Bad Stone VCR: how does it work?

[Taylor]

I'm curious to understand how the Bad Stone's inverter-as-resistor works. We have an oscillator feeding an inverter, and the output of the inverter goes where the variable resistor normally goes in an opamp APF stage. There's a 22k fixed resistor from this point to ground.

Just from looking at the schem I can't really understand the nature of the oscillator. Is it a PWM oscillator, or just a plain jane analog triangle or sine? Since the inverters are in digital mode, I figure it must be a PWM signal, but please correct me if I'm wrong here.

Is the inverter being used as a VCR or more like a switch at an above-audio frequency?

[StephenGiles]

I had this explained to me once by one of the EH UK electronics guys, but never wrote it down. I do remember he used the word "trick" a couple of times!!

[Taylor]

  Oh... the +v supply is being modulated by the same oscillator... hmm.

[slacker]

I think to understand it you need to look at the internal schematic of the invertors, it's basically being used as a bunch of mosfets acting as VCRs.

[Taylor]

Yes, you're right about that. Unfortunately these kinds of tricks resist being simulated properly so I can't play around with the idea except with breadboarding I guess.

I'm wondering if it's necessary to modulate the supply voltage for this to work, or if that just gets the resistance into the ideal range.

[Taylor]

I guess I'm talking to myself here, but maybe somebody will find this interesting. Here's a limiter using a variation of this idea:

http://www.aronnelson.com/gallery/main.php/v/Caseys-Vintage-Effect-Page/album30/album60/ETILimiter_p3.jpg.html?g2_imageViewsIndex=1

Here the ground and power pins are both grounded, not modulated, the control signal goes into the inputs only, and three inverters are paralleled to make each "resistor". I've seen another schem for an old phaser project which does the VCRs like the Bad Stone, but without modulating the supply voltage, so I guess that is not necessary.

The reason I'd like to figure this out is that I'd like to make a tremolo using some unused inverters in a 4049 that is otherwise in use. I thought it would be fun to make a multieffect using only a single active part. The oscillator is simple, but it seems tough to make the volume modulation work. I think I can use up to 3 inverters for this.

[Lurco]

simply search or google long enough for Hoshuyama and Irwin or Osamu and Vsat.

[Taylor]

I guess this is what I'm looking for?

http://www5b.biglobe.ne.jp/~houshu/synth/VcfMos0206.GIF
http://www5b.biglobe.ne.jp/~houshu/synth/CmosOta.GIF

Good info, not sure this will work because I want to use a single 4049 for regular purposes and for this purpose. But I think it's enough to get me going on the breadboard.

Edit:

http://electro-music.com/forum/phpbb-files/hex_vca_312.jpg

This is really cool. Not applicable for me, though.