"Whether disc-based or photographically based, all successful motion-picture recording and reproduction systems used electrical amplification. But photographic systems -- that is, sound-on-film -- additionally required thermionic photoelectric cells. Recording sound on film depends on a highly controllable light source. Photoemissive cells, among many experimental uses, were gradually adapted for recording this kind of sound track. These gas-filled vacuum tubes transform the electric current produced by microphones into a pulsating light source which exposes photographic film stock in proportion to the intensity of the original sound. General Electric scientists working between 1911 and 1913 discovered that filling these tubes with heavy gases such as argon caused them to give off light when charged. Lee de Forest, probably influenced by this research, made a light cell he called the Photion, using a gas-filled tube. De Forest tried to adapt the instrument for recording sound on a photographic negative. The glowing light exposed the moving film, but the results were not very good. He then learned of an improved light emitter developed by Theodore Case and his collaborator Earl I. Sponable in which a quartz tube containing an oxide-coated wire, a metal plate, and helium gas glowed proportionately to the voltage supplied. Case had named his invention Aeo, the acronym for alkaline earth oxide. Unlike the filament of an ordinary light bulb or the Photion, the Aeo light’s short glow decay made possible more accurate recordings of higher sound frequencies. These tubes could be installed inside a camera to record picture and sound simultaneously (single system) or inside a separate “sound camera” which could be synchronized with the picture camera (double system). When the moving film stock was exposed by the pulsating intensity of the light, the result was a “variable density” photographic sound track. On 23 July 1926, after Case had broken with de Forest, he teamed up with the movie magnate William Fox to exploit the Aeo light in what would be called the Fox-Case Movietone system. The tube was replaced in 1929 by the Western Electric light valve (a nonthermionic device discussed later)."
I had sometimes wondered how sound was recorded onto film and played back. The genius and ingenuity of these pioneers is phenomenal.
