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

Richie Burnett rburnett at richieburnett.co.uk
Wed Oct 31 01:03:42 CET 2012


Roland's SVC-350 vocoder used a whole bunch of CMOS analogue switches as 
cheap VCAs back in the 70's.  A quad analogue switch and quad comparator (to 
generate the PWM control signals) must have been a lot cheaper than four 
BA662s!  The filters surrounding the "VCA" inherent to the vocoder 
application automatically attenuate the PWM carrier as well.

The main problem with this type of VCA is that its dynamic range is limited. 
The carrier frequency must necessarily be high so that it can easily be 
filtered out, but then this requires incredibly small pulse widths to be 
generated reliably when you want large amounts of attenuation like 60dB. 
Once the pulse width gets too small the switch will stop responding and just 
remain in the off state.

-Richie,

PS. CMOS hex-inverters are also put to use as analogue switches in the 
TR-909 drum machine's triangle VCOs for kick, snare tom sounds.


----- Original Message ----- 
From: "Russell McClellan" <russell.mcclellan at gmail.com>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Tuesday, October 30, 2012 11:40 PM
Subject: [sdiy] Synth applications of CMOS inverter as SPDT switch?


>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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