Polyphonic AftertouchEdit
Polyphonic aftertouch is a form of expressive control in MIDI-equipped keyboards and controllers that allows a separate aftertouch value for each pressed key. In practice, it means a performer can bend, brighten, or otherwise modulate multiple notes independently after they are struck, rather than applying a single shared pressure to all notes on a channel. This capability expands the musical vocabulary of electronic instruments by making per-note pressure a real-time control parameter for dynamics, timbre, and articulation. For context, aftertouch exists in multiple forms within the broader MIDI framework, and polyphonic aftertouch is distinct from channel pressure in how it assigns pressure data to individual notes. See MIDI for the overarching standard and aftertouch for the general concept.
Polyphonic aftertouch relies on hardware capable of detecting pressure on each individual key (or finger position) and on software that can route that data to per-note modulation targets. The result is richer per-note control, enabling expressive techniques such as per-note vibrato depth, per-voice filter sweeps, or per-note amplitude and envelope modulation. The concept is closely related to, but technically separate from, monophonic aftertouch, which applies a single pressure value to all notes held on a given channel. See polyphonic key pressure and channel pressure for the two canonical forms of aftertouch in MIDI.
Technical background
What is aftertouch?
Aftertouch is pressure information that is transmitted after the initial note-on event. It serves as a secondary dimension of control, complementing velocity and other per-note parameters. In MIDI, there are two main forms: polyphonic key pressure (polyphonic aftertouch) and channel pressure (monophonic aftertouch). See aftertouch and MIDI for foundational details.
MIDI messages and data flow
- Polyphonic aftertouch (polyphonic key pressure) delivers a pressure value for a specific note, allowing each sounding note to carry its own expression data. The MIDI data path treats each note as an independent voice when aftertouch is active on that note. In the MIDI specification, this form is implemented as polyphonic key pressure messages.
- Channel pressure (monophonic aftertouch) sends a single pressure value that affects all notes on a given channel, creating a shared expressive gesture across multiple notes.
Because polyphonic aftertouch data scales with the number of simultaneously pressed keys, instruments and software must manage per-voice modulation targets in real time. See MIDI and polyphonic aftertouch in context with the related messages such as monophonic aftertouch and channel pressure.
Expression mapping and practical limits
Per-note aftertouch is most expressive when synths and effects are capable of mapping each note’s pressure to distinct parameters (e.g., per-voice vibrato depth, per-voice filter cutoff, or per-voice amplitude). However, it also demands more processing power and memory in the instrument’s voice architecture and can generate higher data workloads on MIDI streams. This trade-off helps explain why polyphonic aftertouch has been less ubiquitous in early hardware and remains more common on high-end or specialized instruments. See MPE for an approach to broader per-note expression.
Implementation and usage
Hardware considerations
A keyboard or controller must have per-key pressure sensing and the internal routing to emit polyphonic aftertouch messages. Some instruments provide both polyphonic aftertouch and channel pressure, while others support only one form or approximate polyphonic control via alternative expression features. See polyphonic key pressure for terminology and the hardware implications of per-note sensing.
Software and mapping
Synthesis engines and effects processors must expose per-voice modulation routes that can receive and discriminate per-note aftertouch data. This often involves per-voice envelopes, LFOs, or parameter mapping that can independently respond to each note’s pressure. In modern setups, systems designed around polyphonic expression—such as those employing MPE—facilitate richer multi-note control across a broader expressive spectrum.
History and adoption
MIDI includes two canonical forms of aftertouch, with polyphonic key pressure providing per-note data and channel pressure offering a single per-channel value. The capacity for per-note pressure has long existed in the MIDI specification, but practical adoption varies by instrument family and production era. In the late 1990s and early 2000s, manufacturers began expanding hardware capabilities to support more complex expressive control, and the later rise of expressive formats like MPE helped standardize and popularize per-note expression across both hardware and software ecosystems. See also MIDI and MPE for broader context on expressive control in modern music technology.