The MM-980 is a dual-function Analog Multiplier, featuring 2-quadrant and 4-quadrant amplitude modulation circuits. Or in short, Amplitude and Ring Modulation - great for those bell-like tones, robot voices... all your lovely 60s and 70s weird electronic sound effects probably came from something like this!

4-Quadrant A.M

Input two signals into X and Y, modify their gain with the X/Y Fixed factor controls, and you'll get an output signal that is... checks notes ... the sum and the difference of the input frequencies. Ring Modulation, essentially.

2-Quadrant A.M

Input two signals into X and Y, modify the gain with the X Fixed factor controls, and you'll get an output signal that is... checks notes ... the sum and the difference of the input frequencies as well as the original frequency. Amplitude Modulation.