Chorus effect

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A chorus effect is:

A condition in the way people perceive similar sounds coming from multiple sources.

A simulation of this effect created by signal processing equipment.

A signal processing device designed to produce this effect.

Methods

To produce the effect, either naturally or in simulation, individual sounds with roughly the same timbre and nearly (but never exactly) the same pitch converge and are perceived as one. When the effect is produced successfully, none of the constituent sounds are perceived as being out of tune. Rather, this amalgam of sounds has a rich, shimmering quality which would be absent if the sound came from a single source. The effect is more apparent when listening to sounds which sustain for longer periods of time.

The chorus effect is especially easy to hear when listening to a choir or string ensemble. A choir has multiple people singing each part (soprano, tenor, etc.). A string ensemble has multiple violinists and possibly multiples of other stringed instruments. When individual singers or violins play the same part, the chorus effect can be heard. Some instruments produce the effect all on their own. Examples include:

Piano. Each hammer strikes multiple strings tuned to nearly the same pitch. The chorus effect is so intrinsic to the timbre of a piano that it is difficult to recognize.
12 string guitar. The guitarist fingers two strings where only one string would be fingered on a standard 6 string guitar. Each pair of strings is tuned to nearly the same pitch, though the G, D, A and low E pairs are actually tuned an octave apart.
Synthesizer. By assigning multiple, slightly detuned oscillators to a voice, the effect is achieved.

The chorus effect is enhanced when the sounds originate from slightly different moments in time and/or from different physical locations. Such additional variation is typical of a choir or a string ensemble, but would be lacking on a piano, for example, because a piano hammer strikes all its strings at the same instant and the difference in string location is negligible.

Artificial chorus effect

The chorus effect can be simulated by signal processing equipment. The signal processor may be software running on a computer, a ROM-encoded effect in a digital effect processor, or an analog effect processor. If the processor is hardware-based, it may be packaged as a foot pedal, a rack-mount module, a table-top device, or built in to an instrument amplifier.

Regardless of the technology or form factor, the processor achieves the effect by the taking an audio signal and mixing it with one or more delayed, pitch-shifted copies of itself. The resulting sound simulates the sound of several instruments or voices where there is really only one.

Stereo chorus effect processors produce the same effect, but it is varied between the left and right channels. The effect is thereby enhanced because sounds are produced from multiple locations in the stereo soundspace.

Commercial chorus effect devices often include controls that enable them to be used to also produce delay, reverberation, or other related effects that use similar hardware, rather than exclusively as chorus effects.

External links

[Musicrow Chorus FX Software]
[Kjaerhus Audio - Free Chorus VST Plugin]
[DOD 690 Chorus review], mentioning also the landmark Roland Boss CE-1.

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