ELI5, movies were made with photos that played very quickly, but how was sound added to that when it did?
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Originally, they didn’t Early movies were just the video, with either live musical accompaniment (rare) or a soundtrack played from a record. This obviously had a lot of issues, live music was expensive and didn’t allow for any sound other than music, and sound on disc was difficult to synchronize with the film. Even a fraction of a second off was a huge problem with talkies, so a system needed to be devised that allowed perfect synchronization.
Which brings us to your question about how they went about getting the sound onto the film. There were several different processes, with a patent filed as early as the 1880s, but the technology didn’t really become viable until the 1920s. In short, the process involves encoding the sound from a microphone directly onto a special strip down the side of the film. A light on the projector would then read that strip as it played, decoding it and playing the sound in a process essentially similar to playing a cassette tape, but coded using light instead of magnets.
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