[sdiy] Chris Synths polysynth

[Mike Bryant]

Good links.  They also led to this one where the writer suggests bit-reversing the counter to give a simple form of PDM replacing PWM.  Definitely an idea I'll bear in mind.

https://www.edn.com/three-paths-to-a-free-dac/



-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Steve via Synth-diy
Sent: 22 October 2021 14:24
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] Chris Synths polysynth


I found these articles interesting, but have not tried in practise

https://www.edn.com/cancel-pwm-dac-ripple-with-analog-subtraction/

https://www.edn.com/fast-pwm-dac-has-no-ripple/




Am 22.10.2021 um 12:15 schrieb Vladimir Pantelic via Synth-diy:
> 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-circuit
> s/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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