[sdiy] Unstability of oscillators and psychoacoustic qualities

[Magnus Danielson]

From: media.nai at rcn.com
Subject: Re: [sdiy] Unstability of oscillators and psychoacoustic qualities
Date: Sun, 22 Sep 2002 12:01:23 -0500

> 
> >If you recall, much have been discusses about the unstability of
> >>oscillators and that this or that oscillator sounds "cold" where as this
> >>and that oscillator sounds "warm".
> 
> Yes, for about 10 years, and in the course of that time, most of them have
> been based entirely on subjective opinion, biased or otherwise.  In
> general, people will defend the oscillators they own, and attack
> oscillators they don't.  Such seems to be human nature.

Well... it is allways interesting to bring some order in these matters... but
it seems like a hard thing to capture, so a new approach on it was what maybe
is needed, I don't know...

> >Now, in one end we know that unstability in frequency is a bad thing, since
> >then we need to retune the damn machine. The classic is the MiniMoog which
> >>you had to turn on at least half an hour before the gig start, and if you
> >>forgot you have a hell to keep it in tune all of the time.
> 
> Yes, which is one reason why my MiniMoog is going into storage unless
> someone shows up with a wheelbarrow full of money :)

On the other hand. It is known that new style VCO boards at least have a
design-flaw which makes them less stable than there electronics need them to
be... the Tempco resistors is not siting anywhere near the expo-transistors!!!
A friend (Polarn-P) modified his MiniMoog by just moving them over to the
right place and themically connect them. MUUUUCH more stable. Another of the
misssteps in the art of crafting the perfect analog synth.

Otherwise... I know a home lacking a Mini... ;O)

> >Another aspect is that a certain unstability is claimed to make the sound
> >>more "alive" or "warm". The lack of this unstability is also claimed to
> >>make the oscillator "cold".
> 
> Imho, that is an over generalization -- stable analogue often sounds fine,
> and shaky digital almost always sounds terrible.

Indeed. There is a limit to everything. So, this is the question:

Which amounts of phase noise, and of which frequencies will cause an oscillator
to feel "warm"?

> >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.

That's not quite true... the filter is a multi-edged sword. It does not only
messes with your amplitude spectra, it also messes with your phase. A filter
will change its phase resonce on change of frequency, Q, responce curve, i.e.
any of the controls that effects the characteristics of the sound. Noise in a
filter could cause the phase noise of the signal to increase, since you indeed
is able to modulate the phase of the signal.

Non-linearities in a filter causes overtones. Maybe there is intersting
interplays between the allready excisting overtones and these generated
overtones, I don't know...

> >However, what is the goal of measurement? Well, I would like to learn
> >what form of unstability which sounds "cold" and which sounds "warm".
> >I.e. it would be nice to quantify by both reasoning and by
> >>measurement/listening-test what is "cold" and what is "warm". Interesting
> >>would also to know when things go overboard from "warm" to "too damn hot".
> 
> I would qualify "warm" as having the characteristic of acoustic
> instruments, and use them as your basis of quantitive analysis rather than
> "warm" electronic instruments.  I would define "too damn hot" as out of
> tune.

Well... out of tune really means that your average frequency has drifted off
the mark. You can have a horrendous amount of phase noise while still on the
average be right on the mark (given that your averaging have been given enought
time to make a good judgement).

> >I've considered doing this for quite some time, but I just discovered
> >that I got alot of free time ahead of me, so why not do something usefull
> >>of it?
> 
> The most useful goal would be to come up with mathematical explanations,
> for that seems the only way to have any chance of settling the issue.

Indeed. But I first wanted to make a set of measurements in order to better
define the sweetspot area. Once it is better known it can be compared with
math and eventurally a clearer picture could possibly emerge. On the other hand
it can totally flopp. Who knows? ;O)

> I look forward to hearing your results.

Goodie!

We'll see if I get my act together and actually measure things...

Cheers,
Magnus