EG design

[John P]

Grant Richter wrote:
> 
> >
> > Is there any point charging an envelope generators timing cap with a
> > constant current source, variable of course, to generate a linear change
> in
> > output pd. Or do exponential rises and falls sound more realistic. mmm,
> > there's a thing, realism, I'm not sure thats the right word. My D-50
> (shock
> > horror) seems to have linear EGs, at least the diagrams on the front
> > suggest that, and they sound very odd on a slow attack/decay setting.
> >
> > Any one built a analogue linear EG?
> >
> The Buchla and Serge EGs are all linear slopes, as is the Wiard. The
> thinking goes like this:
> 
> If you have a linear VCA then you need an exponential envelope to get it to
> sound right. This is what Moog did. But the problem is if you want to use
> the envelope for something else. Take panning for instance - you don't want
> an exponential pan envelope, you want a linear one. Buchla and Serges
> response was to use a linear envelope and make the VCAs exponential. That
> way, the envelopes are more generally useful in controlling any parameter.
> 
> To summarize:
> Linear VCA - Exponential envelope
> Exponential VCA - Linear envelope


The Serge envelope generators also have expo VC with a +/- processing
pot.  You can patch the output of the EG into its VC input (feedback). 
With the processing pot set on the + side (noninverting), the envelope
becomes exponential.  With the pot set to the - side (inverting), you
get a logarithmic envelope.  Intuitively enough, the exponential
envelopes sound more percussive and plucky using an expo VCA.

        ....                  ....                  ....
      .      .                .  .               .        .
    .          .             .    .            .            .
...              ...     ...        ...     ...              ...

      linear env            expo env              log
env                                       
This is more of a subtle effect rather than an earth-shattering effect.

This feedback technique can also be used to change the waveshape on a
VCO.  

John P.