In audio production, chorus is one of the two standard audio effects defined by the Musical Instrument Digital Interface ( MIDI ). The other effect is reverb (reverberation). Chorus adds a swirling property to a sound that it is applied to, thickening the sound. Chorus is commonly used with instruments like the electric piano and guitar and with synthesizers. Most sound cards that have an onboard MIDI sound set contain both chorus and reverb.
A digital signal processing ( DSP ) algorithm that combines digital delays and at least one low-frequency oscillator (LFO) produces the chorus effect. The delays add a time offset and the LFOs vary the pitch. Varying the number of delays used in the design of the chorus DSP algorithm changes the quality of the chorusing effect. In addition, the number and speed of the LFOs used in the algorithm design also contribute to the overall effect. In general, the more individual delay lines and LFOs that are part of the DSP algorithm, the thicker and more complex the effect can be.
Here's how it works: An audio signal that is to be processed with chorus is first delayed by a small amount. (Typical delay times are small, in the range of 5 to 40 milliseconds.) This produces a doubling effect. Each delayed signal is then sent to the LFO. The LFO takes the delayed signal and moves its pitch up and down, changing the tuning from sharp to flat. The LFO usually runs at a slow speed; 1 to 5 oscillations per second are typical. The output of the delayed and pitch-altered signal is then mixed in with the original audio. This blending completes the chorusing effect.