[sdiy] Blackmer VCA cell

[Richie Burnett]

> A linear EG into a exponential VCA should sound very natural...

Doesn't the attack part of the envelope end up being the wrong way around if you do that?  You end up with "exponential growth" to some peak amplitude, where what you want is "exponential decay" from zero going upwards and settling at the peak amplitude.

I don't know much about the physics but I think the greatest rate of change in amplitude is right at the start of the attack phase when a pipe is blown or a string is bowed, and the amplitude eventually settles at some equilibrium level where energy input equals energy lost.

-Richie,

Sent from my Xperia SP on O2

---- Brian Willoughby wrote ----

>The simplest envelope generator is exponential, based on pots and switching. But these are more difficult to voltage-control the parameters of the EG itself (at least without varistors). Great for cheap systems where the pots on the front panel are the only way to control the EG.
>
>A linear EG should be possible in analog using op-amp integrators and current sources, which are more easily voltage-controlled. These are probably more expensive, but I'm not familiar with the costs.
>
>A linear EG into a exponential VCA should sound very natural. Varying between linear and exponential would be difficult in the analog domain, though.
>
>Brian
>
>
>On Oct 23, 2021, at 14:41, Mike Bryant wrote:
>>> Would it be possible to redesign the CV envelope generator curve to work with an exponential control VCA?
>> 
>> Yes it is.  I think Neil was being a little bit sweeping as quite a few synths used to have log VCAs with linear envelope generators.  But the accuracy of the log VCA doesn't need to be that good so an expensive solution would be overkill.  In my digital designs I always allow the envelope to be varied anywhere between pure linear and pure log anyway (easy in digits) as they can give quite differing nuances to the sound.  
>> 
>> 
>> -----Original Message-----
>> From: Brian Willoughby
>> Sent: 23 October 2021 22:26
>> 
>> On Oct 23, 2021, at 13:53, Neil Johnson wrote:
>>> Their control function is exponential, linear-in-dB which is great for audio fading so they are not suitable for the traditional envelope generator that needs a linear control VCA.
>> 
>> Would it be possible to redesign the CV envelope generator curve to work with an exponential control VCA?
>> 
>> I haven't thought this through, but I can imagine different ways to generate an envelope - both linear and logarithmic. What I don't know is whether you'd actually need anti-log CV to get the right results out of an exponential control VCA for envelope purposes.
>> 
>> Brian
>
>
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