MOTM

I'll vote for one each.


>From: "Tkacs, Ken" <Ken.Tkacs@...>
>Reply-To: motm@onelist.com
>To: 'MOTM Forum All' <MOTM@onelist.com>
>Subject: [motm] Quadtrature Osc / Shepard Function Generator
>Date: Thu, 9 Sep 1999 09:32:13 -0400
>
>From: "Tkacs, Ken" <Ken.Tkacs@...>
>
>
> >>  ...any interest/plans for a quadrature oscillator, or better yet, a
>Shepard function generator?
>
> >   Since you brought these up, I will ask you (and others) to offer up 
>some
>observations on what these two modules can do and why we would want them in
>our MOTM system. I am still quite modular stupid myself since MOTM is my
>first modular experience.      Larry H.
>
>
>Sure, I'd be glad to elaborate. Forgive me if I drone... these two devices,
>and their uses, are difficult to explain, even simply. This is one of those
>cases where, if you heard the effects, you would understand them almost
>immediately, but to explain them (without diagrams) is a bit of a chore....
>
>First, for those that don't know what a quadrature oscillator is:  Picture
>the Sine output of an oscillator, and then tap off that and run it through
>an inverter. You now have two locked Sines-one with "0-degree shift" (the
>original Sine) and a "180-degree shifted" (inverted) Sine output.
>
>Now, picture shifting the original Sine only 90-degrees, and then taking an
>inverted tap off that. You now have four, 'phase-locked' Sines 90-degrees
>apart (thus, quadrature).
>
>This arrangement is normally used in LFOs, rather than audio oscillators,
>because it can produce some wild stereo/quad panning effects. It can also 
>be
>used to drive the modulator input of a frequency shifter (too big a nut to
>attempt here). I was first introduced to the effect way back in my college
>days when a Teaching Assistant showed me how to take an LFO's Sine, put it
>through a passive Lowpass Filter (using it more as a lag integrator) to
>squeeze the Sine to a 90-degree shift... Then he balanced levels and used
>the 90-degree shifted Sines to feed the modulator inputs of two Ring
>Modulators. The effect, as I remember it, was indescribable! There was a
>strange auto-panning between the two speakers, and the very slight spectral
>shift from the two Ring Mods, mixed back with the original signal, produced
>a very subtle, unearthly shimmer.
>
>This is only one application of an LFO in quadrature-anywhere you can think
>of using an LFO, let your mind wander as to how you could supplement the
>effect with a different phase of the same modulation source. The TA in my
>story above used a very carefully tuned integrator to create the phase
>shift, but for any real work (and for voltage control of the LFO's
>frequency) you need to have the oscillator provide you with the quadrature
>outputs.
>
>I have only ever heard quadrature outputs as Sines (or triangles), and only
>from LFOs, not audio oscillators. Perhaps getting this to work in the audio
>range is too difficult, or perhaps someone discovered along the way that it
>was useless!? I can imagine, though, that, having an audio oscillator with
>multiple waveshape outputs, and some of them in quadrature, would allow one
>to mix the phase-shifted outputs back into a richer waveform to alter
>specific harmonics. I'm thinking of this only on paper; as I said, I've
>never heard the effect.
>
>Now, *Shepard functions* are very difficult to explain without a pencil, 
>but
>the concept is related to the mess I just typed, above. To understand the
>Shepard Function, you have to really understand the quadrature oscillator
>concept--four sine (or triangle, whatever) waveforms, 90-degrees apart. 
>Now,
>fill in another phase between each of those, giving you eight 45-degree
>shifted phases of the same Sine wave (you can use more, but eight
>["octature"?] is a functional minimum for this effect to properly work).
>
>Okay, NOW... for each of those eight Sines, you need a Sawtooth wave that
>shares the exact phase. So you have 8 Sines 45-degrees apart, and 8 Saws
>45-degrees apart.
>
>What can you do with this?
>
>The most famous use is the "barberpole" effect. If you're new to Shepard
>functions, you really have to bear with me here and visualize this.
>
>Send a signal (in your mind) to eight filters (phase shifters are a 
>classic,
>but anything voltage-controllable may be used). The outputs of these 
>filters
>each goes to a VCA, and the eight outputs of these are mixed together (they
>don't have to be, but let's keep it simple).
>
>Now, take the output of each of the Sawtooths from your SFG and use that to
>control the frequency of each of the filters. Then, take the Sines and run
>them to each of the VCAs, making sure that the Sine/Sawtooth phase pairs go
>to the associated VCF/VCA pairs.
>
>What happens? As the Sawtooth rises (or falls), for each filter, the
>associated VCA for each brings the gain up from zero, to full, and then
>gently back down again. The overall gain is constant, because the outputs
>from all of the VCAs are driven by the "same" control signal spaced evenly
>apart (45-degrees). Each VCA reveals a gently timed window on its own
>filter, letting the signal pass only as the sawtooth rises (falls,
>whatever). By the time the Sawtooth wants to 'snap' back to its lowest
>point, that particular VCA has shut off the output completely, and the 
>other
>7 VCAs are revealing their own swept filters at some point on the Sawtooth
>OTHER than the hard 'vertical.'
>
>What this sounds like is an 'infinite' effect, a 'barberpole' effect, where
>the effect is swept in one direction forever-it never seems to cycle around
>the way a normal phaser/flanger does when swept by a Sine or Triangle.
>
>I first heard this in the Seventies where eight VCO/VCA pairs were used. 
>The
>effect that this created was that of an infinitely rising tone. It just
>seemed to go up & up & up in pitch and never go supersonic, never 'fall,'
>and never end. A very strange sonic illusion.
>
>I know that this is pretty bizarre if you've never heard of it before-it 
>may
>take several reads through. I also realize that it is VERY module intensive
>(but weren't we all going to buy 20 of each MOTM module anyway...? :-)  ).
>But it is a VERY cool effect. Plus, you don't need to use all of the 
>Shepard
>outputs all of the time-every other output is a quadrature output, for
>instance. Odd combinations of outputs can create very bizarre effects. What
>happens if you rig something like the above, but scramble the associations
>between the Saws & Sines? I don't know! I want to find out!
>
>
>
>
>
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