This page: http://www.srm.com/qtma/davidsmidispec.html http://www.srm.com/qtma/davidsmidispec.html (which is quite obviously highly unofficial) mentions both 0x

This page: http://www.srm.com/qtma/davidsmidispec.html http://www.srm.com/qtma/davidsmidispec.html (which is quite obviously highly unofficial) mentions both 0xAx and 0xDx, calling them "Key Pressure" and "Channel Pressure", respectively. The explanation says: Some keyboards can detect a change in pressure on each key, while they are held by the player; these keyboards can report "key pressure" over MIDI. Some keyboards may sense the overall pressure on the device, such as with a weight sensor beneath the entire keyboard. These devices can report "channel pressure." Sounds pretty nice.