[sdiy] VCO Jitter, Slop ...

Magnus Danielson magnus at rubidium.dyndns.org
Mon Jun 10 22:37:07 CEST 2013

It's not bullshit, phase-noise (aka jitter) will be there. The question 
is if it is relevant, which is a completely different question.

The phase-noise of an oscillator will follow the Leeson-model, as 
published in Feb 1966. A synthesizer VCO does not differ significantly 
from the high quality oscillators modeled as to make the model 
completely irrelevant. The Q-value (so to speak) will however vary over 
the years, pushing the break-up point lower and lower.

So, 1/f³ and white phase noise will be there, but I won't speculate on 
there to be 1/f² or 1/f noise without spending some quality time on the 

Wither this is relevant or not for "fatness" or any other property can 
be discussed and it takes some serious listening tests in the 
double-blind category and is as usual a generally messy thing.

But phase-noise there is, since it is an oscillator, it's in the physics 
of it.


On 06/10/2013 09:49 PM, David G Dixon wrote:
> I'm going to go just a little bit further than Andre and state that I think
> all this talk of VCO jitter is complete bullshit, unless someone can prove
> otherwise with soundclips and scopeshots.
>> -----Original Message-----
>> From: synth-diy-bounces at dropmix.xs4all.nl
>> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of
>> Andre Majorel
>> Sent: Monday, June 10, 2013 9:57 AM
>> To: synth-diy at dropmix.xs4all.nl
>> Subject: Re: [sdiy] VCO Jitter, Slop ... (was: Re: Smith's
>> Evolver Desktop's spikey VCO waveforms)
>> On 2013-06-09 03:00 +0200, Michael Zacherl wrote:
>>> On 9.6.2013, at 00:11 , Andre Majorel wrote:
>>>>> VCOs suck.  That's why they're interesting.
>>>> I can't prove it but I'm pretty sure it's their jitter that makes
>>>> VCOs interesting, more than their waveforms. For human ears, all
>>>> waveforms are equally boring if perfectly periodic.
>>> if you had to put it into numbers, what would be the amount
>> of jitter,
>>> you think?
>> Hi Michael. Don't know. I experimented with modulating the
>> frequency of an MS-20 VCO with white and pink noise. As the
>> FM sensitivity knob was turned clockwise, it went from no
>> audible effect to unpleasantly warbly. I couldn't find a
>> setting that made it sound "even fatter" or anything like that.
>> My guess is that there is a rather narrow range of jitter
>> between "sterile and brittle" and "blurry and weak" and by a
>> happy accident, the state of electronics in the sixties and
>> seventies led to VCOs which fell right in the middle.
>> Recently read a very interesting bit about how much mains
>> frequency ripple there is on power rails inside synths. Wish
>> I could remember where and who.
>> Some years ago, Scott Gravenhorst posted here an MP3 of one
>> of his digital synth projects. I thought it sounded
>> surprisingly good (i.e. not DCO-like). He said he used a
>> small amount of pink noise FM on the oscillators. Maybe he
>> could tell us more.
>>> et al ... and, in general, something like a slop parameter on DSI
>>> synths for their oscillators, is that just for the
>> frequency and how
>>> much? Never had a play with it.
>> Since you can hear the difference between VCO and DCO on a
>> single note, I don't see how temperature drift&  scaling
>> errors could be the source of "fatness". They may contribute
>> to it, but they're not the essential ingredient.
>> --
>> André Majorel http://www.teaser.fr/~amajorel/
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