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

[cheater cheater]

Here's what I guess is happening:
the vco core, whether it is a saw or triangular core, is using a
comparator to trigger the peaks. This comparator is of course using,
as reference, the voltage between ground and the voltage rail. This is
not going to be a stable voltage because of noise and hum. If you only
had white noise of some amplitude, then you would see a thick line
around 490 Hz.  This is why: sometimes the comparator would have a
lower ref voltage (and hence trigger sooner) and sometimes it would
have a higher ref voltage (and hence trigger later). This might appear
on the time domain graph as a waveform where the peaks are sometimes
higher and sometimes lower. But I cannot explain why the distribution
of your plot looks like that. Notice that FM by a 50Hz sine wave would
be, at this frequency, only a vibrato, but it is difficult to figure
out what's going on, and I don't have the right software to look at
your flac's right now unfortunately. Maybe try disconnecting the noise
source completely from the synth, and see what happens. I think it
might be putting some noise into your ground, but it's just a guess.

D.

On Sat, Feb 20, 2010 at 21:00, Andre Majorel <aym-htnys at teaser.fr> wrote:
> On 2010-02-20 17:25 +0100, Magnus Danielson wrote:
>> 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?
>
> There are three .flac files in
> http://www.teaser.fr/~amajorel/ms20/files/ corresponding to the
> first three graphs. This is how they're processed :
>
> #!/bin/sh
> tmp1=.flac2fvst.tmp
> for f; do
>  flac -cd "$f" | aufreq0 -M >"$tmp1"
>  (
>    echo "set yrange[480:505]"
>    printf 'set title "f vs. t for %s"\n' "$f"
>    printf 'plot "%s" using 1:2 with dots\n' "$tmp1"
>  ) | gnuplot -persist
> done
>
> aufreq0 is the frequency counter. It reads signed shorts. -M makes
> it write to stdout the frequency for each cycle.
>
>> 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?
>
> http://www.teaser.fr/~amajorel/misc/aufreq-standalone-2008-01-01.tar.gz
>
> It integrates the signal and declares a cycle has passed every
> time the integral becomes positive again. The integrator is
> slightly leaky.
>
> In order to estimate the period with better precision than 1/Fs,
> the counter computes a fractional frame number for the zero
> crossing by linear interpolation.
>
> If it didn't, the graphs would look very different.  There would
> be a band at 490 Hz, one at 44100 / ((44100 / 490) + 1) = 484.6 Hz,
> one at 44100 / ((44100 / 490) - 1) = 495.5 Hz and not much else.
>
>> >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.
>
> You're saying the frequency spends more time at the extremes
> because a 50 Hz sine spends more time at the extremes ? Mmm...
> Guess I'll have to trust you on that.
>
> --
> André Majorel http://www.teaser.fr/~amajorel/
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>