Aftertouch is a term coined by Yamaha to describe the touch sensitivity of its earliest synthesizers. They used the names initial touch for velocity and after touch for what was commonly called pressure. The massive popularity of the Yamaha DX7 led to the word "aftertouch" becoming the most popular generic name for this type of modulation control.
A pressure sensor (called a touch sensor by ARP, a force sensor or force bar by Moog, etc.) is a device that runs along the width of a keyboard, under the keys. When the player pushes down on a key after playing a note, the sensor sends a control voltage that gets higher as the player presses harder.
The first synthesizer with a pressure sensor was the ARP Soloist, soon replaced by the more reliable ARP Pro Soloist. It quickly became a vital element in the keyboard rigs of many musicians, with a sound and feature set that were highly sought after. (How highly? So highly that Cherry Audio made a software version – and there was much rejoicing! Yaaaay!)
Pressure sensitivity was a much-appreciated adjunct to, or replacement for, other forms of note modulation like pitch benders or modulation wheels. Remember that at the time (and even today), many synthesizers were stacked on top of organs or acoustic or electric pianos – still the primary instrument for most pop keyboardists. A pressure sensor meant that instead of playing with the right hand and working the wheels with the left hand, you could add modulation in a very natural way with the hand that was actually playing the notes, freeing up the other hand – left or right – to play your other keyboards.
From the very beginning, instruments with pressure sensors offered many different ways the sensor could affect the timbre. For example, the Pro Soloist had a row of six switches, each of which assigned a different sort of modulation to the touch sensor, and the Sequential Circuits Prophet T8 offered a choice of seven destinations with an overall Amount knob.
In keeping with this tradition of aftertouch flexibility, ODC 2800 makes it easy to assign up to 3 aftertouch modulations at once, each with its own destination and positive/negative scaling.
Click the Assign button under any slider to bring up the Aftertouch Modulation Menu:
Unlike the Modulation Source Menu accessed from other Assign buttons on the front panel, this menu contains modulation destinations to be controlled by aftertouch. From the menu and submenus, you can assign any of the following destinations:
Volume
VCF Cutoff
VCF Resonance
Pitch, Pulse Width, or Volume of any of the three VCOs
Rate or Pulse Width of either of the two LFOs
VCO vibrato, modulated by either of the two LFOs
Pulse Width vibrato, modulated by either of the two LFOs
VCF vibrato, modulated by either of the two LFOs
VCA tremolo, modulated by either of the two LFOs
This modulation adjustment can be either positive (pressure turns the modulation up from its preset value) or negative (pressure turns the modulation down from its preset value).
As noted previously, the ASSIGN label underneath the button is replaced with a label showing the selected destination.
One last thing: there are actually two different types of aftertouch, and ODC 2800 supports them both, depending on whether your controller transmits them.
Monophonic Aftertouch (monoAT), called Channel Pressure in the MIDI specification, originated with the ARP Soloist and was featured on hundreds of other synthesizers that came after it. In monoAT, there's a single sensor placed under the keybed that reaches from the lowest to the highest note. It's activated when you press down on any key, and the pressure on one key will affect the sound for all keys being played at any given time.
For a monophonic synth, monoAT is obviously not a limitation at all. On polyphonic synths, a single aftertouch modulation for all notes isn't any more "limiting" than having only one mod wheel or pitch bender for the whole keyboard, and players have been composing and performing with it happily for over 50 years now.
Polyphonic Aftertouch (polyAT), called Key Pressure in the MIDI spec, was pioneered* on the Yamaha GX1 and CS-80 in the 1970s, appeared on a few synths like the Yamaha DX1 and the Sequential Circuits Prophet-T8 at the dawn of the MIDI era in the early 1980s, was quickly abandoned (largely for technical reasons) by all but a stubborn few companies like Ensoniq and GEM, and was out of production and virtually forgotten by the 1990s.
Recently, new technologies have helped polyAT make a big comeback on the keyboards of instruments and controllers like the ROLI Seaboard, Linnstrument, ASM Hydrasynth, and Waldorf Iridium. A polyAT keyboard has a sensor under each key, so that you can vary the pressure and change the sound on individual notes while leaving other notes alone.
ODC 2800 will respond to monophonic aftertouch and polyphonic aftertouch, provided your USB/MIDI controller is capable.
(*We're not counting the clavichord and orphica from the 1600s because for those instruments -- and the Mellotron! -- polyAT was an accidental side effect that wasn't very good for the instrument if you did it too much.)
