Some of the most engaging, energetic synth lines consist of short, pulsing notes that at first listen, sound as if they’re changing pitch. But when you listen analytically, you’ll hear that, in fact, the fundamental note never changes. What sounds like changing pitches is actually the filter resonance and cutoff being raised and lowered, changing the harmonic content, and giving the effect of pitching the notes. There are many great filters out there, but check out the Waves MetaFilter for one that offers a built-in sequencer for modulating the parameters to create exactly this type of effect.)
When you raise the cutoff frequency and increase the resonance of an analog style 2- or 4-pole filter, you’re making your sound brighter and more trebly. When you lower the cutoff and resonance, you remove the harmonics, emphasizing the fundamental. Make cutoff and resonance changes rhythmically, and it can sound like you’re playing different notes.
This is a great way to take a sequenced synthesizer part and create a performance out of it. Many plug-ins let you adjust both resonance and cutoff with one click on a graphical display. You can also assign both parameters to the same physical control on a MIDI controller. However you choose to control the cutoff and resonance, your performance is key to making an energetic but monotone synth line sound alive — and as if it consists of multiple pitches, even when it’s the same repeated note.