In a polyphonic synth, the complete set of circuity for playing a single note (excluding effects, which are usually common to all notes in a given patch). Usually, there is a one-to-one correspondence between voice circuits and the maximum number of notes that the synth is playing at one time. However, some synths provide the ability to layer multiple timbres on a note, which requires multiple voices, so the maximum number of notes may be less than the number of voice on such synths. All synths which are not fully polyphonic -- that is, they do not have enough voices to sounding every note on the keyboard at once (which is most of them) -- must have a method for deciding which voice circuits will play which notes, which is referred to as voice allocation.
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