question for theorists: zeros & phase

[Magnus Danielson]

From: Haible Juergen <Juergen.Haible at nbgm.siemens.de>
Subject: RE: question for theorists: zeros & phase
Date: Mon, 27 Sep 1999 13:12:46 +0200

> Let me put it this way: The whole concept of "phase" makes assumptions
> that idealize reality. In many cases it's close enough to real life, and
> that's where
> it works and does a great job for calculation. 

Oh Martin, how did you plan to handle phase at DC? I have not yeat been
given a 45 deg DC feed, -48V has happend, polarity is _another_ separate
issue.

Hint: You are saved by the bell (the formulas) ;)

> But the more you leave real conditions, the more you get strange results.
> Does phase "jump" ? Not in real life of filter technology. (And let's not
> talk
> quantum physics here (;->)) Phase can *rapidly* change polarity at the
> maximum
> of a high Q filter. Now the theorist comes and says it "jumps" in a filter
> with "infinite Q". And maybe he forgets for a moment that "phase" only makes
> sense on a periodic signal. But when a signal is periodic, it doesn't change
> it's form in infinite time, i.e. there is no way to approach the resonant
> peak
> of a filter from one side and cross it, with such a signal. Unless you
> approach
> it with "infinite slow change of frequency", of course. But then you'd spend
> an infinite amount of time, and a phase difference of 360 degrees might
> be swallowed by that.

Rigth. Let's not forget that this piece would be infinitely booring to listen
to and that the Universe as we would know it would be just gone before you
come mid-piece. But even with such a piece you have broken the conditions
for using the Fourier transform, so it's wide use is a bit questionable as
THE way to characterize an audio system.

At work I have this picture stuck up in my room. The picture is from Bob Pease
"Troubleshooting Analog Circuits". It's a Ziggy strip with a man visiting his
doctor with a larchis animal chewing on his foot while the doctor concerned
says "I'll have to run some tests, but I'm pretty sure that it's either an
alligator or a crocodile." (the viewer can see the same). The point is, some
things are not worth analysing further, since it doesn't help you in the
problemsolving (in this case actually remove the alligator/crocodile from
the poor mans leg). I think it applies as well to your phase trouble.

Cheers,
Magnus