[sdiy] DCO amplitude with filtered PWM

[Tom Wiltshire]

The PWM rate is going to affect your maximum modulation rate, but otherwise there’s no problem.

It’s quite possible (easy, even) to get high accuracy CVs from PWM outputs. On the Frequency Central TransEuropa transposer module, we used one 4-bit output to provide octave CVs and another 4-bit output to provide semitone CVs. Added together the two gave a wide range, and we didn’t even use all of it. Calibrating octaves and semitones is simple, and PWM is intrinsically linear if you stay away from the extremes. The advantage of the 4-bit PWM was that the update rate could be very high (250KHz) which made the filtering easy.

http://www.frequencycentral.co.uk/?page_id=1311

I expect Roland worked out that 7-bit control was about the minimum detectable for amplitude, so if you can do 8-bit control, go for it. My experience has been that you can usually get away with more than the theory suggests you should be able to get away with! E.g. Don’t rule it out until you’ve tried it - it probably doesn’t sound as bad as you think it does.

It sounds to me like you’ve pretty much worked out that this is worth a shot anyway.

T.

> On 28 Aug 2018, at 10:30, Declare Update <declareupdate at gmail.com> wrote:
> 
> 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? 
> 
> Chris
> Sent from my iPhone
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