[sdiy] VCO Jitter, Slop ... (was: Re: Smith's Evolver Desktop's spikey VCO waveforms)

Richard Wentk richard at wentk.com
Tue Jun 11 00:00:59 CEST 2013

I've experimented with most of these features using digital synthesis. It's actually easy to add individual frequency drifts to individual overtones with a controllable mod rate/depth using (say) SuperCollider.

I haven't found any reliable way to make a digital oscillator sound fat yet. I can make some awesome stacked oscillator sounds, especially if each note has a different detuning with an exponential distribution around the nominal pitch. But 'fat' - not so much.

There are a few things I haven't tried, but I'm fairly sure that bandpass filtered noise modulating the frequency won't transform a digital sound into an analog-like one.

My suspicion is that in addition to the tanh distortion generated in a filter, analog op-amps added a lot of colour with intermodulation distortion and limited bandwidth. A lot of early analog circuits use poor (by modern standards) op amps, and the sound isn't at all clean or simple. It's also not very bright - often there's not much happening at the top end.

Simulating these effects requires some fairly scary Volterra expansions, so I haven't gotten very far with modelling it yet.

It's also worth remembering that analog waveshapes are often non-textbook. A cubic sine approximation has a lot more character than a perfect clean function generator sine. The sawtooth in the early Moog 901s was nothing like a good saw. The rounded shape varied with frequency and added extra emphasis on the low partials, which made the sound bigger and warmer.

And so on. Basically perfect VCOs sound boring. And a lot of early designs were well short of perfect - in interesting ways.


On 10 Jun 2013, at 22:07, Tom Wiltshire <tom at electricdruid.net> wrote:

> I totally agree with Bruce. "Fatness" is a concept related to multiple oscillators. I don't think the human ear  (most people's anyway) have anything like the accuracy required to hear a little drift in a single oscillator.
> I also totally agree with Brian. We *really* need to test some of these hypotheses to get this nailed down once and for all. There is a widespread perception that analog oscillators (and some more than others) have some level of drift or noise inherent in their design that improves their sound, particularly with other oscillators. Digital technology has reached a point where we could/can model this property *if we can identify it*. Plenty of effort has already been made in this direction.
> My own experiments in this area have shown me that a little linear phase noise in a waveform isn't a bad thing, and that a little linear frequency wobble (if that's a different thing, which it only just is) helps too. Whether these effects are what's going on in "proper" analogue oscillators I've never been able to show. I only have CEM3340 oscillators around, and they don't show these effects, as far as I can determine.
> One important thing to remember is that even quite minor shifts in phase have a significant audible effect on the higher harmonics if you mix a pair of oscillators. Even if the main pitches remain very close, the highest harmonics will drift in and out of phase. We can easily hear this, especially on the incredibly harmonically rich synth waveforms we're typically talking about. Thus you can put in a little bit of "motion" whilst barely affecting the pitch.
> T.
> On 10 Jun 2013, at 21:25, BRUCE DUNCAN <modcan at sympatico.ca> wrote:
>> I don't think VCOs on their own can can increase their levels of obesity
>> by drift or unstable tuning or whatever. Probably only when multiple VCOs 
>> are played together that these attributes become interesting.
>> One of JHs main points was how Moog VCOs are more detuned in the low ranges
>> adding animation in the bass notes while keeping higher frequencies more or less in tune.
>> This does seem to work as I have tried it on a digital triple VCO with good results.
>> It does sound fat to my ears anyway.
>>> From: tom at electricdruid.net
>>> Date: Mon, 10 Jun 2013 21:10:09 +0100
>>> To: aym-htnys at teaser.fr
>>> CC: synth-diy at dropmix.xs4all.nl
>>> Subject: Re: [sdiy] VCO Jitter, Slop ... (was: Re: Smith's Evolver Desktop's spikey VCO waveforms)
>>> JH argued that linear drift/offset was a significant part of "fatness" in oscillators. A detune of a few 1/10ths of hertz gives motion, without sounding out of tune. Doing this with a V/Oct CV leads to errors becoming increasing large as you go up the octaves, rapidly becoming unpleasant.
>>> http://www.jhaible.com/living_vcos/jh_living_vcos.html
>>> So however much drift you apply, perhaps it should be linear FM, rather than into an exponential FM input.
>>> T.
>>> On 10 Jun 2013, at 17:57, Andre Majorel <aym-htnys at teaser.fr> wrote:
>>>> 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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