Aftertouch explained
Aftertouch – also known as pressure – is another type of MIDI controller message that keyboard controllers can generate. Like pitchbend, aftertouch isn’t grouped in with the continuous controllers that constitute CC numbers 1-127 (like pan or MIDI volume), but instead has its own classification as an independent type of MIDI signal. It produces information based on how hard you press the keys down after you’ve initially played them, and it’s this data that can be assigned to various parameters in your synth to control things – eg, changing the LPF cutoff frequency for making more realistic brass swells, altering the pitch of a guitar sample for string bend effects, or increasing the depth of an LFO for adding vibrato.
There are two types of aftertouch, the most common of which is channel aftertouch. This is when a series of force-sensing resistors beneath the keys of your controller measures the average pressure being applied to the keybed and produces a MIDI control signal based on this changing value. Effectively, this means that the harder you press, the higher the value of the generated signal. The second type, known as key or polyphonic aftertouch, relies on the same principle, but this time each key on the controller generates its own signal, outputting a value that corresponds to that key, rather than the average of the keyboard as a whole. This increases the expressive potential considerably, as every key can send different values as you play. This type of aftertouch used to be quite rare, owing to the amount of data it can generate, but is now supported by an increasing number of devices like Keith McMillen Instruments’ QuNexus and CME’s Xkey USB Mobile MIDI Keyboard.
Many synth presets will already have some kind of aftertouch response built in, but knowing how to add this facility to tailor the behaviour of a particular sound that doesn’t already feature it is definitely worth knowing.