[sdiy] Good Expo curve for VCA use..

Mattias Rickardsson mr at analogue.org
Mon Oct 15 21:13:57 CEST 2012


On 15 October 2012 21:00, Tom Wiltshire <tom at electricdruid.net> wrote:
> Hi JP,
>
> I agree with Mattias; a typical "expo" VCA uses curve Expo2. This includes the SSM2164 or anything by THAT Corp.
>
> However, the typical synth VCA fed by a expo ADSR is a *linear* VCA (thinking Minimoog, Pro-One, etc). The expo effect is caused by the cap charge/discharge curves on the envelope, rather than by the VCA directly.

Yes, the decay & release portions are exponential discharge curves like Expo2.

But note: the attack portion is an exponential charge curve as in your Expo1.
This sounds good for sounds that start quickly, but is not useful for
slowly fading in a sound in a constant fading manner.

> It seems to me that including an expo input on synth VCA chips got a lot more common (though by no means ubiquitous) with the later CEM stuff that was intended to be used with 1980's microprocessors and therefore with linear envelopes (CEM3378/9 for example).

Sounds very probable.

/mr


> On 15 Oct 2012, at 17:02, Mattias Rickardsson wrote:
>
>> Expo2 if you want to fade in/out slowly and evenly...
>> in a dB sense, for your hearing. :-)
>>
>> /mr
>>
>> On 15 October 2012 17:20, Jean-Pierre Desrochers <jpdesroc at oricom.ca> wrote:
>>> Hi List,
>>> Suppose one have a VCA with only linear control voltage
>>> inputs but want to feed it with an Expo control voltage..
>>> What would be the good expo curve to use ?
>>> See this picture for answer:
>>> http://www.arcenson.com/public/VCA%20curves.JPG
>>>
>>> Expo1 or Expo2 ?
>>> A standard ADSR would generate Expo1 to this VCA
>>> but I want to make sure..
>>> Thanks
>>> JP
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