Introduction to Sound Synthesis Effects
Chorus
Chorus mixes a signal with a copy delayed by a few tens of milliseconds, and moves the delay slowly with an LFO. The moving delay shifts the copy's pitch slightly up and down, so the two drift in and out of tune like two instruments playing the same part.
Two people playing the same part do not play exactly together: their timing differs by a few milliseconds, and their tuning wanders by a few cents. A chorus effect imitates that by adding a copy of the signal through a delay of around 15 to 30 milliseconds, too short to be heard as an echo, and moving the delay time slowly with an LFO. While the delay is getting longer, the copy is being played back slightly slower than it was recorded, so its pitch is slightly lower; while the delay is getting shorter, it is slightly higher. The copy therefore drifts a few cents either side of the original, and the two together sound wider and fuller than either.
The tool below runs one or two chorus voices. With two, each has its own delay line, sweeping in opposite directions, one on each side of the stereo field. The plot shows each voice's delay time and the pitch shift that the changing delay produces.
A chorus
A chord or a series of plucks is mixed with one or two copies through delays of about 20 milliseconds, each moved up and down by an LFO. Rate and depth set how fast and how far the delays move.
Listen for With the mix at zero the chord is dry and narrow. Bring the mix up and the chord sounds wider, and it wavers slightly as the copy drifts in and out of tune with the original. Two voices spread the sound across the speakers. A fast rate and high depth turn the wavering into an obvious vibrato, which makes the pitch shift that produces the chorus effect easy to hear.
Why a moving delay changes pitch
A delay line plays back what was written into it a set time earlier. If that time is growing, each moment of playback reaches back a little further than the last, so the playback runs slower than real time and the pitch drops; if it is shrinking, playback runs faster and the pitch rises. The size of the shift depends on how fast the delay changes, not on how long it is: a delay swinging 1 millisecond either side of its centre once a second shifts the pitch by about 11 cents at its fastest. This is the same effect as the change in pitch of a passing siren, and it is why a chorus with a high rate or depth sounds like vibrato.
Voices and width
One delayed copy mixed with the original gives the basic effect. Two or three copies, with their LFOs at different phases, give a denser ensemble sound with no single moment where all the copies line up. Sending different copies to the left and right speakers makes the sound wider, because each ear hears a slightly different mixture. Roland's Juno-60 and Juno-106 synthesisers put a stereo chorus of this kind after their voices, built with analogue bucket-brigade delays, and it is a large part of their sound. The same structure with a delay of a few milliseconds and feedback is a flanger.
Common questions
How does a chorus effect work?
It mixes a signal with a copy delayed by around 15 to 30 milliseconds, and moves that delay time slowly up and down with an LFO. A delay that is getting longer lowers the copy's pitch slightly and one that is getting shorter raises it, so the copy drifts in and out of tune with the original, like a second player.
What is the difference between chorus and flanger?
Both mix a signal with a moving delayed copy. A chorus uses a longer delay, around 15 to 30 milliseconds, and little or no feedback, which is heard as a slight detuning. A flanger uses a delay of a few milliseconds, often with feedback, which is heard as a sweeping comb filter.
Why did synthesisers like the Juno have a built-in chorus?
Each voice of Roland's Juno-60 and Juno-106 has a single oscillator, so there is no second, slightly detuned oscillator to make the sound wider. Both synthesisers add a slowly detuned copy instead, through a stereo chorus placed after the voices, and that chorus is a large part of their recognisable sound.
Related modules
Published Glossary