[sdiy] Waveforms (was dx, chorus and Spock)

harry harrybissell at prodigy.net
Sat Aug 17 04:50:22 CEST 2002


I'd say the rise time of that perfect "square" wave is very important to the
sound quality...

My PAiA 2720 series PWM output had a 100% better sound than my
professional Aries VCO... otoh one went from 0 - 0.5V...the other from 0 -10V...

so slew rate was a major factor.

I like to use CMOS output stages to buffer the sqware waves... and BTW
if you run a 2X VCO and use a divide by 2 circuit... you get a beautiful square
wave. The hollowness is very dependent on wave symmetry.

Try it and find out what a lucky man you are....  ;^P

H^) harry

Gene Stopp wrote:

> This is exactly the kind of response I was fishin' for! I knew I would open
> a can 'o worms if I used the word "better"!
>
> In my minimoog exploits a few days ago searching for oscillator bleedthru I
> did notice the shape of the waveforms and they were distinctly "warped" from
> ideal. The minimoog is what it is, and it is certainly what many could
> consider "musical". It has almost a "tube" warmth to it. Now whether or not
> the "musicality" of the minimoog is a contextual thing, meaning I like it
> because I'm so used to it, or I've heard it in so many recordings that when
> I play one I think "yeah, that's the sound!", is an interesting thought
> experiment in cognitive processes. But there is no right or wrong in these
> things - we're talking about art.
>
> I guess you could say that textbook waveforms could sound "different" rather
> than "better". I remember working on the Big Moog (that was 10 years ago -
> wow!) and I was quite surprised at how crystal clear the sounds were. Both
> brittle and ballsey at the same time. It was just so different than I was
> used to - it shimmered and sparkled and shook the walls. My words at the
> time are in Mark Vail's book. I think I said that anything else sounds like
> there are pillows in front of the speakers. I wonder if the 921 waveforms
> are just more "ideal"? Or was it some kind of "warping"? Or was it that in
> combination with the 904A? And then the 902's? Or the fact that the output
> went straight to the PA? I'm sure it was a combination of many things.
>
> But on to engineering. It seems that the circuits we use for wide-sweep
> subaudio/audio VCO's may not have the same waveshaping capability of an
> 11-Mhz-capable function generator like the HP3325. We're more interested in
> 0.01 to 25Khz range with good exponential response than invisible vertical
> lines and straight sawtooth slopes. Does a squarewave with perfectly flat
> low levels and high levels and infinite slopes between sound more "hollow"?
> When you detune two or more together, do they make a better "Lucky Man"
> sound? ("better" in quotes, of course). I know that the 4151-based charge
> pump VCO in the Rhodes Chroma has a large flat spot in the sawtooth - how
> "good" does the Chroma sound? Should we attempt to idealize waveforms when
> designing, or just take what we get and delight in the occasional accidental
> gem?
>
> This makes me want to put the scope on everything I own. Or maybe I should
> just play and enjoy...
>
> - Gene
>
> From: Scott Gravenhorst
>
> If things like *perfect* sawtooth waves sound "better" (highly
> subjective statement I think), then why in God's name would we EVER
> want to filter?  Using a filter changes the harmonic structure such
> that in most cases, the resulting wave is not a perfect anything.  But
> we do use filters.  Why?  Because doing so gives us a sound that is
> correct for our purpose at the moment.
>
> Waves are waves.  Geometric ones aren't any "better" than ones that
> don't look perfect on a scope, they just have names that describe their
> shape on a scope screen.  Some waves with warts sound good, others may
> not.  I think that what a wave looks like on a screen has nothing to do
> with whether it sounds "better" or not.  Harmonic content is what makes
> a sound, and only harmonic content.  How many of us sit there and tweak
> up a patch while watching a scope?  Special purposes notwithstanding,
> I'd say most of us just use our ears and noodle 'til we smile.  I've
> looked at waveforms after doing that and honestly, they can look pretty
> nasty.
>
> Perfect waveforms do have their purpose, such as sweep generators, or
> slope generation for accurate measuring circuits.  But this is more in
> the field of instrumentation, not instruments.
>
> I'd say Gene had a damn nice filter that was doing a great job of
> isolating the harmonics.  Other repeating waveforms whether a "perfect"
> something or not would still be made up of those same harmonics, just
> different levels and phase relationships.  I'd bet that Gene's filter
> could have isolated those harmonics in a lumpy weird looking wave too,
> as long as it was frequency and phase stable and noise free.

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