[sdiy] Innovative Oscillator Design (or not ?)

René Schmitz synth at schmitzbits.de
Tue Apr 9 14:55:17 CEST 2024


FWIW: DCOs do change the charging current, else the amplitude will 
decrease with frequency.

Typically this current comes from a DAC via a voltage controlled current 
source.

Often using range switching because the DAC resolution alone is not 
sufficient to cover wide frequency/current ratios.

The trade off is that the current doesn't have to be super precise, 
because a small error turns into an amplitude error that is less audible.

(Other than a free running VCO where that turns into a frequency error.)


The difference of the presented idea to a DCO is how the reset works:

A DCO gets reset from a Timer (divided from a higher clock). This idea 
is free running.

And how the charging current works:

A DCO modulates charging current, this idea modulates charging time of a 
fixed current instead.


Best,

  René


Am 09.04.2024 um 13:20 schrieb Roman Sowa via Synth-diy:
> Typical DCO does not modulate charging current for integrator but 
> provides constant one. So the idea is not to apply varying voltge or 
> current, but switch it in time between one value and zero.
> Obviously the switch to be used here is the biggest challenge to make 
> it work good, but that's not the point.
>
> Roman
>
> W dniu 2024-04-09 o 11:52, cheater cheater pisze:
>> I'm having trouble understanding how it is different from a juno alpha
>> style DCO. i get that it's different, i just don't know how.
>>
>> On Tue, Apr 9, 2024 at 10:45 AM Roman Sowa via Synth-diy
>> <synth-diy at synth-diy.org> 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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