Pallophotophone

The pallophotophone (coined from the Greek root words pallo, to oscillate or shake; photo, light; and phone, sound, therefore literally meaning "shaking light sound") was a photographic sound recording and playback system developed by General Electric researcher Charles A. Hoxie circa 1922. The RCA Photophone sound-on-film system for motion pictures was later derived from it.

The pallophotophone was an optical sound system which could record and play back audio tracks on a strip of 35 mm black-and-white photographic film. Separate recording and playback units were employed. In recording, the sound waves vibrated a tiny mirror which reflected a ray of light through a narrow slit onto the moving film, creating a "sound track" that encoded the audio-frequency variations in air pressure as variations in the width of the track. After the film was developed, each track could be played by running it between a slit illuminated by a steady light and a photoelectric cell, converting the variations in track width into variations of light intensity and a similarly modulated electrical signal, which was electronically amplified and used to drive a loudspeaker or other device.

Surviving examples of pallophotophone recordings have several tracks recorded in parallel on each strip of film, and Hoxie's system has therefore been called the world's first multitrack recording system, as it predates magnetic tape multitrack recording by several decades. However, unlike later multitrack optical, magnetic, and digital sound recording systems, multiple tracks on pallophotophone films are not known to have been used for later mixdown or similar post-production purposes, or for simultaneously recording two or more channels for stereophonic sound reproduction. Multiple narrow tracks, recorded one at a time in separate passes through the device, simply made much more economical use of the medium by multiplying the total recording time possible on a given length of 35 mm film running at a given speed.

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