How do audio signals recorded onto motion picture film as waveforms get converted into audio that can be heard, like in a movie, especially with different channels like stereo, and surround sound.
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Are you asking about actual analog film or modern digital movies?
For modern digital movies you have tons of separate audio tracks, and if say 5.1 surround sound then you can select which audio tracks go to which speakers. Dolby Atmos surround is more advanced and instead of which speakers you tell it where in 3D space the sound should come from and then depending on which of the dozens/hundreds of possible Atmos speaker layouts the listener has their own processor decides how to play those 3D positions (you still have bed/anchored positions, like dialogue is almost always locked to the center channel).
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For waveforms on film, you can read more on Wikipedia, but a light is shine through it and a reader captures that light, [here’s a diagram](https://www.granularsynthesis.com/hthesis/svdl7dl9.jpg), and based on how the waveform is it’ll change the light that passes thru, so the reader/decoder will know what the audio should be. It’s kinda ingenious (like imagine just one day thinking this up).
Heyo, film projectionist here!
The waveform-looking sound on 35mm film previously used in theaters is read by a soundhead that projects a slit of light through the film onto a light detector (there’s many different kinds, but you can think of it as a little solar panel). At any moment in time, there’s an amount of light going through the soundtrack, which the light detector picks up, and with the film moving at a constant speed, a continuously varying signal over time can reproduce the waveform on the film. With some basic amplification, that’s enough for some very simple mono audio!
Modern film usually has 2 waveforms on the film, which can produce surround sound through some very clever encoding and decoding using an analog matrix calculation. You can read about that matrix magic Dolby figured out [here](https://en.wikipedia.org/wiki/Dolby_Stereo#The_Dolby_Stereo_Matrix). It’s simultaneously pretty simple and also quite complicated.
Dolby also came up with the black magic that is Dolby Digital sound on film, as well as the application of noise reduction to the standard analog waveforms on the film to make it sound better.
On analog films, if you look closely at one, you’ll see a small stripe down one or both edges with areas of varying darkness and transparency. This is a sound wave recorded on film. When a light shines through it, the variations in light intensity are picked up by a photoreceptor on the other side, and converted into an electrical signal, which is amplified and played through speakers.
This was a common technique used in those 16 mm educational films shown in school classes in the 1960s through the 1980s.
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