I've Got Tone!

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 Dean wrote:

 HackettFan wrote:

 Dean, as you know, I perused your sustainer pickup thread quite awhile ago. I was wondering if you had been following Paul Vo's Vo Guitar. He showed it off at this year's Namm festival. He uses a digitally controlled sustainer pickup mounted on/in an acoustic guitar not just to vibrate the string but to control the vibration so that it produces a square wave or a saw tooth (and with the desired amount of sustain too of course). It controls what harmonics are present and can arpeggiate the harmonics even. It's a whole new type of synthesis he calls acoustic synthesis; purely physical - no signal processing or PV conversion. Talk about tone! They're not available yet. I'm waiting, though. I thought you might also get excited by the technical achievement of it. See www.paulvo.com (if you haven't already).

I had seen his Wond but not the Vo-96. Interesting. There is signal processing and conversions going on, they're just part of the feedback process instead of being post-processing.  Unfortunately the demo video was as dull as dishwater as I didn't care what the presets do or any of that so I didn't watch it to the end, I was more interested in the pick-up/exciter. I had considered a pick-up/exciter per string for my sustainer but that was too complicated for what I was building. Thinking about how the Vo-96 must work and judging by its size, I surmise that he has used (at least) two pick-ups per string so he can control the harmonics and create the tone-arpeggios, giving a total of 24 individual coils.

This has got me thinking... sustainers work by feeding back the picked-up note to a driver coil such that it is in-phase with the note fundamental, the Vo-96 uses two (or more) driver coils so the secondary driver can be a little out of phase with the fundamental and thus control the harmonics. In sustainers such is the Sustainiac and Ebow the drivers are continuously pushing and pulling the string, and unless told otherwise I'm guessing that this is also how the Wond and Vo-96 work (based on the Wond, but he says the technology is the same) but perhaps it isn't... Because this isn't how we play a guitar, we apply the initial impetus to start the vibration and the string oscillates at its fundamental frequency, and without further stimulation 

 This is so interesting, but a lot for a layman to grapple with. I don't understand phase modulation, if that's what you're talking about. One time I was fiddling on paper with combining waves that were out of phase by various amounts and managed to get something on paper that started resembling a sawtooth, but that was only after adding together multiple waves. I'm most likely looking at it all wrong, but I don't understand though how one modulating wave out of phase wave can so thoroughly tweak the harmonics of another wave. Of course they can, but there must be some math I don't know how to apply, or something.

 Dean wrote:

Now consider pushing a child on a swing... here we don't continuously run backwards and forwards while holding on to the swing's seat to keep it swinging, we apply a single push at the top of its backward swing and allow physics to do the rest. This impulse function is more efficient and requires less energy from us to keep the kid amused. In fact we don't even need to give this extra impetus on every back-swing, we can do it once every 2nd, 3rd or nth back-swing and the swing will maintain its oscillation.

So what if we applied this technique to a guitar string... this is tricky in pure analogue sustainers but in digital sustainers it would be simple as the DSP analysis has already analysed each string for pitch, phase and amplitude. So rather than driving the exciter-coil with a reconstituted analogue signal (from a D to A, which I'm pretty certain is how Paul Vo does it), "digitally" pulsing the exciter-coil at the peak point in each vibration cycle would be a doddle, we could even pulse the string 180° out of phase and thus dampen the string. This opens up another (technical) possibility - since the coil is only being driven for a brief time during the note's cycle, the rest of the time the string's vibration will be inducing current in its windings, therefore the exciter can  be used as a pick-up to give continuous feedback to the control circuitry.

hmm... food for thought.
 I'm not clear if your analogy to a child on a swing is a supposition about how Paul Vo is achieving wave form control, or if you are brainstorming your own ideas. I had been presuming that the Vo sustainer pickup was applying continuous control, but I'm far from knowing anything. 

Edited by HackettFan - September 25 2015 at 09:14