Aftertouch

Aftertouch is a control signal used in electronic keyboards and synthesizers that conveys additional pressure on a key after it has been struck. It adds an expressive dimension beyond the initial key velocity, allowing real-time modulation of sound parameters such as pitch, timbre, or volume as the finger continues to press.

In the MIDI standard, there are two forms of aftertouch. Channel aftertouch (also called mono aftertouch) provides a single pressure value for the entire MIDI channel, affecting all notes on that channel. Polyphonic aftertouch reports pressure per individual key, enabling per-note modulation. Channel aftertouch is common on many keyboards, while polyphonic aftertouch requires per-key pressure sensing and consumes more MIDI data, making it less common on older gear.

Operation and usage: A keyboard with an aftertouch sensor detects the continuing pressure after the initial strike and sends corresponding aftertouch messages to a receiving device. The destination instrument or software can map this data to various expressive parameters, such as vibrato depth, filter cutoff, volume, or envelope stages. Aftertouch is distinct from velocity, which measures initial strike force, though many patches link the two for cohesive expression.

History and compatibility: Aftertouch was defined as part of the MIDI specification in the late 20th century. Channel aftertouch is widely supported across devices, while polyphonic aftertouch remains less prevalent due to higher hardware and bandwidth demands. Users should check individual instrument capabilities when aiming for per-note expressive control.

See also: MIDI, modulation, expression control, polyphonic aftertouch, channel pressure.