[sdiy] Virus envelopes (was Re: Fast envelope generation)

[Neil Johnson]

Tom Wiltshire wrote:
>
> On 19 Dec 2012, at 01:55, Russell McClellan wrote:
>
>> I remember being quite impressed with envelope section in the Access
>> Virus digital synth manual - nominally, it only has linear envelopes,
>> but you can use the modulation matrix to produce exponential or log
>> curves by routing the envelope level back to the attack, decay, or
>> release parameters.  I think that's a clever and intuitive UI for
>> users and it must have been relatively easy to implement.
>>
>> Did this idea come from some analog device?  I'd be quite surprised if
>> it didn't and it very much has the feel of buchla or serge or some
>> banana system.
>
> The Oberheim Xpander/Matrix enabled something similar. This is mentioned in the Owner's manual ("Creating very long or unusual envelopes"), so it's clearly something Oberheim thought about.

Mathematically that is exactly how an RC circuit works, giving the
classic log-like curve.  Think of the control input not as a fixed
voltage but as a current, then as the capacitor charges up the voltage
across the resistor reduces and so the current reduces, steadily and
increasingly-slowly heading towards the final value.

You can do the same thing with an op-amp integrator.  Without feedback
the integrator will produce a linear slope until the op-amps hits the
rail.  Now feed back some of the output to the input and it will ramp
inverse-exponentially (to be precise) to the some value.  And low and
behold, its a low-pass filter.

Neil
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