[sdiy] vactrol transfer function

[David G. Dixon]

> 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.