[sdiy] Unstability of oscillators and psychoacoustic qualities

[Glen]

At 10:27 PM 9/21/02 , Magnus Danielson wrote:

>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 think your assumption is probably correct. Rodgers electronic church
organs (when they were still designed with analog oscillators) were
designed with circuitry that purposely introduced frequency "jitter" into
the oscillators. This was an effort to emulate the "warmth" and
"naturalness" of the sound of air turbulence (and a resulting frequency
jitter) in a traditional wind-blown organ pipe. 

They also purposely used a separate oscillator per musical pitch, and they
avoided divider circuitry like the plague. Larger organ models had multiple
sets of oscillator banks, including banks of "celeste" oscillators which
are purposely tuned slightly sharp for a special effect. Of course, the
celeste oscillators were only used for special "celeste voices", which were
sort of the pipe-organ equivalent of a twelve string guitar. In other
words, for each note you played on the instrument, your ear heard two
slightly different pitches simultaneously. Such voices are considered
especially "warm" or "lush" in nature by most organ people.

I think that you'll find a "warm" oscillator has some form of frequency
"jitter" inherent in its nature. What the best form or style of "jitter"
might be, is something that I couldn't say for sure. You might end up
making a spectral plot of the frequencies and amplitudes of the various
deviations from perfect pitch exhibited by a given oscillator, and be able
to display its inherent warmth or coldness as a 2D image or 3D shape. Such
"electronic signatures" might be of some use in characterizing elusive
audible properties of oscillator designs, both old and new.

At least, it's something that will keep you busy for awhile.  :)


later,
Glen