Keyboard expression
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Keyboard expression often shortened to expression is the ability of the keyboard of a keyboard instrument to respond to the dynamics of the music.
For example, the piano responds exceptionally well to the force with which the keys are initially pressed; It is velocity sensitive. Several of its predecessors, such as the harpsichord, were less velocity sensitive than the piano, which is one of the key advantages of the piano.
The clavichord and some electronic keyboards also respond to the force with which a key is held down after the initial impact; They are pressure sensitive. This can be used by a skilled clavichord player to slightly correct the intonation of the notes when playing on a clavichord, and/or to play with a form of vibrato known as bebung.
There is some confusion relating to the term pressure sensitive, with some using it as a synonym for velocity sensitive. To avoid this confusion, pressure sensitivity is sometimes called aftertouch.
Both velocity and (true) pressure sensitivity are supported independently by the MIDI standard.
In general, only the top of the line electronic keyboards implement true pressure sensitivity, while most (not all) recent electronic keyboards support velocity sensitivity. This has led to an unfortunate tendency for manufacturers and distributors of some inferior keyboards to describe their purely velocity sensitive instruments as pressure sensitive.
A third form of sensitivity is displacement sensitivity. Displacement sensitive keyboards are often found on organs. Most mechanical organs, and some electrically actuated organs, are displacement sensitive, i.e. you can press a key part way down, and the corresponding note (pipe, reed, or the like) in the organ will produce some lesser amount of sound than when the key is fully depressed. Small tabletop organs and accordions often respond similarly, with sound output increasing as keys are pressed further down. Even the small circular accompaniment ("one button chord") keys found on accordions and on some organs exhibit this phenomenon. Accordingly, some electrically actuated organs have retained this form of keyboard expression:
A 34-rank organ located in the Swiss village of Ursy is equipped with hi-tech features from Syncordia including the first non-mechanical action in history to directly control the opening of a pipe organs pallets in direct proportion to the movement of the keys thus ostensibly combining the virtues of electric action with the intimate control of tracker action.
Other more sophisticated forms of sensitivity are common in organ keyboards. For example, both the Pratt Reed and Kimber Allen 61-key (5-octave) keyboards have provision for installing up to nine rails, so that they can sense various amounts of displacement, as well as velocity in various regimes of distance from the top to the bottom of the key travel of each key.
Some modern instruments such as the Continuum, a midi controller for keyboards, have extremely sophisticated human interface schemes, allowing dynamic control in three dimensions.
It should be noted that, in principle, displacement can be differentiated to get velocity, but the converse is not entirely practical, without some amount of baseline drift. Thus a displacement sensing keyboard may have greater versatility when it is desired to have both organ and piano behaviour in an input device.
