[sdiy] DCO amplitude with filtered PWM

Declare Update declareupdate at gmail.com
Tue Aug 28 11:30:19 CEST 2018

Howdy list,

I’m on a long train in Japan, just thinkin’ about synths, as usual.

Does anyone have any experience using filtered PWM to control the amplitude of a “normal DCO”, ie with a transistor shorting the cap on an integrator? Looking at the Juno 6 schematics, I see they used a 7-bit DAC to feed the integrator. Some napkin/train math tells me I could use 8-bit PWM at about 90khz (easy on stm32), filter it at 9khz, and control the amplitude just fine. I’ve been using real DAC outputs of fancier chips to do this recently, which works great, but if PWM is fine then I could do 2-4 voices on $0.50 chip ¯\_(ツ)_/¯ 

anything totally wrong or silly about this? any experience here? 

Sent from my iPhone

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