[sdiy] LFO VC Skew?

[Harald]

Juergen Haible did an implementation of Don's with his interpolating 
scanner. Don's article is still available on his website.

Am 18.02.20 um 09:08 schrieb ackolonges fds:
> Thanks so much everyone for your input.
> 
> Those that mentioned digital options - yes indeed it's quite straightforward to implement such a thing via digital means, but I would like to try for the analogue route.
> 
> Thanks a lot for that link to the SNICster Ian, that's a heck-of-a waveshaper!
> 
> It would indeed be great to find out how the Electrik Multiwaves implements this function, maybe if Vinicius see this he might be willing to let us know his approach.
> 
> Also a big thanks to Ben for alerting us to the Wave Morph website. They mention that the VCS3 uses such a circuit, but I was unable to find info on that because the link was broken. He has a hand-written sketch by Don Tillman, that I can't fully parse (does anyone know if anyone ever built that one?), and mentions that Sam Houshu had a design of such a function but that link is also broken. His Wave Morph looks to do just what I'm after, but uses the RC4200, a long out-of-production chip. I'd much rather use the LM13700 OTAs that I have plenty of, or a modern drop in replacement for the RC4200 if anyone knows if such a thing exists. He mentions an OTA version, but alas, another wrong link.
> 
> So, I'm waiting with baited breath to see the magic that David has in mind!
> 
> ________________________________
> From: David G Dixon <dixon at mail.ubc.ca>
> Sent: Tuesday, February 18, 2020 6:53 PM
> To: 'ackolonges fds' <ackolonges at hotmail.com>; 'SDIY List' <synth-diy at synth-diy.org>
> Subject: RE: [sdiy] LFO VC Skew?
> 
> 
> Nevermind.  I think I might have figured it out.  And the beauty of it is that I think I can do it by adding one very simple component to a linearized 2164 VCA circuit.
> 
> 
> 
> The trick is that if the log of X is log X, then the log of 1/X is –log X.  Look at Mike Irwin’s article and see what I saw, and how it can easily be done.
> 
> 
> 
> Now I feel even more stupid for not seeing that three years ago, when I was struggling with this.  Now I should have a design ready by tomorrow.  Stay tuned.
> 
> 
> 
> ________________________________
> 
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of David G Dixon
> Sent: Monday, February 17, 2020 6:37 PM
> To: 'ackolonges fds'; 'SDIY List'
> Subject: Re: [sdiy] LFO VC Skew?
> 
> 
> 
> I’ve struggled with this mightily and come to no solution in the analog realm.  The problem is that there seems to be no way to implement the magic of a potentiometer fed on both ends with VCAs.  I’ve been through the math, and it seems impossible.  The original circuit (two diodes and a potentiometer in the loop of a tri-square oscillator) is so simple and elegant, and yet so elusive for voltage control, other than physically rotating the damn pot with a servo.
> 
> 
> 
> It’s easy to get saw-to-tri-to-ramp with VCAs, but the trick is to maintain the same frequency when you do so.  That’s the impossible part.
> 
> 
> 
> It is probably easily done digitally, but where’s the fun in that?
> 
> 
> 
> Unless I’m dumb.
> 
> 
> 
> ________________________________
> 
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of ackolonges fds
> Sent: Monday, February 17, 2020 6:02 AM
> To: SDIY List
> Subject: [sdiy] LFO VC Skew?
> 
> 
> 
> Hi All,
> 
> 
> 
> I'm a big fan of the 'skew' (sweep from sawtooth, through triangle, over to ramp wave) function of some analogue LFOs and find it indispensable. I would love to be able to CV the skew function though it generally doesn't seem to be an option. Some circuits seem to control skew by having a pot control the balance between current flowing each direction via two diodes with opposite polarity (eg. the MFOS VCLFO circuit, or the Nicolas Woolaston LFO circuit), which seems like a particularly tricky thing to be able to voltage-control.
> 
> 
> 
> I was wondering if anyone here knows of any existing circuits which provide that functionality, or have any ideas as to how one may go about making it happen?
> 
> 
> 
> Thanks a lot for any input 🙂
> 
> 
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