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Region Diode Mixer: circuit idea
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Ricko



Joined: Dec 25, 2007
Posts: 251
Location: Sydney, Australia
Audio files: 27

PostPosted: Sat Apr 04, 2020 11:53 pm    Post subject: Region Diode Mixer: circuit idea
Subject description: Progressive tone variation: towards regions and registers
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I few years ago I put up a circuit for MinMaxMix, a diode mixer/waveshaper here. I haven't put it out a Fricko product, because of a niggle that there was some angle missing that it could be used for.

On Matrixsynth this week is a YouTube video from Daniel Riera, which demos my Blip! waveshaper. (I also first put up the basic design for this on this group several years ago.)

Listening to the demo music at the end, I got a circuit/patch idea that I thought I would jot down here, in case it stimulates any other developers in this direction.

What impressed me about the video was that the patch used in his demo piece at the end was not cheesy anywhere in its range: well done!  Very hard to do (as every almost FM solo patch shows...)

Woodwind players naturally experience their instrument in terms of several registers, each of which have slightly dfferent tones. (Indeed, learning how to play a line and cross into the next register without jarring changes is one of the skills of learning to play these instruments.)

His patch showed three quite different registers: an upper one (where Blip tends to a square wave), the middle one (where the constant-breadth wave effect is in place, but on loud breath modulation tends to a square) and a lower one, where the pulse is thin but augmented by the built in fixed lowpass signal from the source.

In the video, he talks of modules being "inspiring", but I think he means this in a completely concrete sense: when you improvise on an instrument that has the extra axis of different notes having different tones and responses (and therefore different "affects"), then you choose notes because of this emotional aspect, not just the melodic one. I think this is more obvious to guitarist, but especially to players of bowed and blown instruments. The extent that it is true would surprise, I think, keyboard players and drummers and GUI-based musicians.


What does this have to do with Blip! ?   What Daniel's performance reinforced to me was that that perhaps we need some way so that any VCOs and other waveshapers or patch can have a convenient way of  altering their tone across a range, one not relying on access to KCV.

Here is a descriptoin of a module I think might help this: basically just a 3-input MinMaxMix with tunable LPF and HPF on inputs two and three: this would be a pretty simple circuit: 4 opamps for IO buffers and 4 or 6 diodes for the mixer.

Input 1: main wave
Input 2: wave for high notes
Input 3: wave for low notes

knob 1: level of input 1
knob 2: cutoff frequency for HPF for input 2
knob3: cutoff frequence for LPH for input 3

Input 2 goes into a simple tunable HPF, say at 1k. Input 3 goes into a sinple runable LPF say at 100Hz.

Input 1 and the filter outputs go through a diode mixer like the Min Max Mix circuit I put up here several years ago. The two-input version of this circuit has this function: if the two inputs are positive, the output is the maximum of them; if the two input are negative, the output is the minimum (the absolute maximum) of them; otherwise the output is the mix of them. What this means is that if you crossfade between them, you don't get a simpler superimposition (addition) where the harmonics smoothly adjust their level and phase from one to the other: you can have other harmonics created during the fade.

So for notes in between the cutoff frequencies, your output is only Input 1. And for very high the output will be the MinMaxMix of Input 1 and 2: so if Input 1 were qute thin, this would allow the higher notes to be fuller. Similarly, with low notes, the tone would be a morph of the Input 1 and Input 2.


Regards, and keep safe everyone!
Rick


<RAMBLE>I used to have a CS-60 (half a CS-80) and it was really clear to me that one of the reasons it was so damn playable was not just the strange voice architecture and envelope generators, nor the touch and ribbon, but because it provided breakpoints on the keyboard, so you could adjust the KCV follow for the VCF of the top and bottom of the keyboard: so that in each region you could have a slightly different tone.

I think Yamaha had a really sophisticated sense of how ensembles of instruments work, how their different spectral centres allow them to combine or separate.  For polyphonic instruments, if you have band-pass filters and they don't track the CV, you get an equaliser (and a static formant): this increases the sludge (If coarse bands) or pulls out different parts (if the resonance is high enough to only pick out individual notes). But if they track the CV you get that same problem that samplers get: the instrument gets larger or smaller (subjectively) and you lose the sense of a cohesive body,; however, it also can gives clarity to chords, because there is less overlap of their frequency bands. 

Other manufacturers tried to solve this with just adjustable amount of KCV,  but I think Yamaha's approach has the advantage of the creating regions more: within each region (top middle, bottom) you get that separation and so clarity, but you avoid the shrinking effect at the extremes (if you want.) 
</RAMBLE>
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