Harmonics question

Stopp,Gene gene.stopp at telematics.com
Thu May 21 18:13:00 CEST 1998


It would be interesting to see the pulse position modulation circuit.

I started out messing with the DPWG as shown in Electronotes, and as I
looked at things on the scope as I messed with the modulation (and
listened on headphones). This circuit uses a pair of comparators in a
window-comparator configuration, with the outputs simply averaged with a
pair of resistors. The modulation voltage goes to one comparator, and
its inversion goes to the other comparator. This provides a
positive-pulse-ground-negative-pulse waveform. I added two more
comparators, each with half the reference (formed by R-R dividers), to
create a five-step window comparator, giving the following output:

ground
halfway-up step
all-the-way-up step
half-way step
ground
half-way down step
all-the-way down step
half-way down step
ground

(for a triangle input, of course)

As the modulation reference is moved from ground up and back down
through ground, the wave output goes from a squarewave to double-pulse
to double-double pulse and back. It sounded great - nice hollow harmonic
sweeps.

 - Gene

 ----------
From: Ian Fritz
To: Stopp,Gene
Cc: 'synth-diy mail list'
Subject: Re: Harmonics question
Date: Wednesday, May 20, 1998 10:07PM

Of all the work I did on novel waveshapers, the double-pulse waveform
generators (DPWGs) were far and away my favorite circuits. There were
actually several different versions, only one of which went to EN.

The latest incarnation is a totally voltage-controlled module in my
hybrid synth. It has two pulses of equal width. The first has positive
polarity and the second is switchable to either positive or negative
polarity.

The pulse widths are under voltage control as is the position of the
second pulse. This allows "pulse position modulation" as well as the
more familiar pulse width modulation. I use this module all the time,
but more for the wide variety of timbres it can produce than for the
sweeps.

  Ian



Stopp,Gene wrote:
>

> Speaking of interesting harmonic sweeps, this reminds me of a pulse
> shaper that I came up with a while back that produced really
interesting
> sounds. It was an expansion of a circuit in Electronotes (the "Dual
> Pulse Waveform Shaper") which used four comparators to create a
> symmetric-around-ground pulse width modulated waveform. If you feed it
> with a triangle wave and sweep the PWM input, you will get a PWM'ed
> sound with only odd harmonics (i.e. satisfying the mirror-image
> requirements). I'd forgotten about this - now I want to build one
again!
>
>  - Gene
>



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