The world of guitar (and synth, and bass…) modulation effects overwhelmed me at first, but I’ve learned that most pedals and plugins are based around just a couple of ideas.
| Time-based effects | Filter-based effects | |
| Base version | Chorus | Phaser |
| Variations | Flange (chorus with feedback) Vibrato (chorus with no dry signal) | Uni-Vibe or “Vibe” (phaser with mis-matched filters) |
Time-based effects are created by 1) copying the input signal, 2) delaying the copy by a small amount (a few milliseconds), 3) automatically changing the delay time (say, from 27 ms to 33 ms and back), and then 4) mixing the input (or “dry,” or “not delayed) and delayed signals together (typically at a 50:50 ratio). The resulting sound is a pseudo-doubling effect with a bit of pitch going up and down, some frequencies of the input signal enhanced, and others diminished. Boom; basic chorus.
Flangers operate on the same principle, with shorter delay times and feedback in the delay signal path (just like a delay or echo pedal), resulting in a more pronounced effect. Flangers with knobs for feedback amount are often great modulation pedals because they can cover a lot of territory from subtle near-chorus to crazy flying saucer flange.
Vibrato – the signal’s pitch moving up and down – is just chorus without the dry signal. When the delay time is getting shorter, the pitch is going up, and vice versa. You can test this out on your own with any delay pedal; play a sustaining note or chord, then turn the delay time faster or slower. You’ll hear the pitch of the delayed signal go up or down accordingly.
Filter-based effects
Filter-based effects are created by 1) copying the input signal (noticing a pattern here?), 2) sending the copy through a series of all-pass filters, 3) automatically changing the center frequencies of the all-pass filters* (say, from 500 Hz to 2000 Hz and back), and then 4) mixing the input (or “dry,” or “not all-passed”) and filtered signals back together. The resulting sound has a series of mid frequency cuts that move up and down. We’re phasing!
* Optional sidebar – Filters are usually used to boost or cut certain frequencies in a signal. All-pass filters don’t boost or cut any frequency, but they delay the frequencies around the center frequency a tiny amount. If an all-pass filter’s center frequency is 1000 Hz the frequencies around 1000 Hz are delayed the most (still a tiny amount), the frequencies around 500 and 2000 Hz are delayed less, the frequencies around 250 and 4000 Hz are delayed less (almost none), and so on.
The all-pass filters in Uni-Vibe or “Vibe” effects are purposefully mis-matched. This was originally done to mimic the sound of a rotary speaker cabinet. Most people don’t consider it a particularly accurate re-creation of that sound, but it has a cool sound of its own (I adore the “Vibe”-type sounds of my Wilson Effects Haze! There’s even an unreleased Mars Lights song named after that pedal).
Other modulation effects
Amplitude (volume) modulation – tremolo – is the other main type of modulation effect. This is simply the volume of a signal going up and down regularly, like a couple of times per second.
Some Fender amps incorrectly call this type of built-in amplitude modulation “vibrato” instead of tremolo.
Real rotary speaker cabinets and re-creations of them have much more complex modulation going on than a chorus or phaser. There’s pitch modulation, phase modulation, amp distortion, speaker breakup, and more, and these changes interact with each other in complex ways. Rotary speaker cabinets are amazing.
Vibrato and “Vibe”-style effects are often and understandably confused for each other due to the similar names. However, they are generated by different types of circuits and have different sounds.
Wah and auto-wah effects are also filter-based, but a different type of filter (resonant bandpass or lowpass) than phasers (all-pass). Wah effects are not usually considered modulation because the player controls the change in sound, rather than it being done automatically by the circuit.