[sdiy] Voltage references in VCO

[Ian Fritz]

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