I have a yamaha p-155 stage piano.
It has a usb- input.
Can i have sound information files on an usb pin, plug it in and make the piano sound like for example a synth? If not, is there any other way to get more "Sounds"?
No, but that's probably a MIDI out. So you could hook it up to a VST/soft synth on your computer and use it as a MIDI controller. You can't hook up a USB stick to the keyboard and get sounds to come out of its speakers, but you can hook it up to a computer and use those sounds.
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