A tendency mask shapes a stochastic parameter by making its random bounds move over time

A tendency mask (a technique from Xenakis and used by Tenney) constrains a random parameter not to fixed limits but to an upper and lower bound that each evolve over time. Notes are drawn uniformly between the two moving bounds, so the cloud of events can narrow, widen, drift, or sweep across the pitch or panning range as the piece unfolds. In SuperCollider this is written by giving Pwhite two Pseg-driven arguments — one Pseg is the moving low bound, the other the moving high bound — each interpolating through breakpoints over stated durations. The same mask idea applies to any dimension: pitch, pan, density, amplitude. It gives directed, sculpted randomness rather than the flat uniform noise of Pwhite with constant limits, and unlike a Pbrown random walk the successive values are independent — only the envelope of possibilities moves.

Examples

Pbind( \midinote, Pwhite( Pseg(Pseq([83, 108, 109, 25]), Pseq([12, 2, 3])), // moving low bound Pseg(Pseq([90, 108, 109, 42]), Pseq([12, 2, 7])) // moving high bound ), \dur, 1/100, \instrument, \saw ).play

Assessment

Write a Pbind whose pitch is a tendency mask that starts as a wide band and converges to a single note over 20 seconds. Collect and plot the generated midinotes. Contrast a tendency mask with a Pbrown random walk over the same range.