Re: [motm] Module Idea -- Harmonic Bias Source
2000-04-13 by J. Larry Hendry
---------- > 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... :)
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> > Thanks for indulging this rave, folks. My vocal chords hurt now, and I > wasn't even talking.... > > > > ------------------------------------------------------------------------ > eLerts > It's Easy. It's Fun. Best of All, it's Free! > http://click.egroups.com/1/3080/3/_/529958/_/955659109/ > ------------------------------------------------------------------------ > >