[sdiy] Interesting f vs. t graphs of pitch instability

[Tom Wiltshire]

Andre,

These are indeed very interesting graphs.

I've been thinking lately about digital modelling of the drift and  
noise in analogue oscillators, since I'm working on a digital  
oscillator design on dsPIC. And then you come along with some  
beautiful images of exactly the sort of noise and variation that I'm  
thinking about. Synchronicity!
Having a knob on an oscillator that goes from cool digital sterility  
at one end, via CEM3340 to drifty moog, and then finishing at Andre's  
wavering MS20 sounds great to me. The question is what signals do you  
need to feed into the oscillator and how to generate them efficiently.
Your graphs represent some actual data, which is very helpful indeed.

Regards,
Tom



On 6 Oct 2008, at 14:56, Andre Majorel wrote:

> The wavering on my MS-20 has become worse lately so I've started
> to look into it. Some interesting graphs have been made in the
> process :
>
>   http://www.teaser.fr/~amajorel/ms20/files/waver1.png
>
> Those are four frequency vs. time plots. On the X axis, the frame
> number. On the Y axis, the frequency in Hz. Each dot represents
> one cycle. Each plot is one minute long so should have about 5400
> points.
>
>> From top left to bottom right :
> - VCO2 driven by the CV from by the keyboard,
> - the same with preset VR4 shunted,
> - VCO2 driven by an external fixed voltage (no wavering !),
> - mathematically generated sawtooth of the same approximate
>   frequency.
>
> Note how much frequency noise there is. It probably isn't an
> artefact of the frequency counter as the graph for the
> mathematically generated sawtooth is clean.
>
> Also note how the dot density is higher at the extreme
> frequencies. I have no idea why. It might be related to the fact
> that most of the time-domain noise is 50 Hz hum...
>
> -- 
> André Majorel <URL:http://www.teaser.fr/~amajorel/>
> Do not use this account for regular correspondence.
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