[sdiy] Unstability of oscillators and psychoacoustic qualities

[Dave Krooshof]

>>It is interesting to note that certain
>>filters is also claimed to be "cold" or "warm", such that a "cold"
>>oscillator can be compensated with a "warm" filter.
>
>Then in that case, I would analyze waveforms and waveform stability, as
>filters have no effect on frequency stability.
Incorrect conclusion to a correct analysis.
Waveform instablitilty *is* frequency instablility.
If not so much for the fundamentals - though you could prove
mathematically the fundamentals are affected as well - it's
very true for the overtones. And since a filter, even an
LPF*, is mostly used to bring out certain overtones and is judged
by it's behavior in doing so, warmness/coldness is bound
to be decribed in the same terms as used for a VCO.

The way waveform instability affects pitch instabilty, is
the same as Jurgen Haible described VCO instability:
>>Phase noise comes into a classic saw VCO core at two points:
>>a) the comparator (or whatever reset-) threshold (affects phase directly) and
>>b) the ordinary CV path that controls VCO frequency (d_phi = d_f * d_t)
>>I think (b) is the more important part.
>>JH.
Imagine ways to detect pitch:
- Time the zero crossings: it will be unstable after passing a warm filter
- Time the peaks: dito
- FFT: you will notice pitches differ from frame to frame.

Pity: frequency is quite hard to measure, as you need a time frame
to do your measurement in. The instablities discussed here happen
to be smaller then a waveform, let alone a timeframe. So you
will need good math, I'm afraid.
One way is to do correlation on the overtones for filter warmness.
Or principal components / factor analysis on FT or even capstrum
analysis. Buy a good book on social statistics to get the neck of
factor analysis. (I did that before studying sonology).

Clint Davis wrote:
>I have to imagine that three strings
>resonating together in such close proximity have a definite effect on each
>other as the pressure waves interact with each other, since each is tuned
>slightly apart.  I believe this would act completely different than just
>having three oscillators tuned slightly apart.  I wonder if anybody has done
>any research on that?

There's a lot of research done in voice analysis. The medical dept of
the university of Utrecht (.nl) is doing a lot of research.
Your ear happens to be a special thing: It manages to ignore
the beating of natural sources, and do some correlation analysis
to figure out it's actually 3 strings that are sounding.
With singers the ear's/brain's analysis works even better.
VCO's are in some respects too short of information to give
the same psycho-acoustic effect as three strings. Beating
will be more severe. With three synths running the same patch
including FM, filters, whatevers and etc the results might
be *very* pleasing.


Dave






*That's a dutch "fundamentalist" political party now.
I know I shouldn't discuss politics here, but hey, I added
the info purely for the two wordjokes.
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