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Chorus Effect

The chorus effect (or ``choralizer'') is any signal processor which makes one sound source (such as a voice) sound like many such sources singing (or playing) in unison. Since performance in unison is never exact, chorus effects simulate this by making independently modified copies of the input signal. Modifications may include

(1)
delay,
(2)
frequency shift, and
(3)
amplitude modulation.

The typical chorus effect today is based on several time-varying delay lines which accomplishes (1) and (2) in a qualitative fashion. Reverb generally provides (3) incidentally. Before digital delay lines, analog LC ladder networks were used as an approximation, beginning in the early 1940s in the Hammond organ [59, p. 731].

An efficient chorus-effect implementation may be based on multiple interpolating taps working on a single delay line. The taps oscillate back and forth about the positions they would have while implementing a fixed tapped delay line. The tap modulation frequency may be set to achieve a prescribed frequency-shift via the Doppler effect. Each tap should be individually spatialized; in the case of stereo, each tap can be panned to its own stereo position.


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About the Author: Julius Orion Smith III
Julius Smith's background is in electrical engineering (BS Rice 1975, PhD Stanford 1983). He is presently Professor of Music and Associate Professor (by courtesy) of Electrical Engineering at Stanford's Center for Computer Research in Music and Acoustics (CCRMA), teaching courses and pursuing research related to signal processing applied to music and audio systems. See http://ccrma.stanford.edu/~jos/ for details.


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