My digital piano has 88 keys and the manual says it provides 128 voices of polyphony, and this seems not a particular exception or unusual feature.
I even cannot imagine how it would be possible to press all 88 keys all at once or even in a close succession. Ok several keys also must keep sounding while I hold the sustain pedal, but 128?
Why possibly it was required to implement that many voices for the instrument?
Modern digital pianos have a number of features that expand the number of notes that might be simultaneously present in the output:
some voices on your digital piano contains more than 1 tone, for example an organ single note may consist of 3 or 4 voice samples (polyphony) so , when you press 10 claviers it will produce about 40 sound samples... 128 voices of polyphony will let you hear your sound fully and without note cancellation.
Besides the possibility to have multiple sounds assigned to a single key, another important point is: use of pedal requires the previously sounding notes to continue as opposed to terminate suddenly just to re-use the channel for the new notes.
If you use your digital piano to play back MIDI, it might be playing more than 88 notes across several different channels.
The advertisement of "128 voice polyphony" essentially just means "100% polyphonic", as within general midi there are only 128 notes that exist.
128 is a very convenient number for computing: it's 2 to the power of 7
That is to say, if you have seven binary characters (which you can represent however you want, os and xs, 1s and 0s, TRUE and FALSE) there are 128 possible combinations: ooooooo oooooox oooooxo oooooxx etc. for 128 times until you get to xxxxxxx
Midi is designed around the number 128 (or 127 for values where "0" is significant like velocity). The reason for this is that it's built around a protocol that transmits "bytes" of data. A byte of data consists of 8 bits (8 binary units). Usually this consists of 1 bit specifying the type of value, and 7 bits consisting of the value itself.
MIDI includes 128 notes for this reason: in the MIDI protocol, the first bit tells the instrument whether it is a "status byte" or a "data byte" (not important for this answer) and, for a "note on" or "note off" message, the remaining 7 bits tell you which note the message refers to. So every time you press a key on a midi controller, in a simplified sense, a 0, then a 7 bit integer (a number between 1 and 128 represented by 7 1s and 0s) is sent through the wire.
Now, it might seem a bit wasteful to use a whole bit of a byte on this data type for every note press, but there are good reasons why this is a convenient way to design the protocol, and 128 notes (more than 10 octaves) is enough for pretty much all applications. The next option up would be 256 notes, this would make the engineering more difficult (no status/data indicator bit) and 256 notes puts us well outside the range of human hearing.
'Voices' are largely a marketing term. It doesn't necessarily equate to 'polyphony'. Does playing a note use one, two or more 'voices'? We don't know and we aren't told. You're fairly safe in assuming that when anything but the cheapest digital piano is playing a piano sound you won't be troubled by note cut-offs due to an inadequate number of voices. Maybe you could set up an experimental situation where this would happen. Enjoy!
