[sdiy] VCO Jitter, Slop ...

Fast TriggerFish fasttriggerfish at gmail.com
Tue Jun 11 11:41:15 CEST 2013


On 11/06/2013, Fast TriggerFish <fasttriggerfish at gmail.com> wrote:
> What if you started on the assumption that the sound of classic VCOs
> can be closely replicated by feeding a perfect VCO into one saturation
> stage followed by a linear filter and a third saturation stage ?
> Then parameterise each section ( say input and ouput levels for 2*tanh
> sections, and a 4 band parametric eq for the filtering stage ).
> Feed a few periods of various perfect waveforms through it ( with
> adequate oversampling ) at various key frequencies and regress the
> parameters against classic VCO waveforms so that the output of the
> final stage matches the classic waveform in a well chosen measure ( I
> have to admit I'm not sure which one, maybe the least squares of the
> ampitudes and phases of an FFT ? )
> Maybe also regress some form of bleed of 50/60hz and harmonics into
> the oscillators.
> Then once you have regressed all your parameters for a set of key
> frequencies, decide on a way to interpolate them.
> Finally, check if the ears agree with your resulting VCO, and maybe
> tweak things by ear a little.
> If it works you may be able to store the final "regressed VCO" output
> into tables so that you don't have to actually compute these stages
> each time.
>
> Just thinking aloud here, If I was a DSP developer with access to
> hardware and wanted fat oscillators that's probably thing that I would
> try first.
> Does that sound reasonable ?
>
> JT
>
> On 11/06/2013, Terry Shultz <thx1138 at earthlink.net> wrote:
>> Hi Folks,
>>
>> I agree with the premise of setting up a test case and proving or
>> disproving a hypothesis.
>>
>> Plotting results and commenting on what jitter or Phase distortion
>> issues sound best.
>>
>> Listen to a piano and monitor with a strobe tuner. You will see that
>> there is modulation between strings. Some is okay but at what point does
>> it interfere with performance on an instrument.
>>
>> I tuned pianos on occasion for extra money and voicing is a bit of art
>> of listening.
>>
>> If you tune a piano by way of strobe tuner, the piano sounds terrible.
>>
>> Use a tuning fork and use your ears and the tuning will usually sound
>> better as all pianos are not exact in nature. I am sure this is well
>> known by our list of folks. Determining if a Digital Osc. vs. Analog VCO
>> is better or worse is subjective to the listener.
>>
>> I find it interesting that many on this list are preferring Analog VCO
>> design. I can easily build reliable and stable and perfectly in tune on
>> my DSP or ARM products. I found that they sounded sterile in most cases
>> and still enjoy going back to my analog synths.
>>
>> I found synths having a lower frequency modulation made a much fuller
>> sound and fatter sound .
>>
>> I was playing with my friends synclavier a few weeks back and it never
>> gave me the same lower end phase modulation as my E-mu/Moog/Oberheim
>> vco's do. You can try all types of experiments and jitter redux, but in
>> the end your ears will tell you what you need for a particular effect.
>>
>> I saw a note earlier about jitter and such. I tend to think jitter may
>> be so small that I doubt you can notice it. I may be wrong, It might be
>> a good experiment to try with a blind series of tests with folks and
>> perhaps Dr. Diana Deutsch at UCSD can comment on this the next time I
>> see her whilst I am in San Diego next.
>>
>> (I am in Germany again) interviewing for a new position and will do some
>> testing when I return back to my office later next week.
>>
>> Phase modulation and Psychoacoustic illusion's are Dr. Deutsch's work
>> area . Shepard tones etc.
>>
>> http://deutsch.ucsd.edu/psychology/pages.php?i=101
>>
>> Hopefully I am not too off topic as I cannot sleep well yet do to time
>> change and so forth.
>>
>> best regards to you all,
>>
>> Terry Shultz
>>
>> On 6/10/2013 3:00 PM, Richard Wentk wrote:
>>> 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.
>>>
>>> Richard
>>>
>>> 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 synth-diy.org
>>>>>> 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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