[Zhao Yitian]

This sounds very strange, because the velocity message is always with MIDI note message, one note has one velocity message, and the transmission will not change the message, how can that be different? Maybe you can use the MIDI monitor to check on your iPad, for example you can compare the velocity message from WIDI BUD and USB MIDI and see whether they are different (you can do same for iPad Bluetooth and USB MIDI too.).