Chorus is a popular effect that simulates the phenomenon of multiple voices sounding the same note, as is common in choir music. Because the exact tone produced by each singer has a slight, fluctuating deviation from the target frequency, the resultant summed signal involves time-varying constructive and destructive interference. This is simulated by blending a signal with pitch-shifted copies of itself, whose pitch ratio with the dry signal varies over time.
The chorus effect is produced using a modulated delay line. By modulating the delay time of a copy of the signal, a pitch shift is created via the doppler effect. Increasing delay times mimic a sound source traveling away from the listener, resulting in a decrease in pitch, while decreasing delay times simulate the source moving towards the listener, resulting in an upward pitch shift.
Most choruses have a depth parameter that determines a center delay time with delay times modulating between 2 * depth and 0 (no delay). Another chorus parameter is rate, which sets the frequency of the oscillation.