Intro to Synthesis, Part 3: Using Filters to Add Shape and Character to Your Tone

Last week, we launched our new, six-part Intro To Synthesis series that promises to guide anyone interested in synths through the building blocks of the instruments. Our series began with two videos that explain the basics of oscillators—the sound generators that serve as the foundation of a synthesizer’s tone.

Today, we turn our attention to filters, or the tone shapers that will add character to an oscillator’s generated soundwave.

Reverb’s resident synth head Justin DeLay begins by explaining what happens when using a low-pass filter, which cuts out higher frequencies while allowing low frequencies to "pass" through. As you turn your low-pass filter knob, you can determine how many higher frequencies are being cut.

As Justin demonstrates, you can see the effect the low-pass filter has on a complex soundwave. A square wave’s higher frequencies, or upper harmonics, are what make it more complex than a sine wave—which contains only the fundamental frequency of a note. If you use a filter to cut the high-end out of a square wave, it will sound (and, in a waveform analyzer, appear) more like a sine wave.

Tweaking a synth's filter manually

The resonance of a filter accentuate the frequencies right at the cutoff, adding gain at that can bring different qualities to your tone, from nasal or vocal to sharp, piercing, laser-like sounds. By twisting the filter’s resonance knob, you can perform a resonance sweep.

"A synthesizer is going to create a huge amount of sound. Even when you’re playing one note, you’re going to be creating a bunch of upper-order harmonics and frequency content that’s going to give a very, very rich sound," Justin says. "As you filter down, you’re focusing more and more of that synthesizer sound on the root note that you’re playing."

But you don’t just have to manually twist knobs to bring movement and character to your synth’s sound. There are a number of ways you can automate filter moves. By using a synth’s key tracking function, your synth will open or close the filter’s cutoff settings while you play higher or lower on the keyboard.

Envelopes are "another fundamental way to shape your filter and your synth," Justin says. Envelopes open and close a filter over time, simulating the natural decay of an acoustic instrument’s sound—making the a note get duller over time, just like the striking and fading of a violin or drum.

Using an envelope to modulate a filter

You can also shape your filter with automated modulation—using an LFO to open and close your filter at different rates. Setting a slow LFO speed can help make a repetitive part interesting, as it brings different timbres to the notes as the filter slowly sweeps.

Because so much character comes from a synth’s filter, different makes and models of synths are prized for the qualities of their individual filters. Justin demonstrates the differences in sound in the filters of a Korg Mono/Poly, a Moog Minimoog Model D, and a Roland Juno-6, which, in addition to a low-pass filter also contains a high-pass filter, which blocks low frequencies while allowing highs to pass through.

Next week, in the fourth part of our series, we’ll be exploring all of the various ways envelopes can be can applied to your synth to bring excitement, movement, and character. Be sure to check back next week.

Full Intro to Synthesis Series
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