This applet simply adds the top wave to the bottom wave and displays the result below. It also generates the Lissajous figure that starts/stops the phase drift on Wave B (a.k.a. the animation). The user is free to change all wave parameters, however caution is advised to at least meet the Nyquist Criteria (i.e. keep the sample rate at lest twice the frequency) or absolutely nothing of any consequence will happen.
Changes in amplitude are hidden by the auto scaling of the displays, but the right ratio should bring them out. You can also alter the wave's direction and the animation's speed by changing the Animation Phase Step. Changes in the FFT parameters are independent of each other since they are generated from the time sampled data for each wave. If you increase the fft size and get a blank screen, there might not be enough data to fill the buffer. Try increasing the time or sample rate or both. The FFT is a modified form of the "Numerical Recipes in C" algorithm. Errors in conversion are all mine.