[sdiy] Timbral musings

[Magnus Danielson]

From: Ian Fritz <ijfritz at earthlink.net>
Subject: RE: [sdiy] Timbral musings
Date: Mon, 17 Feb 2003 07:51:04 -0700

> Hi Paul --
> 
> At 07:07 AM 2/17/2003, Paul Perry wrote:
> 
> >..wave shapers are often 'disappointing' because the
> >harmonics produced have a constant phase relationship
> >(exactly what a real-world instrument DOESN'T have).
> >I'm not knocking wave shapers, they are a terrific tool,
> >but running a static program source into them is not
> >terribly useful... they rally shine for LFO modification, though!
> 
> Let's see if I follow what you are saying.  You mean that as a waveform is 
> changed dynamically by a shaper that the harmonics change in amplitude but 
> not in phase? I must admit  I never even thought about that.  Certainly, 
> when you sweep a filter there are strong phase changes.  Maybe this is part 
> of why filter sweeps are "stronger" than shaper sweeps.

It's actually not correct either.

Let's take PWM for instance. Depending on weither your PWM is made from a
sawtooth or a triangle you have different changes in phase. Sawtooth based
PWM has significant phase-modulation with the CV, where as triangle based
PWM has none, it only has changes in amplitude of the harmonics.

This phase-shift can be understood by the movement of center of the waveform
as it is being modulated, where as the triangle case has a stable phase just
as the Fourier series derivation usually gives it. Also, since it gets worse
by frequency (linearly), it will become a pitch shift and not a frequency
shift.

The phase modulation of sawtooth based PWM can cause pitch shifts during
modulation.

Naturally, similar examples of CV-controlable waveshapers which may or may not
have phase-shifting properties could be analysed. Since there is a tradition
to only view the harmonic strengths but not phases, so old derivations may
need carefull reevaluation before being judged.

So, phase-shifts are there and at work and even cause for problems, so I think
it's not as easy as pure phase-shift. A phase-shifter is interesting because
the way it modulates the overtones over time and brings life to them, but it
also contributes to phase-modulation and the total phase-noise of the signal.

Cheers,
Magnus