind of quantizer. Maybe this

function

> can work its way into any quantizer designs that might be on the drawing
> board out there. As a stand-alone module, it might be esoteric, but if

this

> could be a feature of another module, it would be more economical. It

would

> be neat if there was some kind of switch: in one position, it quantizes

in

> 1/12 volt steps, like you would ∗expect∗ a quantizer to, and in other

switch

> positions, an effect like the above could be implemented. Because really,
> what I'm describing is a quantizer for Just-Intoned tuning, if used that
> way, which is an important compliment to equal-tempered tuning. If the
> module were _uP_ based, I would think this would be a low-part
> implementation. Just a little extra programming and a switch. (Am I
> wrong...?)
>
> I'm raving about this out loud because I'm not an engineer, and can't
> implement this nut-bag idea on my own, otherwise I would just build it

and

> demonstrate it. I understand acoustic physics a hell of a lot better than

I

> understand the nitty-gritty of the electronics. I just wanted to see if
> there was anyone out there who thinks this could be as cool as I do. Can

I

> get anyone to chant along with me...?
>
> As you can tell, I don't use my modular just to do two-VCO leads over

guitar

> pads... I'm trying to use it to create worlds.... I have weird needs...

:)

>
> Thanks for indulging this rave, folks. My vocal chords hurt now, and I
> wasn't even talking....
>
>
>
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