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Subject: Re: [motm] Module Idea -- Harmonic Bias Source

From: "J. Larry Hendry" <jlarryh@...>
Date: 2000-04-13

> From: Tkacs, Ken <ken.tkacs@...>
> To: 'MOTM Forum All' <>
> 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
> Mr. Phelps---it's always your choice).
> I've been thinking about an idea for a module that I'd like to throw out
> the group for merciless comment. This discussion probably wont interest
> 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
> additive/'Fourier-fashion'... but if created, I think many people would
> the value in it once they heard it.
> Anyway:
> This idea sprang out of Mr. Hendry's recently posted concept for an
> 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
> is a bad idea for several reasons. First of all, it ruins your warranty
> and collectibility, but also, having the VCO as it currently exists is
> often desirable. So you don't want to mess with it; a better solution is
> 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
> 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,
> for that big fat analog sound. But what if you wanted to find ∗other∗
> intervals. What would these be? The Partial relationships between the
> fundamental tone (VCO #1, let's say) and "harmonic #n" (played by VCO
> etc.! Reinforcing harmonically-related partials is very cool. After all,
> that's why tuning to ∗octaves∗ is so cool---the octave is a strong
> 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,
> or not.]
> AAAs some of you may have noticed from all my bitching about "partials"
> 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
> your VCOs to unison (for simplicity let's say there are only two VCOs)
> this Harmonic Biasing module set at the null position, #1. > At position
> 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
> position is octave-fifth (2nd > harmonic), then 2-octaves,
> 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
> be f/2, f/3, f/4...&c.]
> (Please hold your criticism until the end... I'm building this idea in
> 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
> > subtle. Henry Cowell wrote about this in his book "New Musical
> 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
> 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?
> you turned the pot, the voltage would jump the VCO's pitch in >
> 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
> > this is... <tah-dah!> ∗Voltage control!!∗ A CV could change the
> > biasing as well as the front panel control. What could you do with
> 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
> 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
> it---on a percussive envelope, the EG would push the second VCO up to
> higher harmonic of the first VCO, and then VCO #2 would fall to some
> still harmonic relationship. This is similar to what happens in nature
> 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
> 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
> 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
> And, of course, it need not be used just with VCOs. If instead of
> 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,
> 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
> the same time subtle because you are dealing with the natural harmonic
> series, not just crude 'organ stop' relationships. The difference would
> 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
> to simple octaves and fifths to build a sound, which does start to sound
> 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
> > offsets for four VCOs. The four pots on the right are attenuators for
> incoming CVs to modulate them. Four > jacks are CV ins, and 4 are the
> outs.
> > >
> Okay, next.
> The weak link is the bank of trim pots, one per harmonic relationship.
> 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
> the harmonic relationships > could be programmed into a PIC chip. That
> they can be exactly specified > and theoretically would not drift. And,
> course, no calibration would be needed---you could lose the 64 trim pots
> channel, a huge expense and pain in the ass. > > Several people on this
> are PIC gurus who may be able to answer this question. >
> As the above idea evolved, it resembled less of the original octave
> and more of a really unique, specific kind of quantizer. Maybe this
> 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
> could be a feature of another module, it would be more economical. It
> be neat if there was some kind of switch: in one position, it quantizes
> 1/12 volt steps, like you would ∗expect∗ a quantizer to, and in other
> 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
> demonstrate it. I understand acoustic physics a hell of a lot better than
> 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
> get anyone to chant along with me...?
> As you can tell, I don't use my modular just to do two-VCO leads over
> 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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