[sdiy] Unstability of oscillators and psychoacoustic qualities

[Magnus Danielson]

From: Don Tillman <don at till.com>
Subject: Re: [sdiy] Unstability of oscillators and psychoacoustic qualities
Date: Sat, 21 Sep 2002 10:57:39 -0700

> Hey Magnus,

Hi Don!

> I'll suggest that the most typical cause of "coldness" in an
> oscillator is an unnatural synchronization.
> 
> A classic example would the be Top Octave Generator chips that keep
> the pitches in a synchronization with each other that you just don't
> find in nature.  Or any divide-by-N situation for that matter.

Well yes... an no!

What you get by a TOG is lack of drift between two notes. But what if you
only play one note?

My point is, yes, when you play two notes you either have common overtones
exactly (which the TOG gives you). However if you look at it, TOGs still give
you the common overtone between two notes not being an octave apart high up in
the overtone specra. So when you play two tones they add their separate spectra
up to the common overtone, which can be in the range of 50-200 (depending on
the TOG divider chain numbers). These two spectras is distinct for most of the
fundamental range, with the difference that they are indeed frequency locked
to each other. It is correct that we do not percieve any wandering effect.

Now comes the question, if we have two VCOs playing the same notes, how will we
know they are distinct? If "coldness" would be due to synchronisation, then
we should at least be able to explain it in terms where some pair of overtones
is near each other, or? If one oscillator locks to another by synchronisation,
each time it synchronises we should have a jump in frequency, thus giving a
modulation. However, then "coldness" would be due to modulation. How do we
then handle a single VCO case where we can label it as "cold" or "warm"?

My point is that the way the oscillator is being phase and frequency modulated,
as being by external control, such as other oscillators and signal, or by
internal noise like that of thermal noise is of interest. We must be able to
explain both the single oscillator and dual/multiple oscillator cases.

You can ask these questions:

Does my oscillator sound "warm" or "cold" by itself?
Does any particulat modulation of an oscillator make it "warmer" or "colder"?
Does a pair of oscillators sound "warm" or "cold" when playing the same note?
Does a pair of oscillators sound "warm" or "cold" when playing the same note
but an octave apart?
Does a pair of oscillators sound "warm" or "cold" when playing a diff notes
not being on octave steps apart?

I may have an incorrect personal assumption that "warm" oscillators have some
sort of phase noise in them. This causes them to wander about the note
frequency. I also think that near equal frequency tones feel "warmer" than any
of those notes on their own. However, I expect that tones distant from each
other does not sound "warmer" than any of those notes separate. The perceived
delta-frequency between the notes becomes low and is perceived as a slightly
modulated note. That is, low frequency modulations helps to make the perception
warm. This is my assumed model. I would like to learn the aspects of the
modulations in existence and which amounts of modulation and by which frequency
forms some border between "coldness" and "warmness".

Do we design our oscillators with low phase noise as a matter of design
concern? Do we design them with _any_ phase noise concerns? Not what I have
ever heard off in this buissness.

The number one issue for accurate oscillators (such as crystal oscillators)
is phasenoise, aging and drift. We can probably forget about aging for being
a major aspect in this case, drift will certainly be a part of it (retuning
while heating up) and phasenoise is to be discovered but under suspect.

Any modulation will cause phase noise of some sort.

Now, any theory may be good on paper, but testing it against reality one way or
another is a different thing. This is especially true when one tries to model
behaviours based on observations, a good model will then predict new
observations also for conditions not tested in the base analysis material.

You guys have however helped out by pointing in a few different directions.
More points to care about, such as synchronisation and dual tone aspects.

Does any of this make sense? Ah, maybe you should not answer that one! ;O)

PS. I do _not_ want to confuse things by even bringing BBD into the picture!

Cheers,
Magnus - allways in tune