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.
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Content below with reference to Wikipedia
Aftertouch in it's original meaning has to do with a standard piano, either a grand or a vertical. The keystick (what one sees when looking at a piano) is a lever which activates the whippen. The whippen is the double escapement mechanism that actually includes the hammer which of course is what strikes the strings to make the noise. The whippen rests on the back portion of the keystick which pivots on a center rail containing guide pins appropriately called the Balance Rail. As one depresses the keystick at the front the rear lifts activating the whippen which ultimately swings the hammer to strike the strings. There is also a Front Rail which also has guide pins to keep the key motion straight and to regulate the amount of travel of the key. Aftertouch is the amount of key travel after the whippen releases the hammer to fly towards the strings and rebound into the check position. Often it is the regulating of the Aftertouch that gives the pianist the kinesthetic pleasure and interactive feedback when performing or practicing-of course this is in conjunction with fine regulation of the rest of the motions of the whippen.
[="http://www.rennerusa.com/VirtualActionModel.asp"]Here[/]="http://www.rennerus…"]Here[/] is a link to an interactive image of a typical grand piano key/note.
And this flash image is for a [[url=http://="http://www.koehler…"]vertical[/]="http://www.koehler…"]vertical[/] piano action.
A piano responds to the force with which the keys are initially pressed by changing how hard or fast the hammers strike the strings, which in turn changes the tone and volume of the resulting note or chord; it is velocity sensitive. Several of its predecessors, such as the harpsichord, were not velocity sensitive like the piano. 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 professional-quality electronic keyboards support velocity sensitivity. Most inexpensive electronic keyboards, such as toy electronic keyboards and basic learning keyboards made by Casio and Yamaha in the $100 USD price range do not have velocity sensitivity. Some manufacturers use the term "touch sensitive" in their advertising. Even on a touch-sensitive keyboard, not all digital instrument sounds may incorporate velocity sensitivity into the sound's envelope; for example, the digital pipe organ sound often has no velocity sensitive effects, in imitation of the real instrument. The manufacturers and distributors of some inferior keyboards incorrectly describe their purely velocity sensitive instruments as pressure sensitive.