Digital and analog audio for movies is commonly recorded on film via optical means. Pretty freaking clever... I'm actually a little amazed it works so well!
http://en.wikipedia.org/wiki/Sound-on-film
Quote:
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Edge of a 35mm film print showing the soundtracks. The outermost strip (left of picture) contains the SDDS track as an image of a digital signal; the next contains the perforations used to drive the film through the projector, with the Dolby Digital track between them. The two tracks of the analog soundtrack on the next strip are variable-area (RCA Photophone), where amplitude is represented as a waveform. At present, these are generally encoded using Dolby SR matrixing to simulate four tracks. Finally, to the far right, the timecode used to synchronize with a DTS soundtrack CD is visible.
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What did you learn today?