[sdiy] Innovative Oscillator Design (or not ?)

Roman modular at go2.pl
Tue Apr 9 18:19:36 CEST 2024


I was talking about "constant current" at specific pitch. It does not change as long the pitch is not changing. In contary to this new PWM aproach. 

Roman 

---- Użytkownik René Schmitz napisał ----

>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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>
>-- 
>--
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>http://schmitzbits.de
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