What is MIDI aftertouch?
Aftertouch is the MIDI message a keyboard sends when you press harder into a key that is already down. It carries a pressure value from 0 to 127 and keeps updating for as long as you lean on the key. There are two kinds. Channel pressure sends one value for the whole keyboard, and polyphonic pressure sends a separate value for every key.
Both are part of the MIDI standard. I make InputConfig, a free Mac app that can use aftertouch as a trigger, so the end of the page covers that.
Read the two messages
| Kind | Status (hex) | Data bytes | Meaning |
|---|---|---|---|
| Channel pressure | D0 to DF | One: the pressure | One value for every note on the channel |
| Polyphonic pressure | A0 to AF | Two: the note, then the pressure | A value for one key |
D0 55 is channel pressure at 85 on channel 1. A0 3C 40 is pressure 64 on middle C alone. Channel pressure follows the hardest-pressed key, and polyphonic pressure follows each finger.
InputConfig is free on the Mac App Store. macOS 14 or later, no account, no tracking, open source.
Know which kind a keyboard sends
- Channel pressure. The common kind. One sensor strip under the keys reads the hardest press.
- Polyphonic pressure. Rarer, because it needs a sensor for every key.
- Neither. Many small and low-cost keyboards send no aftertouch at all.
- MPE controllers. They put each note on its own channel, so channel pressure becomes a value for each note.
If pressing harder into a held key changes nothing, the keyboard may simply not send it.
See whether yours sends it
- Open the MIDI tester in Chrome, Edge or Firefox.
- Press Start listening and allow MIDI.
- Hold a key down, then lean into it.
- Read the log: aftertouch lines mean channel pressure, and key pressure lines mean polyphonic.
No new lines while you lean on the key means no aftertouch.
Know what it usually controls
- Vibrato. Lean in and the note starts to waver.
- Filter and brightness. More pressure opens the sound up.
- Volume swells. A held chord grows as you press.
- Nothing, until you assign it. A synth only responds if its patch routes aftertouch to something.
Aftertouch sends many messages while a key is held, so a recorded part can fill with them. If a part does not use aftertouch, filtering it out keeps the recording small.
Use aftertouch as a trigger
In InputConfig, aftertouch can be the input of a row: it fires when you press harder into a held key. That gives a keyboard a second action under every note, such as a shortcut that runs only on a firm press. The Live Visualizer's MIDI layout has an aftertouch meter, so you can see the pressure arrive. A firm press can be a different button from a light one. MIDI to keystroke on a Mac lists every kind of control a row can listen to.
The short version
- What. Pressure on a key that is already down, 0 to 127.
- Channel. One value for the keyboard, status D.
- Polyphonic. One value for each key, status A.
Velocity is how hard a key was struck, and aftertouch is how hard it is being held.
Questions
What is the difference between channel and polyphonic aftertouch?
Channel aftertouch sends one pressure value for the whole channel, following the hardest-pressed key. Polyphonic aftertouch sends a separate pressure value for each key.
Is aftertouch a MIDI CC?
No. It has message types of its own: channel pressure, status D, and polyphonic key pressure, status A. Neither has a CC number.
What is the difference between velocity and aftertouch?
Velocity is measured once, when the key goes down. Aftertouch is measured for as long as the key is held and changes as you press harder or ease off.
Do all MIDI keyboards have aftertouch?
No. Many small and low-cost keyboards send none. Of those that do, most send channel aftertouch.
Why does aftertouch do nothing on my synth?
Either the keyboard does not send it, or the patch does not route it to anything. Check with a MIDI monitor first, then assign aftertouch to vibrato or the filter in the patch.
What is aftertouch used for?
Adding vibrato, opening a filter or swelling the volume of notes that are already sounding, without taking a hand off the keys.
Related
Free. macOS 14 or later. No account, nothing leaves your Mac. Open source on GitHub.
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