By Craig Anderton
Synthesizer keyboards are basically a series of on-off switches, so wresting expressiveness from them is hard. There’s velocity, which produces dynamics based on how fast the key goes from key up to key down; you also have mod wheel, footpedal, pitch bend, and usually one or two sustain switches, all of which can help with expressiveness. But some keyboards have an additional, and powerful, way to increase expressiveness: Aftertouch, also called Pressure.
THE TWO KINDS OF AFTERTOUCH
Aftertouch is a type of MIDI control signal. Like pitch bend, it’s not grouped in with MIDI Continuous Controller signals but is deemed important enough to be its own dedicated control signal. It produces an output based on how hard you press on the keys after they’re down. There are two types of aftertouch:
Channel aftertouch (or pressure). This is the most common form of aftertouch, where the average pressure being applied to the keys produces a MIDI control signal. More pressure increases the value of the control signal. From a technical standpoint, the usual implementation places a force-sensing resistor under the keyboard keys. Pressing on this changes the resistance, which produces a voltage. Converting this voltage to a digital value produces MIDI aftertouch data.
Key (or polyphonic) aftertouch (or pressure). Each key generates its own control signal, and the output value for each key corresponds to the pressure being applied to that key.
Fig. 1: Keith McMillen Instruments QuNexus is a compact keyboard with polyphonic aftertouch.
The late, great synthesizer manufacturer Ensoniq made several keyboards with key aftertouch, but the company is no more. Another concern is that key aftertouch is data-intensive, because every key produces data. In the early days of MIDI, this much data often “choked” MIDI sequencers running on old computers that couldn’t keep up. Although many virtual synthesizers (and even hardware ones) can accept key aftertouch data, most likely you’ll be using a keyboard with channel aftertouch.
Back then even channel aftertouch could produce too much data, so most MIDI sequencers included MIDI data filters that let you filter out aftertouch and prevent it from being recorded. Most DAWs that support MIDI still include filtering, and for aftertouch, this usually defaults to off. If you want to use aftertouch, make sure it’s not being filtered out (Fig. 2).
Fig. 2: Apple Logic (left) and Cakewalk Sonar (right) are two examples of programs that let you filter out particular types of data, including aftertouch, from an incoming MIDI data stream.
Depending on the keyboard, the smoothness of how the aftertouch data responds to your pressure can vary considerably. Some people refer to a keyboard as having “afterswitch” if it’s difficult to apply levels of pressure between full off and full on. However, most recent keyboards implement aftertouch reasonably well, and some allow for a very smooth response.
A final issue is that many patches don’t incorporate aftertouch as an integral element because the sound designers have no idea whether the controller someone will be using has aftertouch. So, most sounds are designed to respond to mod wheel, velocity, and pitch bend because those are standard. If you want a patch to respond to aftertouch you’ll need to decide which parameter(s) you want to control, do your own programming to assign aftertouch to these parameters, and then save the edited patch.
Now that you know what aftertouch is and how it works, let’s consider some useful applications.
Add “swells” to brass patches. Assign aftertouch to a lowpass filter cutoff, then press harder on the keys to make the sound brighter. You may need to lower the initial filter cutoff frequency slightly so the swell can be sufficiently dramatic. You could even assign aftertouch to both filter and to a lesser extent to level, so that the level increases as well as the brightness.
Guitar string bends. Assign aftertouch to pitch so that pressing on the key raises pitch—just like bending a string on a guitar. However, there are two cautions: Don’t make the response too sensitive, or the pitch may vary when you don’t want it to; and this works best when applied to single-note melodies, unless you want more of a pedal steel-type effect.
Introduce vibrato. This is a very popular aftertouch application. Assign aftetouch to pitch LFO depth, and you can bring vibrato in and out convincingly on string, guitar, and wind patches. The same concept applies to introducing tremolo to a signal.
“Bend” percussion. Some percussion instruments become slighter sharp when first struck. Assign aftertouch to pitch; if you play the keys percussively and hit them hard, you’re bound to apply at least some pressure after the key is down, and bend the pitch up for a fraction of a second. This can add a degree of realism, even if the effect is mostly subliminal.
Morph between waveforms. This may take more effort to program if you need to control multiple parameters to do morphing. For example, I use this technique with overdriven guitar sounds to create “feedback.” I’ll program a sine wave an octave or octave and fifth above the guitar note, and tie its level and the guitar note’s level to aftertouch so that pressing on a key fades out the guitar while fading in the “feedback.” This can create surprisingly effective lead guitar sounds.
Control signal processors. Although not all synths expose signal processing parameters to MIDI control, if they do pressure can be very useful—mix in echoed sounds, increase delay feedback, change the rate of chorusing for a more randomized effect, increase feedback in a flanger patch, and the like.
I’d venture a guess that few synthesists use aftertouch to its fullest—so do a little parameter tweaking, and find out what it can do for you.
Craig Anderton is Editor Emeritus of Harmony Central. He has played on, mixed, or produced over 20 major label releases (as well as mastered over a hundred tracks for various musicians), and written over a thousand articles for magazines like Guitar Player, Keyboard, Sound on Sound (UK), and Sound + Recording (Germany). He has also lectured on technology and the arts in 38 states, 10 countries, and three languages.