A means of generating a control signal based on how much pressure is applied to the keys of a MIDI keyboard. Most instruments that support this do not have independent pressure sensing for all keys, but rather detect the overall pressure by means of a sensing strip running beneath the keys. Aftertouch may be used to control such functions as vibrato depth, filter brightness, loudness and so on....
OB-Xa
An enhanced version of the OB-X, the OB-Xa also was "available in four-, six-, and
eight-voice configurations, offered 32 program memories, polyphonic portamento, and
polyphonic sample-and-hold", and in addition to that, it "offered the ability to split the
keyboard, or double up the voices so that two voices would sound for each key that was
depressed."