[sdiy] Chris Synths polysynth

[Vladimir Pantelic]

Arturia uses a dual PWM scheme in all of the their Keystep and Beatstep 
products, basically following the circuit laid out here:

http://www.openmusiclabs.com/learning/digital/pwm-dac/dual-pwm-circuits/index.html

it works nicely, BUT also introduces a >4ms slew on all CV outputs, no biggie if 
driving an analog synth and it gets hidden in the attack, but a real nuisance if 
it gets sampled by a digital module...


On 10/21/21 11:34 AM, Tom Wiltshire wrote:
> Here’s the correction:
> 
> Imagine you have a DAC with 4-bit resolution. If we use one with a step-between-values of 1V, and then add the output of another with a step-between-values of 1/16th of a volt, we can make an 8-bit DAC. That’s basically what you do with the PWM. The process is helped by the very good linearity of digital-counter-based PWM as a method of producing a voltage.
> 
> I’ve used a 3-bit PWM scaled to produce 1V per step and a 4-bit PWM scaled to produce 83mV per step to produce Pitch CVs. It worked very well and produced good CVs across the 8 octaves. The reduced bit depth of each PWM DAC means that the output frequency can be higher without the PWM clock frequency having to be super-high, and this improves the effect of filtering and the responsiveness of the CV to changes.
> 
>> On 21 Oct 2021, at 09:54, cheater cheater via Synth-diy <synth-diy at synth-diy.org> wrote:
>>
>> Correct me if I'm wrong, but I think summing two PWMs will just add 1
>> bit of depth.
> 
> 
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