A chorus is a sound effect that imitates a choir or adds thickness to the sound by creating the impression that the sound comes from multiple sources. The chorus effect does so by producing one or more repetitions of the input signal usually with the same amplitude, with very short amounts of delay, and usually with variations in the delay to imitate the differences between the same pitch being produced by different voices.
The delays between the original signal and the repetitions are very short (e.g., around 20 ms) so that the repetitions are almost indistinguishable from the input signal. The delay for each one of the repetitions changes (delay sweep) to imitate differences both in timing and in pitch (the pitch of the repetition changes slightly because of the change in delay; when the delay is being shortened, the pitch is slightly higher, when the delay is being increased, the pitch becomes slightly lower).
Typical chorus parameters
Most chorus effects will have at least two parameters:
- Modulation rate: The modulation rate is the rate of change in the repetition delays. It is specified in Hz and represents the number of times the delays will complete a cycle (from minimum to maximum and back) in one second. Typical modulation rates are between 1 Hz and 10 Hz (see, for example, the recommended chorus default parameters in MIDI under MIDI Global Parameter Control message). The modulation rate may also be called the rate of low frequency oscillation or LFO.
- Modulation depth: The modulation depth is the maximum delay in milliseconds. Typical modulation depths are between 5 ms and 25 ms.
A chorus may use a single modulation rate and depth or a combination of multiple ones to avoid a noticeable wave in the sound.
Other chorus parameters
The following are other possible chorus parameters.
- Voices: Some chorus effects use only a single repetition (single voice) or several repetitions (multi voice). Some chorus effects allow the user to control the number of repetitions.
- Stereo: Some chorus effects will apply the same modulation rate and depth on each channel (mono) or different parameters in different channels (stereo). The term stereo chorus as well as the term surround sound chorus is also used to describe a chorus that inverts the phase (inverts the polarity) of the repetitions to create a different spatial perception.
- Wet and dry mix: The chorus may allow the user to control the mix levels between the input (dry signal) and the repetitions (wet signal).
- Feedback: Some choruses allow the repetitions (with some decay) to be fed back into the input signal. When the decay is small (e.g., 80% of the repetitions are fed back into input) and the delay is very short (e.g., 5 ms), the chorus becomes a flanger with feedback.
- High pass filter: Some choruses remove the bass portion of the signal or decrease the amplitude of that portion, so that the bass is not repeated and does not "muddy" the chorus. This type of chorus is called a bass chorus. The cutoff frequency of the high pass filter is usually between 500 Hz and 2 kHz.