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

cheater00 . cheater00 at gmail.com
Sun Jun 9 10:29:52 CEST 2013

Hi Michael,
you usually specify jitter by its frequency spectrum. Jitter is
basically phase modulation by a noise source. So phase modulation by
noise that's got a gaussian spectrum centred high above your VCO's
frequency with the 2nd std dev still above it is not going to make a
lot of difference for treble waveforms, it is going to do a bit for
bass waveforms though. Then, PM  with a centre frequency around your
VCO's frequency is gong to sound like what Andre is looking for, most
likely. PM with a centre frequency far below your VCO's frequency is
going to sound like VCO instability.

With today's ultra-stable VCOs, I think it would be in good form to
have a new function, which sets this quantity. It could be a tri-axial
potentiometer which sets 1. a band pass filter's frequency related to
the VCO's frequency 2. the band pass filter's bandwidth (note: not the
same as Q; we're talking about the fall-off behaviour not the
behaviour at resonant freq; most likely well done as  a pair of 6 dB
filters, think "width") 3. amount; 4. switch (push-button) which says
whether the frequency is relative or absolute. I'd call that parameter
"phase error"

In addition to that, I believe one would greatly benefit from being
able to add this noise to the comparator voltage, so that sometimes
the VCO will trigger late, and sometimes it'll trigger early. I'd call
that parameter "reset error". So maybe quad-axial ;) I've been using
too much Tektronix stuff lately...

I also believe a LOT of organic quality can be had if VCOs had an
extra pitch input which can be slew-limited (while the original input
can still be used for fast FM etc). A lot of old keyboards sound very
cool because their VCOs have exactly that - a fixed slew limiter at
the input. It's to prevent clicking noises and EMI pickup from making
the VCOs unstable. Usually, this filter was set fairly low but you
could actually not hear it because few synths were played together
with other synths and they all just had 1 VCO. I'd call that parameter

On Sun, Jun 9, 2013 at 3:00 AM, Michael Zacherl <sdiy-mz01 at blauwurf.info> 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?
> 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.
> m.
> --
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