[sdiy] Innovative Oscillator Design (or not ?)

mjbauer at iprimus.com.au mjbauer at iprimus.com.au
Wed Apr 10 03:42:01 CEST 2024


Thanks, René, you are mostly correct, but not all DCO's function the 
same. My concept relies on tweaking the integrator slope from cycle to 
cycle to get the required pitch accuracy. The output amplitude remains 
constant. The resulting timing jitter may or may not be a 
problem...(TBD).  The design could be modified to use an MCU timer 
module to set the output period, as in some existing DCO designs. As you 
noted, the resulting amplitude error is negligible.

MJB

On 9/04/2024 10:55 pm, René Schmitz wrote:

> 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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