exponential rising (was Re: definition of Syncing)

[Mark Smart]

> curious question (possibly stupid, but who knows?): what would it sound like
> if you had an exponentially rising voltage across that same cap? interesting
> sounding waveform or no?
> 
> mitch

Well the ARP Omni's (here I go again with the Omnis) tone generation
circuitry uses exponentially decaying sawtooth waves. These are
obtained by running each square wave from the polyphonic divide-down
circuitry through its own RC differentiator circuit. The RC circuits
are crudely tuned to each square wave's frequency (in some cases by
chopping off part of the ceramic disc cap and covering up the exposed
electrolyte with fingernail polish!). This produces a waveform like this:


       |.
       | ..
-------|   -------| ..--------
                  |.
                  |

This wave then goes thru a diode clipper circuit with two selectable clip
points. One selection chops off the entire bottom half of the wave,
producing the exponentially decaying sawtooth. This is the wave which
sounds more string-like and which is used on the Omni most of the time when
you hear it.

The other clipping selection just chops off a small chunk of the bottom of
the downward-going part of the waveform, making a little flat place at the
bottom (I hope my technical jargon is not too sophisticated for you!!).This
is called "hollow waveform" on the front panel, and has a much more
square-wave-like sound.

I REALLY like the sound of the string-like waveform, especially when it is
run through the Omni's great triple analog chorus unit.

************************************************
*     Mark Smart                               *
*     Network Technician                       *
*     University Communications Inc. (UCI)     *
*     smart at medusa.nn.com                      *
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