For this tutorial, I'll be using TAL's NoiseMaker, a freeware synth that should work fine on both Mac and PC. So go grab it if you don't have it already, and follow along.
1. When you first open NoiseMaker, it defaults to an initialized patch, which is just 1 oscillator, a sub oscillator, with the filter wide open and no modulations applied. We'll be using this "blank slate" as our starting point. The first thing we want to do is to close the CUTOFF in the FILTER section. If you play a few notes, you won't hear anything. This is because we've filtered out all the frequencies in our sound. We're going to use LFO modulation to fix that.
2. In the upper lefthand corner of the GUI, you'll see a purple box marked LFO 1. This is one of two LFO's offered in NoiseMaker. First things first, click the SYNC button until it lights up. This is telling the LFO to run in sync with your host's tempo, letting you match the speed of the LFO to specific note divisions.
3.) Twiddle the RATE knob next. If you look at the CONTROL section at the bottom right of the GUI as you do this, you'll see the DISPLAY readout changing to different note divisions. Set the knob so the LFO will cycle at 1/8th notes.
4.) Next, we need to tell the LFO what it should modulate and by how much. So right beneath the SYNC button, you'll see a drop-down menu that reads OFF. This is where you set your modulation's destination. Click on this and select FILTER. The LFO now knows to modulate the FILTER CUTOFF.
5.) Next, click the KEY TRIG button at the bottom of the LFO. This will cause a key press to trigger the LFO.
6.) You still won't hear anything until you turn up the DEST 1 knob next to the modulation destination menu. Nudge it up and play a few notes and you'll probably hear something that sounds like a subdued dubstep wobble (and indeed, LFO's are key to those types of sounds). Go ahead and crank the knob all the way up and the sound will become brighter and the modulation more extreme. It still pretty much sounds like a dubstep bass, though.
7.) Any well-appointed LFO will offer a variety of WAVEFORMS to choose from. These waveforms simply allow different shapes of modulation. The reason this sounds like a wobble is because NoiseMaker's LFOs default to a sinewave. This would be great for vibrato, but we need something a bit more abrupt to simulate the retriggering of notes. So click the little green waveform display for the LFO and drag up and down until you see the SQUAREWAVE, which looks a bit like the top of a castle wall. This waveform produces a modulation that is either on or off, essentially. The effect would be less extreme with a smaller DEST 1 value, but with it set all the way up, the LFO is opening the filter all the way and then snapping all the way shut at a speed that just happens to be the speed of an 1/8 note at your host tempo. Play a few notes and you should hear a pulsing 1/8th note that repeats as long as you hold down a key.
Now, obviously, this isn't the same thing as a full-fledged sequencer or arpeggiator, but it can produce some useful effect and be fun to play around with, especially once you start layering on additional modulations. A couple of notes before I go...
• If you have a synth that allows the LFO to modulate an OSCILLATOR'S AMPLITUDE, you can achieve a similar effect, with the advantage that it frees up the filter to do something else, allowing more complex sounds. NoiseMaker just doesn't have this capability right now.
• If you have a synth that offers a descending sawtooth waveform on its LFOs, try using this instead, as it produces a sharper, but more subtle sound. On some synths, modulating the filter with a squarewave can produce clicks, but a descending sawtooth doesn't have this problem. It just so happens that this isn't yet an option on NoiseMaker.
• If you're feeling ambitious, try assigning LFO 2 to modulate OSC 1 (the pitch of OSC 1). Assign it to a squarewave, make sure SYNC and KEY TRIG are selected, set the RATE to 1/4 notes, and the DEST 1 amount to .3987. Play a key and you'll hear the note value changing. Experimentation will yield more musically-pleasing results, but hopefully you get the theory behind it.