A chorus (or ensemble) is a modulation effect used to create a richer, thicker sound and add subtle movement. The effect roughly simulates the slight variations in pitch and timing that occur when multiple performers sing or play the same part.
Chorus is a common guitar pedal effect that gives a clean electric guitar a "dreamy" quality. It's also widely used on acoustic guitar, electric piano, and clavinet. On strings and synth pads, chorus creates a richer, more complex sound. Stereo chorus effects also can be used to widen a stereo image.
How does it work?
The chorus effect is based upon a short delay. Incoming audio is split and run through the delay, then mixed with the original audio and sent to the effect's output. With short delay times (20 to 50ms), the delayed audio blends with the original audio instead of creating a distinct echo.
To add movement, chorus effects slowly modulate the delay time with a low-frequency oscillator (LFO). As the LFO cycles the delay time up and down, the delayed audio shifts up and down in pitch by a little bit.
|Apple's Chorus for GarageBand|
GarageBand's plugin is a typical
single-voice stereo chorus. Its controls
adjust the speed and depth of the LFO,
and the wet/dry mix is fixed at 50%. Left
and right stereo channels are modulated
180 degrees out of phase to create a
wide stereo image.
Most chorus effects include knobs to adjust the LFO speed (a.k.a. rate or period) and depth (a.k.a. amplitude or intensity). LFO speeds are usually in the range of natural human vibrato (up to about 10 Hertz).
Some plugins include control of the wet/dry mix. At 100% wet, the pitch modulation of a chorus effect sounds like vibrato. To blend the modulated audio with the original, the wet/dry mix is often set at 50% of each.
Mono, stereo, and surround chorus
A mono chorus operates on a mono input, or sums its inputs to mono before running them through a single modulated delay. The output of the effect may be stereo. Guitar pedals are usually mono effects generating stereo outputs.
A stereo chorus applies the effect to left and right stereo inputs, or to a duplicated mono input, using two independent delays. The two delays may share the same LFO, but the delays are often modulated 180 degrees out of phase. This makes the left channel cycle up while the right channel cycles down, and vice versa, creating a richer chorus and a wider stereo image.
A surround chorus applies the effect to each surround channel using independent delays. Like a stereo chorus, the surround chorus may use the same LFO for all of the delays, but modulate the delays out of phase to create a wider surround image.
Single and multi-voice chorus
|Apple's Ensemble for Logic|
Logic's plugin is a multi-voice stereo and
surround chorus that uses a single delay
modulated by three summed LFOs. LFO
phase offsets for the stereo or surround
outputs may be adjusted to narrow or
widen the stereo/surround image.
A single voice chorus uses a single delay that creates a single modulated duplicate of the incoming audio. Basic chorus effects and inexpensive guitar pedals are often single-voice.
A multiple voice chorus uses multiple modulated delays to create a richer sound with more movement. Some chorus effects use the same LFO to modulate all of the delays in sync, but at different points in the LFO's cycle. Other effects use multiple LFOs to modulate the delays independently. The latter creates a richer, less obviously cyclical effect, but with added complexity in the user interface and a higher CPU use.
A single LFO chorus can create a noticeable up-and-down pitch wobble. To make the wobble less obvious, a plugin may use multiple LFOs summed together. When each LFO has a different speed and depth, the resulting modulation waveform is complex and the wobble less obvious.
|Audio Damage's Vapor|
Audio Damage's plugin uses a diffuse
reverb algorithm instead of a delay. Plugin
controls adjust the LFO speed and depth,
the delay time, the wet/dry mix, and the
amount of reverb diffusion. A low-cut
filter removes low frequencies that can
muddy the diffusion.
Some chorus effects replace the delay with a diffuse reverb tail algorithm. Instead of a single echo to double the sound, reverb creates a diffuse scattering of echos that are pitch shifted up and down randomly.
Delay time and feedback
Some multi-effect plugins include controls to adjust the delay time and delay feedback. Neither of these are strictly necessary for chorusing, which always uses a short delay time and little or no feedback. The additional knobs allow the plugin to perform a wider range of effects. For instance, a short delay time and high feedback creates flanging. A deep LFO and no delay creates vibrato.
Unison mode on a synthesizer creates a bigger sound by playing multiple slightly detuned notes each time a keyboard key is pressed. The effect can be similar to chorus.
Flanging is the same as chorus except that it uses a shorter delay time and delay feedback. This creates a distinctive "zipper" noise as the effect's LFO modulates the delay time back and forth.
Phasing is similar to chorus except that it uses multiple all-pass filters instead of a delay. When the filtered audio is mixed with the original audio, it creates comb filtering effects. When modulated, phasing creates a "swirling" feel to the sound.