[sdiy] VCO reset time

[Ian Fritz]

Hello All --

I hadn't looked at this question closely before, so I decided to set up an 
experiment.  My double pulse shaper:
http://home.earthlink.net/~ijfritz/sy_cir5.htm ,
being an independent module, can be driven by any signal, so it is easy to 
switch quickly between tri and saw input.  With the "timber" control at its 
extreme, only a single pulse is produced.

This simple-sounding experiment is actually a bit tricky to set up.  You 
have to be careful that the same span of pulse widths is covered in the two 
cases, which will not be true if the driving waveforms do not have the same 
amplitudes and offsets, or if the shapes are not ideal.  I got this as good 
as I could manage.

What I hear is that the two signals sound the same for slow modulation 
speeds, but that small but definite differences build up as the modulation 
rate moves into the low audio range.

I suppose this is what is expected, based on the fact that static phase 
differences cannot be heard but dynamic ones may.

   Ian


At 01:02 PM 6/6/2004, JH. wrote:
>The two of you have convinced me.
>So there is no angle modulation involved.
>What I remembered was that there are sidebands on the harmonics
>(not the fundamental) created - much less than in the saw pwm case,
>but still there. Of course the sidebands are completely explained
>by AM - no FM/PM needed.
>
>The "moving notches" (comb filter effect) can certainly be traced back to
>AM as well.
>
>The sidebands which are created from modulation (as opposed to
>simple waveform / sound changes of a very slow, quasi static
>PW change) are responsible for the remaining chorusing / detuned
>effect. It's more prominent at low notes (for a fixed PWM rate
>and depth), as the relative frequency deviation from sidebands
>of harmonics to original harmonics is bigger there.
>
>
>JH.
>
>
>
>----- Original Message -----
>From: "Don Tillman" <don at till.com>
>To: "JH." <jhaible at debitel.net>
>Cc: <cfmd at bredband.net>; <synth-diy at dropmix.xs4all.nl>
>Sent: Sunday, June 06, 2004 6:25 PM
>Subject: Re: [sdiy] VCO reset time
>
>
> >    > From: "JH." <jhaible at debitel.net>
> >    > Date: Sun, 6 Jun 2004 14:14:12 +0200
> >    >
> >    > I only was partially surprised, because tri based PWM still
> >    > _sounds_ like having some angle modulation components; it's just
> >    > surprising when you look at the modulated waveform and see that
> >    > its symmetry is is never changed.  I would have to dig this up,
> >    > but I'm sure Magnus you are faster developing the formula
> >    > yourself than me finding the old calculations. If memory serves,
> >    > the fundamental of a tri based pwm has no angle modulation
> >    > component, but the higher harmonics still do.
> >
> > Email posts are not the best medium for this, so we'll just have to
> > pretend we're all sitting around a table with a large pad of paper, a
> > few pens, and a pitcher of "Fat Tire Amber Ale".
> >
> > The topic is PWM from a triangle vs. PWM from a sawtooth.
> >
> > Magnus just reached over and pulled out the equations for triangle-
> > based PWM, and, strangely enough, the fundamental and all the
> > harmonics are all in phase.
> >
> > Triangle-based PWM sounds great.  It's like the harmonics are all
> > phasing around.  Yet remarkably, the phases of the harmonics are not
> > actually changing.
> >
> > What's happening is a comb filter effect.  At this point I take the
> > pad of paper and draw out the harmonic spectrum of a pulse stream, and
> > it shows that the strength of the harmonics take the overall shape of
> > a decaying full-wave-rectified sine wave.  [scribble-scribble...]
> >
> > As the pulse width narrows, the FWR sine shape spreads out.  And for
> > the theoretical case of an infinitely narrow pulse, the spectrum is
> > flat and the harmonics all have the same level.  [scribble...]
> >
> > As the width of the pulses increase, going toward a square wave, the
> > FWR sine shape compresses.  And for the case of a symmetrical square
> > wave there's a null for each of the even harmonics and we get the
> > classic all odd harmonics sound.  [scribble...]
> >
> > It's the moving shape of the spectrum that we hear as the
> > triangle-based PMW sound.
> >
> > Now...
> >
> > Triangle-based PWM and sawtooth-based PWM are essentially the same
> > thing.  The difference is that with sawtooth-based PWM, everything is
> > moving back and forth in time with the modulating signal compared to
> > the triangle version.  So the phase of the fundamental is changing,
> > and the phase of each harmonic is changing by multiples of that.
> >
> > For an audio VCO, triangle-based PWM is more "correct".  The duty
> > cycle of the waveform is changing and nothing else.  An audio VCO with
> > sawtooth-based PWM has all these other phases and timings changing;
> > that may sound better for some applications, and it makes a great
> > optional feature, but it's not what was asked for.
> >
> > For an LFO, folks care less about phase, it's more about where the
> > waveform starts.  So for an LFO it may be more reasonable to have
> > falling-sawtooth-based PWM because the effective start of the
> > waveform, the rising point, stays in place as the modulating
> > voltage changes.
> >
> >   -- Don
> >
> > --
> > Don Tillman
> > Palo Alto, California
> > don at till.com
> > http://www.till.com
> >
> >