Additive vs subtractive synthesis confusion

[mickev]

Hello all,
I'm currently trying to learn som synthesis basics. I'm a bit stuck on these 2 basic forms of synthesis. Hope someone can sort out my confusion!

As I've understood, additive synthesis is the form of synthesis where you combine simple waveforms (sines) generated by oscillators to get a more complex wave (e.g. a square wave).

My question is how a subtractive non-additive analogue synth can produce anything but a sine wave? I don't see how a single oscillator manage to create all these harmonics required for a square (or any other "more-complex-than-a-sine" waveform).

As I see it at the moment, the way to produce the square wave is by additive synthesis where one oscillator creates the fundamental and then one oscillator for every odd harmonic is required?

Hope someone can sort out the confusion for a newbie .

 

[krushing]

I think the confusion arises from the fact that different oscillator types can already produce more complex waveforms than sines - can't say I understand the precise electronics that well but the basic premise is that there's a transistor in the oscillator that changes its output to a more harmonically-rich waveform such as a square wave, and the subtractive part - filters, LFOs, envelopes - happens after that. And of course, when you have more than one oscillator in a subtractive synth the combinations multiply.

So theoretically you're somewhat right in your assumptions, it's just that the oscillators can be a bit more complex

 

[bandcoach]

As Krushing says, its about the electronics. Generating a pure sine wave is difficult in standard analog electronics as the circuits will be naturally unbalanced due to imperfections in component values due to manufacturing tolerances - most resistors are manufactured at 55 tolerance so can vary up or down in value by that much. Same with capacitors. Transistors are the same.

Simple way to think of it though. Power supplies start with alternating current usually. They then treat this current by rectifying (changing the waveshape) so that it becomes direct current (fixed voltage source). By applying a half rectifier to an sine wave oscillator, you can create an effective square wave, that with a bit of cleaning up is pretty close. Similarly, other rectifying circuits using resistor-capacitor networks allow you to generate ramp (sawtooth) and triangle waveforms.

These outputs are then able to modified by the filters and envelopes as needed.

I'll find some links to typical circuits for you to explore.