Sound on film is fascinating, and demonstrates a funny tension in analog formats: there's visible grain in the frames themselves, and yet a precise light source can encode massive amounts of data in just the spare emulsion between the sprockets.
Some old "Super 8" 8mm handheld movie cameras recorded audio magnetically on the side of the film, which is weird because the film itself is not magnetic at all. There's just a thin magnetic strip running along the edge. I've seen it on recordings from the 1970s, but it probably dates to the 1960s.
There was also stereo optical recordings but it was typically mono. You could hold the film up to a light source and see the inverse of the waveform. There was one place in town that I was aware of that had a stereo optical on their telecine unit. It wasn't that hi-fi though.
> The optical soundtrack on a Dolby Stereo encoded 35 mm film carries not only left and right tracks for stereophonic sound, but also—through a matrix decoding system (Dolby Motion Picture matrix or Dolby MP[1]) similar to that developed for "quadraphonic" or "quad" sound in the 1970s—a third center channel, and a fourth surround channel for speakers on the sides and rear of the theater for ambient sound and special effects. This yielded a total of four sound channels, as in the 4-track magnetic system, in the track space formerly allocated for one mono optical channel. Dolby also incorporated its A-Type noise reduction into the Dolby Stereo system.
That sounds like you might be thinking of the earliest form of surround sound, as was used for 2001 and some other films. This was developed to become Dolby Stereo in the late 70s, and Dolby Pro Logic for home cinema in the 80s.
Makes you wonder what resolution of video could be encoded digitally if you used the entire frame. With compression algorithms and a good encoding method, surely the image quality could be improved over the standard analogue print.
There is actually a reason to do this - and people do. For long term archival film is king - it's store and forget, unlike hard drives that require migration every few years. So people write digital data to film, either in the form of text documents as images, or some digtal encoding of the data, and then it's put into long term storage.
GitHub did this with its Archive Program https://archiveprogram.github.com/, and Piql https://www.piql.com/ is a company that specialise in doing this. They're the company that created the Cinevator, a high speed data to film transfer machine that was originally used for creating film prints, but since film is kinda dead these days, they've pivoted to archival storage.
