[sdiy] VCA response - continously variable between linearand exponential

David G Dixon dixon at mail.ubc.ca
Sun Mar 3 04:36:41 CET 2013


I think we're having an "Emily Litella moment" here:

We're talking about VCAs, not VCOs!
 

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of 
> rsdio at sounds.wa.com
> Sent: Saturday, March 02, 2013 6:11 PM
> To: neil harper
> Cc: Synth-Diy
> Subject: Re: [sdiy] VCA response - continously variable 
> between linearand exponential
> 
> Why would you need continuously variable curves? Are you just 
> talking about shifting away from the standard 1 V per octave, 
> but still being based on the octave standard?
> 
> In my experience, a VCO either responds to CV with a linear 
> relationship between voltage and frequency (simpler circuits, 
> easier to get precise), or it responds to CV with a 
> logarithmic relationship between voltage and frequency 
> (allowing a pitch bend CV to apply the same relative pitch 
> change regardless of octave). The latter happens to have a 
> linear relationship between voltage and octave or semitones. 
> I cannot think of any reason to have a response other than 
> one of those two. Note that the relationship between pitch (Hz or
> period) and octave is not linear.
> 
> As I mentioned, the linear response is more basic, and 
> because it is linear you can make it as precise as you want. 
> Unfortunately, a pitch bend CV applied in one octave will 
> have a different relative pitch offset than in any other 
> octave, making it impossible to pitch bend a chord on a 
> polyphonic keyboard (unless you specifically desire 
> inharmonious results). Thus, the only reason for non-linear 
> CV of pitch is to allow mixing multiple CV sources (e.g. from 
> keyboard, pitch bend, octave/footage selector, etc), and in 
> that case the only logical choice is the specific curve of an 
> octave relationship. The only disadvantage of the latter is 
> that it is very difficult to create this non-linear curve. 
> It's easiest to use, but hardest to create. Thus, the only 
> two logical choices seem to be a Hz scale or an octave scale, 
> e.g. V/Hz, Hz/V, V/octave, octave/V, V/period, period/V. 
> There are lots of variations, but they still basically fall 
> into just two categories in terms of the exponential or 
> linear aspect, and I can't see the utility of anything 
> "between" linear and exponential.
> 
> I imagine that the only useful variation would be the value for "n"  
> in the expression, "n V per octave." The industry has mostly 
> settled on 1 V per octave, with some using 1.2 V per octave, 
> but my point is that it's always something per octave unless 
> it's totally linear.
> 
> By the way, I'm sure everyone here understands the above, and 
> I don't intend to insult anyone by spelling it all out, but I 
> really want to make sure there's not something I'm missing. 
> It seems like there was an implication that a cubic or 
> quadratic or higher power exponential curves might be 
> desirable, but I can't see why. I can see why 2 V per octave 
> or 0.5 V per octave might be incredibly useful, because it 
> would allow a different range of pitch for the same power 
> supply. But unless I'm using the terminology incorrectly, 
> that's not continuously variable between linear and exponential.
> 
> Brian Willoughby
> Sound Consulting
> 
> 
> ----- "neil harper" <metadata at gmx.com>
> > hey all,
> > i'm looking for some advice/pointers on how to get a VCA response 
> > curve to be continuously variable between linear and exponential. i 
> > can do switchable between the two, but is there any trick 
> to getting 
> > everything inbetween?
> >
> > if it helps, my VCA is lm13700 based but i don't mind starting with 
> > something else.
> 
> 
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