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

[Andre Majorel]

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/>
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