Aftertouch

Keyboard expression—often shortened to expression—is the ability of a keyboard musical instrument to respond to change tone or other qualities of the sound in response to velocity, pressure or other variations in how the performer depresses the keys of the musical keyboard. Expression types include:

Keyboard instruments offer a range of expression types. Acoustic pianos, such as upright and grand pianos, are velocity-sensitive—the faster the key strike, the harder the hammer hits the strings. Baroque-style clavichords and professional synthesizers are after-touch-sensitive—applied force on the key after the initial strike produces effects such as vibrato or swells in volume. Tracker pipe organs and electronic organs are displacement-sensitive—partly depressing a key produces a quieter tone.

The piano, being velocity-sensitive, responds to the speed of the key-press in how fast the hammers strike the strings, which in turn changes the tone and volume of the sound. Several piano predecessors, such as the harpsichord, were not velocity-sensitive like the piano. Some confuse pressure-sensitive with velocity-sensitive. To avoid this confusion, pressure sensitivity is often, perhaps usually, called aftertouch. The MIDI standard supports both velocity and aftertouch.

In general, only high-end 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 manufactured by Casio and Yamaha in the US$100 price range, do not have velocity sensitivity, but instead a manually-adjusted note volume.

Some manufacturers' advertising incorrectly uses the term "touch-sensitive" for velocity sensitivity. 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.

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