[sdiy] VCO - Saw and Ramp Outputs

[David G. Dixon]

Jerry,

I think that VCO output should be centred about ground, and most VCO
designers will take significant pains to ensure this outcome.  This way the
signal can be amplified without having to worry about clipping.

For CV, I'm not sure it matters that much, since CVs are almost always added
to other signals (to provide FM or AM, for example), and the DC level is
ultimately fixed by the voltage setting of whatever the CV is being added
to.  Of course, if you run out of room on the negative end of the control
pot, and you still haven't gotten down to the desired range, then this can
be a problem, which is why I prefer LFO outputs (and subaudio VCO outputs)
centred about ground.

Most envelopes are 0 to 10V, and LFOs can be just about anything.  My custom
LFO puts out -5 to +5V saw-triangle-ramp and pulse waves, but the pulse
waves can be changed to either 0 to 5V or -5 to 0V with a switch.  This is
pretty useful for trilling effects (hook the LFO pulse output to the Linear
FM input of an oscillator and adjust the attenuator so that the pulse wave
is creating, for example, a semitone.  Then, the positive pulses give an
upward trill and the negative pulses give a downward trill).

I'm thinking about building a very simple module which takes a control
signal input, and puts out the inverse and half-wave rectified versions of
both polarities.  It would probably also be desirable to be able to apply a
DC offset to the incoming signal, and perhaps have access to the offset and
non-offset versions of all the outputs simulataneous.  Basically, it would
have one knob and a bunch of jacks -- probably a dual module on a 1U-wide
MOTM-format panel.  My main motivation for doing this is to be able to take
the positive and negative parts of a pulse wave (with variable pulse width)
and use them separately as gates for things, to give something called
"pulsed mixing" (so-called on page 44 of Allen Strange's book, "Electronic
Music: Systems, Techniques and Controls", 1972 ed.).  I'm very fond of a
certain track by the Chemical Brothers called "Playground for a Wedgeless
Firm" from their first album, "Exit Planet Dust" and this seems like a good
way to achieve some of the effects on that track.

Those are my thoughts at the moment.  They may differ as I gain more
experience with all this synth business.

> Hello All,
> 
> Should the Saw and Ramp waveforms forms from a VCO used as the Audio
> Signal
> source be centered on Ground (i.e. -5v to +5v) or is it just as well that
> they run in the 0v to + 10v range?
> 
> What about a Low Frequency VCO used as a CV source, is it the same setup?
> 
> Jerry
> 
> 
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