After Touch is normally monophonic, (A single pressure bar covers all the keys) thus pressing another key harder will affect any voice that is using pressure control. (Vibrato etc)
Very few keyboards of any description are polyphonic due to the mechanical complexity of reliably implementing it.
Before anybody mentions that dynamics of a keyboard are polyphonic, dynamics is easier to control, as it is calculated by how fast any key goes from off to on. (It’s not a constant like pressure)
Bill
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