two sine tones, summed and pushed through a nonlinear stage (a speaker driven hard, the inner ear, a fuzz circuit all shrug the same way), and pitches appear that neither oscillator is playing. Giuseppe Tartini heard the difference tone |fA−fB| on solo violin double-stops in 1714 and called it his "terzo suono," the third sound. in this signal the cubic tones 2A−B and 2B−A come out strongest, about 12 dB above the rest at the default gains, and the sum fA+fB matches the difference exactly, because both come from one squared term · in the ear the sum is the hardest to hear. none of it is synthesized separately · it is in the signal, the same intermodulation math your ear does to any two loud tones.
0 ghost = clean linear mix, no ghosts possible, and the readout above goes empty to match · the two gains are scaled so their sum never passes full scale, so 0 stays clean at any setting · turn it up to fold the sum through a soft asymmetric curve · all four derived tones arrive together, the moment it leaves 0 · "stop" (or esc) fully kills both oscillators and closes the audio graph