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

[cheater cheater]

Might I add: I think this might be the single most important reason
why old synth VCOs sound 'fuller'.

D.

On Sat, Feb 20, 2010 at 23:08, cheater cheater <cheater00 at gmail.com> wrote:
> 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/
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>