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<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Ugh, gawd….. Mine’s gonna rock. I’m
gonna put the finishing touches on the design in about 15 minutes, once the
fentanyl wears off just a little bit more (although it didn’t prevent me from
practicing a Haydn sonata just now).<o:p></o:p></span></font></p>
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<p class=MsoNormal><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'> Tom Wiltshire
[mailto:tom@electricdruid.net] <br>
<b><span style='font-weight:bold'>Sent:</span></b> Tuesday, February 18, 2020
4:24 AM<br>
<b><span style='font-weight:bold'>To:</span></b> David G Dixon<br>
<b><span style='font-weight:bold'>Cc:</span></b> ackolonges fds; SDIY List<br>
<b><span style='font-weight:bold'>Subject:</span></b> Re: [sdiy] LFO VC Skew?</span></font><o:p></o:p></p>
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<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>On 18 Feb 2020, at 02:37, David G Dixon <<a
href="mailto:dixon@mail.ubc.ca">dixon@mail.ubc.ca</a>> wrote:<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=2 color=navy face=Arial><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.<u1:p class=""></u1:p></span></font><o:p></o:p></p>
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<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Many of the same problems you have when doing it analoguely (that’s the
equivalent of “digitally”, right?!) turn up in the digital version.<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>I used phase distortion to produce the modified waveforms. Essentially,
you have a ramp core (an NCO) and you apply a wave shaper. To get the phase
distortion, you run the first half of the NCO’s cycle at a different frequency
to the second half. To avoid frequency wobble when altering the shape, the two
sets of frequency increments have give the same overall frequency as one steady
increment. This means they needs to be pretty accurate (how accurate depends on
audio versus LFO and how much you care).<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>This accuracy requirement also gets worse as you get to the extreme
ends (because you finish up with a division that approaches division by zero).
Using integer math, you reach a point where the results overflow the available
accuracy. For this reason, I limited the Distort CV on my chips to 4% to 96%
duty cycle. This means you *can’t* get a really vertical saw waveform edge if
you start by distorting a triangle wave.<o:p></o:p></span></font></p>
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12.0pt'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>It’s funny how the difficulties remain the same, even using a
completely different method and technology.<o:p></o:p></span></font></p>
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