[sdiy] VCO reset time

[mark verbos]

How about doing both and then feeding the outputs into an XOR based ring 
mod, or any kind of logic gate array. You could get some new sounds 
maybe. Just a dumb thought... has anyone done that?

Mark



Don Tillman wrote:

>   > 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
>
>  
>