[sdiy] Innovative Oscillator Design (or not ?)

mjbauer at iprimus.com.au mjbauer at iprimus.com.au
Wed Apr 10 03:32:21 CEST 2024


Hello Roman,

Thanks for your thoughtful reply.

You may be correct with some of your criticisms, but I think perhaps I 
did not give enough detail about the role of the MCU firmware, leading 
you to some false assumptions. The firmware would not need to make "huge 
corrections from cycle to cycle"... just very minor corrections. The 
"steps" that you referred to are at the PWM carrier frequency (16kHz 
minimum), so they would not be audible. Sure, there would be some timing 
"jitter" in the output signal, but my intuition tells me it would be 
negligible... like harmonic distortion. Anyhow, I guess there's only one 
way to find out... I must build the thing and prove the concept. Also I 
am not expecting to achieve a 10kHz range. I would be happy if it worked 
over a 4 octave range with musical precision.

Thanks also for your suggestion about using a sigma-delta DAC. I will 
look into this idea. There is also the possibility to use a 
digital-to-resistance type DAC, but an objective was to minimize 
external components.

Kind regards,
MJB

On 9/04/2024 6:42 pm, Roman Sowa wrote:

> This is not doable with simple PWM. You talk about closed loop control, 
> but then it will have to make huge corrections from cycle to cycle, 
> leading to blurry, modulated waveform. Assuming 10kHz range, the low 
> end using 8-bit PWM is like 40Hz, and next step is one octave higher. 
> To make it work you have to constantly measure the VCO output voltage 
> and not just the cycle period and adjust PWM multiple times during 
> single slope.
> The slopes of the triangle will not be smooth anyway, but steppy. The 
> steps will have the slope the same as the highest frequency triangle, 
> that is when PWM is at 100%.
> PWM frequency is independent of VCO frequency, so at some frequencies 
> you might expect intermodulation that will blend all over audio band.
> 
> Interesting idea and I don't hink I have ever seen it, although it 
> highly resembles switched capacitor filter. Who knows, maybe there is a 
> way to use commercially available switched cap filter ICs to work as 
> VCO. But IMHO there is a lot more other simpler options to make VCO.
> 
> How about using common $2 Sigma-Delta DAC to control the integrator 
> switch? It makes it so much easier. Wide range precise control of 24 
> bits, every note is kept stable with single write to the DAC. The MCU 
> only writes the DAC on pitch change, and possibly check tuning from 
> time to time but it will not drift at all if you use proper integrator 
> parts.
> 
> Roman
> 
> W dniu 2024-04-09 o 08:52, M J Bauer via Synth-diy pisze:
> 
>> Attn: Synth-DIY community...
>> 
>> A while back (2022), I had an idea for an oscillator design which 
>> could be used in modular or hybrid synth's, but I have not yet tested 
>> the concept. The oscillator may be digitally controlled or voltage 
>> controlled, or both at once. I'm not sure if this technique has 
>> already been tried and proven. I haven't seen any evidence of it, so I 
>> would be grateful if anyone with a greater knowledge of the art can 
>> tell me if this is a novel idea, or a concept that has appeared 
>> before. (See attached summary.)
>> 
>> M.J. Bauer
>> 
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