Modulation & Filter

What moves you?

Chorus, tremolo and vibe run on an LFO: the differences are waveform, number of delay lines, and whether the modulation affects pitch, amplitude or phase. Wah and pitch shifting are the two that put your treadle ("expression pedal") to work, and the wah page also covers the envelope-driven version, for when your picking hand should do the sweeping instead of your foot.

How we got here

Modulation started as an attempt to fake a rotating speaker: the Uni-Vibe stacked four staggered allpass stages, each swept by a lamp shining on a photocell. The lamp's thermal lag is why it lurched rather than glided, and people loved what they heard.

Phasers cleaned that up in the mid-1970s with matched FET stages and an even sweep, and chorus arrived from a different direction entirely: Roland lifted the circuit out of the Jazz Chorus amp and put it in a box, a short bucket-brigade delay with its clock modulated so pitch drifts against the dry signal. Tremolo predates all of it, having been built into amplifiers since the 1950s either by wobbling a tube's bias or by using that same lamp-and-photoresistor technique.

The wah has a stranger origin: it began as a mid-boost circuit in a solid-state amp, got a treadle bolted to it, and was marketed for trumpet before guitarists took it. What it does is move a resonant peak through the midrange, and by 1972 the Mu-Tron III was doing the same sweep from an envelope follower instead of a foot, which makes it a genuinely different instrument rather than a wah on autopilot.

Editorials

• Chorus — One delay line or six: wobble versus width
• Tremolo — Amplitude modulation, from optical cells to bias trem
• Uni-Vibe and Phaser — Swept allpass: throb versus glide
• Wah — Treadle and auto-wah filter sweeps
• Pitch Shifter — Treadle-controlled pitch shifting