Oscillator 2 and basic modulation
First, the obvious application: two oscillators playing together in unison generally sounds thicker and fatter than one oscillator.
First, here's a bassline played with just a straight saw wave from one oscillator.
Now, here's the same riff with two oscillators playing in unison - it sounds richer, thicker, and more interesting than the solo oscillator.
If you listen closely, you'll notice a slight phasing effect in that second mp3 - this is because the two oscillators cannot be perfectly in tune with each other, so there are all sorts of tiny fluctuations and phase relationships at work. Older synths like the early Moog modulars had notoriously unstable oscillators - they'd drift in and out of tune and were much more prone to fluctuation. Many theorize that this is a major source of "fatness" in analogue synths, so many modern digital synthesizers even include an "analogue drift" option that introduces minor tuning instabilities among their oscillators.
Intentionally tuning the oscillators further apart is generally called "fat tuning" - many synthesists will use three oscillators, and tune one slightly above and the other slightly below the middle one in order to get a fatter, thicker sound spread.
But what if we want to go further? The two oscillators can be tuned in any relation to each other. Here's the second oscillator tuned two octaves above the first one. It's a subtle effect, but one that might make a lead stand out more. Now I've tuned the second oscillator to a fifth above the first. It kind of reminds me of an old Commodore 64 game sound or something. With a third oscillator, we could even start to make basic three-note chords triggered from one key.
While it's fun to layer two sounds together, a much more interesting and obvious use of two oscillators together is modulation! By tuning the second oscillator down into lower sub-audio frequencies, I can use its output as a modulation source, patching it to modulate the pitch of the first oscillator for a subtle vibrato effect - or I can crank up the amount of modulation applied to the oscillator's pitch to take it straight into outer space.
Starting with a basic single-oscillator sawtooth wave, I patch the low-frequency triangle wave from oscillator 2 to the linear pitch input of the first oscillator. As the little riff plays, I start to crank up the amount of signal applied to the oscillator's pitch input - it begins as a little warble, and as I turn the knob, the sound oscillates more wildly and over a greater range until it's in outer space territory.
Here's the same thing, but with the square wave. I also vary oscillator 2's frequency (speed) in this clip, from the normal starting speed to significantly faster and significantly slower speeds. You can hear what a dramatic difference that merely varying the speed and amount of modulation applied can have on the sound.
And finally, the same as before but with the ramp wave. I can also use the sine wave and sawtooth shapes, but they're variations on the ones that I've just displayed, so I chose to stick with the most dramatic examples.
And finally, we can't forget pulse width modulation. Unlike the other waveforms generated by the oscillator, the pulse wave's actual waveform can be shaped and modulated, which I talked about the basic idea of the pulse wave and pulse width modulation in one of the early entires in this blog. By patching to oscillator 1's pulse width input, I can modulate the shape of its pulse wave. Here's some basic pulse width modulation - oscillator 2's low frequnecy triangle wave is modulating the pulse width of oscillator one's pulse wave. Like before, I vary the speed and amount of modulation just to show off some different sounds. The pitch doesn't change like in the previous examples, but the shape of the pulse wave rapidly changes from normal square to thin notch and back in time with oscillator 2's triangle wave. It's a cool effect, and one of my favorite synthesizer sounds for melodic leads.
Well, those are just some basic examples of modulation - either or both of those oscillators can be kicked into low frequency mode and then used to modulate anything else that can be modulated on any other module, so there is really a world of possibility - modulating the filter for rhythmic sweeps of the cutoff or resonance, modulating the amp to cut the sound in and out for a tremolo or gating effect, and so on. There's also a whole oscillator sync function that I'll get into when I write up the applications of having a second envelope generator.