[sdiy] Voltage references in VCO

[elmacaco]

Right, but I think you can have the great sound without tuning drift, but
the other drifting may be a factor.

I remember playing a very stable minimoog and the older roland SH synths and
tuning was very stable yet they have that something.

Kevin Lighter identified some jitter that appears to be slight noise
modulation of the pitch, I wonder if dialing in a small amount can really
result in that sound.  I'll have to try it on some of the weaker oscillators
around here and see if it helps.

This maybe of interest, it's where Kevin has a short video clip of a scoped
moog oscillator showing this jitter.

http://matrixsynth.blogspot.com/2006/01/why-moog-sounds-like-moog.html


----- Original Message -----
From: "Ian Fritz" <ijfritz at earthlink.net>
To: "René Schmitz" <uzs159 at uni-bonn.de>; "elmacaco" <elmacaco at nyc.rr.com>
Cc: "'Synth DIY List'" <synth-diy at dropmix.xs4all.nl>
Sent: Friday, April 14, 2006 6:36 PM
Subject: Re: [sdiy] Voltage references in VCO


> At 05:00 PM 4/14/06, René Schmitz wrote:
> >Hi all,
> >
> >elmacaco wrote:
> >>That sounds interesting,
> >>It would be cool if you could use it to modulate osc tracking as well,
for
> >>authenticity ;)
> >>But that's assuming it is the drift in the oscillators that is what is
> >>pleasing.
> >
> >I think its not that simple. But drift is part of it. Here's how I see
it:
> >The thing is, what is drift actually? It might be thermally, this is what
> >we are mostly focussed on, but there are surely other factors. Like 1/f
noise.
> >
> >In essence a voltage reference, an opamp, comparator or any other
> >component has its own noise, some of that with so low frequencies that we
> >can measure it on a timescale that aproaches minutes or even days or
> >weeks. (This is then what you'll find as long term stability on
> >datasheets.) This doesn't have to do with temperature as such, since such
> >measurements would be made in a thermostated environment. The lower the
> >noise, the better the reference. (Neglecting thermal influcence for now.)
> >
> >If my interpretation is right, then for a controlled modulation of an
> >ideal VCO (if that were possible) you would need a source of randomness
> >that has a zero DC effect, and a spectrum which falls off sharply below
> >some fractions of Hz. So all influences that you might interpret as
> >detuning, cancel out on average.
> >Say a rise in pitch of 10 cents that would average out in weeks wouldn't
> >be acceptable, but say 1 cents over timescales of 10s of seconds would
perhaps.
> >
> >Of course temperature fluctuations have a similar spectrum on similar
> >timescales. But you have another variable than only time, and you can
> >measure that. As Ian has said, you can cancel the net effect of thermal
> >influences. For example with some sensor and appropriate scaling
circuitry.
> >
> >>I don't know what it is, but I find older oscillators much more
interesting
> >>straight tone wise then newer ones.   Moog, older Roland (system 100,
> >>SH-09, -2,-5, & -7) pleasing in a similar way yet the rolands don't have
> >>much drift in my experience.
> >
> >I think part of that has to do with the modern components. Low offset
> >drift OPs mustnot have much 1/f noise, so the "interesting" range of
> >"drift" is also reduced. With noisier components you buy the
"interesting"
> >and the "annoying" in one package. The tradeoff to me seems to be to have
> >enough to be interesting and little enough not to be annoyed.
>
> OK, so to add to my TCDS idea you could make a module that puts out that
> old-timey random opamp noise.  Just make a chain of 741 amplifiers with
> input to ground through a carbon comp resistor.  Maybe add a filter to
> shape the spectrum.
>
> My Roland rompler has a parameter for "analog feel", and I have it turned
> on most of the time.  Doesn't sound too bad.  So I don't disagree with
> having some irregularity.  I just hate slow tuning drift.  In fact I think
> I hate it as much as Harry hates B**s.
>
>    Ian
>