[sdiy] LFO VC Skew?

[Quincas Moreira]

The Multiwaves has one digital LFO/VCO and two analog LFOs, all of which
have the VC tilt function. However, the analog ones do NOT preserve the
frequency. I still find them useful anyways. The digital one does preserve
the frequency, and in VCO mode that makes for some beautiful faux filtering
sounds, when you tilt that sine, cosine or triangle wave...  I'l ask him to
come pitch in here ....

On Tue, Feb 18, 2020 at 5:54 PM David G Dixon <dixon at mail.ubc.ca> wrote:

> Well, the circuit is pretty simple.  However, there is one major snag,
> which
> I will only discover upon trying it...
>
> The maximum stated (amplification) gain of a 2164 VCA is 22dB.  My circuit
> requires 40dB of gain.  Even at that gain, the VCA only has to process
> about
> 133uA of current, so it should be within the realm of possibility if the
> gain limitation is really a limitation on output current (which I was led
> to
> believe was closer to 300uA) and not on actual multiplication factor.  If
> the gain is truly limited to 22dB, then the saw/ramp flybacks are going to
> be fairly slowish -- possibly still useful, but...  If I can actually
> squeeze 40dB out of the beast, then we're in business.  Either way, I'll
> have a test circuit finished before I go to bed.
>
> If it works well, then yes, I will give Danjel van Tijn first dibs on
> taking
> the design for Intellijel (which he has already suggested he may want to
> do)
> and will not be sharing the tricks with y'all (sorry -- my loyalties are
> clear).  I won't discourage you from figuring it out yourselves though,
> based on the hints I've given.
>
> Of course, if the 2164 only gives me 20dB and I need 40dB, it's tempting to
> just string two 2164s together sharing the same VC voltage.  However, this
> won't work.  It will increase the frequency towards the middle of the morph
> (where the triangle lives).  Because I'm working with logarithmic math, the
> arrangement is pretty darn unforgiving.
>
>
> -----Original Message-----
> From: Jason Proctor [mailto:jason at redfish.net]
> Sent: Tuesday, February 18, 2020 1:48 PM
> To: David G Dixon
> Cc: Tom Wiltshire; SDIY List
> Subject: Re: [sdiy] LFO VC Skew?
>
> 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:
> >
> > 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).
> >
> >
> >
> > ________________________________
> >
> > 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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-- 
[image: QMA]

Quincas Moreira
Director | QMA
mobile:  5534988825
site:  quincasmoreira.com
email:  quincas at gmail.com
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