Design of the exPressure Pad
When we started to develop the exPressure Pad we made up the following criterion: “How can we explore and control complex electronic sound spaces in improvisation, retaining the millisecond interaction that is taken for granted in acoustic improvisation, but has somehow gotten lost in electronic music?”
The design of our instrument makes use of commercially available equipment: the M-audio Trigger Finger is a midi controller, which consists of an array of sixteen pads that sends velocity and pressure data, in addition to a number of faders and knobs, while mapping and sound generation take place within a Clavia Nord Modular G2.
In order to design an exploratory instrument such as the exPressure Pad, one must think in potential, rather then trying to imagine all possible combinations of parameter values. Therefore, we choose a vector implementation operating in a multidimensional musical space. Essentially, the design consists of a set of fifteen randomized vectors in a fourteen dimensional synthesis parameter space. Each individual pad (1-15 in Figure 3) on the interface are assigned a particular vector. All vectors add up in order to arrive at a single point in the parameter space of a monophonic sound. It is possible to explore a parameter space around the current point in all directions. Sound morphology, such as attack and decay times, are under direct control from designated knobs and faders.
Another dimension mention worth is pitch. Initially we mapped pitch from one vector component, which resulted in a lot of glissandi and did not made musical sense. Therefore, we designed an additive pitch algorithm, and superimposed it on the mapping engine; consecutive pads are assigned a chromatic scale, which start at the bottom left and increasing to the right and upwards. Simultaneously pressed pads (= intervals) add up and form a result interval, similar to the valves of a trumpet. This solution is compatible to our notion that higher pitches require more effort (engage more fingers), and the design of clavier instruments where low to high pitches goes from left to right. The interval sum is scaled by a secondary controller, which allows a continuum from no pitch control, via micro tuning to chromatic pitch.
The sound engine (Figure 4) consists of two intermodulating oscillators, high and low pass filters, comb filter, amplitude control, and reverb. Important musical parameters are controlled by vector components, which sum up to approx 30 synthesis and control parameters respectively. However, some parameters are under direct control from assigned knobs and faders, such as oscillator waveforms.