Increasing filter resonance to its conventional limit sharpens audio, giving it presence and bite. Pushing resonance beyond that limit can generate wild and delightfully unstable wails, groans, and ululations.
Resonance is the tendency of a physical body to vibrate at one or more specific frequencies. Resonant filters simulate physical resonance by introducing feedback, which boosts a frequency band around the filter's cutoff frequency. The resonance-amount setting usually affects the width of the boosted frequency band: low settings boost a wider band, and high settings boost a narrower band.
FIG.1: On the left are the Filter 1 and Output settings for the original Arp Epiano preset; on the right are the settings for the high-resonance variant. Note that the level is lowered, the cutoff is down, and resonance is up for the high-resonance version.
Some resonant filters will begin to oscillate once the resonance reaches a certain level. This self-oscillation can be useful, but it can also generate hideous ear-scrunching screams. Turn the output level way down when experimenting with high-resonance patches. Once you're convinced the sound is stable, adjust the volume to taste.
For my examples, I've used Applied Acoustics Systems Ultra Analog VA-1 (www.applied-acoustics.com), but most synths with analog-style resonant filters will do the trick. Ultra Analog's filters have five modes, but high-resonance effects work best in lowpass, bandpass, and highpass modes. Resonance is audible but less effective for the notch filter, and the resonance control (labeled Q) for the formant filter actually changes vowel formants rather than affecting resonance.
A resonant filter needs a stimulus, and harmonically rich stimuli offer more frequencies with which the filter can resonate. Starting with Ultra Analog's Mono preset from the Lead bank, turn off both oscillators, turn on the Noise module, set the Filter 1 Drive to Off, and set the Cutoff Env1 and LFO1 knobs fully counterclockwise. You'll now have white noise feeding Filter 1, which is in lowpass 4-pole mode. Set Filter 1's cutoff to 42, crank its Q knob to maximum, and center the Cutoff Kbd knob. The resulting timbre will be a typical breathy sort of sine wave.
Next, turn the level way down, turn off the Noise module, turn on the Oscillator 1 module, and play with the filter's Cutoff knob. You'll notice that resonance peaks occur at the harmonics of the sawtooth. You now have two good starting points for creating your own high-resonance patches. Don't forget to try the other filter types and to bring Filter 2 into the action in series or parallel with Filter 1.
From a Preset
You can easily modify Ultra Analog presets to get interesting high-resonance sounds. I created three quick patches this way by lowering the cutoff frequency, raising the resonance, and lowering the output level (see Fig. 1).
Load the Arp Epiano preset from the Guided Tour bank and lower Filter 1's cutoff frequency from 44 to 5, raise its resonance from 36 to 112, and lower the main output level from 43 to 20. Arp Epiano uses the arpeggiator, so simply hold a chord and play with the filter cutoff frequency to vary the color. For performance, you might want to assign a MIDI continuous controller to modulate filter cutoff. This high-resonance patch sounds like the original e-piano preset played underwater (see Web Clip 1).
My second high-resonance patch is a variant of the Arpeg Aggressive preset, another arpeggiated sound from the Guided Tour bank. This time I lowered Filter 1's cutoff from 59 to 41, raised its resonance from 22 to 127, and lowered the main output from 75 to 38. Using maximum resonance produces a wild glistening sound, particularly on note attacks (see Web Clip 2).
The Glass preset from the Ambient bank provides an opportunity to get both filters into play. It starts with two sine waves an octave apart, and the output of Filter 1 is mixed with Oscillator 2 in Filter 2. Lower the main output, and set the cutoff and resonance respectively to 37 and 108 for each filter. That places the resonance roughly between the low and high oscillator pitches.
You can use LFO 2 to add some motion to this patch. Center the Cutoff LFO2 knob, change LFO 2's shape to Random 1, and increase its rate to 96. That will cause the resonant peak of Filter 2 to jump around, occasionally hitting each of the oscillator pitches to create a percussive thwop (see Web Clip 3).
rachMiel composes deliriously experimental electronic and acoustic music.