[sdiy] LOG VCA?

[Michael Ruberto]

>From: Metrophage <c0r3dump23 at yahoo.com>

>How about envelope generators in which any number of splined segments
>are possible? Not that I have any idea how this would be done!
>

Please define a splined segment. are you referring to a multistage rate 
level EG?
Now that would be a project... with different attack + decay slopes - hmmmm!

>
>--- Paul Perry <pfperry at melbpc.org.au> wrote:
>
> > Aaron, as it happens, I am just this moment trying for a "one knob"
> > control to go from log thru linear to antilog
> > (over a 0 to 10 volt range say).
> > I guess there is a family of curves that at least LOOK something like
> > this? Any advice gratefully received.
> > Application is (among other things) reshaping envelopes. And
> > distortion.
> >

Distortion? Then it must be built! ;-)
With Irwin's VCA we have the log converter implemented by putting a negative 
biased SSM2164 gain block in the feedback of an opamp. So alternately we can 
make a similar converter to provide an expo converter - just change the 
signs. Since we now know we can't just add those 2 for a linear result it 
would make sense to have something to crossfade from the log output to the 
original applied CV and then to the Expo output. The output of the 
crossfader is then fed to a linear VCA.

Now for the crossfader. Maybe it could be done with some special type of 
pot, not sure. To crossfade between 3 sources of CV we could use the amazing 
interpolating scanner. It would mean adding another 3 linear VCAs. But, 
being able to use a CV to modulate between slopes - sounds tasty!!

M.A. Ruberto