The intent is to loosely emulate the pitch envelopes from the Acorn BBC microcomputer, but most people haven't heard of that nowadays (still not sure why I was given one for work, especially in the US). So here is a brief description of how envelopes work.
The pitch changes in steps; the length of time a step takes is adjustable using the Step Length slider. At each step, the pitch changes by the value specified in the Step Size 1 slider. The starting pitch is 89, which is middle C, and the pitch can go as low as 0 (about 72 Hz) to 255 (about 2890 Hz), and it wraps around if it is incremented above 255 or below 0. A pitch difference of 48 equals an octave.
After some number of steps (specified by the Step Count 1 slider), we finish section 1 and start section 2, where the Step Size 2 and Step Count 2 sliders control the size and number of steps. Then we go to section 3, and when that is finished we go back to section 1. If the step count for a specific section is 0, then we just skip that section.
Just play around with the sliders and you'll figure it out.
The sound is output using a pulse waveform. The Pulse Width slider controls the pulse width.
2/7/09: v1.1 fixed to work on java 2.