The EM Poll

Resonant Filter Sweeps Reborn

Oct 1, 2005 12:00 PM, By Peter Schwartz

           

Resonant filter sweeps are as old as synthesis itself, and they often tag a track as dated and synthy. Nevertheless, a good old-fashioned eeeeowww is often just the thing to cut through the mud and grab a listener's attention. Here are a few ways to add interest to that time-honored sound.

FIG. 1: The purple and green peaks -indicate the cutoff frequencies of two parallel lowpass filters at three different times -during a downward filter sweep.

Start by selecting or creating a monaural patch on your favorite soft synth with a typical downward resonant filter sweep (see Web Clip 1). Insert the same synth and patch on a second track, panning one track hard left and the other hard right. Next, set up your MIDI routing to simultaneously play both patches. If the sound is being generated from a synth with free-running oscillators — meaning that the phase of the oscillator waveform is not reset each time a note is triggered — the part may already have a nicely animated stereo spread and a fuller overall sound.

The animated stereo effect comes from differences in the phase of the free-running oscillators. If you're using a single-oscillator synth patch, you'll probably need to slightly offset the pitch of each synth to avoid phase-cancellation effects. You can also avoid that problem while achieving thicker textures and a more animated stereo effect by using detuned two-oscillator patches.

Keeping Track

Although this simple trick can be highly effective, it is often avoided because it eats up two tracks. You can make the sound even more interesting and perhaps justify burning that extra track by slightly offsetting one synth's filter-cutoff frequency (see Fig. 1). That will increase the stereo separation and add harmonic interest as the two synths' filters sweep through different resonant harmonics (see Web Clip 2).

Although that effect may sound as though it's out of phase, it is perfectly mono compatible. If worse comes to worst, you can retrieve the extra track by bouncing to mono without losing the new timbral quality. Bear in mind that a slight difference in the cutoff frequencies produces a subtle timbral change that lends new character to the sound, but too great a difference results in distinct, hard-panned sounds rather than a cohesive stereo effect.

As an alternative to using two mono tracks, you can often produce the same effect with a single stereo synth patch. The Moog Voyager is particularly well suited to that task. While the cutoff frequencies of its parallel filters are adjusted with a single control, the Voyager's Spacing control creates an offset between their cutoff frequencies. Other synths are equally capable of creating these effects, though some may require a little more work than others to set up.

As a variation on that theme, keep the filter cutoffs set identically and vary the decay time of one filter's envelope, making it a little faster or slower than the other (see Web Clip 3).

Different Mothers

You can create that effect with hardware synths that do not offer multiple outputs by recording one pass of the sound. Play the recorded part panned hard to one side while making your pitch, filter, and decay adjustments to the live part, panned to the other side. Be sure to use the same MIDI data for each part.

You can even create the effect with a sampler playing back a sampled filter sweep. The trick is to detune one of the samples; but take care with the tuning, because too slight a tuning difference will produce flanging when rendered in mono (see Web Clips 4 and 5).

Peter Schwartz is a composer, arranger, and keyboardist living in upstate New York. His analog synth programming is heavily featured in the factory patches of the new Korg OASYS.