novice and the oscillator

[Daniel Gendreau]

The basic concept of most VCOs is based on a simple saw tooth oscillator
controlled by an input signal called a CV (Control Voltage). A standard CV
usually represents 1Volt per octave of pitch. The saw tooth oscillator will
sweep linearly upwards from something like 0V to 5V. When it reaches 5V, it
snaps back to 0V and continues. The input CV determines the rate at which
the VCO saws upward and thus the frequency of the VCO.

>From there, you can use a simple opamp comparator (comparing at 2.5V) to
convert the saw tooth waveform to a square wave. When the saw is below 2.5V,
the comparator outputs 0V, and when its above 2.5v, the comparator outputs
5V. Optionally, some VCOs allow you to adjust this compare point which
changes the duty cycle of the square wave, also known as Pulse Width
Modulation.

These are generally the 2 simplest waveforms to generate using analog
circuits.

>From there, you can use flipflops on the square wave to generate 1/2 and 1/4
frequency square waves called Sub oscillators. These can be used in
different proportions to add more bass harmonics to a waveform.

You can also use an opamp integrator circuit on the square wave to generate
a triangle wave, but the problem here is that the amplitude of this triangle
wave is inversely proportional to the VCO frequency. I think you have to use
some sort of compressor circuit which tracks the amplitude of the triangle
wave and adjusts its gain so the output is always 0V to +5V. Most synths do
not go to all this trouble because a triangle wave is not very rich in
harmonic content as compared to the Saw and square waves. IMHO, Its very
similar in sound to a sine wave and does not filter very well.

I have also seen an exotic scheme which uses an ADC to sample a VCO saw
waveform, look up the sampled value in a ROM chip and then use a DAC to
output the analog value from the ROM. This allows you to generate very
complex waveforms using a simple saw oscillator. I believe it was called
waveform city, but I cant remember the site I saw it on... Anyone?


Check out some of the Synth-DIY web sites and especially Tom G's site from
some good projects to experiment with.
http://www.mindspring.com/~vco/content.html

Happy experimenting!,
-Dan G.