[sdiy] Unstability of oscillators and psychoacoustic qualities

[jhaible]

> 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"?

As long as the VCOs are beating there is some "warmth". If you tune them
close enough so they will lock -> no more beating, "cold". (or "thin" ?)
If a modulation kicks the VCOs out of lock periodically -> "warm" again.
Different from free running, though.

> Does my oscillator sound "warm" or "cold" by itself?

I say "cold", as long as you listen to it "dry" on headphones. In practice,
however, room ambience will make delayed versions of one VCO
signal interact, and if the frequency is modulated, the phase of the
delayed signal will wander against the phase of the direct signal,
causing some "warmth".

(You can get a remarkable amount of "warmth" from the most cold
and digital sound source by running it thru a modulated delay line.
BUT this needs _time_, and thus you loose "direct-ness". I think
the big plus of analog is not the capability to be "just warm", but
to be "warm" _and_ "direct" at the same time.)


> 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.

Yes, of course! Phase noise and random frequency modulation are two
aspects of the very same thing.


> I also think that near equal frequency tones feel "warmer" than any
> of those notes on their own.

Certainly.


> 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.

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.

Some VCOs use noisy components and have that modulation built-in,
some VCOs avoid noisy components and give the choice between
stable and warm, by choosing external modulation.

We should also keep in mind that modulation does not necessarily mean
noise only. (And the noise part does not mean unfiltered "white" noise.)
IMO the magic of a "warmth" in groups of VCOs lies in a mix of
deterministic and random modulation.

JH.