Polyphony refers to the maximum number of notes that a keyboard or sound module can produce at one time. For instance, if you were to play a 3-note chord with a 1-note melody, you'd need at a keyboard capable of at least 4-note polyphony. Many of today's keyboards and sound modules have 64-note or even 128-note polyphony, and while few keyboardists are going to play that many notes at once, many people want the capability to play complicated, multi-part layers that demand much more polyphony than a single instrument sound would ever need. Keep in mind that on some keyboards or sound modules, polyphony can be adversely affected by how heavily a sound is layered. For example, if you are playing a rich, layered sound made up of 4 simpler sounds, you may only have 16 notes of polyphony (or less) on a keyboard with maximum polyphony of 64-notes (64 divided by 4 equals 16). Also keep in mind that some high end Yamaha instruments can use 8 elements per voice. And this is before we start layering the voices.
What happens when the maximum polphony of an instrument is exceeded? That depends on what sounds are layered and how the customer is using the instruement. For example, let's say we have 4 layers; 1 acoustic piano, 1 electric piano, 1 string pad and one vocal pad. If we step on the sustain pedals and repeatedly play the same notes, we begin to hear the sound building, getting thicker, as layer upon layer are continuing to play because the pedal is held down. The piano is a percussive sound (electric piano too), so even though the pedal is held down, these notes are decaying. They may not be gone, but the earliest played notes are now now decayed to the point that they are buried in the wash of other notes. But the string and vocal pads will continue to sustain as long as the pedal is held. The result is that, when the polyphony limit is exceeded, it seem that string sounds are chopping off randomly and the sound gets thinner as new notes are played.
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