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Thanks so much everyone for your input. <br>
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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.
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Thanks a lot for that link to the SNICster Ian, that's a heck-of-a waveshaper!</div>
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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.</div>
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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.
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So, I'm waiting with baited breath to see the magic that David has in mind!<br>
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<div id="divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" face="Calibri, sans-serif" color="#000000"><b>From:</b> David G Dixon <dixon@mail.ubc.ca><br>
<b>Sent:</b> Tuesday, February 18, 2020 6:53 PM<br>
<b>To:</b> 'ackolonges fds' <ackolonges@hotmail.com>; 'SDIY List' <synth-diy@synth-diy.org><br>
<b>Subject:</b> RE: [sdiy] LFO VC Skew?</font>
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<font size="2" face="Arial" color="navy"><span style="font-size:10.0pt; font-family:Arial; color:navy">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.</span></font></p>
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<font size="2" face="Arial" color="navy"><span style="font-size:10.0pt; font-family:Arial; color:navy">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.</span></font></p>
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<font size="2" face="Arial" color="navy"><span style="font-size:10.0pt; font-family:Arial; color:navy">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.</span></font></p>
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<b><font size="2" face="Tahoma"><span style="font-size:10.0pt; font-family:Tahoma; font-weight:bold">From:</span></font></b><font size="2" face="Tahoma"><span style="font-size:10.0pt; font-family:Tahoma"> Synth-diy [mailto:synth-diy-bounces@synth-diy.org]
<b><span style="font-weight:bold">On Behalf Of </span></b>David G Dixon<br>
<b><span style="font-weight:bold">Sent:</span></b> Monday, February 17, 2020 6:37 PM<br>
<b><span style="font-weight:bold">To:</span></b> 'ackolonges fds'; 'SDIY List'<br>
<b><span style="font-weight:bold">Subject:</span></b> Re: [sdiy] LFO VC Skew?</span></font></p>
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<font size="2" face="Arial" color="navy"><span style="font-size:10.0pt; font-family:Arial; color:navy">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.</span></font></p>
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<font size="2" face="Arial" color="navy"><span style="font-size:10.0pt; font-family:Arial; color:navy"> </span></font></p>
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<font size="2" face="Arial" color="navy"><span style="font-size:10.0pt; font-family:Arial; color:navy">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.</span></font></p>
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<font size="2" face="Arial" color="navy"><span style="font-size:10.0pt; font-family:Arial; color:navy">It is probably easily done digitally, but where’s the fun in that?</span></font></p>
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<font size="2" face="Arial" color="navy"><span style="font-size:10.0pt; font-family:Arial; color:navy">Unless I’m dumb.</span></font></p>
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<b><font size="2" face="Tahoma"><span style="font-size:10.0pt; font-family:Tahoma; font-weight:bold">From:</span></font></b><font size="2" face="Tahoma"><span style="font-size:10.0pt; font-family:Tahoma"> Synth-diy [mailto:synth-diy-bounces@synth-diy.org]
<b><span style="font-weight:bold">On Behalf Of </span></b>ackolonges fds<br>
<b><span style="font-weight:bold">Sent:</span></b> Monday, February 17, 2020 6:02 AM<br>
<b><span style="font-weight:bold">To:</span></b> SDIY List<br>
<b><span style="font-weight:bold">Subject:</span></b> [sdiy] LFO VC Skew?</span></font></p>
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<font size="3" face="Calibri" color="black"><span style="font-size:12.0pt; font-family:Calibri; color:black">Hi All,</span></font></p>
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<font size="3" face="Calibri" color="black"><span style="font-size:12.0pt; font-family:Calibri; color:black">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.</span></font></p>
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<font size="3" face="Calibri" color="black"><span style="font-size:12.0pt; font-family:Calibri; color:black">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?</span></font></p>
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<font size="3" face="Calibri" color="black"><span style="font-size:12.0pt; font-family:Calibri; color:black">Thanks a lot for any input
</span></font><font face="Calibri" color="black"><span style="font-family:Calibri; color:black"><span id="x_🙂">🙂</span></span></font><font face="Calibri" color="black"><span style="font-family:Calibri; color:black"></span></font></p>
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