Software-based musical instruments have controls for input, a sound synthesizer for output,
and mappings connecting the two. An effective layout of controls considers how many degrees of
freedom each has, as well as the overhead of selecting each one while performing. An isolated
mapping from one control to one synthesis parameter needs an appropriate choice of proportional,
integral or derivative control (the control's value, or that value's rate of change, drives the
synthesis parameter's value, or that value's rate of change). Beyond this, a compound mapping
cross-coupling several controls and synthesis parameters can surprisingly increase the performer's
intuitive understanding of the instrument.