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

[Magnus Danielson]

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.

Do you have the frequency or time data available in suitable text form?

I would like to do some analysis just for fun.

By the looks of it, you have both excessive white FM (1/f² phase noise) 
and flicker FM (1/f³ phase noise).

> 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.

Usually you see white and flicker phase noise pollute the data, but it 
depends on the details of the counter. What counter did you use?

> 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...
> 

Sounds very reasonable, looking at the histogram of a sine you would see 
something very similar to the histogram of this data.

Cheers,
Magnus