[sdiy] Blackmer VCA cell

[Gordonjcp]

On Mon, Oct 25, 2021 at 09:18:30AM -0400, mskala at northcoastsynthesis.com wrote:
> On Mon, 25 Oct 2021, Gordonjcp wrote:
> > On Sat, Oct 23, 2021 at 06:18:37PM -0400, Matthew Skala via Synth-diy wrote:
> > > A capacitor charging from a voltage source through a resistor will change
> > > its voltage fast at first, when there's a large drop across the resistor
> > > and therefore a large current, and then slower as it approaches the target
> 
> > It's a little more complicated than that because in a "real" envelope
> > you have the timing cap charged off the full positive rail (either 12V
> > or 15V) but the comparator that flips to the decay phase set for 10V.
> 
> What I wrote is true - it's faster at the start and slower toward the end.
> The fact that the voltage source used for charging is beyond the target
> voltage does make the difference between the start and end smaller; it's
> less concave down, and closer to linear, than it would be with a voltage
> source nearer the target voltage.  With the voltage source exactly at the

Right, but the "shinty stick" part of the the exponential charging curve is very much lopped off by stopping charging at 10V.  When you do that, then the attack portion is audibly indistinguishable from linear.  Indeed, the Juno 106 which has famously punchy envelopes uses a totally linear attack phase and expo decay and release.

-- 
Gordonjcp