SV: Re: SV: Re: [sdiy] Noise as one unsteady tone!

[Magnus Danielson]

From: karl dalen <dalenkarl at yahoo.se>
Subject: SV: Re: SV: Re: [sdiy] Noise as one unsteady tone!
Date: Tue, 6 Sep 2005 16:04:08 +0200 (CEST)
Message-ID: <20050906140408.93231.qmail at web25510.mail.ukl.yahoo.com>

This answer has been somewhat delayed, sorry!

> 
> --- Magnus Danielson <cfmd at bredband.net> skrev:
> 
> > > --- Richard Wentk <richard at skydancer.com> skrev:
> > > 
> > > > At 12:07 05/09/2005, Magnus Danielson wrote:
> > > > 
> > > > >If you have a very selective filter you will find an unstable "tone"
> > > > anywhere
> > > > >you look, but what you hear is nothing but a narrow filter stimulated by
> > > > noise
> > > > >and not a characteristic of the noise itself. Not that it is not
> > usefull,
> > > > but
> > > > >it is a different matter.
> > > > 
> > > > This is one of those times where theory collides with the real world.
> > > > 
> > > > In reality if you patch together enough sine oscillators - like a few 
> > > > thousand - spread randomly across the frequency band, you get a
> > reasonable 
> > > > approximation of noise.
> > > 
> > > The reason i asked  was that i recently did some laborating with
> > > PM, two VCOs and a delay line, this gives pass zero modulation, 
> > > but strangely enough it sounds better then two pass zero oscillator,
> > > one modulating each other, more dynamic, the wave looks different too.
> > > 
> > > In the PM example modulate enough and noise are created wich would be 
> > > the same as your suggestion, ie at strong modulations the wave folds
> > > over so many times and by doing so the amount of dissplaced overtones
> > > in the end creates noise. Supricingly it seams, at least on the scope,
> > > the power spectrum of the noise seams quite uniform!
> > > 
> > > So, then i comes to this, i have two VCOs and a delay line both
> > > a reproducing a steady tone, one are used as carrier wave the other
> > > modulates the time delay, enough modulation and i have noise, 
> > > the fundamental and all the overtones move up and down in 
> > > frequency to the frequency of the modulator wich tells us
> > > that noise can be of different origin, tousands of steady 
> > > sine waves or a simple mowing frequency PM setup!....No? 
> > 
> > Well, almost. When you do excessivly deep PM with a delay-line, you "open up"
> > enought Bessel terms of side-tones that they create a fairly dense spectrum.
> 
> But,,,,,,,there has been discussions in the past on the 
> difference between PM and FM and as i can remember lin FM 
> are PM, vice versa!

This is not true. They are very similar, but they are not the same thing.
A very simple example:

If you raise the input of a PM input by one volt, you will shift the phase of
the signal by some amount. Let's say that 1 V is equalent of 360 degrees of
phase-shift. During the transition from 0V to 1V we will experience a
modulation of the signal, where as when it stays at 1V it is the same sound as
when it stayed at 0V, unless we have some flaw in the design shifting the
character of the sound.

If you do the same to a linear FM input, you will shift the frequency of the
signal by some amount. Let's say that 1 V is equalent of 10 Hz of frequency
shift. During the transition from 0 V to 1 V we will experience a modulation of
the signal, but we will also experience a modulation as we stay put at 1 V
which is different from when it stayed at 0 V. Given the scaling we have, we
will shift the phase 360 degrees 10 times per second at a constant rate when at
1 V.

So, if you take the linear FM input signal, integrate it in a perfect
integrator, and insert it into a perfect PM modulator, then the PM will behave
as the direct FM. The difference is the perfect integrator on the input.

Now, what does this mean? Well, for a modulating sine of some frequency, you
need to double the amplitude of the sine as you double the frequency to get
the same modulation effect with FM as you would from PM when just doubling
the frequency.

> I can se that to, but what would be the rason that you get more bessel
> terms in a delay line then pushing the phase in a pass zero osc, i dont 
> get that at the moment? (no maths please) :-)

OK, the point is that in FM will high frequency material be suppressed by the
-6 dB/Oct slope as compared to the PM. So, with PM you will be "driving" the
Bessel stuff harder. Does that makes sense?

> In wich book did you read this?

Read? Book? What's wrong with just good old plain thinking every now and then?

There are books which cover these things, but I have no good reference at hand
right now.

> > Here you will notice the difference between PM and FM, so if you where doing
> > the same thing with linear FM it would behave quite different. If you insert
> > an integrator between the modulating oscillator and the delay line CV input
> > you have the FM case.
> 
> Hmmm, it seams to me that even in the old days it could have been a cheap
> way to get PM/Lin FM by using two sine/triangle models and a very short 
> delay cell.

Indeed.

> But why would the integrator funktion cause FM if the integarator
> are absolutely linear wouldent that result in to the same, ie
> it integrates whatever shape are passign to its input.

I hope my examples above helps to explain. Just because you *can* use an
integrator like that does not necessarilly mean it is a good idea. Just to
show that they are similar but different in details.

> > A dense set of sines do in some degree behave as noise but also as a tone
> > with
> > chorus. It all depends.
> 
> It would be interesting to test the tousand sine example to se what it ends!

Please, go ahead, we are not stopping you! ;O)

Cheers,
Magnus - who today contemplated on a method to generate sine out of squarewaves