Fast analog sw. as VC resistor

Mark Smart smart at
Tue Jan 7 17:41:52 CET 1997

> Yes. Ok, I didn't understand that you want 3 terminal pots. Were do you
> use them in the envelope generator?

I haven't actually hooked it up to an envelope generator circuit yet, but I
was planning to use ARP's envelope generator. 3-terminal pots are needed
for the sustain and release adjustments; two-terminal pots are used for
attack and decay. I also just would like to find a really good general way
to do this which will work for floating pots of different values. It kind
of amazes me that there are not chips which do this. If there are, I want
to know about it.

> I spent some time in the weekend trying to figure out what you have in mind.
> I include a uuencoded GIF of the result. There are 2 solutions.
> The first uses 3 VCR and 1 multiplexer; the second uses 1 VCR and 2 multiplexers.
> The design is not complete, as you can see. The control logic requires at
> least one more input from the attack threshold comparator (not shown in the
> picture).
> I would go for the first solution (3 VCR and a multiplexer) because the design
> is safer. Moreover if I use switched capacitors as VCR I don't need the
> multiplexer because the VCR can be turned off (resistance = infinity) by
> gating its input clock.

Wow, great! I will check these out.

> > It's almost totally linear as long as R is a lot bigger than the on-resistance 
> > of the analog switch.  I tested this circuit out last night with a couple of
> > 100K resistors and a 4066, which has Ron of about 130 ohms. It worked great once
> > I got a good pwm'ed clock for it. 

I said all this shit before I had actually measured the linearity of the
circuit! Can you tell?

> This is true if you use it as a voltage divider with no load at the output.
> I know you use it as a pot but be careful if you use it as a variable resistor
> in an RC network.
> Your version of a VCR is a resistor with a switch in parallel. Right?
> Now, if we assume that the switch is ideal (Ron = 0) then the capacitor is
> charged immediately when the switch is closed, no matter how wide the control
> pulse is.

Hmm. I'll have to think about this. When I hooked it up like this:

        |   ---
 disch \    ---   10 uF
        |    |
             /    VCR
            ---   Gnd
it seemed to work ok, but I didn't measure it very closely.
Thanks for the ideas. I'll check out your GIF.

> +-------------------+----------------------------------+
> | Paolo Predonzani  |  email: predo at |
> +-------------------+----------------------------------+

*     Mark Smart                               *
*     Network Technician                       *
*     University Communications Inc. (UCI)     *
*     smart at                      *

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