[sdiy] VCO Jitter, Slop ...

Terry Shultz thx1138 at earthlink.net
Mon Jun 10 23:09:09 CEST 2013

Hi all,

Jitter on a VCO is more of a problem on PLL design and A/D-D/A codec 
clocking in most use cases.

I have not put my VCO's on my Audio Precision to see what they measure 
out to.

The VCO's I build are identical to Oberheim/E-Mu VCO sub-Module/Module 
design as I was previously employed at Oberheim/ E-Mu and kept the same 
discete designs.

I am curious in plotting out my Moog VCO vs the E-Mu design to see what 
linear / non-linear error/features might show up.

Most of the time I use my ears for what I think sound best. Paul 
Schreiber may have some comments as well as he has don an excellent job 
in building up product over the years.

I do believe thermal drift on tuning is always an issue in some designs 
and I am more concerned about tuning stability than jitter issues.

Having said that, I do build a VC Clock module that is pretty solid for 
sequencer clocking. Again what is solid in 1970's design would not pass 
my needs today in ARM/DSP clocking needs.

Perhaps a bit of work can be done to plot out some successful products 
to compare against using an Audio Precision or similar tool.

best sound always,


On 6/10/2013 1:37 PM, Magnus Danielson wrote:
> 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 analysis.
> 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.
> Cheers,
> Magnus
> 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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