[sdiy] Linear response VCOs?

[Roman Sowa]

You can safely assume EVERY vintage Korg and Yamaha synth was linear VCO.

Check the most classic linear VCO from Korg MS-20 for an inspiration. It 
was used widely in many synths.

I've posted it (and bragged about it) several times already, but since 
you asked, here's 5V single supply linear VCO:
sowa.synth.net/schem/vco8/

Roman

W dniu 2026-03-20 o 22:18, Thomas Hudson via Synth-diy pisze:
> Thanks to everyone on this thread. I am currently working on a MIDI to 
> Hertz program for the Raspberry Pi (though I can probably move to an 
> ESP32). Two last questions. Can anyone tell me which commercial synths 
> were linear response? I know about the Moog Taurus / Behringer Toro.
>
> Lastly, can anyone suggest a good design / schematic / kit for a 
> linear VCO?
>
> TIA
>
>> On Mar 20, 2026, at 7:39 AM, Mike Bryant <mbryant at futurehorizons.com> 
>> wrote:
>>
>> Well yes I was actually doing frequencies like 1004Hz (a telecomms 
>> test freq) at 8MHz sampling, but as you say the secret with PDM is to 
>> stay well away from the peak positive and peak negative values.  
>> However a little bit of high frequency noise injection (from a look 
>> up table) added to the digital sinewave helped as this also gets 
>> filtered out.
>> ------------------------------------------------------------------------
>> *From:*Tom Wiltshire <tom at electricdruid.net>
>> *Sent:*20 March 2026 10:37
>> *To:*Mike Bryant <mbryant at futurehorizons.com>
>> *Cc:*brianw <brianw at audiobanshee.com>; synth-diy at synth-diy.org 
>> <synth-diy at synth-diy.org>
>> *Subject:*Re: [sdiy] Linear response VCOs?
>> There's also differences in *where* the errors are worst between PWM 
>> and PDM.
>>
>> Generally, you worry most about the fundamental and lowest harmonics 
>> of the pulse output frequency, since they're the hardest to filter. 
>> For PWM, you get the worst fundamental and 3rd harmonic at the 
>> midpoint value, when the output is a 50/50 square wave. You get the 
>> worst 2nd harmonic at the 25% and 75% values, since that's where that 
>> harmonic peaks for pulse waves. This is all fairly "central" and very 
>> likely to be values you're using all the time. In short, if your PWM 
>> output is at 100KHz, then 100KHz is what you'll get, and your 
>> filtering had better be able to deal with that.
>>
>> For PDM, the central values are the best-case, rather than the worst. 
>> With PDM, the worst-case comes as you get to extreme values, so <5% 
>> or >95%. This happens because there's so few pulses going out that 
>> the effective output rate drops. Imagine we're using a 2MHz PDM 
>> output to create a 10-bit DAC. If we output our midpoint value of 
>> 512, we get a lovely squarewave at 1MHz - one period on, one period 
>> off. If we output a value of 1, we get one period on followed by 1023 
>> periods off - a very narrow pulse wave at 2MHz/1024= 1953Hz. That's 
>> terrible! Of course, as you approach these extremes, the amount of 
>> fundamental and lower harmonics in such a narrow pulse drops off 
>> markedly, but still - you probably weren't thinking of a 2KHz output 
>> when you designed your 2MHz PDM DAC.
>> My view is that the secret with PDM is to discard the extremes and 
>> use the good bit in the middle!
>>
>> Tom
>>
>>
>>> On 20 Mar 2026, at 09:30, Mike Bryant <mbryant at futurehorizons.com> 
>>> wrote:
>>>
>>> This is why you use PDM, not PWM.  The pulses are at much higher 
>>> frequency and easier to filter to the correct DC level with less noise.
>>>
>>> ------------------------------------------------------------------------
>>> *From:* Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of 
>>> brianw <brianw at audiobanshee.com>
>>> *Sent:* 20 March 2026 08:22
>>> *To:* synth-diy at synth-diy.org<synth-diy at synth-diy.org>
>>> *Subject:* Re: [sdiy] Linear response VCOs?
>>> The challenge with PWM is that changing a rail-to-rail square wave 
>>> into a steady DC value requires a lot of filtering. That filter must 
>>> remove the sharp rise and fall of the raw PWM output, and thus the 
>>> DC output value cannot sharply rise or fall either. The problem gets 
>>> worse if a single channel needs to feed multiple unrelated CV values 
>>> through a mux+S&H. The slew rate is horrible.
>>>
>>> Brian
>>>
>>>
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>
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