A filter phrasebook

Guitarists haven’t embraced the new wave of filter effects with the same fervor as DJs and remixers, but that just means there’s more room for the filter fiends among us. Here are some tips to help you speak fluent filterese.

Filtering simply means boosting or cutting particular frequencies within a signal. The tone controls on a guitar or amp are filters. But when we speak of filter effects, we usually mean a process where the blend of frequencies changes over time. Consider the wah-wah pedal–the one filtering effect every electric guitarist knows. Wah pedals are bandpass filters, which means they boost specific frequencies within a signal while removing those above and below. Pushing the pedal changes the pitch, or frequency, of the filter. Bandwidth (sometimes called Q) refers to how wide a swath of frequencies is affected. Differences in bandwidth are among the traits that distinguish different wah pedals.

Notch filtering is the inverse of bandpass filtering, as a usually narrow frequency band is removed. Notch filtering is most often used to diminish unwanted noises, but it can also create subtle phase-shifted sounds.

The sweeping sound associated with analog synthesizers is lowpass filtering. Here, only the lower frequencies are allowed through the filter. The cutoff frequency (the highest frequency that can pass through the filter) can be defined by the music’s dynamics or by an envelope generator–a synthesizer circuit that lets you specify how quickly the filter opens, and how long it remains open. Guitarists are most aware of envelope generation through envelope followers–such as the Electro-Harmonix Q-Tron and that ’70s classic, the Mu-Tron III–where the filter envelope (and thus its cut-off frequency) is controlled by the player’s picking dynamics. The usual formula is louder equals brighter, but some devices let you reverse the equation.

Most synths and filter pedals also let you specify the amount of resonance-that is, the amount of emphasis the cutoff frequency receives. Resonance (sometimes referred to as regeneration) can add anything from a subtle edge to a brain-scrambling shriek. If you hear a high, whistling sound that swoops down in pitch as the filter closes, that means the tone has a lot of resonance.

Highpass filtering reverses the lowpass formula-only the upper frequencies make it past the filter. It’s less common as a guitar effect, but it can be great for generating eerie crackling sounds that can cut through any mix.

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