[sdiy] Blackmer VCA cell

[mark verbos]

ah, back here again….

A linear VCA controlled by an RC curved envelope is different from an exponential VCA controlled by a linear envelope. 

RC curves have a convex attack and a concave decay. So hybrid synths with linear digital envelopes are accused of not being “punchy” or “useless for basses” , etc.

A “hyper-exponential” decay made by an RC envelope and an exponential VCA can sound nice for percussion (like in an ARP 2600). A digital envelope should be able to have a “hyper-convex” attack to overcompensate for the hollow attack effect.

Mark




> On Oct 23, 2021, at 11:49 PM, Brian Willoughby <brianw at audiobanshee.com> 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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