[sdiy] Synth applications of CMOS inverter as SPDT switch?

Olivier Gillet ol.gillet at gmail.com
Wed Oct 31 01:08:49 CET 2012


Hi Russell,

I've already seen hacks like that around my Shruthi-1 synth, which
took advantage of the fact that the "control voltages" emitted by the
control board are indeed PWM signals. The PWM signals were fed to 4053
switches to build crude PWM-controlled panners, continuous filter
pole-mixers, or CV crossfaders.

For VCOs and VCFs this won't work well - the reason is that you need a
big dynamic range for the attenuation factor if you want to cover the
range of audio frequencies (a 1 to 1000 ratio in the attenuation
factor). Say you want a control on frequency with a resolution of 2 Hz
and go up to 10kHz. This will require at least 12-bit of resolution
for the PWM ratio. If you want the carrier to be easy to filter,
you'll want it to be at least above 40kHz. This gives a 160 MHz master
clock for the PWM. Ouch! And at this rate, I doubt a CMOS switch will
catch up with such small pulses...

Olivier

On Wed, Oct 31, 2012 at 12:40 AM, Russell McClellan
<russell.mcclellan at gmail.com> wrote:
> I recently read the Jim Williams chapter on his famous "zoo circuit"
> in which he recounts the design process of a high-precision,
> battery-powered VCO.  The critical insight was apparently using a CMOS
> inverter as an SPDT switch - signal A is connected to the Vcc pin,
> signal B is connected to the ground pin, then the inverter input
> selects whether the output is connected to signal A or signal B.
>
> That got me thinking - if you connect the ground pin to ground, and
> vcc pin to a signal, and the input to a high-frequency PWM from a
> microcontroller, and follow it with a low pass filter that passes
> audio but stops the PWM carrier frequency, you'd have in effect a very
> cheap computer controlled voltage gain stage.  This could be used as a
> control element in many synth circuits. One could perhaps very cheaply
> make a full computer-controlled analog synth, using analog
> oscillators, filters, and amplifiers controlled in this way.
>
> Is this a crazy idea?  Would it work?
>
> Thanks,
> Russell
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