ADDITIONAL COMMON MODULATIONS SOURCES
The Mod Wheel - As you probably gathered, the "mod" in "mod wheel" stands for modulation. Virtually every keyboard synth you will run into will have a mod wheel located next to the pitch bend wheel. (Some older Roland and Korg synths have joysticks where side to side movements bend the pitch, and forward movement acts as a mod controller, but the concept is the same.) The mod wheel doesn't have a function until you assign it one, so it can do just about whatever you want it to. Common uses are to trigger a vibrato effect that can be faded in or opening the filter cutoff. But it can be assigned to anything your synth allows you to modulate.
The Pitch Bend Wheel - Since the pitch bend wheel is already hard-wired to change the pitch of your oscillators, it's not something most people use for other modulation purposes that often, but many synths do allow it. If you assign it to open the filter, your sound will rise in pitch and get brighter as you advance the wheel.
Velocity - Velocity is simply a measure of how hard you hit a key. If you play a key on a piano softly, the sound you produce will be someone quiet and subdued. Hit it harder, though, and it's much louder, brighter, and more aggressive. It's this ability that allows us to make instruments that allow for expressive playing just like "real" instruments. The most common destination to modulate with velocity is amplitude, so the harder you play, the louder the sound gets. Filter cutoff is another popular choice. Real sounds tend to get brighter as they are played harder, so you can emulate that, or you can do really complex sounding, modulating synth riffs. Remember, on many synths you can modulate by negative amounts too, so you could create a sound that was loud when you played lightly and quiet when you played it hard if you wanted to. Why you'd want to, I don't know, but you can do it.
Pressure - Pressure allows you to apply modulation to notes you are already sustaining. Play a chord and as you're sustaining the chord, you press harder on the keys to apply the modulation. This is great for pads when assigned to filter cutoff, as it allows you to do really expressive swells. The important thing to remember is that this is not a polyphonic effect. That is to say, it will look at a chord you're playing, and whatever note you're playing with the most pressure will effect the modulation for ALL the notes you're playing.
Channel Pressure - Less common than plain old pressure, channel or polyphonic pressure operates the same way, but it will treat each note individually. Thus, if you're playing two notes at a time, one very lightly, and one very hard, the note triggered with a soft touch will have less modulation applied to it than the one you played hard, which will have lots of modulation applied to it. This is obviously a still more expressive form of modulation. In addition to the popular amplitude or filter cutoff destinations, if you apply it to oscillator pitch, you can emulate the sometimes odd pitch bends within chords on pedal steel guitars.
Key Scaling/Following - This varies the amount of modulation that is applied to the modulator based on the position of the key on the keyboard. With a positive mod amount, the amount of modulation increases the higher the note you're playing is on the keyboard. You can guess what a negative amount would do. This is most commonly used on filter cutoff. Many times, the filter setting that is perfect for the low end of your keyboard is too dark or muffled in the higher notes, so this allows you to compensate for that and have higher cutoff values on them.
PRACTICAL USES FOR MODULATION
Add a vibrato effect - Assign an LFO as your modulation source with your oscillator(s) pitch as the destination. Your modulation amount can be positive or negative, but should be for a relatively low amount to avoid the "crazy siren" effect. LFO waveform should generally be a triangle, although sine can work as well. Using a square wave here with more intense modulation amounts can create trills.
Add a dubstep wobble effect - Set your filter cutoff and resonance to a medium setting. Assign an LFO as your modulation source with the filter cutoff as the destination and a positive modulation amount to taste. If your synth will sync the LFO to note values, try using triplet or dotted eighth notes. Feed it through some distortion and you're good to go.
Add an auto-pan effect - Set an LFO as your mod source, with your sound's PAN position as the destination. Higher modulation amount values will result in more extreme panning, while more moderate values will generally be a bit more useful. If your synth allows your LFO to sync to a note value, try setting it to 2-3 bars.
Use pressure to bend the pitch - Assign pressure (or channel pressure, if available) as your mod source, and your oscillator(s) pitch as the destination.
Use velocity to make a sound brighter as you play harder - Assign velocity as your mod source and filter cutoff as the destination with a positive modulation amount.
Use velocity to make a sound louder as you play harder - Assign velocity as your mod source and the sound's VOLUME or AMP level as the destination with a positive modulation amount.
There's just a few to get you started. Take some time to explore your synth's modulation options. Some will have more than others, and as you get comfortable using modulation, you'll come to really appreciate synths with deep mod options. Remember also that with most synths, you can have multiple modulation sources modulating multiple destinations at once, so you can see the potential for really complex, evolving sounds on a synth with great modulation capabilities. Next post, I'll wrap things up with some advice to keep in mind when you're programming your own sounds.