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

[Gene Stopp]

Good info on the VCO cores, Terry! I think my question now becomes:

If the input waveshape is "textbook perfect", when a resonant VCF is
sweeping through the harmonic series, does it say "nice strong harmonic
here, OK, moving along, hmmmm nothing here, oops, here's another nice strong
one, moving along, nope nothing here..."

In other words are the spectral spikes sharp and distinct and highly
differentiated, and if the waveform is somehow warped they are "smeared" or
"blurred"? I do realize that if every cycle of the waveform is identical to
the previous one and the following one, then the harmonics should fall on
integer ratios only (right?). Spectral garbage between the integer
boundaries would imply some kind of intermodulation or jitter or some other
thing happening at a frequency other than that of the waveform in question,
right?

Perhaps the spikes in the spectrum of a textbook waveform are in "pleasing"
places at "pleasing" amplitudes. Or, perhaps the amplitude of the waveform
as it enters the filter has a "sweet spot", not so low that the signal chain
is noisey, and not so high that the blurring effect starts to happen. Maybe
the filter with resonance needs a *huge* amount of headroom in which to
perfectly pick out the harmonics. A much huger amount than traditionally
found in most synthesizers, with the knobs set to "normal" positions.
Certainly the state-variable hi-Q filters tend to squeal when driven hard!
See: Korg MS-20.

And yes, I do have a tendency to crank the minimoog mixer inputs to 10 and
adjust the master volume instead. Part of the sound. Sometimes I even run
the low output back into the external input and peg the overload lamp on
solid. Maybe I should spend some time being nice to the filter and see how
the harmonic-picking sounds?

Hey - I wonder what a good spectrum analyzer goes for on ebay. Does anybody
know of a good make/model with a comfortable audio-range display?

- Gene


-----Original Message-----
From: Terry Michaels [mailto:104065.2340 at compuserve.com]
Sent: Friday, August 16, 2002 11:13 AM
To: Gene Stopp; synth
Subject: [sdiy] Waveforms (was dx, chorus and Spock)


Message text written by Gene Stopp
> ...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.<

Hi Gene:

The Moog 921 and the Minimoog oscillators both use a similar design, which
has the collector of the expo transistor charging a capacitor, the
capacitor is discharged by a transistor in parallel with the cap.  As the
capacitor charges, and the sawtooth ramp rises, the collector current
decreases slightly, because the collector to base voltage is decreasing. 
This is due to the Early effect.  The result is the sawtooth ramp is not
exactly a straight line, it flattens slightly as it nears the top.  This is
plainly visible on an oscilloscope.  OTOH,  your ASM-1 VCO, and my original
VCO design as published in Electronotes, and probably some others, all  use
an integrator that is reset by a FET.  Because the collector voltage of the
expo transistor in this design doesn't change as the sawtooth ramps up, the
charge rate is constant, and the sawtooth ramp is straight as a ruler.

The Moog 901 uses a true constant current source to charge the cap, so the
ramp from that one should be pretty straight.  I don't have one on hand to
verify that.

I would guess the slightly warped sawtooth will sound a little different
than the perfectly straight sawtooth to a discriminating ear, although I'm
guessing the difference will be subtle.  The flattened sawtooth will
probably have slight differences in the amplitudes of its various harmonics
compared to a "textbook" perfect sawtooth.   It would be interesting to see
a high resolution spectral analysis of all of these well known VCO's of the
past.

Also, the triangular and sine waveforms in these VCO's are derived from the
sawtooth, so a warped sawtooth ramp will cause the triangular waveform to
be slightly warped also, and the sine will have some additional distortion.
  Again, the difference will probably be audible, but subtle.

Terry Michaels