[sdiy] vactrol transfer function

[mark verbos]

On Dec 31, 2010, at 4:44 PM, David G. Dixon wrote:

>> I haven't analyzed it closely, but I don't think that a low pass would
>> quite get you there. With many Vactrols, the turn-on time is shorter than
>> the turn-off time. Maybe a slew with different rise and fall time
>> constants?
> 
> Yes, I had a look at the Perkin Elmer vactrol application sheet (VT500)
> which Danjel van Tijn kindly sent me, and it's fairly obvious what the
> "special character" of the vactrol is.  I think that a slew circuit with a
> couple of diodes and different resistors would do the job nicely.
> 
> Here are the stated response times (on-time to 63% of final R_on, or
> off-time to 100k) for a collection of common vactrols:
> 
> 		on-time	off-time
> VTL5C1	2.5 ms	 35 ms
> VTL5C2	3.5 ms	500 ms
> VTL5C3	2.5 ms	 35 ms
> VTL5C4	6.0 ms	1.5 s
> 
> Based on this, am I correct in assuming that the VTL5C2 is the most useful
> for synth purposes?  Off-times for VTL5C1 and VTL5C3 seem kinda short, and
> the off-time for VTL5C4 seems kinda long.
> 
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There is the added complication of "memory" in the photo cells. It is most noticeable when recording a Buchla into a DAW when you see that the first note is quieter than all the others. Of course, this is probably not a thing you WANT to emulate.

Mark