[sdiy] LFO VC Skew?

David G Dixon dixon at mail.ubc.ca
Tue Feb 18 21:50:19 CET 2020

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



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?



On 18 Feb 2020, at 02:37, David G Dixon <dixon at mail.ubc.ca> wrote:


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.



Many of the same problems you have when doing it analoguely (that's the
equivalent of "digitally", right?!) turn up in the digital version.


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).


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.


It's funny how the difficulties remain the same, even using a completely
different method and technology.

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