[sdiy] Innovative Oscillator Design (or not ?)

Roman Sowa modular at go2.pl
Wed Apr 10 10:48:47 CEST 2024

I agree that "not give enough detail" is the key here, although your 
idea was clearly described in the document. It said even 40kHz PWM if 
I'm not mistaken. But regardless of PWM frequency, there is also its 
resolution. You wanted cheap micros with 8 bit PWM. You could even make 
10bit PWM above audio with small PIC. So assuming top frequency of 
10kHz, the lowest PWM = 1/1024 only gets as low as 9.77Hz. One PWM step 
more and we have 2/1024, that is one octave higher. So that was my 
concern about unusable resolution at low end.

Now you're telling that planned range is only 4 octaves. Then yes, this 
might work with 10-bit PWM, but still it will not sound clean but FM 
modulated at low end, because PWM has to be adjusted every few cycles.

You could add octave range resistors from Vref with a MUX in the 
integrator, so each octave has better resolution of pitch.

And I dare to dissagree with your intuition - those steps will be 
audible, and not as harmonic distortion, but as aliasing whistles. Not 
very loud probably, but will be there. Like aliasing from short 
wavetable DDS with no interpolation.

Of course I encourage you to build it, that concept is interesting and 
I'm sure many people will enjoy seeing the results.


W dniu 2024-04-10 o 03:32, mjbauer at iprimus.com.au pisze:
> 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,
> 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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