Back in the olden days, "multimedia" meant two slide projectors, a dissolve unit (so one image would "dissolve" into the next) and synchronized sound track. But back in 1984 there weren't any dissolve controllers that met my needs, so I did what any self-respecting engineer would do - I designed and built my own circuitry, and programmed my HP-71 calculator (MUCH more powerful than the HP-41 previously mentioned!) in assembly language just to put on a show that moved people emotionally (like this one: https://friedmanarchives.com/peace-child-in-lativia/ ).
The system emplyed a stereo cassette player (not shown); where the audio was on the left track, and the touch tone sounds (the same sounds you hear when dialing on a pushbutton phone) were on the right track. The touch tone sounds, when detected, would tell the computer when to start dissolving and how quickly. The system used continuously-changing pulse-width modulation to control the intensity of the two projector bulbs, and to get it to work fast enough I had to surgically alter the HP-71 to bring out several lines directly from the CPU.
The full details of how it worked were documented in Chapter 10 of my book Control the World with HP-IL.
