How is sound recorded on motion picture film?

The phrase "optical soundtrack" sounds like a contradiction in terms, but it's the secret to how sound is recorded on motion picture film.

How is the audio recorded for a movie? - Quora

THE SOUNDMAN 1940s PRODUCTION SOUND RECORDING FOR MOTION PICTURES EDUCATIONAL FILM 64144

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How is sound recorded on motion picture film?

Bell telephone magazine . se the new method. Recording Sound on Film During the developments leading upto the orthophonic phonograph, otherWestern engineers steadily advancedthe art of recording sound on film. Donald

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ISO 1188:1984, Cinematography - Recorded characteristic for magnetic sound on full-coat 16 mm motion-picture film - Specifications: ISO TC 36/WG 3: : Books

Stereophonic sound - Wikipedia

Kodak Basic Magnetic Sound Recording For Motion Pictures D.D.Teoli Jr. A.C. : D.D.Teoli Jr. A.C. : Free Download, Borrow, and Streaming : Internet Archive

How come it's impossible for me to find sound footage in videos made before the 1920s? Did cameras not have the ability to record sound until the mid-20th century? - Quora