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 full details of how it worked were documented in Chapter 10 of my book Control the World with HP-IL.
