[sdiy] LFO VC Skew?

Jason Proctor jason at redfish.net
Tue Feb 18 22:47:54 CET 2020


Will this mean a fancy new Intellijel LFO? :-)

fwiw, always liked the waveshaping on the venerable MOTM-320 LFO. Verrr nice.


On Tue, Feb 18, 2020 at 12:54 PM David G Dixon <dixon at mail.ubc.ca> wrote:
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> 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).
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> ________________________________
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> From: Tom Wiltshire [mailto:tom at electricdruid.net]
> Sent: Tuesday, February 18, 2020 4:24 AM
> To: David G Dixon
> Cc: ackolonges fds; SDIY List
> Subject: Re: [sdiy] LFO VC Skew?
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> On 18 Feb 2020, at 02:37, David G Dixon <dixon at mail.ubc.ca> wrote:
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> 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.
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> Many of the same problems you have when doing it analoguely (that’s the equivalent of “digitally”, right?!) turn up in the digital version.
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> 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).
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> 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.
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> It’s funny how the difficulties remain the same, even using a completely different method and technology.
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